Ca²⁺ spikes in the flagellum control chemotactic behavior of sperm

The events that occur during chemotaxis of sperm are only partly known. As an essential step toward determining the underlying mechanism, we have recorded Ca²⁺ dynamics in swimming sperm of marine invertebrates. Stimulation of the sea urchin Arbacia punctulata by the chemoattractant or by intracellular cGMP evokes Ca²⁺ spikes in the flagellum. A Ca²⁺ spike elicits a turn in the trajectory followed by a period of straight swimming ('turn-and-run'). The train of Ca²⁺ spikes gives rise to repetitive loop-like movements. When sperm swim in a concentration gradient of the attractant, the Ca²⁺ spikes and the stimulus function are synchronized, suggesting that precise timing of Ca²⁺ spikes controls navigation. We identified the peptide asterosap as a chemotactic factor of the starfish Asterias amurensis. The Ca²⁺ spikes and swimming behavior of sperm from starfish and sea urchin are similar, implying that the signaling pathway of chemotaxis has been conserved for almost 500 million years.