Seed dispersal, germination and fine-scale genetic ...eprints.qut.edu.au/16441/1/Peter_Prentis_Thesis.pdf · Seed dispersal, germination and fine-scale genetic structure in the stream

Queensland University of Technology

School of Natural Resource Sciences

Seed dispersal, germination and fine-scale

genetic structure in the stream lily,

Helmholtzia glaberrima (Philydraceae)

Peter Prentis B.Sc. (Hons).

Submitted in fulfilment of the requirements

for the degree of Doctor of Philosophy

1

July 2005

Abstract

Seed dispersal in aquatic habitats is often considered to be a complex multistage

process, where initial seed shadows are redistributed by water (hydrochory). The roles

of hydrochory in seed dispersal and influencing population genetic structure were

examined in Helmholtzia glaberrima using both ecological and genetic techniques.

Ecological experiments showed that water can redistribute seeds and seedlings over

local scales and that hydrochory can provide the potential for very long distance seed

and seedling dispersal. Patterns of seedling genetic structure were affected by micro-

drainages that direct water flow within populations and influence water-borne seed

dispersal on a local scale. Strong non-equilibrium dynamics and persistent founder

effects were responsible for the patterns of genetic structure observed among

established populations of H. glaberrima. Classical metapopulation models best

described dispersal patterns, while water-borne seed dispersal could potentially explain

patterns of genetic differentiation within a stream system, it could not explain the

distribution of genetic variation among stream systems. The current study found that

although hydrochory influenced seed dispersal and seedling genetic structure within a

population, it had little effect on the spatial pattern of genetic variation among

established populations of H. glaberrima. Moreover, even though prolonged buoyancy

and viability in water provide the potential for long-distance hydrochory, results

presented here do not support the hypothesis that flowing water is an effective long

distance seed dispersal vector for H. glaberrima. Taken together, these results suggest

that the relative importance of gene flow via water-born seed dispersal in H. glaberrima

may be low compared with that of some other riparian species.

2

Keywords: genetic diversity; Helmholtzia; hydrochory; seed dispersal

3

List of manuscripts

1. Prentis, P. J., A. Vesey, N. M. Meyers, and P. B. Mather 2004. Genetic structuring

of the stream lily Helmholtzia glaberrima (Philydraceae) within Toolona Creek,

south-eastern Queensland. Australian Journal of Botany 52: 201—207.

2. Prentis, P. J., N. M. Meyers, and P. B. Mather. In press. The significance of post-

germination buoyancy in Helmholtzia glaberrima and Philydrum lanuginosum

(Philydraceae). Australian Journal of Botany.

3. Prentis, P. J., N. M. Meyers, and P. B. Mather. (In Prep) Seed dispersal and

seedling establishment in the riparian plant Helmholtzia glaberrima. Freshwater

Biology.

4. Prentis, P. J., N. M. Meyers, and P. B. Mather. (In Review) Micro-geographic

landscape features demarcate seedling genetic structure in the stream lily,

Helmholtzia glaberrima (Philydraceae)1. American Journal of Botany.

5. Prentis, P. J., N. M. Meyers, AND P. B. Mather. (In Review) Fine-scale patterns of

genetic diversity and population structure in the stream lily Helmholtzia

glaberrima (Philydraceae) along rainforest streams, south-east Queensland.

Freshwater Biology.

4

Table of Contents

Abstract 1

List of Manuscripts 3

Table of Contents 4

Statement of Original Authorship 7

Acknowledgments 8

Chapter 1: GENERAL INTRODUCTION

Introduction 9

Spatial and temporal dynamics of aquatic habitats 10

Seed dispersal in freshwater habitats 12

Colonisation of isolated habitat patches via long-distance hydrochory 16

Other mechanisms affecting long-distance seed transport 17

Study species and system 22

Account of research progress linking manuscripts 24

References 26 Chapter 2: GENETIC STRUCTURING OF THE STREAM LILY Helmholtzia

glaberrima (PHILYDRACEAE) WITHIN TOOLONA CREEK, SOUTH-EAST

QUEENSLAND

Statement of Joint Authorship 34

Manuscript 1 35

Introduction 37

Materials and Methods 40

Results 43

Discussion 45

References 50

5

Tables and Figures 54

Chapter 3: THE SIGNIFICANCE OF POST-GERMINATION BUOYANCY IN

Helmholtzia glaberrima AND Philydrum lanuginosum (PHILYDRACEAE)


Manuscript 2 62

Introduction 64


Results 72

Discussion 74

References 78


Chapter 4: SEED DISPERSAL AND SEEDLING ESTABLISHMENT IN THE RIPARIAN

PLANT Helmholtzia glaberrima


Manuscript 3 85

Introduction 88


Results 94

Discussion 96

References 100


Chapter 5: MICRO-GEOGRAPHIC LANDSCAPE FEATURES DEMARCATE

SEEDLING GENETIC STRUCTURE IN THE STREAM LILY, Helmholtzia glaberrima

(PHILYDRACEAE)


6

Manuscript 4 106

Introduction 108


Results 112

Discussion 114

References 118


Chapter 6: FINE-SCALE PATTERNS OF GENETIC DIVERSITY AND POPULATION

STRUCTURE IN THE STREAM LILY Helmholtzia glaberrima (PHILYDRACEAE)

ALONG RAINFOREST STREAMS, SOUTH-EAST QUEENSLAND


Manuscript 5 127

Introduction 129


Results 134

Discussion 136

References 140


Chapter 7: GENERAL DISCUSSION General Discussion 147

References 153

7

Statement of Original Authorship

This work contains no material which has been accepted for the award of any other

degree or diploma in any university or other tertiary institution and, to the best of my

knowledge and belief, contains no material previously published or written by another

person, except where due reference has been made in the text.

Signed

Date

8

Acknowledgments

Firstly, I would like to thank my supervisors for your guidance and encouragement in

helping me complete this project. Many thanks must go both to the ecology and genetics

discussion groups for fruitful discussions on sampling designs and data. My parents

deserve many thanks for their support of my chosen path and providing guidance during

the gloomy PhD blues.

A special thankyou to all the people who volunteered to help me with field and lab work,

particularly Ana Pavasovic, Cameron Schulz, Mark Schutze, Doug Harding, Alex Wilson

and Grant Hamilton. Big thanks must go to Dr Graham Kelly the man whose lectures

and infectious enthusiasm are responsible for my interest in plant biology. Lastly and

mostly importantly, I have to thank my partner Ana for her support, encouragement and

cooking without which I would be a lonely, thin man.

9

CHAPTER 1. General introduction

Introduction

How seeds disperse among isolated habitat patches, establish and

contribute to the genetic pool of new plant populations has long fascinated

botanists (Darwin, 1859). This is because dispersal is one of the primary

processes that influence population dynamics and evolution of plants (Nathan &

Muller-Landau, 2000). In particular the spatial and temporal dynamics of plant

populations will be determined chiefly by the movement of seeds within and

among populations (Husband & Barrett, 1998). At a broader geographic scale,

the range at which seed dispersal is effective will influence the possibility that

extirpated populations are recolonised and the probability that new isolated

habitat patches are colonised (Cain et al., 2000).

Plant species are rarely distributed uniformly in space but often occur as

isolated local populations where favourable conditions for successful

establishment exist (Edwards & Sharitz, 2000; Ellison & Parker, 2002). Dispersal

patterns within and among local populations will determine the extent to which

local populations are interconnected via gene flow (Ouborg et al., 1999). Gene

flow will only occur however, if the seeds dispersing among populations establish

and contribute to future reproduction in the new population (Ouborg et al., 1999).

The level of gene flow among patches will determine the distribution of genetic

variation within and among local populations and whether they function

collectively, or as isolated units (Tero et al., 2003). In situations where dispersal

10

rates among populations are high, gene flow will tend to homogenise gene

frequencies among local populations, so they form a single panmictic unit

(Ouborg et al., 1999). Alternatively, if gene flow is very low or absent, local

populations are likely to diverge and evolve independently due to the diversifying

forces of drift and selection (Tero et al., 2003). A number of factors can influence

the level of seed dispersal and gene flow that occurs among isolated local

populations, including temporal and spatial distribution of favourable habitat

patches, the type of vectors that disperse seeds and the individual life-history

characteristics of a plant species (Pannell and Charlesworth, 1999, 2000). This

review will focus primarily factors acting on aquatic plants restricted to freshwater

habitats.

