The Influence of the Breeding System on the Genecology of Thlaspi alpestre L

The Influence of the Breeding System on the Genecology of Thlaspi alpestre L.Author(s): Ralph RileySource: New Phytologist, Vol. 55, No. 3 (Oct., 1956), pp. 319-330Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2429288 .

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THE INFLUENCE OF THE BREEDING SYSTEM ON THE GENECOLOGY OF THLASPI ALPESTRE L.

BY RALPH RILEY

Department of Genetics, University of Sheffield and Plant Breeding Institute, Cambridge

(Received 2I December I955) (With i figure in the text)

The work of Turesson, Gregor, Clausen Keck and Hiesey and many others has clearly demonstrated the widespread occurrence of population differentiation within species of plants. The analysis of the causes of this type of differentiation has, however, in the main been directed to the study of the environmental factors operative in promoting divergence. Less interest has been taken in the influence of other factors, such as the breeding system, the degree of isolation of populations and their age and size. Nevertheless, the nature of the breeding system of an organism and its population structure are generally acknowledged to be of considerable importance in evolutionary divergence.

The influence of an individual on evolutionary divergence depends not upon single characteristics, but upon its overall phenotype inasmuch as this is determined by its total gene complement. Well-adapted phenotypes will be favoured by natural selection but the maintenance, or assembly, of gene complements, the phenotypic expression of which is close to the selective optimum, depends upon the breeding system. This emphasizes the importance in evolution of the role of recombination as a source of individual variation.

Each generation will be genetically similar to the previous one in apomictic or obliga- torily self-fertilizing species. The consequent phenotypic stability will result in the maintenance of fitness in a constant environment, but there will be poor response to changing conditions. Where the progeny differ genetically to a greater or lesser extent from their parents, and there is phenotypic variation from generation to generation, there will be the continual exploration of new gene combinations and that genetic flexibility which is the source of evolutionary change. This applies to those species in which there is a considerable amount of cross-fertilization. The evolutionary potentiali- ties of a species are thus dependent on its breeding system but it is necessary to consider the breeding system at two levels. In the individual the degree of heterozygosity, the amount of genetic recombination and the fertilization system in reproduction are important, but at the group level divergence is also related to the size, structure and history of the population, and to the amount of gene migration within and between populations. In view of the importance of the breeding system of a species on the differentiation of its populations it is proposed to discuss the breeding behaviour of Thlaspi alpestre in this paper so that population differentiation may be subsequently considered against this context.

MATERIAL

Thlaspi alpestre, a species or aggregate of species, in the Tribe Lepidieae of the Cruci- ferae, may be considered to be especially suitable material for genecological work both

3I9



320 RALPH RILEY

because of its extreme variability and because of the way it is distributed in Britain. Its variability was first recognized by Jordan (I846) with the award -of specific names to variants, the constancy of which he had established by garden culture, and has since been noted by the authors of most British floras; most recently by Clapham, Tutin and War- burg (I952). The species is an alpine or sub-alpine, perennial or biennial herb, with Central European affinities, which in Britain has a disjunct distribution. It is confined to a few areas in the Mendips and in Wales, to a series of localities along the Pennines from Derbyshire to Northumberland, and to the Lake District. It also has three stations in Scotland and occurs on the island of Rum. Within each of its major occurrences it is broken up into local populations generally restricted to open habitats of limited area, especially the spoil heaps of abandoned lead mines. There are therefore major disjunc- tions in the distribution of the species between its main occurrences and minor disjunc- tions between its local populations.

In the present work the following populations have been examined:

Di On the waste of disused lead workings at Slaley near Matlock in Derbyshire, Li On the debris of a disused lead mine in the Gwydry forestry plantation near

Llanwrst, Mi On lead mine waste on the miners road between Settle and Malham, MEN At Rowberrow Bottom in the Mendips, T In the Teesdale area seven local populations, all except T7 on lead mine waste:

Ti at Moor House on Moss Burn; T2 at the junction of Trout Beck and the Tees; T3 close to the source of the South Tyne; T4 at Greenhurth mine on Herdship Fell; T5 at Ashgill Head mine, on the road from Forest in Teesdale to Alston; T6 close to Grasshill House on the main Alston road; T7 on igneous rock at Winch Bridge on the Tees.

