Bio 100 - Study Guide 16

http://skepticwonder.fieldofscience.com/2009_09_01_archive.html

Protists and Plants

• There is now considerable evidence

– That much of protist diversity has its origins in endosymbiosis

Endosymbiosis in Eukaryotic Evolution

http://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_03

• The plastid-bearing lineage of protists

– Evolved into red algae and green algae

• On several occasions during eukaryotic evolution

– Red algae and green algae underwent secondary endosymbiosis, in which they themselves were ingested

Cyanobacterium

Heterotrophic

eukaryote

Primary

endosymbiosis

Red algae

Green algae

Secondary

endosymbiosis

Secondary

endosymbiosis

Plastid

Dinoflagellates

Apicomplexans

Ciliates

Stramenopiles

Euglenids

Chlorarachniophytes

Plastid

Alv

eola

tes

Figure 28.3

• Diversity of plastids produced by secondary endosymbiosis

• The conventional model of relationships among the three domains place the archaea as more closely related to eukaryotes than they are to prokaryotes.Similarities include proteins

involved in transcription and translation.

This model places the host cell in the endosymbiotic origin of eukaryotes as resembling an early archaean.

Fig. 28.6

Under one evolutionary scenario, the endomembrane system of eukaryotes (nuclear envelope, endoplasmic reticulum, Golgi apparatus, and related structures) may have evolved from infoldings of plasma membrane.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Where did the Nucleus and ER come from?

Each endosymbiotic event adds a membrane derived from the vacuole membrane of the host cell that engulfed the endosymbiont.

http://www.life.umd.edu/labs/Delwiche/pubs/endosymbiosis.gif

• The chimeric origin of the eukaryotic cells contrasts with the classic Darwinian view of lineal descent through a “vertical” series of ancestors.

– The eukaryotic cell evolved by “horizontal” fusions of species from different phylogenetic lineages.

– The metaphor of an evolutionary tree starts to break down at the origin of eukaryotes and other early evolutionary episodes.

• The conventional cladogram predicts that the only DNA of bacterial origin in the nucleus of eukaryotes are genes that were transferred from the endosymbionts that evolved into mitochondria and plastids.

• Surprisingly, systematists have found many DNA sequences in the nuclear genome of eukaryotes that have no role in mitochondria or chloroplasts.

• Also, modern archaea have many genes of bacterial origin.

Lynn Margulis

http://www.snowballearth.org/images/Lynn_Margulis.jpg

1883 ~ AFW Schimper noted that the plastids of plant cells resembled free-

living Cyanobacteria.

1905 ~ Mereschkowsky proposed a reticulated tree of endosymbiosis for the

origin of algal plastids.

1920s ~ Ivan Wallin suggested a bacterial origin for mitochondria.

1959 ~ Stocking and Gifford discovered DNA in the plastids of Spirogyra, a

green algae.

1960s ~ Lynn Margulis argued the case for endosymbiotic origins of

mitochondria and plastids.

1970 ~ Margulis published her argument for the endosymbiotic origin of

eukaryotes in The Origin of Eukaryotic Cells.

1977~ Carl Woese declared the case for prokaryotic endosymbiosis “clear cut”

and “proven”. Other biologists subsequently declared the endosymbiotic theory

demonstrated beyond a reasonable doubt.

1981 ~ In Symbiosis in Cell Evolution, Margulis argued that eukaryotic cells

originated as communities of interacting entities. She extended the argument to

including endosymbiotic incorporation of spirochaetes that developed into

eukaryotic undulopodia -- flagella and cilia. (This proposal has not gained wide

acceptance because flagella lack DNA and do not show ultrastructural

similarities to prokaryotes.)

History of Ideas Concerning Endosymbiosis

http://endosymbionts.blogspot.com/

• All three domains seem to have genomes that are chimeric mixes of DNA that was transferred across the boundaries of the domains.

