SEGREGATION DISTORTION IN UNSTRUCTURED AND STRUCTURED POPULATIONS - COMPETITION BETWEEN STERILE T-HAPLOTYPE

By means of two simple models we investigate the competition between s ex-specific segregation distorters in unstructured and structured popu lations. The models are motivated by the t complex of the house mouse. Some variants at this gene complex, the t haplotypes, distort Mendeli an segregation in their favour in heterozygous males. The selective ad vantage at the gamete level is counterbalanced by strong negative fitn ess effects at the individual level. A large number of t haplotypes wi th 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. Surpris ingly, a distorter which is inferior at the gamete level and equivalen t in every other respect to a more efficient distorter may well persis t in a population. We argue that rare distorters are inherently favour ed, and that, as a result, fitness considerations alone are not suffic ient to predict the outcome of competition. Since 'sterile' t haplotyp es are not only influenced by gamete and individual selection, but als o by selection at the level of the group, we furthermore study the rel ation between unstructured and structured populations. It is shown tha t the persistence of a seemingly inferior distorter is also possible i n a structured population. In contrast, a single efficient distorter w ith high segregation ratio may not even be able to persist in a struct ured population. Hence, in a metapopulation with migration between loc al demes, the segregation ratio is an even worse predictor of the evol utionary success of a segregation distorter than in an unstructured po pulation.