On the Evolution of Genetic Incompatibility Systems. II. Initial Increase of Strong Gametophytic Self-Incompatibility Under Partial Selfing and Half-Sib Mating

This study addresses the origin of gametophytic self-incompatibility in populations undergoing a mixture of random outcrossing, sib mating, and selfing. First, I describe the effect of the incompatibility system on the genotypic distribution among offspring derived from the three pollen sources. I partition the total change in average additive genotypic value into components reflecting trade-offs in offspring number, parent-offspring relatedness, and gametic selection. Second, the conditions for the initial increase of a self-incompatibility allele with full activity are presented under the assumptions of complete dominance and complete recessivity of stylar expression. Low rates of inbreeding and recessivity of stylar expression of the incompatibility reaction promote the introduction of gametophytic self-incompatibility. I examine the components of selection that contribute to these results. Third, I assess the findings of the analysis with respect to the hypothesis that incompatibility systems function as a means of regulating outcrossing distance as well as avoiding self-fertilization. I review some of the empirical evidence concerning recessivity in the stylar rejection reaction and speculate on the response of inbreeding depression to the introduction of self-incompatibility.