Evolution of homomorphic sporophytic self-incompatibility

Abstract

A population genetic model is described for the evolution of sporophytic self-incompatibility by successive mutations to active incompatibility alleles from an ancestral compatibility, Sf, allele, homozygotes for which are capable of self-fertilisation. For spread of the active incompatibility alleles there must be strong inbreeding depression. It is shown that polymorphism for the ancestral allele and the active alleles is generated, unless the number of active alleles becomes very large. Thus, although there is selection for alleles that cause outcrossing, such polymorphic populations would not be completely outcrossing, but would have selling rates between zero and the selfiing rate of the homozygote for Sf. The equilibrium frequencies of the active alleles depend on whether dominance or independent action is assumed for the alleles in the pistil. In either case the equilibrium allele frequencies are higher than would be the case for gametophytically acting alleles.