The marine brown alga Fucus serratus represents one of the few multicellular plant species in which the process of fertilization can be studied relatively easily. Fertilization marks the onset of a cascade of events associated with egg activation. Fertilization in Fucus serratus bears several superficial similarities to fertilization in several animal systems. The essential features of Fucus serratus egg activation are compared with those of protostome and deuterostome animal systems. Ca2+ is required for egg activation in Fucus serratus and cytosolic [Ca2+] changes can be observed in fertilizing eggs. However, these are small and variable in comparison with those occurring in deuterostomes, and fertilization can proceed normally in the absence of any global cytosolic Ca2+ transients. A model for egg activation in Fucus serratus is presented, invoking a role for both Ca2+ influx and localized propagation of the sperm signal around the plasma membrane by an as yet unidentified mechanism. Polarity in Fucus serratus is acquired a considerable time after fertilization and the role of cytosolic Ca2+ gradients in the acquisition and expression of polarity is discussed. The problem of the signals associated with the onset of the cell cycle in the fertilized Fucus serratus egg is also addressed.

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