A variety of broadcast-spawning marine organisms require sophisticated
sperm guidance mechanisms, named chemotaxis, to locate the egg and fertilize it. In
sea urchins, oocytes release small peptides from the egg outer envelope that bind to
their sperm flagellar receptors and trigger a signaling pathway that results in
intracellular Ca2+ concentration fluctuations. Each transient Ca2+ increase leads to a
momentary elevation of flagellar bending asymmetry which results in a pronounced
turn essential for chemotaxis. In addition, this process needs a precise spatiotemporal
coordination between the Ca2+-dependent turns, the form of chemoattractant gradient
and periods of straighter swimming. Chemotaxis results when spermatozoa are able to
undergo Ca2+-dependent turns when swimming down the chemoattractant gradient,
while they suppress turning events when swimming up the gradient. This chapter
summarizes the sequence of events and known components of the signaling pathway
leading to chemotaxis in sea urchin spermatozoa, and the strategies that are being
employed to unravel this fascinating and fundamental process.
Keywords: Axoneme, Chemotaxis, Cell-cell communication, Chemoattractant
signalling, Calcium signalling, Mathematical modelling of signal transduction,
Sperm motility.