Spatial and temporal dynamics of aquatic habitats

Many studies have reported that habitat patches are often dispersed

heterogeneously across the landscape (Husband & Barrett, 1998; Levins, 1969;

McCauley, 1989). Spatial discontinuities, including mountains, valleys or rivers

can influence significantly the distribution of many organisms (Lawton, 1993).

Most organisms do not exhibit uniform distributions in space (Lawton, 1993).

Instead organisms frequently occur as isolated local populations confined to

suitable habitat surrounded by a matrix of less favourable habitat (Andrewartha &

Birch, 1954; Dejong, 1995; Dupre & Ehrlen, 2002). This is particularly true for

organisms with narrow habitat requirements such as freshwater aquatic plants,

whose preferred habitat is often distributed like naturally occurring islands in a

11

sea of terrestrial habitat (Barrett, 1985; Husband & Barrett, 1998; Santamaria,

2002).

Freshwater aquatic plant species are restricted to permanent, seasonal or

ephemeral aquatic environments, such as rivers, lakes or ponds (Edwards &

Sharitz, 2000; Ellison & Parker, 2002). Most freshwater aquatic habitats occur as

discrete habitat patches often separated by hundreds or thousands of metres of

unfavourable areas (Barrett et al., 1993). As a result, populations of many

aquatic plant species have spatially disjunct distributions. Freshwater aquatic

habitats in most landscapes are also highly heterogeneous at several spatial and

temporal scales, meaning that the distribution of many aquatic plant species can

also be temporally unstable (Husband & Barrett, 1995, 1998).

Most freshwater aquatic habitats are not static as they are created and

replenished by precipitation or runoff (Brock & Rogers, 1998; Hampe, 2004;

Lopez, 2001; Scarano et al., 2003; Van der Valk, 1981). As rainfall is not usually

evenly distributed in space or time, temporal and spatial fluctuations in water

level occur in freshwater habitats particularly in areas with seasonal climates

(Hampe, 2004; Husband & Barrett, 1998). Consequently, many aquatic habitats

are ephemeral or experience dramatic water level fluctuations among seasons

depending on the frequency and duration of rain or runoff from catchments

(Brock & Rogers, 1998). In periods of heavy rain, freshwater habitats may flood

and connect to other isolated wetland patches; however during drought many

aquatic habitats may disappear. As freshwater aquatic habitats can be

12

ephemeral, aquatic plants inhabiting them may experience alternating wet and

dry conditions (Lopez, 2001).

Alternating wet and dry conditions can influence the persistence of aquatic

plant populations in particular survival during the phase from seed release to

seedling establishment (Husband & Barrett, 1998; Lopez, 2001). Due to the

unpredictable nature of habitats occupied by most aquatic plants, individual

species may have only a short window of opportunity for seed dispersal,

germination and seedling establishment (Hampe, 2004). Many wetland plants,

however, have evolved unusual adaptations to water level changes or are

capable of variable responses (plasticity), promoting survival in heterogeneous

environments (Arber, 1920; Dorken & Barrett, 2004a; Santamaria et al., 2003;

Sculthorpe, 1967; Wells & Pigliucci, 2000). Thus seed dispersal, germination and

seedling establishment in aquatic plants may be less constrained by fluctuations

in water levels and alternating wet and dry conditions than had previously been

thought (Lopez, 2001). Traits that favour increased seed floatation during

flooding, seed survival in water and/or seedling establishment under flooded

conditions should be advantageous to aquatic or riparian plants.

Seed dispersal in freshwater habitats

Patterns of seed dispersal and seedling establishment have important

effects on the dynamics and persistence of plant populations (Barrett et al.,

1993). Dispersal of seeds provides an opportunity for individual plants or

populations to recruit because some seeds are likely to arrive at suitable

13

microhabitats for seedling establishment, thus increasing their potential for

survival (Nathan & Muller-Landau, 2000). Seed dispersal is often a complex

multistage process, with seeds known to be dispersed by a variety of vectors

(primary and secondary; sensu Vander Wall & Longland, 2004) over a number of

different spatial scales (local and long-distance) (Chambers & McMahon, 1994;

Hampe, 2004). Local and long-distance dispersal in complex seed dispersal

systems can often affect different components of the demography and dynamics

of individual plants and populations (Cain et al., 2000).

Seed dispersal in aquatic habitats often occurs as a multistage process,

complicated by each individual plant’s life history characteristics (Hampe, 2004).

Broadly we can classify aquatic plants into four life-history categories; 1) semi-

aquatic/riparian species, 2) emergent aquatic species 3) free-floating aquatic

species and 4) submerged aquatic species (Sculthorpe, 1967). For the purposes

of this review we will concentrate principally on emergent and semi-

aquatic/riparian species. Early life-history traits that affect seed dispersal in

emergent and semi-aquatic species can be quite similar (Boedeltje et al., 2003).

For example many emergent and semi-aquatic wetland species exhibit a limited

capacity for primary seed dispersal due to their reliance on gravity as a vector

(Waser et al., 1982).

The mean dispersal distance for the herbaceous wetland perennial

Sarracenia purpurea, was 12.8 cm with 78% of seeds caught within five

centimetres of focal plants (Ellison & Parker, 2002). Similarly short primary

dispersal distances have also been found in many other wetland herbs (Cain et

14

al., 1998). Reliance solely on short-distance primary seed dispersal in these

species should constrain the potential for movement of seeds among patches,

yet propagules are known to have established in isolated sites following rare

dispersal events across large tracts of unsuitable habitat (Ellisson & Parker,

2002). Understanding dispersal mechanisms that facilitate these rare events is

the first step in understanding colonisation, establishment and dynamics of

spread in emergent or semi-aquatic plant species that otherwise possess limited

dispersal capacity.

Many riparian and emergent aquatic species exhibit prolonged seed

buoyancy (Boedeltje et al., 2003). Consequently seeds in wetland species can be

redistributed by water (Hampe, 2004). Redistribution of seeds in fresh water may

affect the spatial pattern of the seed shadow generated by primary seed

dispersal vectors (Hampe, 2004). Prolonged seed buoyancy may enhance the

potential range of seed dispersal, possibly conferring the capacity to colonise

isolated habitat patches. Redistribution of seeds by fresh water however,

depends strongly on the amount of rainfall or runoff received between seed

release and seedling establishment (Hampe, 2004). For example, if seed release

coincides with dry conditions then there is limited or no opportunity for seed

dispersal by water in ephemeral wetlands. Alternatively if rainfall does occur

during times of seed release then redistribution of seeds by water is highly likely.

Dispersal via primary and secondary dispersal vectors are poorly understood,

particularly systems where initial seed shadows are redistributed by freshwater

(Hampe, 2004).

15

The relative contribution that hydrochory makes to secondary dispersal

and establishment of wetland species remains contentious. Most likely, in many

freshwater herbs water-born seed dispersal contributes little to long-distance

dispersal and if it does, it represents a rare event (Ellison & Parker, 2002). Some

investigators have found evidence that hydrochory can result in effective long-

distance seed dispersal at scales of hundreds to thousands of metres (Kinamoto

et al., 2005; Kudoh & Whigham, 1997; Lonsdale, 1993; Nilsson et al., 1991;

Ridley, 1930). This indicates that in at least some emergent or riparian species

long-distance water-borne seed dispersal is possible. Consequently, rare long-

distance hydrochory may explain colonisation of isolated habitat patches in both

emergent and semi-aquatic species (Tero et al., 2003).

