EVOLUTIONARY DYNAMICS OF SPOROPHYTIC SELF-INCOMPATIBILITY ALLELES IN PLANTS

The stationary frequency distribution and allelic dynamics in finite p opulations are analyzed through stochastic simulations in three models of single-locus, multi-allelic sporophytic self-incompatibility. The models differ in the dominance relationships among alleles. In one mod el, alleles act codominantly in both pollen and style (SSIcod), in the second, alleles form a dominance hierarchy in pollen and style (SSIdo m). In the third model, alleles interact codominantly in the style and form a dominance hierarchy in the pollen (SSIdomcod). The SSIcod mode l behaves similarly to the model of gametophytic self-incompatibility, but the selection intensity is stronger. With dominance, dominant all eles invade the population more easily than recessive alleles and have a lower frequency at equilibrium. In the SSIdom model, recessive alle les have both a higher allele frequency and higher expected life span. In the SSIdomcod model, however, loss due to drift occurs more easily for pollen-recessive than for pollen-dominant alleles, and therefore, dominant alleles have a higher expected life span than the more reces sive alleles. The process of allelic turnover in the SSIdomcod and SSI dom models is closely approximated by a random walk on a dominance lad der. Implications of the results for experimental studies of sporophyt ic self-incompatibility in natural populations are discussed.