The sex-ratio trait and its evolution in Drosophila simulans: a comparative approach

Sex-ratio X chromosomes, which prevent the production of Y-bearing sperm, have been identified in a dozen Drosophila species covering a wide phylogenetic range. It has not yet been established whether the same ancestral genetic system underlies this type of meiotic drive across the genus, but the biological characteristics and the evolutionary history of species undoubtedly determine the fate of X-linked drivers. The intragenomic conflict they trigger contributes to geographical variation in D. simulans, which shows a sharp contrast between ancestral-stock derived and recently introduced populations. In the former, sex-ratio X chromosomes are widespread and sometimes reach a high frequency, but they are inactivated by strong Y-linked and autosomal drive suppressors. In recently-introduced populations, sex-ratio X chromosomes are generally rare and suppressors are moderate or absent. We discuss how this pattern could be related to the recent geographical expansion of D. simulans, and consider possible reasons why sex-ratio drive apparently does not occur in D. melanogaster.