Influence of microhabitat heterogeneity on gene frequency distribution and gametic phase disequilibrium in Avena barbata [wild oats, California]

Seeds were collected from 23 sites located within a Napa Valley hillside stand of the slender wild oat, Avena barbata. At the time of collection each site was assigned an environmental rating which ranged from mesic to xeric. In seven of the sites (Sites 1-7) seeds were collected from 250 individuals and each site was divided into six subsites. In the other sixteen sites (A-P) seeds were collected at random from a variable number of individuals. Seedlings from each individual were analyzed for five electrophoretically determined enzyme loci. The results indicated that both environmental and allozyme variation was distributed patchily among the collection sites. Furthermore, allelic frequencies were highly correlated with the environmental conditions; i.e., allele frequencies were predicted by the a priori habitat ratings. Similar associations were found between microhabitat and genetic variation in the subdivisions of Sites 1-7, indicating that the physical dimensions of the biologically relevant microhabitats may be quite small. Large interlocus associations of alleles also exist and subsequent analyses demonstrated the existence of significant gametic phase disequilibrium in all of the sites and most of the subsites studied. Two five-locus gametic types were found to predominate throughout the hillside. Both had much higher frequencies than expected assuming no interlocus association (67% observed vs. 15% expected). These two gametes are the same two that predominate in monomorphic mesic and xeric populations of A. barbata. These results and those of previous studies lead us to the conclusion that natural selection has shaped the genetic structure of A. barbata on this hillside. The action of genetic drift, even combined with gene flow, cannot sufficiently explain the associations between the electrophoretic genotypes and the patchily distributed microhabitats. From this study, however, we cannot determine whether selection is acting on the electrophoretic variants directly or just on the genomes which they mark. Given the predominately selfing mating system and the nearness of large monomorphic populations of both genotypes, it is also difficult to distinguish whether selection acts independently at each locus, or whether selection is epistatic. Although we feel that co-adaptation is likely to exist within the genome of A. barbata, we hesitate to claim that the five allozyme loci studied here belong to such a co-adapted complex.