is essential for targeted sperm elimination by .

Intra-genomic conflict driven by selfish chromosomes is a powerful force that shapes the evolution of genomes and species. In the male germline, many selfish chromosomes bias transmission in their own favor by eliminating spermatids bearing the competing homologous chromosomes. However, the mechanisms of targeted gamete elimination remain mysterious. Here, we show that , a gene required for both segregation distortion and male sterility in hybrids, is broadly conserved in Dipteran insects but dispensable for viability and fertility. In is required for targeted spermatid elimination after the histone-to-protamine transition in the classical system. We propose that functions as a general spermatid quality checkpoint that is hijacked by independent selfish chromosomes to eliminate competing gametes.