Tuning sperm chemotaxis by calcium burst timing

Marine invertebrate oocytes establish chemoattractant gradients that guide spermatozoa towards their source. In sea urchin spermatozoa, this relocation requires coordinated motility changes initiated by Ca2+-driven alterations in sperm flagellar curvature. We discovered that Lytechinus pictus spermatozoa undergo chemotaxis in response to speract, an egg-derived decapeptide previously noted to stimulate non-chemotactic motility alterations in Strongylocentrotus purpuratus spermatozoa. Sperm of both species responded to speract gradients with a sequence of turning episodes that correlate with transient flagellar Ca2+ increases, yet only L. pictus spermatozoa accumulated at the gradient source. Detailed analysis of sperm behavior revealed that L. pictus spermatozoa selectively undergo Ca2+ fluctuations while swimming along negative speract gradients while S. purpuratus sperm generate Ca2+ fluctuations in a spatially non-selective manner. This difference is attributed to the selective suppression of Ca2+ fluctuations of L. pictus spermatozoa as they swim towards the source of the chemoattractant gradient. This is the first study to compare and characterize the motility components that differ in chemotactic and non-chemotactic spermatozoa. Tuning of Ca2+ fluctuations and associated turning episodes to the chemoattractant gradient polarity is a central feature of sea urchin sperm chemotaxis and may be a feature of sperm chemotaxis in general.