ALLELIC GENEALOGIES IN SPOROPHYTIC SELF-INCOMPATIBILITY SYSTEMS IN PLANTS

Expectations for the time scale and structure of allelic genealogies i n finite populations are formed under three models of sporophytic self -incompatibility. The models differ in the dominance interactions amon g the alleles that determine the self-incompatibility phenotype: In th e SSIcod model, alleles act codominantly in both pollen and style, in the SSIdom model, alleles form a dominance hierarchy, and in SSIdomcod , alleles are codominant in the style and show a dominance hierarchy i n the pollen. Coalescence times of alleles rarely differ more than thr eefold from those under gametophytic self-incompatibility, and transsp ecific polymorphism is therefore expected to be equally common. The pr eviously reported directional turnover process of alleles in the SSIdo mcod model results in coalescence times lower and substitution rates h igher than those in the other models. The SSIdom model assumes strong asymmetries in allelic action, and the most recessive extant allele is likely to be the most recent common ancestor. Despite these asymmetri es, the expected shape of the allele genealogies does not deviate mark edly from the shape of a neutral gene genealogy. The application of th e results to sequence surveys of alleles, including interspecific comp arisons, is discussed.