Sperm length and seminal receptacle length do not evolve in populations of Drosophila melanogaster adapted to larval crowding

Pre-adult density can affect a range of adult traits, including body size, fecundity, courtship, and ejaculate characters. However, the effects of larval density on the evolution of sperm size remain unknown. The long flagellum of Drosophila sperm is considered a costly trait which covaries with the length of the female sperm storage organ, the seminal receptacle (SR). Long sperm have a competitive advantage in long SRs. Increased larval density reduces nutritional availability, body size, and SR length, and can therefore select for altered sperm length. In the current study, we used laboratory populations of Drosophila melanogaster selected for adaptation to extreme larval crowding and their controls. We measured sperm length, SR length and body size in the selected and control populations after growing them under high and low larval densities. Our results show that body size and SR length decrease under high larval density, regardless of the population's selection history. However, sperm length remained unchanged in both high- and low-density conditions for both selected and control populations. We found no evidence for the evolution of sperm length or SR length in populations adapted to larval crowding.