All the plants examined in cytological checks of small samples of these populations were diploid with I4 chromosomes.

I,~~~~~~~~~~~~~~~~~~~

.%~~~~~~~~~~~~~.-

a~~~~ ~~~~~~~~~~~~~~~~~~~~~~~ '-

- -

b - C

Fig. i. Drawings of flowers of Thlaspi alpestre with the androecium and gynaecium in continuous lines and all other parts in broken lines. (a) The style has extended to burst through the bud and the stigma is receptive. (b) The petals and filaments have partially extended but pollen has not yet been released. (c) Anthesis occurs at this stage when the anthers are close to the stigma, resulting in self-pollination.



Genecology of Thlaspi alpestre L. 32I

Floral mechanism

T. alpestre has the normal Cruciferous floral structure but the bud is burst by the extension of the style, and the style protrudes from the bud for about three days before any extension of the petals and filaments takes place. During this exposure the stigma is receptive but no pollen is released from the same flower until the anthers are about level with the stigma. At this time the filaments often bend to bring the anthers into contact with the stigma (Fig. i). As a result of this mechanism a stigma is receptive to pollen from other flowers before any dehiscence occurs in the same flower, but following dehiscence self-pollination seems assured.

Artificial pollination Artificial pollination may often reveal information concerning the genetic control of natural fertilizations in plant species, especially the presence of self-incompatibility or of interpopulation cross-incompatibility. It was consequently important when examining the breeding system of a genetically unexplored species, such as T. alpestre, to make controlled pollinations.

During all emasculations inflorescences in which no stigmas were exposed were bagged in cellophane bags. Subsequently, when several stigmas were exposed, but no pollen had been released, the bags were removed and the flowers with receptive stigmas were emasculated, and all the other flowers of the inflorescences were removed. Pollen from a newly opened anther of the chosen male parent was then applied to the stigmas by brushing the anther across them. The pollinated inflorescence was then rebagged.

It is necessary in the interpretation of the results of artificial pollinations to bear in mind the difficulties of maintaining a constant technique. However, all the present results (Table i) were obtained by one person in a limited period and in constant greenhouse conditions. It seems probable therefore that any marked divergences in the fertility of artificial pollinations amongst the present material were the outcome of genetic differences.

Table i. Artificial pollinations

Individ- Individ- Propor- Flowers Total Seed set Pollination Population uals uals tion emascu- seed per stigma

class treated fertile fertile lated and set pollinated pollinated

-T4 10 0 67* 0 Control D i 110 0 -53* 0-

Mi 1 10 0 59* 0-

ContrlT4 I9 8 o.88 49 I03 2.10 Self Di I2 I0 o.83 82 I24 I.5I

MI I0 8 o.8o 57 I71 3.00

Intra- T4 x T4 29 I2 0.4I I5I I4I 0.93 population Di X Di 22 19 o.86 I2I I89 I .56 cross MixMi 20 13 o.65 107 I26 i. i8

Inter- T4 x Di 27 9 0.33 I28 97 o.67 population T4 XMI 22 I3 0-59 I07 I89 I.77 cross Di xMi 24 IO 0.42 II9 II9 I.00

* Not pollinated. C



322 RALPH RILEY

Three types of artificial pollination were possible; selfing, and interpopulation and intrapopulation crossing. Data for intrapopulation crossing and selfing and diallel crossing were obtained for the three populations T4, Di and Mi (Table i). No seed developed in the control plants which were emasculated only, but not pollinated. The results with these plants may also be taken as partial evidence against apomixis, but as pollen stimulus is required in some apomicts the evidence is not complete.

