My research synthesizes the fields of cellular aging and sexual selection in order to shed light on the role of sperm age in sperm competition and use. Sperm age, long recognized in the fields of human medicine and animal breeding as an important factor in determining fertilization success and offspring viability, has been largely overlooked by evolutionary biologists. Given that much of the observed across-male variation in fertilization success remains unexplained by commonly-studied sperm and ejaculate traits like sperm number, motility, and morphology, sperm age is a likely candidate for predicting sperm function.

 

Sperm aging and sperm use patterns in Drosophila

Recent work shows that some female insects delay the aging of stored sperm cells by reducing the cells’ metabolic rates, thereby limiting their production of damaging reactive oxygen species. This mechanism may underlie the complex spatial and temporal patterns of sperm storage and use in the model system Drosophila. Drosophila females have two types of sperm storage organs: the seminal receptacle (SR) and the spermathecae (ST). In D. melanogaster, the SR is used for short-term and the ST for long-term storage. In the closely related D. simulans, the two organ types have opposing fertilization biases: the SR favors the first male to mate, while the ST favor the second male. Sister species D. mauritiana, meanwhile, shows no temporal or mating-order biases in sperm use. Our project investigates how differential modulation of sperm metabolism in female storage, across the two organ types and the three species, relates to these different usage patterns. We are also investigating potential mechanisms of delayed sperm aging in storage, including oxygen restriction and high ion concentration.