Redistribution of seeds via water however may affect seed germination

and seedling recruitment. Water-borne seed dispersal can affect seed

germination or establishment in at least three ways; first, if seeds dispersed by

water are not flood tolerant then immersion in water for prolonged periods may

reduce seed viability and germination (Edwards et al., 1994; Lopez, 2001;

Middleton, 2000; Scarano et al., 2003). Second, redistribution of seeds by water

will determine if seeds are deposited in suitable microhabitat conditions for

germination and establishment (Boedeltje et al., 2004; Ozinga et al., 2004; Van

der Valk, 1981). Last, if water-born dispersal changes the density of the initial

seed shadow, germination and establishment may be negatively or positively

affected by density dependent processes such as mortality (Debussche &

Isenmann, 1994; McMurray et al., 1997; Nathan & Casagrandi, 2004; Orth et al.,

16

2003; Russo & Augspurger, 2004). Therefore, how seeds of emergent or riparian

species respond to water-borne dispersal and where they end up (deposition

site) will influence potential for colonisation of isolated habitat patches (Nathan &

Muller-Landau, 2000).

Colonisation of isolated habitat patches via long-distance hydrochory

Populations of emergent or riparian plant species often occur in spatially

isolated habitat patches (Edwards & Sharitz, 2000). Many hundreds to

thousands of metres may separate individual wetland habitat patches (Ellison &

Parker, 2002). Thus colonisation of suitable isolated patches will require effective

long-distance dispersal. The capacity of seeds from emergent or riparian species

immersed in water to float for long periods of time (Boedeltje et al., 2003), may

represent an adaptation to long-distance water-borne dispersal. For example,

Fridriksson (1975) found that species with adaptations that promoted buoyancy

were more likely to disperse over distances of greater than 20 km on ocean

currents to colonise the newly emerged volcanic island of Surtsey. In fact, over

78% of the vascular plant taxa that arrived on Surtsey between 1963 and 1972

were known to be dispersed by water (Fridriksson, 1975). This demonstrates that

water-borne dispersal can be an effective method for long-distance dispersal in

plants and the colonisation of isolated habitats. It is unknown however, if

secondary seed dispersal by flowing freshwater can disperse seeds over

relatively long distances allowing colonisation of isolated habitats.

17

Experiments have shown that seeds of many wind dispersed species,

often float and remain viable in runoff, streams and rivers (Ridley, 1930). It is

thought in some of these instances, that seeds can be dispersed much greater

distances via water than by wind (Carlquist, 1967). Furthermore, a recent study

by Hampe (2004) found that secondary dispersal by water changed the

distribution of the bird-mediated seed shadow of the Eurasian tree, Frangula

alnus from a negative exponential towards an extended Poisson distribution. This

indicates that secondary seed dispersal by water can result in regular long-

distance seed dispersal for F. alnus. Secondary water-borne dispersal may

therefore represent an important long-distance dispersal vector, aiding the

colonisation of isolated habitat in some emergent or riparian species. More than

one type of dispersal vector however may result in long-distance secondary seed

dispersal.

Other mechanisms affecting long-distance seed transport

There are several ways that long distance seed dispersal events may

occur including; dispersal by water (Kudoh & Whigham, 1997; Lonsdale, 1993;

Nilsson et al. 1991; Ridley, 1930), by biotic influences (Figuerola & Green, 2002;

Fragosa, 1997; Holbrook et al. 2002; Shilton et al. 1999), wind vectors (Bullock &

Clarke, 2000; Nathan et al. 2002) or rafting on other dead or live organisms

(Nathan & Muller-Landau, 2000). Some researchers assume a link between seed

attributes such as morphology and dispersal (e.g. buoyant seeds imply

hydrochory) (e.g. Hughes et al., 1994). A recent review however, recognised that

18

the morphological attributes a seed may possess does not necessarily determine

which vector will disperse it (Higgins et al. 2003). Moreover, this study suggests

that although dispersal types based on morphological traits provide a useful

framework for describing local dispersal processes, they are often poorly related

to the actual mechanisms responsible for long-distance seed dispersal in many

plant species (Higgins et al. 2003).

Many wind and water dispersed seeds from wetland species are known to

adhere to the fur, feathers or feet of some vertebrate species (Carlquist, 1967;

Darwin, 1859; Figuerola & Green, 2002; Higgins & Richardson, 1999; Ridley,

1930; Vivian-Smith & Siles, 1994). This will confer a potential for long-distance

dispersal by mechanisms other than those determined primarily by morphological

traits (Higgins et al. 2003). For example, it is estimated that approximately one

quarter of the plant species that colonised Easter Island and the Juan Fernandez

Islands were transported in mud attached to the feet of migratory birds (Carlquist,

1967). In addition, the distribution of some wetland plants with buoyant seeds are

strongly correlated with the dispersal patterns and preferred habitat of migratory

waterfowl (Frith et al. 1977) suggesting that waterfowl may act as dispersal

vectors for these species. Further evidence supporting the role that biotic vectors

play in dispersing the seeds of aquatic plants over long distances comes from

population genetic studies (Tero et al., 2003).

Studies of the genetic structure of many aquatic species suggest that

dispersal mechanisms other than water have been important in some long

distance seed dispersal events (Dorken & Barrett, 2004b; Godt & Hamrick, 1998;

19

Tero et al., 2003). A study of the genetic structure of the fennel pondweed

(Potamogeton pectinatus) in Europe found strong evidence that long distance

seed dispersal in this species was not related to hydrochory (Mader et al., 1998).

The study established a correlation between genetic and geographic distance

among ponds not visited by swans (a potential dispersal agent). When ponds

visited by swans were included in the analysis no relationship was evident.

Based on this finding the authors concluded that swan-mediated long-distance

dispersal influenced the pattern of genetic structure observed (Mader et al.,

1998). Therefore, molecular markers in combination with well designed

experiments can be used to examine patterns of dispersal in plant species

(Abbott et al., 2000; He et al., 2004; Mader et al., 1998).

Genetic methods are broadly applicable to investigating both patterns of

local and long-distance seed dispersal in plant species (Cain et al., 2000; He et

al., 2004; Kinamoto et al. 2005; Ouborg et al., 1999; Sork et al., 1999). Studies of

seed dispersal in plant species that have applied genetic markers however, have

usually sampled seedlings or older stages of plant development (Cain et al.,

2000). If genetic studies of dispersal are based on samples collected from

seedlings or later stages of development, they not only provide estimates of seed

dispersal but also plant establishment, thus providing a measure of ‘effective’

seed dispersal (Nathan & Muller-Landau, 2000; but see Godoy & Jordano, 2001).

Effective seed dispersal may be the parameter of greater importance in many

ecological and conservation studies, since seed movement to locations in which

20

establishment fails will not influence the population dynamics and genetic

structure of the recipient population (Nathan, 2006).

Several models have been proposed to explain how dispersal can

influence population structure in plant populations. The types of models proposed

include: island models, fragmented models, stepping stone models, classical

metapopulation models and source/sink models (Gaggiotti and Smouse, 1996;

Kimura 1953; Kimura & Weiss 1964; Levins, 1969; Wright, 1931).

In an island model, all populations form a single panmictic unit with free

gene flow across a genetically uniform population (Wright, 1931). This model

assumes that populations have reached equilibrium, that all are of equal size and

that dispersal rate is uniform among populations irrespective of their relative

proximity in the landscape (Wright, 1951). Where panmixia is operating in an

island model, populations should not show any structure (Wright, 1951). A

fragmented model however, assumes that a formerly continuous population has

been fragmented into remnant populations where no contemporary gene flow

occurs among populations (Slatkin, 1985). Strong genetic structuring is expected

therefore among all populations in a fragmented system (Slatkin, 1985).

In contrast to the island and fragmented models, a stepping-stone model

assumes dispersal is limited by distance and occurs at higher rates among

adjacent local populations (Kimura 1953; Kimura & Weiss 1964). At equilibrium in

a stepping-stone model, genetic differentiation among populations should

increase linearly with increasing geographical distance among populations

21

(Kimura & Weiss 1964). Therefore a signature of isolation by distance is

expected under a stepping-stone model (Slatkin, 1985).