Selfed plants were pollinated with pollen from the same individual, and it seems clear from a comparison of the results of selfing with those for intrapopulation crossing that there is no evidence of self-incompatibility in this species. There is indeed a suggestion that selfing may be more successful than intrapopulation crossing. However, a comparison of the summed ratios of individuals fertile: sterile for each of these blocks is not possible since the data for intrapopulation crossing are statistically heterogeneous (contingency ZX2(3) = IO.74; P = 0.02-O.OI). When the results for selfing and intrapopulation crossing are compared population by population there is a statistically significant difference in population T4 ( =X2(1) 25.43; P = <O.OOI), but the differences in Di (EX2(1) o O.I8; P = 0.7-0.5) and in Mi (EX2(l) 2.8I; P = O.I-0.05) are not significant. There is thus no constant significant increase in the fertility of self-pollination over cross-pollination, but the heterogeneity of the data for crossing demonstrates differences between populations in the fertility of intrapopulation crosses.

The development of reproductive isolation in nature must be considered as of out- standing importance in evolutionary divergence, and Clausen, Keck and Hiesey (I945) have considered the artificial production of hybrid seed as a useful criterion of such isolation. However, artificial interpopulation crossing in T. alpestre revealed no complete internal barrier to crossing between the populations tested (Table i). There was, nevertheless, a smaller proportion of fertile individuals in interpopulation crosses. Unfortu- nately the heterogeneity of the results for intrapopulation crosses makes comparison of the summed data of intra- and interpopulation crosses invalid, and the separate components of these blocks are not comparable. It seems reasonable to infer, however, that interpopulation crosses were genuinely less fertile than intrapopulation crosses. It is however possible to compare the summed ratios of individuals fertile: sterile in selfs and interpopulation crosses, both sets of data being homogeneous (interpopulation contingency EX2(3) = 4.35; P 0.3-0.2, and selfing contingency EX2(3) = o.o8; P = >0.9). Comparing the sums of these two classes selfing is found to be significantly more fertile than interpopulation crossing (2X2(l)-I8.8; P<O.OOI). It is thus possible to detect a partial barrier to crossing between these widely separated, and probably long isolated, populations. Such a barrier may have arisen from the genetic divergence of the populations to such an extent that there is some failure of compensation and balance in the genotypes of the interpopulation hybrid zygotes. The unbalance may be so great as to lead to the lethality of some genotypes. Alternatively it may be that the barrier occurs before fertilization, in the failure of pollen-germination or of pollen-tube growth.

Germination rates The proportion of mature seeds which germinates is one of the factors which determine the ability of a species to persist in any habitat. Also, as the proportion of viable zygotes which arise from natural fertilizations, it may reveal the presence of lethal genotypes. Consequently, this important aspect of the breeding system has been examined in T. alpestre.

One hundred and twenty ripe seeds, collected in the field in June, from each of the



Genecology of Thlaspi alpestre L. 323 eleven populations under consideration were sown in sandy loam in seed trays in the following October. In order to test the maximum range of variability of the populations each seed was the progeny of a different seed parent. The mean proportion of germination over all populations was O.9358?0.0775. This extremely high germination rate may well be of survival value to a species such as T. alpestre which occupies a marginal habitat where few seeds will reach situations suitable for plant establishment, and where there will be heavy seedling mortality. Whilst there were statistically significant differences in the germination rates of some populations these were apparently not related either to geographical position or ecological situation.

It is of some interest to compare the germination rates of the seeds derived from the three types of artificial pollinations in populations T4, Di and Mi discussed earlier (Table 2). Unfortunately the ratios of seeds germinating: not germinating are statistically homogeneous, within blocks, only for selfs (contingency EX2(3) = 3.50; P = 0.5 - 0.25) and intrapopulation crosses (contingency ZX2(3) = 2.76; P = 0.5 -0.25). Similar data for natural field pollinations (contingency ZX2(3) - 8.7 I; P = 0.05 - 0.02) and interpopulation crosses (contingency ZX2(3) = 52.66; P = <o.ooi) are heterogeneous. Comparison of the summed data shows that the seeds derived from intrapopulation crosses had a significantly higher germination rate than those from selfs (2X2(l) = I .9I; P = <O.OOI), the respective ratios of germination: non-germination being 2I9: 37 and 226: 96. This result

