BOTTLENECK-INDUCED DISSOLUTION OF SELF-INCOMPATIBILITY AND BREEDING SYSTEM CONSEQUENCES IN ASTER FURCATUS (ASTERACEAE)

Aster furcatus is a rare species with extremely limited genetic variat ion at isozyme loci. We utilized crossing experiments and seed set dat a obtained from natural populations to verify that there is also littl e allelic variation at the self-incompatibility (S) locus. Seed set in several populations was limited by a low number of S-alleles. Associa ted with a low number of S-alleles in populations was the dissolution of the incompatibility system, manifest by individual variation in sel f-compatibility, and by complex dominance relationships among S-allele s. Plant self-compatibility was correlated with mean number of ovules per inflorescence. Thus, self-compatibility appeared to be under parti al environmental influence. Computer simulations revealed that the sha pes of seed set distribution curves of modeled self-incompatible plant populations depend on the number of incompatibility alleles in the po pulations. By varying the number of S-alleles in modeled populations, we generated seed set distribution curves similar to those of natural populations. Genetic bottlenecks reduce the number of S-alleles and th e proportion of compatible matings in populations of multiallelic self -incompatible species. Self-compatible genotypes are at a selective ad vantage in populations that lack a sufficient number of S-alleles to p roduce compatible crosses. Aster furcatus appears to be evolving self- compatibility as a result of bottleneck-induced losses of S-alleles.