The evolution and breakdown of S-allele systems

Abstract

We have examined a model proposed by East (1929) for the evolution of gametophytic self-incompatibility allele systems, starting from populations with a self-fertility allele. The original self-fertility allele is not eliminated from the population when three active S alleles have been incorporated, but self-fertility alleles can coexist with S alleles. In order to examine this coexistence more fully, we studied the invasion of self-incompatible populations containing various numbers of S alleles (all assumed to be equally frequent) by three possible types of self-compatibility factors that could arise by mutation at the S locus. We find that there is always some critical number of active S alleles that ensures the elimination of mutant alleles with no activity in pollen; this number depends on the inbreeding depression. Alleles abolishing the stigma activity can spread, irrespective of how many active alleles are present, provided the inbreeding depression is small enough. Some of the biological implications of these results are discussed. © 1979 The Genetical Society of Great Britain. All Rights Reserved.