Table 2. Germination

Pollination Number Number Proportion class Population sown germinated germinated

T4 1 I20 II7 o.96 Natural, field Di i20 io6 o.88

MI 120 II4 0.95

T4 io6 78 0.74 Self Di 96 6o o.68

M I 120 87 0.73

Intra- T4 62 50 o.8i population D I 97 8 I o.87 cross M I 97 88 0.9I

Inter- T4 x DI 88 6o o.68 population T4 XMI I70 I63 0.95 cross DI XMI 58 57 0.99

may well arise from the phenotypic expression of recessive lethal genes which were heterozygous in the selfed parents. However, it is difficult to equate this interpretation with the high germination rate of seeds from natural field fertilizations, since observations discussed later indicate that such seeds are largely the product of self-pollinations. Alternatively the low level of germination of the seeds from artificial selfing, relative to those collected in the field, may merely reflect the abnormal environmental conditions of their development and ripening following artificial pollination, and the higher fertility of seeds from intrapopulation crosses, in similar conditions, may be an expression of heterosis.

The germination rates of seeds from interpopulation crosses were, in general, as high as those from selfings and from intrapopulation crosses. There is thus no evidence that the partial reproductive barrier, which was detected in the low level of interpopulation cross-compatibility, is expressed as hybrid inviability. This may perhaps be taken to



324 RALPH RILEY

indicate that the partial incompatibility of interpopulation crosses was due to non- fertilization rather than to the genotypic lethality of the hybrid zygotes. Fertilization failure may be due either to poor germination of pollen or to the poor growth of the pollen tubes.

Insect visitors T. alpestre has pollen with the thick, ornamented, walls characteristic of the Cruciferae, and the pollen is too heavy for wind pollination to be effective. It is of interest therefore to know something of the insects that visit the flowers of this species and could act as pollinating agents.

Whilst the individual flowers are small a fully flowering inflorescence is quite con- spicuous, and at this stage insect visits are most frequent. In the experimental garden bees were seen visiting inflorescences with many flowers open and ignoring the interven- ing inflorescences with few open flowers. In the field several species of insects have been observed to visit T. alpestre, apparently preferentially. These included several bees, two species of hover fly, Syrphidae spp. Diptera; a species of tiny moth, Tineidae sp. Lepidoptera; and a fly, Bradycera sp. Diptera. In addition, the ubiquitous pollen beetle, Adrastinae sp. Coleoptera, was often seen on the anthers of T. alpestre but is unlikely to be important in pollination. Nine flies, two Lepidoptera, seventeen bees and one wasp are listed by Knuth (I908) as visiting T. alpestre. It seems therefore that there must be some pollen transfer by insects in T. alpestre, but before it can be assumed that any effective cross-pollination precedes self-pollination it is necessary to determine the pollination rates.

Pollination rates Whilst there is considerable information available on the proportions of self- and cross-polllination in nature, in species in which there is some self-incompatibility, relatively little is known about the detailed breeding systems of self-compatible species. Determinations that have been made on the breeding systems of self-compatible species have been dependent on the use of genetic markers. In this way, for example, it has been shown by Fyfe and Bailey (1952) that the outcrossing frequency is 30 per cent in Vicia faba and by Olsson (I952) that it is 33 per cent in Brassica napus oleifera and 99 per cent in Sinapis alba.

Thlaspi alpestre was found to be completely self-fertile but no genetic marker was available for scoring the breeding system. However, it was possible to obtain an estimate of the proportions of natural selfing and crossing from counts of the number of pollen grains on the stigmas of flowers at different stages of development. The counts were made by microscope on styles stained in lacto-phenol. For the purpose of interpretation the flowers from which the counted stigmas were taken were arranged into three classes: (i) those in which the petals had not expanded, (ii) those with expanded petals but in which no pollen had been released, (iii) those in which pollen had been released.

Similarly the inflorescences from which flowers had been taken were divided into: (i) those with no fully opened flowers, (ii) those with fully opened flowers, but in which no pollen had been released, (iii) those with fully opened flowers and dehiscent anthers. Twenty inflorescences of each class, from population Di, were counted (Table 3).

The mean number of pollen grains on stigmas from inflorescences in which there were no fully opened flowers was I.483, whilst the mean for inflorescences with some fully opened flowers but in which no pollen had been released was 2 (0.8IO + I.73 I) = I.2705.



