Lighting the fuse at fertilization

In most deuterostome eggs, fertilization is marked by an abrupt and transient increase in intracellular calcium concentration. The transient takes the form of a propagating wave and is the signal for the onset of development. For those interested in cell signalling, the two obvious questions to ask are how the wave is initiated and how it propagates through the egg cytoplasm. Answers have come largely from experiments in frog, hamster, mouse and sea urchin eggs. One explanation of signal transduction at fertilization makes an analogy with transmembrane signalling in somatic cells, where a family of G-protein-linked receptors pass activating signals across the plasma membrane. Another, older idea is that it is the fusion of sperm and egg that is responsible for detonating the calcium explosion at fertilization. We discuss the relative merits of the two ideas. Both are plausible; the creative tension between them has led to experiments that broaden our view of signal transduction at fertilization.