Coevolution of the major histocompatibility complex and the t-complex in the mouse. II. Modification of response to sharing of histocompatibility antigens.

Selective pressures imposed by high complementarity associations between the major histocompatibility complex (MHC) and the t-complex on a locus that modifies the expression of prezygotic and postzygotic incompatibility are investigated through the analysis of a quantitative model. Sharing of MHC antigens between mates or between mother and offspring elicits weak inhibition of conception or gestation. In the presence of high complementarity associations between the MHC and the t-complex, weak incompatibility depresses the mean fitness of the population. Nevertheless, natural selection favors the enhancement of the expression of incompatibility if the number of antigens associated with the +-haplotype exceeds the number associated with the t-haplotype by a sufficient margin. Under absolute linkage between the modifier locus and the t-complex, the number associated with the +-haplotype need only be greater than the number associated with the t-haplotype. In the absence of linkage, a twofold difference is sufficient to ensure the initial increase of modifier alleles that intensify the expression of incompatibility.