Genecology of Thlaspi alpestre L. 325 Thus there had been some pollination by pollen from other inflorescences before the very rapid increase, probably due to self-pollination, in the amount of pollen on the stigmas of inflorescences in which some dehiscence had occurred. The mean proportion of stigmas exposed in an inflorescence before any dehiscence was 0.3066+0.0258 (n = 40), and the mean seed set per capsule in population Di was 7.4i66?0.I769.

Table 3. Pollination rate. (Mean number of pollen grains per exposed stigma)

\ Flower I Inflores- \ type Flower expanded, Flower expanded

cence type Unexpanded bud but no pollen released and pollen released

No flowers expanded and no I.483 ?0.2i6 pollen released

Flowers expanded and no pollen o.8io +0.I92 I.73I +0.29I

released

Flowers expanded and some pollen Io.937 ?3.3551 i8.474 ?5.3952 43-375 ?6.992' released

Plus the following foreign pollen: 1 z Compositae, i Gramineae, 2 Leguminosae or Ro3aceae. 2 I unidentified. 3 3 Compositae, i Gramineae, i6 Caryophyllaceae, i Plantago.

From these data the amount of outbreeding may be calculated as:

the amount of pollen from other inflorescences x the proportion of stigmas the proportion of ovules available to outcrossing exposed before dehiscence.

The proportion of outcrossing is therefore estimated as:

7.4766 x 0.3066 = 0.0525.

This estimate of 5.25 per cent outcrossing is open to error in a number of ways, firstly the pollen on the stigma of a flower in an inflorescence with no dehiscence may have come from another inflorescence of the same individual, or the same clone. Pollen germination and the achievement of a successful fertilization are also assumed. Further, no account is taken of the pollen from other individuals which reaches a stigma after release, has occurred in the same inflorescence. Observations suggest that insects are attracted by those inflorescences with many flowers open, that is in which anthers are dehiscent, to a greater extent than by the less significant inflorescences with few expanded flowers. In addition the pollen of other species was found only on the stigmas of the oldest type of inflorescence, and in as much as some of this was the pollen of en- tomophilous species it emphasizes the greater likelihood of insect visits to the older inflorescences. Errors in the estimate are thus likely in both directions, so that whilst this estimate should be treated with caution it may well indicate the approximate level of outbreeding, and the conclusion must be that the amount is low.

The protogynous nature of the floral mechanism therefore permits a low level of outbreeding, but the high level of self-pollination, indicated in the high pollen counts on



326 RALPH RILEY

stigmas of the oldest inflorescences, will maintain fertility. High fertility is no doubt of considerable importance to a species such as T. alpestre which occupies a marginal habitat.

Gene dispersal The maintenance of genetic heterogeneity either within or between populations depends upon those factors of the breeding system which determine the spatial relation- ships of cross-fertilizing individuals and the geographical origins of those individuals. These factors determine the extent, and the rate, of movements of genetic material through the species population. In plant species this gene migration may be by pollen or seed.

It has been shown that there is some cross-pollination in T. alpestre and since the pollen is relatively thick-walled and heavy this must be mainly effected by insects. In order to obtain some estimate of the extent of pollen movement, a series of groups of five to seven individuals of T. alpestre were planted out in an area near Matlock, from which the species was absent, at known distances from a natural colony. This situation resembled a common form of the distribution of the species in nature since individuals are often found in groups separated from one another by several yards. Initially buds and flowers with exposed stigmas were removed from the experimental plants. For the following five days as each bud loosened it was emasculated before any pollen was released, care being taken to avoid damage to the petals. When the experiment ended each individual had one or more inflorescences with several fully opened flowers, such as might have been attractive to insects. The styles were removed from the emasculated flowers and taken to the laboratory, in 70 per cent alcohol, where the number of pollen grains on each stigma was counted.

