Genetic coordination of sperm morphology and seminal fluid proteins promotes male reproductive success in Drosophila melanogaster

Spermatozoal morphology is highly variable both among and within species and in ways that can significantly impact fertilization success. In Drosophila melanogaster, paternity success depends on sperm length of both competing males and length of the females primary sperm storage organ. We found that genes upregulated in long sperm testes are enriched for lncRNAs and seminal fluid proteins (Sfps). Transferred in seminal fluid to the female during mating, Sfps are secreted by the male accessory glands (AG) and affect female remating rate, physiology, and behavior with concomitant advantages for male reproductive success. Despite being upregulated in long sperm testes, they have no known function in testis tissue. We found that Sex Peptide and ovulin (Acp26Aa) knockouts resulted in shorter sperm, suggesting that Sfps may regulate sperm length during spermatogenesis. However, knockout of AG function did not affect sperm length, suggesting that AG expression has no influence on spermatogenic processes. We also found that long sperm males are better able to delay female remating, suggesting higher Sfp expression in AG. These results might suggest that long sperm males have a double advantage in sperm competition by both delaying female remating, likely through transfer of more Sfps, and by resisting sperm displacement. However, we also found that this extra advantage does not necessarily translate to more progeny or higher paternity success. Thus, we found that multiple components of the ejaculate coordinate to promote male reproductive success at different stages of reproduction, but the realized fitness advantages in sperm competition are uncertain. Significance StatementThe ejaculate is comprised of sperm produced in the testis and seminal fluid primarily produced in the male accessory glands (AG). These complementary components are both critical for male reproductive success, but they are largely considered to be functionally, genetically, and developmentally independent. In a quest to understand genetic mechanisms of sperm length variation, we found that genes upregulated in long sperm testes are enriched for lncRNAs and seminal fluid proteins (Sfps). Knockout of two Sfps, Sex Peptide and ovulin, results in shorter sperm, though knockout of AG function has no effect. Moreover, long sperm males delay female remating longer. These results suggest sophisticated testis-AG coordination that amplifies male reproductive success, with implications for evolutionary integration of sexually selected traits.