Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom.

ABSTRACT

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²âº) in all animals that have been examined, and such Ca²âº elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²âº transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²âº dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²âº signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²âº signals are typically propagated as global waves that depend on Ca²âº release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP3). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²âº transients are also examined. In addition, the importance of fertilization-induced Ca²âº signals for activating development is underscored by noting some major downstream effects of these signals in various animals.