ASSORTATIVE MATING AND NATURAL-SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic v ariation among flowering plants and their progeny were examined to ass ess the levels of assortative mating and selection in a hybrid populat ion of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of paren tal genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking inte rmediate genotypes. Early in the season only I. fulva genotypes produc ed Bowers, but as flowering in these plants decreased, the hybrid geno types and I. brevicaulis genotypes began flowering, resulting in a 24- d period of coincidental flowering. The genotypic distribution of seed s produced during the period of flowering overlap contained a high fre quency of intermediate genotypes that were not present in the adult ge neration. The degree of effective assortative mating was examined by c omparing the observed progeny genotypic distributions with expected di stributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowe rs on the day the recipient Bower was receptive. The observed genotypi c distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribu tion. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate g enotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were abor ted, indicating that intermediate genotypes have lower viability. On t he basis of the observed progeny genotypes and genetic disequilibria e stimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry i s most likely due to pollen-style interactions that reduce the fertili zation ability of genetically dissimilar pollen, or preferential abort ion of genetically intermediate zygotes by I. fulva-like genotypes. Th e lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genoty pes.