COMPETITION BETWEEN SEGREGATION DISTORTERS - COEXISTENCE OF SUPERIOR AND INFERIOR HAPLOTYPES AT THE T-COMPLEX

By means of population genetical models, we investigate the competitio n between sex-specific segregation distorters. Although the models are quite general, they are motivated by a specific example, the t comple x of the house mouse. Some variants at this gene complex, the t haplot ypes. distort Mendelian segregation in heterozygous males in their fav or. The selective advantage at the gamete level is counterbalanced by strong negative fitness effects at the individual level (male sterilit y or even lethality in both sexes). A plethora of different t haplotyp es has been found, both in the field and in the lab. Up to now, howeve r, models have focused on the equilibrium frequency of a single t hapl otype. In contrast, we explicitly model the competition between severa l t haplotypes, A deterministic model for a large, well-mixed populati on predicts a surprisingly high degree of polymorphism. Haplotypes wit h seemingly inferior fitness characteristics may easily coexist with ' 'superior'' haplotypes. For instance, a lethal haplotype with a low se gregation ratio may stably coexist with a sterile haplotype with a hig h segregation ratio, Stable coexistence is even possible for haplotype s with a segregation disadvantage. A simple stochastic model shows tha t the same principles apply in the context of a structured metapopulat ion. Although counterintuitive at first sight, all our results can be explained by the fact that segregation distorters have an inherent adv antage when they are rare. We conclude that fitness comparisons are no t sufficient to predict the outcome of competition when selective forc es are acting at different levels.