IS THE PRODUCTION OF MULTIPLE SPERM TYPES ADAPTIVE?

Abstract

Males of many species concurrently produce more than one sperm type, now called sperm heteromorphism. In the Drosophila obscura group, all species examined to date produce multiple sperm types that differ in sperm length. Short sperm types in at least three obscura group species do not participate in fertilization, leading to questions regarding the adaptive value of sperm heteromorphism. The common and pervasive inheritance of this trait in the obscura group, however, may indicate that sperm heteromorphism is phylogenetically constrained and therefore does not reflect an adaptive response to selection pressures. I measured interspecific variation in sperm length and determined the number of sperm types produced in 10 obscura group species. I subsequently tested if interspecific variation in sperm length is significantly associated with phylogeny by using an autoregressive comparative method. All obscura group species examined produce two visually distinct sperm lengths, short and long. Phylogenetic autoregression analyses indicated that 22% of the interspecific variation in long sperm is related to phylogeny, whereas short sperm are not significantly correlated with phylogeny. These results suggest different selection pressures on the two sperm length types; long sperm have evolved in response to fertilization demands and short sperm have been decoupled from these requirements.