The signalling pathway and the behavioural strategy underlying chemotaxis of sperm are poorly understood. We have studied the cellular events and motor responses that mediate chemotaxis of sperm from the sea urchin Arbacia punctulata. Here we show that resact, a chemoattractant peptide, initiates a rapid and transient rise in the concentration of cyclic GMP, followed by a transient influx of Ca2+. The binding of a single resact molecule elicits a Ca2+ response, and 50-100 bound molecules saturate the response. The ability to register single molecules is reminiscent of the single-photon sensitivity of rod photoreceptors. Both resact and cyclic nucleotides cause a turn or brief tumbling in the swimming path of sperm. We conclude that a cGMP-mediated increase in the Ca2+ concentration induces the primary motor response of sperm to the chemoattractant.

The events that occur during chemotaxis of sperm are only partly known. As an essential step toward determining the underlying mechanism, we have recorded Ca 2 þ dynamics in swimming sperm of marine invertebrates. Stimulation of the sea urchin Arbacia punctulata by the chemoattractant or by intracellular cGMP evokes Ca 2 þ spikes in the flagellum. A Ca 2 þ spike elicits a turn in the trajectory followed by a period of straight swimming ('turn-andrun'). The train of Ca 2 þ spikes gives rise to repetitive loop-like movements. When sperm swim in a concentration gradient of the attractant, the Ca 2 þ spikes and the stimulus function are synchronized, suggesting that precise timing of Ca 2 þ spikes controls navigation. We identified the peptide asterosap as a chemotactic factor of the starfish Asterias amurensis. The Ca 2 þ 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.

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Sperm are attracted by chemical substances which are released by the egg. This process is called chemotaxis. Several molecules that are involved in chemotactic signaling of sperm from marine invertebrates are described and a model of the signaling pathway is presented. We discuss the motor response during chemotaxis and propose a model of the navigation strategy of sperm.

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