POLYMORPHISM FOR Y-LINKED SUPPRESSORS OF SEX-RATIO IN 2 NATURAL-POPULATIONS OF DROSOPHILA-MEDIOPUNCTATA

In several Drosophila species there is a trait known as ''sex-ratio'': males carrying certain X chromosomes (called ''SR'') produce female b iased progenies due to X-Y meiotic drive. In Drosophila mediopunctata this trait has a variable expression due to Y-linked suppressors of se x-ratio expression, among other factors. There are two types of Y chro mosomes (suppressor and nonsuppressor) and two types of SR chromosomes (suppressible and unsuppressible). Sex-ratio expression is suppressed in males with the SRsuppressible/Y-suppressor genotype, whereas the r emaining three genotypes produce female biased progenies. Now we have found that similar to 10-20% of the Y chromosomes from two natural pop ulations 1500 km apart are suppressors of sex-ratio expression. Prelim inary estimates indicate that Y-suppressor has a meiotic drive advanta ge of 6% over Y-nonsuppressor. This Y polymorphism for a nonneutral tr ait is unexpected under current population genetics theory. We propose that this polymorphism is stabilized by an equilibrium between meioti c drive and natural selection, resulting from interactions in the popu lation dynamics of X and Y alleles. Numerical simulations showed that this mechanism may stabilize nonneutral Y polymorphisms such as we hav e found in D. mediopunctata.