In both source/sink and classical metapopulation models, populations are

not permanent and are subject to extinction/colonization dynamics (Pannell and

Charlesworth, 1999). Source/sink metapopulation models assume that

individuals disperse from more permanent, high-quality source subpopulations

into low quality ephemeral sink populations and that sink populations are likely to

go extinct without ongoing dispersal from source populations (Gaggiotti and

Smouse, 1996). Therefore sink populations are expected to possess lower levels

of genetic diversity relative to source populations. Classical metapopulation

models assume that all patches are ephemeral and may be founded by different

numbers of individuals from single or multiple source patches (Giles and Goudet,

1997). This model also assumes that only limited dispersal occurs among extant

populations. Consequently, levels of genetic variation may decrease or increase

randomly within and among populations under a metapopulation model (Pannell

and Charlesworth, 1999, 2000).

These models provide a number of testable hypotheses about how

different modes of dispersal can influence the distribution of genetic variation

within and among subpopulations. Therefore this genetic information permits

inferences to be made about patterns of dispersal within and among populations.

In conjunction with ecological experiments genetic studies can be used to

elucidate patterns of local and long-distance seed dispersal.

22

Study species and system

The Philydraceae is a small family of chiefly erect herbs that is restricted

to Australia and south east Asia (Clifford, 1983; Hamann, 1998). Family

Philydraceae is closely related to the Pontederiaceae and Haemodoraceae in the

O. Commelinales (Graham et al., 2002). Species within this family are only found

in saturated areas like permanently waterlogged soil, swamps or soaks (Clifford,

1983). The Family Philydraceae comprises a total of six species distributed in 3

genera (Adams 1989). The 3 genera include the montane rainforest genus

Helmholtzia (3 species: H. glaberrima, H. acorifolia and H. novoguineensis) and

2 lowland genera Philydrella (2 species: P. drummondii and P. pygmaea) and

Philydrum (monotypic: P. lanuginosum) (Adams, 1989). The species examined in

this study was H. glaberrima a semi-aquatic riparian plant (Hamann, 1998).

H. glaberrima is a long-lived perennial herb (Philydraceae), (2n =34) that

is known to live for at least ten years and which grows to 2.5 m (Adams 1989).

The species has low levels of protracted flowering throughout the year and

flowers are produced on either single or multiple large pink panicles which are

believed to be pollinated by small insects (Adams 1989). Numerous 4-5 mm long

seeds are produced in capsules throughout the infructescence after pollination

(Adams 1989). Currently, no dispersal vectors are known that disperse the seed

of this plant, but hydrochory has been suggested as a possible dispersal

mechanism (Hamann, 1998).

The stream lily, H. glaberrima (Philydraceae), is a rare but occasionally

locally abundant, riparian species confined to high-elevation dendritic streams in

23

temperate and subtropical rainforest of the McPherson Ranges on the border of

Queensland and New South Wales (Clifford, 1983). Within its range of

approximately 100 km, H. glaberrima is restricted to riparian wetlands and moist

gullies along high elevation creek banks (Hamann, 1998). As H. glaberrima is

restricted to permanently waterlogged areas, it occurs in groups of spatially

isolated patches along stream systems. Consequently, populations at the top of

stream systems are thought to be less isolated from each other than downstream

populations. Although extant populations are spatially isolated from one another

they are interconnected by permanent stream systems (Adams 1989).

In the McPherson Ranges amount of rainfall received decreases with

altitude. Altitudes above 1000 m receive ≤ 3000 mm in annual rainfall, while

areas between 800 – 1000 m receive ≤ 1800 mm annually and areas below

800m receive < 1300 mm of annual rainfall (BOM, 2003). Consequently, the

preferred habitat of H. glaberrima declines with altitude as presumably do the

number and size of H. glaberrima populations.

Rainfall is also highly seasonal in south east Queensland and northern

New South Wales with most rain falling over the summer period between

December and March (BOM, 2003). Many patches occupied by H. glaberrima

are subject to periodic inundation and soil saturation in summer that usually

recedes to moist soil conditions over winter, the same time that many species

disperse their seed in this area (Prentis, personal observation). Therefore H.

glaberrima seeds have a strong possibility of being redistributed by water during

periods of heavy rain.

24

A combination of genetic and ecological approaches were used to explore

the role that hydrochory plays in the dispersal of H. glaberrima seeds over local

and long distances. Ecological approaches were used to examine the potential

for water-borne seed dispersal to transport seeds over local and long distances.

Estimates of genetic diversity among established populations of H. glaberrima

and of seedlings within a population were examined to elucidate the actual role

that water-borne seed dispersal may have on population genetic structure over

local and long-distance scales. These data were used to address the five aims of

this project:

1. To define what model of dispersal can best explain the observed

population structure (manuscript 1)

2. To examine potential for water-borne seed dispersal over local and

long-distance scales (manuscript 3)

3. To investigate the role that post-germination buoyancy plays in

seed dispersal and seedling establishment (Manuscript 2)

4. To test whether microtopographic landscape features that direct

water flow can influence seed dispersal and seedling genetic

structure in local populations (manuscript 4)

5. To quantify the influence that water-borne seed dispersal has on

observed population genetic structure (manuscript 5)

Account of research progress linking manuscripts

25

The first manuscript (chapter 2) examines the model of dispersal which

best explains the observed pattern of population genetic structure in Helmholtzia

along a single stream system. The pattern of genetic structure observed along

this stream system was best explained by classical metapopulation dynamics

and a complex seed dispersal strategy which was hypothesised to be the result

of hydrochory. The subsequent two manuscripts (chapter 3 & 4) investigated the

role of hydrochory to ‘effectively’ disperse seeds and seedlings over local and

long distances. The fourth and fifth manuscripts (chapter 5 & 6) then evaluated if

the hydrochory actually influenced the pattern of genetic structure in Helmholtzia

over a local (seedling genetic structure) and long-distance scale (among stream

systems).

26

References

Abbott, R.J. Smith, L.C. Milne, R.I. Crawford, M. Wolff, K. & Balfour, J. (2000) Molecular

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34

Statement of Joint Authorship

Manuscript 1

Prentis, P. J., A. Vesey, N. M. Meyers, and P. B. Mather 2004. Genetic structuring of the

stream lily Helmholtzia glaberrima (Philydraceae) within Toolona Creek, south-eastern

Queensland. Australian Journal of Botany 52: 201—207.

A re-print of the journal article is presented in Appendix 1.

Peter Prentis (Candidate)

• Wrote the manuscript and acted as corresponding author

• Designed and formulated sampling design and experimental protocols

• Undertook all field and laboratory work, analysis and interpretation of data

Amanda Vesey

• Undertook all field work and some lab work

• Provided editing and contributed to the structure of the manuscript

Noel Meyers

• Co-supervised the sampling design and experimental protocols

• Assisted in the interpretation of data


Peter Mather




35

CHAPTER 2. Genetic structuring of the stream lily Helmholtzia glaberrima

(Philydraceae) within Toolona Creek, south-east Queensland

Peter J. PrentisA,B, Vesey A.A, Meyers N.M.A and Mather P.B.A

A. School of Natural Resource Sciences, Queensland University of Technology, GPO

Box 2434, Brisbane, Qld 4001, Australia.

B. Corresponding author; email: [email protected]

Suggested running title: Population structure in Helmholtzia glaberrima.

36

Abstract. The distribution of genetic variation among five isolated sites of the riparian

species Helmholtzia glaberrima (J.D. Hook) was examined in Toloona Creek (280 13’ S,

153007’E) using dominant AFLP markers. From the 137 fragments assessed, analysis of

molecular variance (AMOVA) showed that most genetic variability occurred within sites

(68%), although high (32%) variation also occurred among sites. Highly significant

pairwise θ estimates among all sampled sites suggests gene flow is restricted in H.

glaberrima. Levels of within site diversity were intermediate and differed significantly

across the sampled sites. Significant levels of linkage disequilibrium were detected at all

sites except TC3. Differences in linkage disequilibrium and genetic diversity among the

sites suggest that sites may be founded by different numbers of colonists. Mantel tests

found no correlation between geographic and genetic distance and significant levels of

linkage disequilibrium were detected at the total site level. This result supports a non-

equilibrium model of population structure. The observed pattern of non-equilibrium

population structure and genetic diversity in H. glaberrima are best explained by a

classical metapopulation model.