Table 4. Pollen dispersal Distance from Number Number Pollen per stigma pollen source of of

plants stigmas exposed Mean S.E. 5 yards 7 66 I-747I 0.2642

i oyards 6 56 o.8666 0.44I4 25 yards 6 52 0.7II6 0.2017 55 yards 7 62 O.I942 o.o984

I05 yards 5 39 0.0000 0.0000

About twice the amount of pollen travelled 5 yards as was carried iO yards (Table 4), but there was no significant difference in the amount carried IO yards and 15 yards. Very little travelled 55 yards or beyond. Under these conditions therefore the majority of cross-fertilization was between closely adjacent individuals, and relatively little between those more widely separated. However, in conditions in which plants or groups of plants were less widely spaced it may be that crossing between distant individuals would be more likely owing to the passage of the pollinating insects along the series of inter- vening plants. This may be a source of error in those experiments on the pollen contamination of crops in which stringers of plants are arranged to radiate outwards from a central source of contamination. Bateman (I947) found that contamination was not altered in Raphanus sativus and Brassica rapa by increasing the spacing of plants, and suggested that insects distribute themselves over the nectar-collecting area at densities determined by the availability of nectar. This would imply that in T. alpestre populations, insects would move about less where individuals were closely spaced than where they



Genecology of Thlaspi alpestre L. 327 were widely spaced. In situations with diff-erent spacings from those in the present experiment there should then be a similar fall-off in the amount of cross-pollination with distance, but the actual distance of pollen transport would be less with closer spacing.

It seems that the small amount of cross-pollination in T. alpestre must generally be between near neighbours, but that nevertheless more widely separated individuals may occasionally cross-pollinate. In the main, however, gene migration through pollen movement must be slow, and slight in extent, and it seems improbable that the largest populations are internally panmictic.

The germination of seeds some distance from the seed parent may lead to the establishment of plants with genotypes new to the area of establishment. Seeds from these plants may in turn germinate in the immediate vicinity, or be transported away, so as to increase the genetic variation in that area, or to carry it further. In this way gene migration may result from seed dispersal.

Whilst the seeds of T. alpestre are light they are unlikely to be blown far across the disturbed surface of the soil, or amongst the ground layer of vegetation, and there is no specialized mechanism of dispersal. However, the ripe raceme is very light and the stem brittle and whole fruiting inflorescences may occasionally be blown some distance. On two occasions complete racemes with some unbroken capsules were found more than one hundred yards from the nearest plants of T. alpestre. Thus whilst individual seeds travel only slight distances racemes with several seeds may be distributed over a much greater area. This may have important implications on the genotypic variability of newly established populations.

In the neighbourhood of lead mines it is noticeable that T. alpestre frequently grows on the rough roads for some distance before the main colony is reached. This may be related either to the suitability of the open habitat of the track or to the transport of seeds by users of the track. Certainly it seems likely that the establishment of populations of this species on the debris of so many distinct mines was due to the carriage of seeds from place to place by miners.

Thus whilst no precise data are available, it is probable that the majority of seeds remain in the vicinity of the seed parent, but a few may be carried by men or animals for unpredictable distances, and a very few may be dispersed for moderate distances without release from their capsules, in entire racemes. It is thus difficult to determine the influence of seed dispersal on gene migration, since that which takes place over the greater distances may be of an importance far in excess of that indicated by its infrequent occurrence in the establishment of new populations.

Population size In theoretical studies Wright (I940) has related the random fixation of genetic factors to the size of the breeding population. Further, the larger the population the greater the differences between the environments that different members are likely to encounter, so that, as a result of selection, the gross genetic variability of a population must be related to its size. In addition, the potential genetic variability of the progeny of any seed parent depends upon the range of variation, and the number, of potential pollen parents. The larger the population then the greater the number of cross-pollination combinations that are possible, so that in general a greater range of genetic variability may be tested each generation. For these reasons population size is a factor of the breeding system which may have considerable influence on the nature and speed of



328 RALPH RILEY

population differentiation, therefore some estimates have been made of the sizes of the Teesdale populations of T. alpestre.

In populations of limited area the total number of individuals was counted, and where tussocks coalesced the number of separate individuals was estimated. In large populations the number of plants was counted in i-yard quadrats taken at random throughout the area occupied. When there was difficulty in counting due to the density of cover the combined tussocks were dug up and each plant separated out. It is, however, the size of the breeding population, rather than the total size, which determines the probability of genetic drift, so that in addition to the total number in each quadrat the number of breeding individuals, either fruiting or flowering, was separately counted. From these counts the mean number of individuals, and the mean number breeding, per square yard were calculated (Table 5). The total area occupied by a population was estimated after its boundaries had been mapped. From these data the size of a population was calculated as the product of the mean number of individuals in i square yard and the number of square yards occupied. The standard error of this estimate was

Vs' individuals per square yard x (area in square yards)2.

