Selfish chromosomes exploit a germline checkpoint to eliminate competing gametes

Spermatid individualization is a common stage of spermiogenic failure suggesting that a male germline checkpoint may act at this stage during sperm development in Drosophila. However, the molecular identity of such a male germline checkpoint has remained elusive. Here, we show that Overdrive (Ovd), a gene dispensable for male fertility, is required for the selective elimination of post-meiotic spermatids targeted by selfish chromosomes. Gamete elimination occurs during the individualization stage of spermiogenesis, following histone-to-protamine transition failure of targeted spermatids. We show that Ovd is necessary for the selfish behavior of segregation distorters across distantly related Drosophila species, indicating that independent selfish chromosomes use common mechanisms. Our study suggests that the normal function of an Ovd-mediated germline checkpoint involves the elimination of abnormal gametes during male germline development, which is exploited by selfish chromosomes into eliminating competing sperm.