Evolution of driving X chromosomes and resistance factors in experimental populations of Drosophila simulans

Sex-ratio drive is a particular case of meiotic drive, described in several Drosophila species, that causes males bearing driving X chromosome to prod uce a large excess of females in their progeny. In Drosophila simulans, dri ving X chromosomes and resistance factors located on the Y chromosome and o n the autosomes have been previously reported. In this paper, we report the study of the dynamics of sex-ratio factors in experimental populations. We followed the evolution in frequency of driving X chromosomes in the absenc e of resistance factors and the evolution of resistance factors in the pres ence of driving X chromosomes. The driving X chromosome was lost, contraril y to theoretical expectations that predict its rapid invasion. Autosomal re sistances increased in frequency, and resistant Y chromosomes invaded the p opulation very quickly, as predicted by theoretical models. Fitness measure ments showed that the loss of the driving X chromosome was due to a strong deleterious effect that was expressed only when distorting males were in co mpetition with standard males. However, the spread of autosomal resistances reduced this deleterious effect. Implications for the maintenance of polym orphism in natural populations are discussed.