• This has lead some researchers to suggest replacing the classical tree with a web-like phylogeny

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 28.7

• Protists are an extremely diverse assortment of eukaryotes

• Protists are more diverse than all other eukaryotes and it is hard to generalize about their Characteristics

– a. eukaryotes

– b. not fungi, plants or animals

– c. most are unicellular but some are colonial or multicellular

Evolutionary Trends – Protists and Plants

Most Protists are aquatic - habitats are also diverse including freshwater and marine species

Figure 28.2a–d

100 m

100 m

4 cm

500 m

The freshwater ciliate Stentor,

a unicellular protozoan (LM)

Ceratium tripos, a unicellular marine dinoflagellate (LM)

Delesseria sanguinea, a multicellular marine red alga

Spirogyra, a filamentous freshwater green alga (inset LM)

(a)

(b)

(c)

(d)

Protists, the most nutritionally diverse of all eukaryotes, include

– Photoautotrophs, which contain chloroplasts

– Heterotrophs, which absorb organic molecules or ingest larger food particles

– Mixotrophs, which combine photosynthesis and heterotrophic nutrition

– Often discussed by their ecology – feeding patterns

Reproduction and life cycles

–Are also highly varied among protists, with both sexual and asexual species

–exhibit “evolution” of mitosis and meiosis

–Exhibit all three types of life cycles.

• The most complex life cycles include an alternation of generations

–The alternation of multicellular haploid and diploid forms

Alternation of Generations

MEIOSIS FERTILIZATION

n

n

n

n

n

2n

Haploid multicellular

organism

Mitosis Mitosis

Gametes

Zygote

(c) Most fungi and some protistsFigure 13.6 C

• In most fungi and some protists

– Meiosis produces haploid cells that give rise to a haploid multicellular adult organism

– The haploid adult carries out mitosis, producing cells that will become gametes

MEIOSIS FERTILIZATION

nn

n

nn

2n2n

Haploid multicellular

organism (gametophyte)

Mitosis Mitosis

Spores

Gametes

Mitosis

Zygote

Diploid

multicellular

organism

(sporophyte)

(b) Plants and some algaeFigure 13.6 B

• Plants and some algae

– Exhibit an alternation of generations

– The life cycle includes both diploid and haploid multicellular stages

• In animals

– Meiosis occurs during gamete formation

– Gametes are the only haploid cells

Gametes

Figure 13.6 A

Diploid

multicellular

organism

Key

MEIOSIS FERTILIZATION

n

n

n

2n2nZygote

Haploid

Diploid

Mitosis

(a) Animals

• Dinoflagellates

– Are a diverse group of aquatic photoautotrophs and heterotrophs

– Are abundant components of both marine and freshwater phytoplankton (some are heterotrophs

– Have internal plates of cellulose

– Have 2 flagella in perpendicular grooves, spin

– Responsible for red tides – neurotoxin

– Symbionts in coral polyps – coral bleaching

– Are abundant components of both marine and freshwater phytoplankton

Dinoflagellates – Clade Alveolata

• Each has a characteristic shape

– That in many species is reinforced by internal plates of cellulose

• Two flagella

– Make them spin as they move through the water

Flagella

• Rapid growth of some dinoflagellates

– Is responsible for causing “red tides,” which can be toxic to humans

http://serc.carleton.edu/microbelife/topics/redtide/general.html

http://www.miamipoison.org/x59.xml

• Diatoms are unicellular algae

– With a unique two-part, glass-like wall of hydrated silica

Diatoms – Clade Stramenopila

Figure 28.15

• Diatoms are a major component of phytoplankton

– And are highly diverse

Figure 28.1650 µm

• Accumulations of fossilized diatom walls

– Compose much of the sediments known as diatomaceous earth

http://www.cndiatomite.com/de.htm

http://www.riograndefarm.org/programs-events/community-garden/garden-guide/may-2010/

Include many of the species commonly called seaweeds

Are the largest and most complex algae

Are all multicellular, and most are marine

Have the most complex multicellular anatomy of all algae

Figure 28.18

Blade

Stipe

Holdfast

Brown Algae – Clade Stramenopila

• Red algae and green algae are the closest relatives of land plants

• Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont

– And the photosynthetic descendants of this ancient protist evolved into red algae and green algae

Clade - Viridiplantae

• Red algae

– Are usually multicellular; the largest are seaweeds

– Are the most abundant large algae in coastal waters of the tropics

– Red algae are reddish in color due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll

Figure 28.28a–c(a) Bonnemaisonia hamifera. This red alga

has a filamentous form.

