Sperm production responds to perceived sperm competition risk in male Drosophila melanogaster

- We investigated changes in sperm production in male Drosophila melanogaster.
- Males were maintained either alone or with a male rival for 7 days.
- Sperm from the males' seminal vesicles were stained to detect live and dead heads.
- Males pre-exposed to a rival were found to have a greater proportion of live sperm.
- D. melanogaster may adjust sperm production in response to sperm competition cues.

Postcopulatory sexual selection arising from female multiple mating leads to the evolution of ejaculates that maximize a male's reproductive success under sperm competition. Where the risk of sperm competition is variable, optimal fitness may be achieved by plastically altering ejaculate characteristics in response to the prevailing sperm competition environment. In the model species Drosophila melanogaster, males expecting to encounter sperm competition mate for longer and transfer more accessory proteins and sperm. Here we show that after being housed with a single rival for one week, the seminal vesicles of male D. melanogaster contain a significantly greater proportion of live sperm than those of males maintained alone, indicating adaptive adjustment of sperm quality in response to the perceived risk of sperm competition. This effect is due to an increase in the number of live sperm produced, indicating that males upregulate sperm production in response to the presence of rivals. Our data suggest that males show plasticity in the rate of spermatogenesis that is adaptive in the context of a fluctuating sperm competition environment.