37

Introduction

Tero et al. (2003) propose several models to explain how dispersal may influence

population structure in linearly arranged riparian plant subpopulations. These models

include: an island model, a completely fragmented model, a stepping stone model, a

classical metapopulation model and a source/sink model. The island model predicts

populations will exhibit genetic uniformity over all spatial scales, while the fragmented

system model suggests all populations should show significant differentiation due to a

lack of recurrent gene flow. In the stepping stone model, populations in close proximity

should be more genetically similar than populations at greater geographical distance.

While, the classical metapopulation model recognises that patches are ephemeral and

can be founded by differing numbers of individuals from single or multiple source

patches. Consequently, genetic variation may decrease or increase randomly within

patches while high levels of linkage disequilibrium may also be present. The source/sink

model predicts that genetic diversity will be lower in the source populations relative to

the founder patch.

Relatively few studies have examined how dispersal may influence the genetic structure

of riparian angiosperm species. Those studies that address this question report results

that do not always conform to the proposed models. For example, Hibiscus moscheutos

did not conform to the models because the moderate differentiation found in a stream

system was related to the distance from the stream channel and did not conform to the

proposed models (Kudoh and Whigham 1997). Alternatively, Keller (2000) found

extremely low levels of differentiation among sampled populations of Phragmites

australis in the Charles River watershed attributed to gene flow among populations. An

island model of population structure best explains the pattern of genetic structure found

in P. australis. A recent study on Silene tatarica along the Oulankajoki River found high

38

levels of genetic differentiation among sites, which did not conform to an isolation by

distance model (Tero et al. 2003). The authors suggest that the observed population

genetic structure resulted primarily from “classical” metapopulation dynamics and that

populations were not in drift/gene flow equilibrium.

In this paper we studied the genetic structure of a rare but locally abundant, riparian

species H. glaberrima along a single stream to evaluate which models of population

structure best fit this species. H. glaberrima is confined to high-elevation dendritic

streams in the subtropical rainforest of the McPherson Ranges on the border of

Queensland and New South Wales. H. glaberrima is restricted to permanently

waterlogged soil along creek banks, resulting in spatially structured patches that become

increasingly more isolated with distance downstream. Patches of H. glaberrima comprise

dense aggregations of up to 1.6 individuals/m2 (Prentis, Unpublished data). An added

complication is that H. glaberrima is rhizomatous over short distances, so ramets may

contribute to the expansion of patches after colonisation of new sites by propagules

(Harden 1993). Seed is presumably first dispersed via gravity and secondarily by water.

These factors may disperse seeds over very different spatial scales. Gravity dispersal

should increase genetic structure among populations as dispersal by this mechanism is

likely to occur over very small spatial scales. While hydrochoric dispersal may spread

seed over much greater distances, seed movement will be unidirectional resulting in

either increasing or decreasing genetic diversity downstream depending on how patches

are colonised.

The study aimed to test the suitability of various population models (Tero et al. 2003) to

explain the patterns of genetic variation among riparian populations of H. glaberrima

because previous work indicates exceptions to these models (Kudoh and Whigham

39

1997). H. glaberrima was chosen as a model species since populations occur in discrete

patches and are restricted to specific habitat along creek systems. We also sought to

determine whether vegetative growth in H. glaberrima could influence patterns of genetic

structure and which model best suits population structure in this species.

40

Materials and methods

Study species

The stream lily, H. glaberrima (Philydraceae), (2n =34), is a perennial herb, which is

endemic to the understorey of subtropical, warm and cool-temperate rainforest in the

McPherson Ranges with a range of less than 100 km. Within this range the species

exhibits a restricted distribution to high-elevation creek banks. H. glaberrima is a long-

lived (at least 10 years) species that grows to 2.5 m and has low levels of protracted

flowering throughout the year with most individuals flowering between August and

February. Flowers are produced on either single or multiple large, many-branched

panicles which Adams (1989) believes are pollinated by small insects. Numerous 4-5

mm long seeds are produced in capsules throughout the infructescence after pollination.

No dispersal vectors are known to disperse the seed of this species, although Hamann

(1998) suggests hydrochory as a possible dispersal mechanism.

Study site and plant collections

Samples were collected from five populations of H. glaberrima located in the headwaters

of Toolona Creek (Fig. 1) in Lamington National Park (280 13’ S, 153007’E). This single

creek system allowed sampling of both high and low altitude sites, not possible in other

adjacent stream systems. Sampled populations ranged in size from 118 to 651 adult and

juvenile plants (Table 1). In all populations, sampling was based on a concentric circle

design with distances varying from a known reference individual. The 20 m diameter of

this circle was large enough to encompass all individual plants within sites. Samples

were collected from adult plants at intervals of 1, 2, 4, 7, and 10 m in each quadrant of

the circle yielding a total of 21 individuals from each population for genetic analysis.

41

DNA extraction and AFLP analysis

Genomic DNA was extracted from 1.2 g of finely ground plant leaf tissue according to

the protocol of Doyle and Doyle (1988) with slight modifications.

AFLP procedures were performed according to the restriction ligation, PCR reactions

and gel analysis protocols of Adjome-Mardsen et al. (1997) with slight modifications.

Modifications included using the Tru91 (Roche) restriction enzyme (isoschizomer for

Mse1) in place of Taq1 as the dominant cutter. The selective PCR was conducted using

the selective nucleotides combinations: (1) E-AAG/M-AG, (2) E-AAG/M-GA and (3) E-

ATG/M-AG. The AFLP procedure was trialled twice on four samples initially, using the

same primer sets and reproducible loci were detected for this species.

Data analysis

Statistical analyses using variation in AFLP phenotypes were based on the assumptions

that AFLP markers are dominant diploid markers, which conform to Hardy-Weinberg

equilibrium and where no co-migration of fragments occur. A Dice similarity matrix

(Sneath and Sokal 1973) was used to compare the similarity and clonality among

individuals within sites using SPSS (2002). Mean genetic diversity estimates within

populations were calculated three ways: (i) as the percentage of polymorphic loci (P%),

(ii) Shannon’s index of phenotypic diversity (IS) (Lewontin 1972) and (iii) Nei’s (1978)

unbiased expected heterozygosity (HE) using POPGENE Version 1.32 (Yeh et al. 1997). To

lend support to measures of genetic diversity found within sites average Dice similarity

within sites was also calculated and subtracted from one. A Kruskal-Wallace test was

used to examine whether levels of genetic diversity were consistent among sites. To

investigate the influence that patch size and isolation from nearest neighbouring patch

had on diversity indices, a Spearman rank correlation was used. A Monte Carlo

42

simulation process was used to examine linkage disequilibrium between AFLP

phenotypes within and among sampling sites using Lian 3.1(Haubold and Hudson 2000).

The significance of departures from linkage equilibrium in these tests were generated

over 5000 iterations. Linkage disequilibrium was examined to asses if populations had

gone through bottlenecks or were formed by different levels of admixture.

Principal Coordinates Analysis (PCA) was used to visualise how individuals from

sampling sites clustered using Genalex (Peakall and Smouse 2001). A UPGMA

dendogram was constructed using Nei’s unbiased genetic distance (Nei 1978) to

visualise how sampling sites clustered together with TFPGA (Miller 1997). To evaluate

the extent of among population genetic differentiation, data were first analysed using

analysis of molecular variance (AMOVA) in Arlequin 2.000 (Schneider et al. 2000).

Estimates of θ were obtained for each pair of sites using coancestry distance (Reynolds

et al. 1983) in TFPGA. Using the general framework of Hutchinson and Templeton

(1999) we inferred the importance of genetic drift and migration among sampling sites by

constructing scatter plots using pairwise θ comparisons against both in-stream and

straight geographic distance. A Mantel test (Mantel 1967) was then used to evaluate

whether genetic and in-stream or linear distance matrices were significantly correlated in

TFPGA using 1000 perturbations.

43

Results

AFLP polymorphism and patterns

Three primer pairs were used to screen 137 unambiguous AFLP loci. One hundred and

sixteen of these fragments (92%) were polymorphic across the entire survey, with the

primer pairs one, two and three producing 33 (80.5%), 41 (89.1%) and 42 (84%)

fragments, respectively. Five individuals did not amplify yielding a total of 100 individuals

across five sites for the analysis.

