Segregation Distortion in Unstructured and Structured Populations: Competition Between 'Sterile' t Haplotypes

Abstract

By means of two simple models we investigate the competition between sex-specific segregation distorters in unstructured and structured populations. The models are motivated by the t complex of the house mouse. Some variants at this gene complex, the t haplotypes, distort Mendelian segregation in their favour in heterozygous males. The selective advantage at the gamete level is counterbalanced by strong negative fitness effects at the individual level. A large number of t haplotypes with varying degrees of segregation distortion has been found. In order to address this phenomenon we explicitly model the competition between two t haplotypes which induce male sterility when homozygous. Surprisingly, a distorter which is inferior at the gamete level and equivalent in every other respect to a more efficient distorter may well persist in a population. We argue that rare distorters are inherently favoured, and that, as a result, fitness considerations alone are not sufficient to predict the outcome of competition. Since 'sterile' t haplotypes are not only influenced by gamete and individual selection, but also by selection at the level of the group, we furthermore study the relation between unstructured and structured populations. It is shown that the persistence of a seemingly inferior distorter is also possible in a structured population. In contrast, a single efficient distorter with high segregation ratio may not even be able to persist in a structured population. Hence, in a metapopulation with migration between local demes, the segregation ratio is an even worse predictor of the evolutionary success of a segregation distorter than in an unstructured population.