Sperm length and seminal fluid proteins promote male reproductive success in D. melanogaster.

Spermatozoal morphology is highly variable both within and among species, often corresponding to variation in the shape of the female sperm storage organs in ways that can significantly impact fertilization success. In an effort to understand genetic mechanisms of sperm length variation, we compared gene expression patterns in the testes of Drosophila melanogaster males that produce either long or short sperm. We found that genes upregulated in long sperm testes are enriched for long noncoding RNAs (lncRNAs) and seminal fluid proteins (Sfps). Transferred in seminal fluid to the female during mating, Sfps are secreted by the male accessory glands and affect female remating rate, physiology, and behavior with concomitant advantages for male reproductive success. While sperm and Sfps are both critical for male reproductive success, they are largely considered to be functionally, genetically, and developmentally independent and despite being upregulated in long sperm testes, Sfps have no known function in testis tissue. We found that knockouts of two Sfps upregulated in long sperm males, Sex Peptide (SP) and ovulin (Acp26Aa) resulted in shorter sperm, which altogether suggests that Sfps may play a role in the development of sperm length during spermatogenesis. Consistent with this, knockout of accessory gland function did not affect sperm length, suggesting that accessory gland expression had no influence on spermatogenic processes. We also found that long sperm males were better able to delay female remating. 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 found that the delay in female remating does not necessarily translate to more progeny or higher paternity success. Thus, we found that multiple components of the ejaculate promote male reproductive success at different stages of reproduction, but the realized fitness advantages in sperm competition are uncertain.