Within-site variation

All individuals screened showed unique AFLP banding profiles inferring that individuals

possessed a unique genotype and were not clonal. Table 1 shows the mean average

dice similarity, Nei’s expected heterozygosity, Shannon index of phenotypic diversity and

percentage polymorphic loci at each site. Similarity values among individual plants

ranged between 0.039 and 0.269, while percentage polymorphic loci ranged between

46.8% to 68%. Nei and Shannon indices varied from 0.16 to 0.236 and from 0.239 to

0.352 respectively. A Kruskall-Wallace test showed that Nei and Shannon indices were

significantly different among sampled sites (P<0.05), but differences in diversity levels

among sites were not correlated with either patch size or distance to neighbouring

patches (P>0.2).

Significant levels of linkage disequilibrium were found at all sampling sites except for

TC3, where the result was marginally within the 95% Monte-Carlo confidence interval for

linkage equilibrium. This indicates that sampled sites show different levels of linkage

disequilibrium which is supported by the observed levels of linkage (VD) in Table 2.

Significant linkage disequilibrium was also detected at the total site level (Table 2).

44

Significant linkage disequilibrium suggests populations may have gone through

bottlenecks or have been formed by different levels of admixture.

Among-site distances

PCA analysis showed relationships among populations, with the first two axes

accounting for 28.2% of total variation. This analysis revealed minimal overlap among

sampling sites as individuals within sites tended to cluster together, with sites 4 and 5

clustering independently from sites 1, 2 and 3 (Fig. 2). The UPGMA dendrogram also

illustrated a similar pattern as sites 4 and 5 clustered together and sites 1, 2 and 3

clustered together (Fig. 3). AMOVA results indicate that although the major proportion of

genetic variation was found within populations (68%), a significant proportion was also

distributed among populations (32%). Pairwise θ estimates also confirmed that all sites

were highly different (Table 3). Neither the scatter plots (Fig. 4) or Mantel tests revealed

any significant relationships between genetic and in-stream or linear distance among

populations within the stream system.

45

Discussion

Within-site variation

All Helmholtzia glaberrima individuals sampled possessed unique multilocus genotypes.

This result demonstrates that vegetative growth contributes little to within population

structure or that the effect occurs over a spatial scale of less than one metre. An

alternative explanation is that infrequent mis-scoring of phenotypes led to the false

interpretation that no clones were present. This interpretation is unlikely however, as any

difficult to interpret or ambiguous loci were excluded from the analysis to eliminate this

problem and a minimum of 5 loci were different between any two individuals.

Helmholtzia glaberrima exhibited moderate levels of polymorphic AFLP loci, and

diversity compared to other plants (Gaudeul et al. 2000; Despres et al. 2002, Tero et al.

2003). Two other herbaceous perennial species, Eryngium alpinum and Trollius

europaeus (European globe flower) exhibited similar diversity levels to H. glaberrima, but

unlike these species, no relationship occurred between genetic diversity and population

size or degree of isolation (Gaudeul et al. 2000; Despres et al. 2002). Like H.

glaberrima, other plant species with non-equilibrium dynamics often exhibit a poor

correlation between genetic diversity and population size or isolation (Schmidt and

Jensen 2000; Tero et al. 2003).

Linkage disequilibrium tests showed that four of the five sampled populations of H.

glaberrima were not in equilibrium. High levels of linkage disequilibrium within some, but

not all sites, suggest that patches may result from different dynamics. Theory predicts

that equilibrium dynamics can be disrupted by different levels of admixture and

bottlenecks during patch formation (Tero et al. 2003). Also, patches founded by

individuals arising from a greater number of source patches will exhibit greater levels of

46

genetic diversity relative to other patches (Whitlock and McCauley 1990). Differences in

the level of population admixture or the severity of bottlenecks during patch formation

are two potential causes for observed patterns of linkage disequilibrium and genetic

diversity among sampling sites of H. glaberrima.

Among-site distance

Highly significant genetic differentiation occurred among sampled sites for H. glaberrima

(φst =0.32), in spite of the fact that most genetic variation was present within sites.

Therefore, sampled sites do not form a single panmictic unit. This result demonstrates

that the species does not conform to an island model of population structure.

Similar patterns of variation over small spatial scales have been reported with dominant

markers in other plant species (Hogbin and Peakall 1999; Gaudeul et al. 2000; Despres

et al. 2002). Restricted dispersal among geographically isolated patches has often been

used to explain this pattern of variation. An isolation by distance pattern of population

structuring is likely to develop under low levels of localised gene flow (Hutchison and

Templeton 1999). However, H. glaberrima populations demonstrated strong

differentiation, irrespective of their geographical proximity by linear or in-stream distance.

This suggests that most gene flow among H. glaberrima individuals occurs within

patches. Moreover, it demonstrates that gene flow which occurs among patches is not

related to patch proximity.

Both UPGMA and PCA results indicate that sites TC2 and TC3 and sites TC4 and TC5

respectively cluster more strongly than other sampled sites. The two clusters are also

the most divergent from each other, even though sites TC3 and TC4 are closer to each

other than is site TC2. This relationship indicates that one and two dimensional stepping

47

stone models of dispersal cannot explain H. glaberrima’s population structure.

Furthermore, scatter plots of coancestry distance versus geographic distance (Fig. 4)

show a random scattering of points, indicating that recurrent gene flow has less

influence than drift or non-equilibrium dynamics on the observed pattern of genetic

structure. Two likely explanations may account for this pattern of fine-scale structure: (1)

genetic drift predominates because dispersal is low or absent among sites and/or (2)

different numbers of individuals arising from different numbers of source populations

contribute to founding patches.

Short-distance seed dispersal may result in the strong genetic differentiation among

sampled sites. Seed dispersal among patches may be rare because H. glaberrima seed

lacks specialised structures for long distance dispersal, except potentially by water. In

this situation, genetic drift may cause the random fixation or loss of alleles within

geographically isolated patches if effective population sizes are low. Consequently,

genetic differentiation should increase among patches as private alleles evolve or alleles

are lost within patches. No private alleles were detected, however, among the sampled

sites. In addition the highest frequency of null alleles in any patch was 5%. Thus genetic

drift within sites would seem to be quite weak and the observed population structure

cannot be adequately explained by a fragmented model. Non-equilibrium dynamics

associated with patch formation or dispersal represent more likely causes for the

observed population structuring.

Chance colonisation events of the limited suitable habitat occurring along stream banks

represent a likely explanation for potential differences in population founding numbers in

H. glaberrima, causing populations to be out of equilibrium. Because of the low

probability that seeds with similar genotypes will found a patch, when different H.

48

glaberrima sites are compared highly divergent gene frequencies among sites may

occur. Also the occurrence of linkage disequilibrium at the total population level indicates

that H. glaberrima patches have been colonised by non-equilibrium dynamics. Highly

divergent gene frequencies and high levels of linkage disequilibrium are often associated

with typical non-equilibrium dynamics of both source/sink and classical metapopulations

(Whitlock and McCauley 1990; Boileau et al. 1992). However as genetic diversity did not

decrease downstream as the source/sink model predicted but varied randomly in the

stream system it indicates that classical metapopulation models can best explain the

observed pattern of population structuring in H. glaberrima.

In conclusion, the present study has shown that patches of H. glaberrima display

intermediate levels of genetic diversity even in small patches and that vegetative growth

does not influence patch structure. The random pattern of genetic variation among

patches and high levels of linkage disequilibrium within some sites indicate populations

do not conform to an equilibrium population structure model. Moreover, classical

metapopulation models best explain patterns of genetic variation and differentiation.

Future work should investigate whether H. glaberrima population structure follows

metapopulation or non-equilibrium models in other stream systems. This information will

assist our understanding of the population dynamics in riparian plant species.

49

Acknowledgments

The authors thank Grant Hamilton and Shaun Meredith for assistance with collections

from field sites and Vincent Chand and Natalie Baker for their help with lab work. We are

also grateful to Ana Pavasovic and Dr Grahame Kelly and two anonymous reviewers

who read earlier drafts of this manuscript.

50

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Harden GJ (1993) ‘Flora of New South Wales’ (New South Wales University Press:

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522.

