The risk of sperm competition and the evolution of sperm heteromorphism

Abstract

Members of theDrosophila obscuraspecies group exhibit sperm heteromorphism in which males simultaneously produce two different morphologies of sperm, short and long. Short sperm represent at least 50% of the ejaculate in several species but do not function in fertilization and thus, the evolutionary significance of this phenomenon is unknown. Selective pressures associated with the risk of sperm competition have been suggested to explain the maintenance of sperm heteromorphism. Here I test: (1) whether males alter their behaviour or ejaculate characteristics in response to the risk of sperm competition, and (2) whether short, nonfertilizing sperm serve as ‘cheap filler’ in the female reproductive tract, thereby delaying female receptivity to subsequent matings. Males did not alter tactics based on the risk of sperm competition; copulation durations of males mated to nonvirgin females were unexpectedly shorter than when both sexes were virgin and, contrary to sperm competition predictions, males did not alter the ratio of fertilizing:nonfertilizing sperm relative to female mating status or female age. Short sperm had a small, if any, role in influencing female remating behaviour. The number and proportion of short sperm present in sperm storage organs did not differ between nonvirgin receptive and nonvirgin nonreceptive females. The proportion of short sperm present in the ventral receptacle, however, was lower in receptive than nonreceptive females. Remating behaviour was strongly linked to oviposition patterns. Nonvirgin receptive females oviposited more eggs than nonreceptive females, and female remating interval was positively related to both the number of eggs and progeny produced. Although, oviposition parameters are related to fertilization and thus, the use of long sperm, nonvirgin receptive and nonreceptive females did not predictably differ in the number or proportion of long sperm in storage. These results suggest oviposition per se is probably more important than sperm in determining female receptivity and that sperm heteromorphism may play a marginal, if any, role in affecting female remating.