EVOLUTION OF SEGREGATION DISTORTION - POTENTIAL FOR A HIGH-DEGREE OF POLYMORPHISM

By means of a population genetical model, we study the evolution of se gregation distortion. Most models of segregation distortion focus on a single distorter allele. In contrast, we consider the competition bet ween a large number of distorters. Motivated by systems as the t compl ex of the house mouse or the Sd complex of Drosophila melanogaster, we assume that there is some ''complementation'' between distorter allel es, i.e. that the fitness of individuals heterozygous for two distorte r alleles is higher than the fitness of homozygous individuals. In the presence of complementation, the most efficient distorter allele with the highest segregation ratio often does not outcompete less efficien t distorters. In fact, our results show that coexistence of a large nu mber of distorter alleles is more typical than the competitive exclusi on of less efficient distorters by a single superior allele. We first consider the analytically tractable system where all distorters show t he same amount of complementation. In this case, all distorters with a segregation ratio higher than a certain critical value will persist, resulting in a polymorphic population where the average segregation ra tio is only slightly larger than 0.5. If the degree of complementation varies, there may be more than one stable equilibrium, and the outcom e of competition may depend on the initial conditions. Motivated by em pirical examples, we also consider the case that the distorting abilit y of an allele is negatively related to its effects on individual fitn ess. Interestingly, the outcome of competition depends crucially on de tails of such a trade-off. We conclude that verbal arguments are insuf ficient to predict the evolution of segregation distortion. (C) 1998 A cademic Press Limited.