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distance measures: inferring the relative influences of gene flow and drift on the

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Keller BE (2000) Genetic variation among and within populations of Phragmites australis

in the Charles River watershed. Aquatic Botany 66, 195-208.

Kudoh H, Whigham DS (1997) Microgeographic genetic structure and gene flow in

Hibiscus moscheutos (Malvaceae) populations. American Journal of Botany 84, 1285-

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Mantel N (1967) The detection of disease clustering and a generalised regression

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Schmidt K, Jensen K (2000) Genetic structure and AFLP variation of remnant

populations in the rare plant Pedicularis palustris and its relation to population size and

reproductive components. American Journal of Botany 87, 678-689.

Schneider S, Kuffer JM, Rossli D, Excoffier L (2000) Arlequin, version 2.0: a software for

population genetic data analysis. Genetics and Biometry Laboratory, University of

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Spss (2002) SPSS for windows, release 11.5.0. (SPSS Inc.: Chicago)

Tero N, Aspi J, Siikamaki P, Jakalaniemi A, Toumi J (2003) Genetic structure and gene

flow in a metapopulation of an endangered species, Silene tatarica. Molecular Ecology

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54

Table 1. Population size, isolation of populations from nearest neighbouring

population and within population diversity indices from the sampled H. glaberrima

populations

Populatio

n

Isolation

(m)

Populatio

n size

Nei’s

Index (HE)

Shannon

Index (IS)

Variable

Loci (%)

Average

Similarity

TC1 250 234 0.236 0.352 68 0.174

TC2 68 118 0.177 0.268 54.5 0.087

TC3 40 618 0.159 0.239 46.8 0.105

TC4 32 307 0.195 0.289 54.2 0.131

TC5 21 213 0.178 0.266 49 0.109

55

Table 2. Analysis of linkage disequilibrium within each of the sampled sites and at

the total population level in H. glaberrima

Levels of significance were derived by 5000 iterations using a Monte-carlo simulation

where * < 0.05, ** < 0.025, *** < 0.001

Sites VD VE LMC Significance

TC1 39.3 17.1 21.9 ***

TC2 84 12.5 16.4 ***

TC3 15.6 12.2 15.7

TC4 33.9 14.6 19 ***

TC5 20.6 13.5 17.2 **

Total 60.5 19.4 21.3 **

56

Table 3. Pairwise θ distance values among sampled populations of H. glaberrima

Population TC1 TC2 TC3 TC4 TC5

TC1 0

TC2 0.358 0

TC3 0.312 0.31 0

TC4 0.33 0.381 0.423 0

TC5 0.447 0.539 0.542 0.374 0

57

.

Fig. 1. Map of sampling locations within Toloona Creek

Toolona Gorge

0km 0.5km

N

TC2

TC3

TC1

TC5 TC4 Toolona Creek

Mt Toolona

Mt Wanungara Mt Bithongabal

Lamington National Park

Lamington National Park 1km

58

-40

-30

-20

-10

0

10

20

-20 -10 0 10 20 30

axis 1

axis

2

site 1site 2site 3site 4site 5

Fig. 2. Principal coordinate analysis illustrating genetic differentiation of the H.

glaberrima individuals sampled across the 5 sites. Axis 1 extracted 17.88% of the

variance and axis 2 extracted 10.36% of the variance.

59

Fig. 3. UPGMA dendrogram illustrating sites 1, 2 and 3 clustering with each other and

away from the cluster of sites 4 and 5.

60

Fig. 4. Scatter plots of pairwise θ distance versus in-stream and linear distance

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000 2500 3000

In-stream distance (m)

Coa

nces

try

dist

ance

0

0.1

0.2

0.3

0.4

0.5

0.6

0 500 1000 1500 2000

Linear distance (m)

61


Manuscript 2

Prentis, P. J., N. M. Meyers, and P. B.Mather. In press. The significance of post-

germination buoyancy in Helmholtzia glaberrima and Philydrum lanuginosum

(Philydraceae). Australian Journal of Botany.





Noel Meyers

• Supervised the sampling design and experimental protocols



Peter Mather




62

CHAPTER 3. The significance of post-germination buoyancy in Helmholtzia

glaberrima and Philydrum lanuginosum (Philydraceae)

Peter J. PrentisA, B, Noel M. MeyersA and Peter B. MatherA

A School of Natural Resource Sciences, Queensland University of Technology, GPO Box

2434, Brisbane, Qld 4001, Australia.

B Corresponding author: E-mail: [email protected]

Running title: Seedling floatation promotes establishment

halla

This article is not available online. Please consult the hardcopy thesis available from the QUT Library

84


Manuscript 3

Prentis, P. J., N. M. Meyers, and P. B.Mather. (In Prep) Seed dispersal and seedling

establishment in the riparian plant Helmholtzia glaberrima. Freshwater Biology.





Noel Meyers




Peter Mather




85

CHAPTER 4. Seed dispersal and seedling establishment in the riparian plant

Helmholtzia glaberrima

Peter J. Prentis1, Noel M. Meyers and Peter B. Mather

School of Natural Resource Sciences, Queensland University of Technology, GPO Box 2434,

Brisbane, Qld, 4001 Australia.

1 Corresponding author:

Peter Prentis

School of Natural Resource Sciences,

Queensland University of Technology,

GPO Box 2434,

Brisbane, Qld, 4001 Australia.

[email protected]

Fax: +617 3864 1535

Phone: +617 3864 2186

Running title: Seed dispersal in H. glaberrima

halla


105


Manuscript 4

Prentis, P. J., N. M. Meyers, and P. B. Mather. (In Review) Micro-geographic landscape

features demarcate seedling genetic structure in the stream lily, Helmholtzia glaberrima

(Philydraceae)1. American Journal of Botany.





Noel Meyers




Peter Mather




106

CHAPTER 5. Micro-geographic landscape features demarcate seedling genetic

structure in the stream lily, Helmholtzia glaberrima (Philydraceae)

Peter J. Prentis,1,2,* and Peter B. Mather1

1School of Natural Resource Sciences, Queensland University of Technology, GPO Box

2434, Brisbane, Qld 4001, Australia.

2School of Earth and Environmental Sciences, University of Adelaide, North Terrace, SA

5005, Australia.

* Corresponding author: Peter Prentis

Email: [email protected]

Phone: +618 8303 5594

Fax: +618 8303 4364

Postal address: School of Earth and Environmental Sciences, University of Adelaide,

North Terrace, SA 5005, Australia

halla


126


Manuscript 5

Prentis, P. J., N. M. Meyers, and P. B.Mather. (In Review) Fine-scale patterns of

genetic diversity and population structure in the stream lily Helmholtzia glaberrima

(Philydraceae) along rainforest streams, south-east Queensland. Freshwater Biology.





Noel Meyers




Peter Mather




127

CHAPTER 6. Fine-scale patterns of genetic diversity and population structure

in the stream lily Helmholtzia glaberrima (Philydraceae) along rainforest

streams, south-east Queensland

Peter. J. Prentis, Peter. B. Mather, and Noel. M, Meyers

School of Natural Resource Sciences, Queensland University of Technology, GPO

Box 2434, Brisbane, Qld 4001, Australia.

Corresponding author e-mail: [email protected]

Running head: Population structure in a riparian plant

Keywords : AFLP, genetic diversity, landscape, population structure, riparian plant

halla


147

CHAPTER 7. GENERAL DISCUSSION

A number of recent papers have debated the role of genetic and ecological

approaches to understanding seed dispersal in plant species (Cain et al., 2000;

Nathan, 2005; Nathan & Muller-Landau, 2000; Nathan et al., 2003). The authors of

these papers have suggested that genetic and ecological approaches usually

measure different components of seed dispersal. Ecological approaches are

suggested to measure actual or potential seed dispersal, while genetic approaches

usually measure realised seed dispersal or the phase after seedling establishment

(Cain et al., 2000). Most studies however, only use one approach to examine seed

dispersal, thereby overlooking either potential or realised components of seed

dispersal (Nathan, 2005). Information on both potential and realised dispersal is

needed to understand the dynamics of persistence, spread and distribution of plant

populations. For instance seed may often disperse to among sites over very long

distances, but never establish and contribute to realised dispersal among sites.

