Female-induced remote regulation of sperm physiology may provide opportunities for gamete-level mate choice.

In sedentary externally fertilizing species, direct interactions between mating partners are limited and prefertilization communication between sexes occurs largely at the gamete level. Certain combinations of eggs and sperm often have higher fertilization success than others, which may be contingent on egg-derived chemical factors that preferentially attract sperm from compatible males. Here, we examine the mechanisms underlying such effects in the marine mussel Mytilus galloprovincialis, where differential sperm attraction has recently been shown to be associated with variation in offspring viability. Specifically, we focus on the sperm surface glycans, an individually unique layer of carbohydrates that moderate self-recognition and other cellular-level interactions. In many species egg-derived factors trigger remarkable changes in the sperm's glycan layer, physiology, and swimming behavior, and thus potentially moderate mate choice at the gamete level. Here, we show that sperm glycan modifications and the strength of acrosome reaction are both dependent on specific male-female interactions (male-female combination). We also find associations between female-induced sperm glycan changes and the Ca2+ influx into sperm--a key regulator of fertilization processes from sperm capacitation to gamete fusion. Together, our results suggest that female-induced remote regulation of sperm physiology may constitute a novel mechanism of gamete-level mate choice.