Egg-induced changes to sperm phenotypes shape patterns of multivariate selection on ejaculates

Ejaculates exhibit extraordinary phenotypic diversity and rapid rates of evolution, yet the adaptive value of most sperm traits remains equivocal. Recent findings suggest that to understand how selection targets ejaculates we must recognize that female-imposed physiological conditions often alter ejaculate phenotypes. These phenotypic changes to ejaculates may influence the relationships among sperm traits and their association with fitness. Here, we show that chemical substances released by eggs (known to modify sperm physiology and behavior) alter patterns of selection on a suite of sperm traits in the mussel Mytilus galloprovincialis. We use multivariate selection analyses to characterize linear and nonlinear selection acting on sperm traits in (a) seawater alone, and (b) seawater containing egg-derived chemicals (egg water). Our analyses revealed that nonlinear selection on canonical axes of multiple traits (notably sperm velocity, sperm linearity and percentage of motile sperm) was the most important form of selection overall, but importantly these patterns were only evident when sperm phenotypes were measured in egg water. These findings reveal the subtle way that females can alter patterns of selection, with the implication that overlooking environmentally-moderated changes to ejaculate phenotypes may result in erroneous interpretations of how selection targets phenotypic (co)variation in ejaculate traits.