Ecological data alone in this case would suggest that there is high levels of seed

mediated gene flow among sites, while genetic data alone would suggest there is no

seed dispersal or gene flow occurring among the sites. In combination an ecological

and genetic approach would determine that although a large amount of seeds are

dispersed among the sites, there is no recruitment of these seeds into the breeding

population and therefore no gene flow. Therefore by integrating genetic and

ecological approaches to studying seed dispersal we can determine if seeds are

actually dispersing among populations and if they are then incorporated into the

breeding population at new sites. To this end, the present study investigated seed

dispersal in H. glaberrima using both ecological and genetic approaches.

Ecological approaches indicated primary seed dispersal in H. glaberrima

apparently occurs over short physical distances, which has also been reported in

many other herbaceous species (Cain et al. 1998; Ellison & Parker, 2002). Unlike

148

some herbaceous wetland species, however, the results of the current study support

the contention that water could potentially disperse seeds of riparian or wetland

specialists over both local and long-distances (Ellison & Parker, 2002; Waser et al.

1982). After prolonged periods of immersion in water, nearly all H. glaberrima seeds

tested still floated and over half of these seeds were still viable conferring the

potential for successful long-distance water-borne dispersal. The duration of seed

buoyancy observed here is similar to those observed in other riparian plant species

(Boedeltje et al., 2003; Ridley, 1930).

The capacity of H. glaberrima seeds to refloat after sinking and the prolonged

seedling buoyancy observed may also represent an adaptation to local or long-

distance dispersal via water. Seedlings of some other riparian plants are positively

buoyant (Ridley 1930; Sculthorpe 1967; Van der Valk 1981), which may promote the

probability of seed dispersal or the early anchoring and subsequent establishment of

seedlings (Sculthorpe 1967; Nicol and Ganf 2000). Seedling dispersal would be likely

to occur over a local scale within populations, as long-distance transport in the fast

flowing rocky streams of the McPherson Ranges could damage young leaf tissue.

Nicol and Ganf, (2000) also suggested that seedling dispersal may occur over a local

scale, but only if seedlings dispersed to the permanently moist edges of wetland

habitats.

Seed germination and seedling establishment was strongly reduced by

lowered soil moisture in H. glaberrima, with seedling establishment only occurring

under permanently wet conditions. Hydrological conditions have long been known to

influence seed germination and seedling establishment in aquatic or riparian plant

species (Boedeltje et al., 2003; Keddy and Ellis, 1985; Keddy and Constable, 1986;

Nicol and Ganf 2000). Furthermore, hydrology plays an important role in zonation

and structuring the composition of aquatic vegetation as water levels rise and fall and

soil moisture levels change (Blanch et al., 1999). Therefore, dispersal of seeds in

aquatic or riparian species to areas with unfavourable hydrological conditions is

149

unlikely to result in successful seedling recruitment and establishment.

Consequently, the effectiveness of hydrochory is likely to be limited if seeds or

seedlings of H. glaberrima are not dispersed to permanently wet habitat, as wet/dry

soil conditions inhibit seedling establishment.

Together these ecological data provide evidence that dispersal by water can

move seeds successfully over long distances along streams between isolated

wetland habitats if seeds are dispersed to microhabitat conditions favourable for

seedling establishment. Long-distance seed dispersal along riparian corridors has

been suggested to be an effective method for seed dispersal among streamside

populations in other obligate riparian species (Kudoh & Whigham 1997; Kitamoto et

al. 2005). In fact seeds of some riparian plant species have been dispersed distances

of at least six kilometres in flowing waters along streams (Boedeltje et al., 2003). In

many plant species however, potential long-distance seed dispersal is not realised as

seedling establishment usually does not result. Therefore, genetic data are required

to determine if long-distance seed dispersal has been successful.

Genetic studies showed that classical metapopulation models of dispersal

best explain the pattern of genetic structure observed here in H. glaberrima. Mixed

patterns of local and long-distance dispersal are reported to be common in plant

metapopulations (Jakalaniemi et al., in press). In many cases, most dispersal occurs

on a local scale, but very rare long-distance colonisation or dispersal events do

occur. The patterns of genetic diversity observed in H. glaberrima suggest that nearly

all successful dispersal occurs at a local scale, but that extremely rare long-distance

dispersal events may be responsible for establishment of new patches. Similar

patterns of genetic structure have also been found in the riparian species, Silene

tatarica, which were suggested to arise from mixed patterns of seed dispersal (Tero

et al., 2003). Differences in the amount of local versus long-distance dispersal may

be influenced by the effectiveness of a vector for short or long-distance dispersal or

the presence of more than dispersal vector (Ozinga et al., 2003).

150

On a local scale water-born seed dispersal can strongly influence the pattern

of genetic structure of seedlings within a population, however it had little affect on the

spatial pattern of genetic variation among established populations of H. glaberrima.

Moreover, results presented here do not support the hypothesis that flowing water is

an effective long-distance seed dispersal vector in H. glaberrima. Although ecological

experiments demonstrated seeds of H. glaberrima have many obvious adaptations

for hydrochory including prolonged buoyancy and extended viability in water, water-

borne seed dispersal does not provide effective gene flow among established

populations of H. glaberrima. Recently it has been suggested that dispersal attributes

related to seed characteristics provide a very useful framework for describing local

dispersal processes, but are poorly related to the rarer processes that may move

seeds over long distances (Higgins et al., 2003). The authors refer to the dispersal

vectors responsible for rare long-distance dispersal as non-standard means of

dispersal. It is possible then that long-distance dispersal events responsible for

population founding in H. glaberrima may occur via non standard means.

The pattern of genetic differentiation among stream systems in H. glaberrima,

suggest that rare dispersal events by nonstandard vectors may be responsible for the

colonisation of new patches. The greatest level of genetic variation was found at the

smallest spatial scale, among populations within a single stream system, while only a

small percentage of variation was found among stream systems. If water was

responsible for colonisation of new patches rather than a non-standard vector,

population genetic structure would likely conform to a hierarchical model of gene

flow. As this was not the case, other seed dispersal vectors therefore, must play an

important role in shaping the genetic structure of H. glaberrima in this region and may

prove to be a significant influence on the spatial pattern of genetic variation in this

species as a whole.

More generally, this study has shed new light on the role of hydrochory and

seed dispersal in riparian and wetland habitats. Although the potential for long-

151

distance hydrochory exists in H. glaberrima, it would seem to only occur at a local

scale. This contrasts with previous findings which show that hydrochory is often an

effective long-distance dispersal vector in complex, multistage dispersal processes

(Fridriksson, 1975; Hampe, 2004). These studies however, have only examined

potential seed dispersal and have no genetic data to determine if the plants

dispersing over long-distances have contributed to the gene pool of new populations.

This highlights the importance for incorporating genetic components into studies on

seed dispersal as without this information we gain an insight into potential seed

dispersal. Without a genetic component to the current study, it would be assumed

that hydrochory should maintain gene flow among streamside populations of H.

glaberrima. This is not the case however, as gene flow among established

populations would seem to extremely rare and unrelated to hydrochory.

In H. glaberrima, a non-standard vector would appear to be the most likely

long-distance seed dispersal vector. This highlights the lack of generality that can be

made about seed dispersal in many riparian or wetland systems except to say that

dispersal in these systems is very complex and often involve a number of different

seed dispersal vectors. Currently no mechanistic models of seed dispersal can

explain such complex seed dispersal patterns as those found in many riparian

systems. These models need to be developed in order to understand how seed

dispersal influences spatio-temporal patterns of seedling and population recruitment

in riparian and wetland plants. This information is critically required to gain

knowledge into the persistence of populations and dynamics of population extinction

and recolonisation in riparian and wetland plants. Without an understanding of how

riparian or wetland plants colonise extirpated populations or new patches we cannot

adequately develop management strategies to promote the persistence of these

species in urbanised landscapes. Therefore, H. glaberrima and other riparian species

present a unique opportunity to investigate the processes of population extinction

152

and recolonisation and long-distance seed dispersal by non-standard dispersal

vectors extinction/recolonisation dynamics in plants.

153

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