Intraspecific variation in population gene diversity and effective population size correlates with the mating system in plants.

Abstract

Published data on allele frequencies at isozyme loci in inbreeding and outbreeding plant species were analyzed to examine intraspecific variation in gene diversity and effective population size (Ne). Compared with outbreeders, inbreeding species showed markedly greater variation among populations in average values of Nei's gene diversity statistic. Effective population size was estimated by assuming that the variation observed at isozyme loci is selectively neutral. Inbieeding species showed greater levels of variation in Ne than did outbreoders, although the upper range ofNe was similar in the two classes of species. The results suggest that there may be considerable genetic variation and potential for evolutionary change in some but not all populations of inbreeders. Moreover, these findings are important with respect to the conservation of genetic resources. In particular, that the amount of intraspecific variation in population genetic diversity and Ne differs between inbreeding and outbreeding species should be taken into account in sampling efforts designed to optimize the diversity of germplasm collections.Studies ofgenetic variation in plants have typically used Nei's genetic diversity statistics or Wright's F statistics as tools for describing the extent of differentiation among populations (1)(2)(3). While informative, these statistics do not provide direct information about the amount of among-population variation in levels of polymorphism and gene diversity, a feature that can be of key importance in efforts to conserve the genetic resources of a species. In this regard, a survey of isozyme variation in several plant populations conducted by Brown (4) suggested that inbreeding species will tend to show greater variation among populations in the level of genetic diversity compared with outbreeding ones. Brown (4) proposed that, in inbreeders, among-population variation in the average value of Nei's gene diversity statistic will often exhibit an L-shaped or bimodal distribution. Published data for the predominantly self-pollinating plants Hordeum spontaneum and Lycopersicon pimpinellifolium supported this view (4).In this investigation, we analyze additional published isozyme data from inbreeding and outbreeding plant species in order to explore in detail the nature and implications of variation in genetic diversity among populations of inbreeding and outbreeding species. This is done by examining the level of among-population variation in gene diversity. We have also asked what the observed patterns of variation in diversity imply about the amount of variation in effective population size in species with contrasting breeding systems.MATERIALS AND METHODS Published tables of allozyme frequency data were used in all analyses described below. These data sets were selected according to the following criteria: (i) the mating system of the species in question was known either through progeny testing of open-pollinated seed parents (5), from analysis of genotype frequencies (6), or based on the obs...

Repeatability of adaptation in experimental populations of different sizes

“…However, many isolated microbial populations, such as pathogens initiating an infection, will have their effective population sizes in the range investigated here following environmental change or colonization of new habitats. While they are of the same order as species such as Caenorhabditis elegans (with an estimate of 8 Â 10 4 ) [54] and many plant populations (with estimates of 10 3 -10 4 ) [55], our results are probably only directly relevant to asexual populations without standing genetic variation.…”

A Metapopulation Perspective on Genetic Diversity and Differentiation in Partially Self-Fertilizing Plants

“…The maintenance of genetic variation at the metapopulation level is a function of (among other things) levels of differentiation among local populations (Whitlock and Barton 1997). Population bottlenecks are also expected, on theoretical grounds, to be more extreme in inbreeding plant species where, in principle, a single seed can establish a new population (Baker 1955;Schoen and Brown 1991). Thus, it is possible that metapopulation dynamics (i.e., recurrent extinctions and recolonizations) can explain the observed reductions in genetic diversity within local populations of selfing species (Charlesworth et al 1997;Liu et al 1998;Pannell and Charlesworth 2000).…”

Molecular Evolution of the GapC Gene Family in Amsinckia spectabilis Populations That Differ in Outcrossing Rate

“…That the tests are significant only in the noncoding regions is likely a reflection of the very low levels of polymorphism in exons 6, 7, and 8. While reduced effective population size associated with inbreeding, bottlenecks that occur during colonization, and background selection are all more likely to occur in selfing as opposed to outcrossing taxa, and thereby render the detection of purifying selection more difficult (Schoen and Brown 1991;Charlesworth et al 1993;Hill and Robertson 1966), these statistics do not discriminate among the effects of background selection, selective sweeps, and other factors such as population history. Values for other, unlinked loci would be useful to rule out or confirm population dynamics as an important force in determining levels of polymorphism in this system.…”

“…In the extreme case, within-population diversity should be reduced by one-half in completely selfing versus completely outcrossing populations at mutation-drift equilibrium. Plant allozyme variation studies show that selfing is generally associated with a reduction in genetic variation (Hamrick and Godt 1990;Schoen and Brown 1991). Apart from this direct effect, selection at linked sites may also have an influence on levels of diversity at neutral loci in selfing populations.…”