Drivers of Female Sperm Storage Organ Evolution in Drosophila

Evolution of long sperm in D. melanogaster is driven by sperm competition within long seminal receptacles (SR), such that long sperm outcompete short sperm, but only in long SRs1. This male-female interaction means that SR length is a mechanism of cryptic female choice, and longer SRs are more selective for sperm length. Sperm and SRs are also coevolving both across species within Drosophila2 and within species1,3, likely driven by a genetic correlation4 and fitness benefits for both sexes5. Consistent with Fisherian runaway sexual selection of male traits and female preferences, it is assumed that SR length drives evolution of sperm length, but nothing is known about what drives the evolution of SR length. The strength of sexual selection can be approximated using female remating rate, or the average time a female waits between her first and second mating. Previous work examined whether sexual selection is associated with the evolution of SR length across 17 species, finding no association between remating rate and SR length. However, time to sexual maturation is correlated with SR length, opening up the possibility that long-SR species were not sexually mature when assayed for remating rate. Here, we included a very long SR species, D. hydei, and allowed at least a week before allowing females to mate, to determine if this made a difference in the relationship between SR length and remating rate.