The Y chromosomes of Drosophila simulans are highly polymorphic for their ability to suppress sex-ratio drive

The sex-ratio trait, known in several species of Drosophila including D. si mulans, results from meiotic drive of the X chromosome against the Y. Males that carry a sex ratio X chromosome produce strongly female-biased progeny . In I). simulans, drive suppressors have evolved on the Y chromosome and o n the autosomes. Both the frequency of sex-ratio X and the strength of the total drive suppression (Y-Iinked and autosomal) vary widely among geograph ic populations of this worldwide species. We have investigated the pattern of Y-linked drive suppression in six natural populations representative of this variability. Y linked suppressors were found to be a regular component of the suppression, with large differences between populations in the mean level of suppression. These variations did not correspond to differences i n frequency of discrete types of Y chromosomes, but to a more or less wide continuum of phenotypes, from nonsuppressor to partial or total suppressor. We concluded that a large diversity of Y-Linked suppressor alleles exists in D. simulans and that some populations are highly polymorphic. Our result s support the hypothesis that a Y-chromosome polymorphism can be easily mai ntained by a balance between meiotic drive and the cost of drive suppressio n.