Dulse (Palmaria palmata). This edible

species has a “leafy” form.(b)

A coralline alga. The cell walls of

coralline algae are hardened by calcium

carbonate. Some coralline algae are

members of the biological communities

around coral reefs.

(c)

• Green algae

– Are named for their grass-green chloroplasts

– Are divided into two main groups: chlorophytes and charophyceans

– Are closely related to land plants

Green Algae

• Chlorophytes include

– Unicellular, colonial, and multicellular forms

Volvox, a colonial freshwater chlorophyte. The colony is a hollow

ball whose wall is composed of hundreds or thousands of

biflagellated cells (see inset LM) embedded in a gelatinous

matrix. The cells are usually connected by strands of cytoplasm;

if isolated, these cells cannot reproduce. The large colonies seen

here will eventually release the small “daughter” colonies within

them (LM).

(a)

Caulerpa, an inter-

tidal chlorophyte.

The branched fila-

ments lack cross-walls

and thus are multi-

nucleate. In effect,

the thallus is one

huge “supercell.”

(b)

Ulva, or sea lettuce. This edible seaweed has a multicellular

thallus differentiated into leaflike blades and a rootlike holdfast

that anchors the alga against turbulent waves and tides.

(c)

20 µm50 µm

Figure 28.30a–c

Figure 28.31

Flagella

Cell wall

Nucleus

Regions

of single

chloroplast

Zoospores

ASEXUAL

REPRODUCTION

Mature cell

(n)

SYNGAMY

SEXUAL

REPRODUCTION Zygote

(2n)

MEIOSIS

1 µm

Key

Haploid (n)

Diploid (2n)

+

+

+

+

• Most chlorophytes have complex life cycles

– With both sexual and asexual reproductive stages

In Chlamydomonas,

mature cells are haploid and

contain a single cup-shaped

chloroplast (see TEM at left).

1In response to a

shortage of nutrients, drying

of the pond, or some other

stress, cells develop into gametes.

2

Gametes of opposite

mating types (designated

+ and –) pair off and

cling together. Fusion of

the gametes (syngamy)

forms a diploid zygote.

3

The zygote secretes

a durable coat that

protects the cell against

harsh conditions.

4

After a dormant period, meiosis

produces four haploid individuals (two

of each mating type) that emerge from

the coat and develop into mature cells.

5

When a mature cell repro-

duces asexually, it resorbs its

flagella and then undergoes two

rounds of mitosis, forming four

cells (more in some species).

6

These daughter cells develop flagella

and cell walls and then emerge as

swimming zoospores from the wall of

the parent cell that had enclosed them.

The zoospores grow into mature haploid

cells, completing the asexual life cycle.

7

Defining the Plant Kingdom

Plantae

Streptophyta

Viridiplantae

Red algae Chlorophytes Charophyceans Embryophytes

Ancestral algaFigure 29.4

• Land plants evolved from green algae

• Researchers have identified green algae called charophyceans as the closest relatives of land plants

• Comparisons of both nuclear and chloroplast genes

– Point to charophyceans as the closest living relatives of land plants

Genetic Evidence

Chara,

a pond

organism

(a)10 mm

Coleochaete orbicularis, a disk-

shaped charophycean (LM)(b)

40 µm

Figure 29.3a, b

• Many characteristics of land plants

–Also appear in a variety of algal clades

• Multicellular, eukaryotic, photosynthetic

• Cell walls of cellulose

• Chloroplasts with chlorophylls a and b

Morphological and Biochemical Evidence

• Five key traits appear in nearly all land plants but are absent in the charophyceans

– Apical meristems

– Alternation of generations

– Walled spores produced in sporangia

– Multicellular gametangia

– Multicellular dependent embryos

Derived Traits of Plants

• Additional derived units

– Such as a cuticle and secondary compounds, evolved in many plant species

The End