Some Genetic Consequences of Being a Plant. In: Brussard PF, editor. Ecological Genetics: The Interface. New York: Springer; 1978. pp. 189-212. [DOI: 10.1007/978-1-4612-6330-2_11]

Some sexual consequences of being a plant

Plants have characteristic features that affect the expression of sexual function, notably the existence of a haploid organism in the life cycle, and in their development, which is modular, iterative and environmentally reactive. For instance, primary selection (the first filtering of the products of meiosis) is via gametes in diplontic animals, but via gametophyte organisms in plants. Intragametophytic selfing produces double haploid sporophytes which is in effect a form of clonal reproduction mediated by sexual mechanisms. In homosporous plants, the diploid sporophyte is sexless, sex being only expressed in the haploid gametophyte. However, in seed plants, the timing and location of gamete production is determined by the sporophyte, which therefore has a sexual role, and in dioecious plants has genetic sex, while the seed plant gametophyte has lost genetic sex. This evolutionary transition is one that E.J.H. Corner called 'the transference of sexuality'. The iterative development characteristic of plants can lead to a wide variety of patterns in the distribution of sexual function, and in dioecious plants poor canalization of reproductive development can lead to intrasexual mating and the production of YY supermales or WW superfemales. Finally, plant modes of asexual reproduction (agamospermy/apogamy) are also distinctive by subverting gametophytic processes. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

THE EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION IN PLANTS. II. EMPIRICAL OBSERVATIONS

A bimodal distribution of outcrossing rates was observed for natural plant populations, with more primarily selfing and primarily outcrossing species, and fewer species with intermediate outcrossing rate than expected by chance. We suggest that this distribution results from selection for the maintenance of outcrossing in historically large, outcrossing populations with substantial inbreeding depression, and from selection for selfing when increased inbreeding, due to pollinator failure or population bottlenecks, reduces the level of inbreeding depression. Few species or populations are fixed at complete selfing or complete outcrossing. A low level of selfing in primarily outcrossing species is unlikely to be selectively advantageous, but will not reduce inbreeding depression to the level where selfing is selectively favored, particularly if accompanied by reproductive compensation. Similarly, occasional outcrossing in primarily selfing species is unlikely to regularly provide sufficient heterosis to maintain selection for outcrossing through individual selection. Genetic, morphological and ecological constraints may limit the potential for outcrossing rates in selfers to be reduced below some minimum level.

Natural selection of allozyme polymorphisms: a microgeographical differentiation by edaphic, topographical, and temporal factors in wild emmer wheat (Triticum dicoccoides)

Allozymic variation in proteins encoded by 47 loci was analyzed electrophoretically in 1983/4 and 1984/5 in 356 individual plants of wild emmer wheat, Triticum dicoccoides, from a microsite at Tabigha, north of the Sea of Galilee, Israel. Each year the test involved two 100-meter transects, each equally subdivided into basalt and terra rossa soil types, and comparisons were based on 16 common polymorphic loci. Significant genetic differentiation, genetic phase disequilibria, and genome organization according to soil type were found over very short distances. Our results suggest that allozyme polymorphisms in wild emmer wheat are partly adaptive, and that they differentiate at both single and multilocus structures primarily from environmental stress of such ecological factors as soil type, topography, and temporal changes, probably through aridity stress.

Impact of population structure on the apparent outcrossing rate of grain sorghum (Sorghum bicolor)

Population structure, the physical arrangement of related and unrelated individuals, can have profound effects on the apparent outcrossing rate ([Formula: see text]) in a plant population. However, detailed experimental investigations of the impact of population structure on [Formula: see text] are few. We compared the apparent outcrossing rates of experimental populations of grain sorghum with seed families spatially arranged in stratified and overdispersed treatments. Using alcohol dehydrogenase allozymes as genetic markers, [Formula: see text] was calculated for each of the treatments at two locations over three years. For all six comparisons, the overdispersed treatment yielded significantly larger apparent outcrossing estimates than the stratified treatment. In one case, the difference was over five-fold. Site-specific and time-specific differences were small compared to treatment differences. Whether natural structuring plays a role in altering the effective outcrossing rates of natural populations has been addressed by only a few descriptive studies; structuring effects appear to have an impact in only about half of such studies. The sample is still too small to make any generalizations.Population structuring can also be of significance in plant breeding programs. Controllable variation in [Formula: see text] values of the magnitude reported herein may be useful in optimizing selection methods for quantitative characters in experimental plant breeding populations. Further work is under way to determine the effects of the variation in apparent outcrossing rates on genetic gains from selection.