RECOMBINATION SYSTEM AND POPULATION STRUCTURE IN OENOTHERA.

Abstract

Reproduction involves one of three recombination systems-outcrossing, selfing, or cloning-or some combination of the three (Fryxell, 1957). How these reproductive modes affect the population structure of a species has been the subject of considerable discussion (Fryxell, 1957; Grant, 1958, 1971; Baker, 1959; Wright, 1969; Levin and Kerster, 1971; Jain, 1976). The theory generated by this discussion predicts that different recombination systems ought to strongly influence how the genetic variation of a species is organized. Patterns of heterozygosity and population differentiation may be influenced by the recombination system (Baker, 1959; Wright, 1969; Levin and Kerster, 1971; Jain, 1976; Lokki, 1976; Brown, 1979). Populations of predominant selfers are expected to have lower levels of heterozygosity and intrapopulation polymorphism, but higher levels of interpopulation differentiation than obligate outcrossers (Wright, 1969; Jain, 1976). Selfing species as a whole are expected to be less heterozygous and polymorphic than outcrossers (Jain, 1976). On the other hand, clonal species should have higher levels of heterozygosity than outcrossing species, both within populations and species-wide, since they can retain mutant alleles by sheltering them from segregation (Lokki, 1976). But, like selfers, clonal species are expected to have low levels of within-population polymorphism and high interpopulation differentiation (Baker, 1959; Levin and Kerster, 1971). The ideal test of these hypotheses would be to compare population structure of three closely related, sympatric species: an obligate outcrosser, a selfer, and a clonal species. However, studies attempting to determine the influence of recombination system on population structure have compared related taxa representing only two reproductive modes; in most cases, the related taxa were allopatric. Comparisons among animals have been those of obligate outcrossing to unisexual species (Parker and Selander, 1976; Atchley, 1977; Parker et al., 1977; Schultz, 1977; Suomalinen et al., 1977). Conversely, students of plant population structure have concentrated mainly on comparing outcrossers with selfers (Solbrig, 1972; Gottlieb, 1973, 1977; Levy and Levin, 1975; Levin, 1978; Brown and Jain, 1979). But no one has yet investigated sympatric congeners, each with one of the three different reproductive modes. A model system for the study of how reproductive mode affects population genetic structure is provided by three closely related, geographically and ecologically sympatric species of Oenothera subsection Raimannia (Stubbe and Raven, 1979): Oenothera grandis Smyth, 0. mexicana Spach, and 0. laciniata Hill. Each of these annuals utilizes a different recombination system: Oenothera grandis is a self-incompatible outcrosser (Cleland, 1968); Oenothera mexicana exhibits a high degree of selfing (Ellstrand, 1978); and Oenothera laciniata is a permanent translocation heterozygote (Cleland, 1968), functionally a clonal species (see Cleland, 1972, for a detailed description of this unusual syndrome). These three species cooccur in disturbed areas of south and central Texas, often in mixed populations (see Ellstrand, 1978 for distribution). The purpose of this study is to determine the population genetic correlates of different re-