Females play larger role in sperm selection

Females play a larger role in determining paternity than previously thought -- at least in fruit flies, U.S. biologists say. Lead author Stefan Lupold, a research assistant professor in the Department of Biology at Syracuse University, and colleagues said the findings have major implications for the study of sexual selection, sexual conflict and the co-evolution of male and female reproductive traits. "Our studies show that female flies don't just provide a static arena for sperm competition; they also influence who fathers their offspring," Lupold said in a statement. "This is indicated by various means, including the re-mating interval; progeny production rate; sperm-storage organ morphology; and the way females store and use sperm." Lupold and his team mated female flies with two groups of males, the latter of which were distinguished by green- and red-tagged sperm heads. "The colored heads allowed us to better study the physical displacement of the 'resident' sperm by the second male from the female's storage organs. They also helped us witness the female's ejection of the sperm and the biased use of competing sperm for fertilization," said Scott Pitnick, a biology professor at Syracuse University and an expert in the evolution of reproduction. Females of most species mate with multiple males within a reproductive cycle, intrasexual competition and intersexual choice can continue in the form of sperm competition and female choice, the researchers said. The study found the timing of the female ejection of sperm was genetically variable, and, thus, influenced the amount of sperm competing for fertilization. "The longer a female waited to eject the sperm, the more time it had to enter her storage organs and displace the sperm from her previous mate," Pitnick said. The study was published in the Proceedings of the National Academy of Sciences.