The sizes of the Teesdale population of T. alpestre varied considerably, and in some cases were of the order at which the Sewall Wright effect could operate (Table 6). The apparent influence of differences in population size on population differentiation will be considered when this subject is discussed in another paper.

Table 5. Mean number of plants per quadrat of one square yard Number Number flowering Total number of plants

Population of quadrats Mean Variance Mean Variance

Ti 32I 7.398 258.030 I6.376 I2I0.000 Tz 293 2.266 I8.I40 6.I30 20I.i6o T5 242 2.669 I7.3 I I I I .340 3 I8.045

It is of interest that the variance of the number of individuals per square yard always considerably exceeded the mean (Table 5), so that the samples do not fit a Poisson distribution. The distribution of individuals within population areas is therefore not at random, and since the variance exceeds the mean, individual plants are under-distributed. That is plants occur more frequently in groups than they would if randomly distributed. This situation may in part arise from a limited range of seed dispersal, so that each new generation is established, on the whole, in the immediate vicinity of its seed parent. Ob- servation in the population areas, however, indicated the importance of the localization of suitable micro-habitats for plant establishment.

Table 6. Estimate of the total size, and the effective breeding size, of populations Number Area in Breeding size Total size

Population of square quadrats yards Mean S.E. Mean S.E.

Ti 32I 5,987 44,29I i5,367.5 98,046 ? I2,297.0 T2 293 14,224 32,232 ?4,697.6 87,395 ? I I,785.0 T3* 393 72z T5 242 3,200 8,542 ? 855.8 36,293 i 868.9 T6* - 235 - 764 T7* I -I 4I4

* Sizes obtained by direct counts.



Genecology of Thlaspi alpestre L. 329

DISCUSSION

Thlaspi alpestre has a protogynous floral mechanism, several insect visitors and an estimated out-pollination rate of 5.25 per cent. However, it is self-compatible and the floral mechanism promotes very liberal self-pollination should cross-pollination not have taken place. There is thus some outcrossing and genetic segregation and recombination will result, although an estimate of the range of pollen dispersal suggests that most cross-pollination will be between neighbouring and, because of limited seed dispersal, probably genetically similar, individuals. However, the safety device of high self- pollination will ensure high seed production. High fertility, the genetic stability resulting from inbreeding, and the high germination rate make the species well adapted to maintain itself in its marginal habitat. The breeding system may thus be regarded as a selective adaptation to this type of environment, and the 5.25 per cent level of outbreeding may either represent the failure of complete adaptation, or may be selectively main- tained. Selective retention seems more probable, and this would imply that in the breeding system the compromise between the needs of genetic constancy and regulated change, discussed by Mather (I943) and Darlington (1946), had been resolved in favour of the former. However, whilst a high level of self-pollination will favour stability and the perpetuation of the population, the small amount of outbreeding may be all- important in the production of genetic variation upon which selection may operate.

The limited gene migration must result in a considerable degree of reproductive isolation between populations of T. alpestre, even between those which are topographically close to each other. Since such a situation is likely to lead to genetic differentiation it is interesting to note evidence of this in the partial cross-incompatibility of plants from distinct occurrences of the species. The diversification likely to result from the absence of inter-population gene-flow may be further increased in T. alpestre by the colonial nature of the distribution of the species in suitable habitats of limited area. Thus the species is broken up into many colonies, small in area, and largely reproductively isolated.

