Sex-ratio meiotic drive in Drosophila simulans is related to equational nondisjunction of the Y chromosome

The sex-ratio trait, an example of naturally occurring X-linked meiotic dri ve, has been reported in a dozen Drosophila species. Males carrying a sex-r atio X chromosome produce an excess of female offspring caused by a deficie ncy of Y-bearing sperm. In Drosophila simulans, such males produce similar to 70-90% female offspring, and 15-30% of the male offspring are sterile. H ere, we investigate the cytological basis of the drive in this species. We show that the sex-ratio trait is associated with nondisjunction of Y chroma tids in meiosis II. Fluorescence in situ hybridization (FISH) using sex-chr omosome-specific probes provides dir-ect evidence that the drive is caused by the failure of the resulting spermatids to develop into functional sperm . XYY progeny were not observed, indicating that few or no YY spermatids es cape failure. The recovery of XO males among the progeny of sex-ratio males shows that some nullo-XY spermatids become functional sperm and likely exp lains the male sterility. A review of the cytological data. in other specie s shows that aberrant behavior of the Y chromosome may be a common basis of sex-ratio meiotic drive in Drosophila and the signal that triggers differe ntial spermiogenesis failure.