Sperm competition and the dynamics of X chromosome drive in fi nite and structured populations

Of the several deleterious consequences that sex chromosome meiotic drive can have for a population, extinction is the most severe. Several studies of this phenomenon have suggested that males carrying a driving X chromosome may be disadvantaged during sperm competition. Deterministic modeling indicates that sperm competition can maintain a balanced polymorphism of driving and non-driving X chromosomes, but that if the frequency of the driving chromosomes exceeds some threshold value, then these chromosomes will spread to fi xation and the population will go extinct. In this article we present the results of individual-based simulations of a stochastic model of X chromosome drive in fi nite and structured populations. We show that in large populations the balanced polymorphism can be maintained for hundreds of thousands of generations, but that reductions in population size and certain forms of population structure promote fi xation of the driving chromosome.