It is of some interest to attempt to visualize the genetic structure of populations of T. alpestre in terms of the breeding system. However, this must be speculative since there is no evidence of the operation of selection upon various components of the populations. Thus although the Fl's of intrapopulation crosses should comprise about 5 per cent of the population, assuming the random establishment of seedlings, the composition of the remaining 95 per cent must depend upon the relative reproduction rates of F1's and their increasingly homozygous later generation derivatives, and various inbred lines. These factors, and the natural occurrence of topcrosses and backcrosses are unknowns in a natural population, and no more than a vague outline of the genetic structure may be discerned. Assuming genotypic variability at the establishment of a population, the segregants from intrapopulation hybrids and the inbred derivatives of the founding biotypes would provide genetically based phenotypic variability upon which selection could operate. Lines with selectively favoured phenotypes would quickly become established, and those derived from the later generations of outcrosses would be composed of more or less homozygous individuals. Thus the population would consist of a number of inbred lines of well-established adaptive value. The mean phenotypes of the different lines would be distributed more or less at random about the selective optimum, the differences between lines being genetically determined. Variabilitywithin a line would be almost entirely phenotypic. In an old established population there would also be the early generation derivatives of hybrids between different inbred lines. In populations of



330 RALPH RILEY

this genetic structure certain lines would continually have a selective advantage and others would fail and disappear. New inbred lines would generally be established from hybrids of which at least one of the parents was a member of a line which already had some selective advantage.

The genetic structure of the species may perhaps be regarded as an enlarged version of that of its populations, its essentials being a large number of inbred lines, each with a considerable degree of homozygosity, and differing from each other genetically. Popula- tions will differ from one another according to the distribution of the phenotypes of the inbred lines of which they are composed. The high rate of inbreeding which results in the production of inbred lines within populations will also maintain differences between populations.

In T. alpestre, then, the adaptive compromise between fitness and flexibility has been resolved in such a way as to severely limit outbreeding. It would seem that each population may, within its resources of genetic variability, become adapted to its own habitat without the reduction of this adaptation by contamination with the pollen of genotypic- ally unadapted parents. Paradoxically, therefore, the extensive genetic diversity of the species, as revealed by its taxonomic difficulty, appears to be due to adaptations of the breeding system selected to reduce the variability of each population.

It is a pleasure to acknowledge the help and advice of Dr. J. M. Thoday in all aspects of this work.

SUMMARY

(i) Thlaspi alpestre has a protogynous floral mechanism and artificial pollinations have shown that it is self-compatible and freely cross-compatible within populations, although there is some incompatibility in interpopulation crosses.

(2) It is estimated that about 95 per cent of natural pollinations are self-pollinations; and the germination rate is high. It is suggested that these two factors may be of importance to a species which occupies a marginal habitat because of the resulting high fertility and genetic stability. The possible significance, in the evolution of the species, of even the low rate of cross-pollination which was estimated is recognized.

(3) Visits of insects to the flowers of T. alpestre are adequate to ensure some cross- pollination. However, it is estimated that pollen is transported only short distances, so that gene migration by pollen is slight. The influence of seed dispersal on pollen migration is discussed.

(4.) The possible influence of the breeding system on the evolution and intraspecific differentiation of T. alpestre is considered.

REFERENCES BATEMAN, A. J. (I947). Contamination of seed crops. I. Insect pollinations. J. Genet., 48, 257. CLAPHAM, A. R., TUTIN, T. G. & WARBURG, E. F. (1952). Flora of the British Isles. Cambridge. CLAUSEN, J., KECK, D. D. & HIESEY, W. M. (1945). Experimental taxonomy. Carnegie Inst., Washington,

Yearbook, No. 44, 7'. DARLINGTON, C. D. (1946). The Evolution of Genetic Systems. Cambridge. FYFE, J. L. & BAILEY, N. T. J. (1951). Plant breeding studies in leguminous forage crops. I. Natural

cross-breeding in winter beans. J. Agric. Sci., 41, 371. JORDAN, A. (I846). Observations sur plusiers plantes nouvelles, rare ou critique de la France. II. Ann.

Soc. Linnegenne de Lyon. KNUTH, P. (I908). Handbook of Flower Pollination. Oxford. MATHER, K. (I943). Polygenetic inheritance and natural selection. Biol. Rev., 18, 32. OLSSON, G. (1952). Undersoking av graden av korabefruktning hos vitserap och raps. Sv. Utsadesforenings

Tidskrift, 62, 31 '. WRIGHT, S. (1940). The statistical consequences of Mendelian heredityi n relation to speciation. The New

Systematics (ed. J. S. Huxley), p. i6i.



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The Influence of the Breeding System on the Genecology of Thlaspi alpestre L