Sperm length, sperm storage and mating system characteristics in bumblebees

Multiple insemination induces sperm competition and may select for longer, faster moving sperm in species where sperm is short-lived and egg fertilization takes place almost immediately after ejaculation. Here we report the first detailed analysis of sperm length in social insects with long-term storage of sperm, using three bumblebee species with different mating systems as models. We show that individual males produce only one size-class of sperm, but that sperm length is highly variable among brothers, among unrelated conspecific males, and among males of different species. Males of Bombus hypnorum, a species with multiple-mating queens, have longer sperm than males of B. terrestris and B. lucorum whose queens are single mated. Although the sample size on the species level was too small to perform a phylogenetic analysis, this finding supports the hypothesis that, all other things being equal, multiple mating may select for longer sperm. Sperm length was positively correlated with male body size in B. terrestris and possibly in B. hypnorum, but not in B. lucorum. The variance of sperm length within single B. terrestris males before mating was consistently higher than the variance of ejaculated sperm that was stored in a queen's spermatheca. Both longer sperm and shorter sperm could be preferentially stored, depending on the colony in which the males and queens were born and raised. These results indicate that the genotype of males may affect sperm length and that cryptic female choice of sperm length may have a genetic component as well.