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Unformatted text preview: INTRODUCTION The members of a species consist of a discrete reproductive community that is variably maintained by pre-mating (habitat, mate discrimination, physiognomy), mating (fertilization) or postmating (hybrid lethality, sterility) determinants (ORand, 1988). Most commonly, species arise following physical separation and genetic evolution (allopatry) that results in reproductive isolation even when the two species are reunited (Coyne, 1992). Some species, however, arise from sympatric speciation in which distinct reproductive communities are formed in the absence of physical barriers (Schliewen et al., 1994; Shoemaker and Ross, 1996). Although the macro- molecules that mediate sperm-egg interactions can serve as a cross-breeding block between species (Metz and Palumbi, 1996), this seems unlikely to be of critical importance for mammals which fertilize internally. Rather, the value of highly specific sperm-egg interaction to reproductive efficiency may lie in limiting egg access to a single sperm of the correct species and therefore, minimizing the risk of polyspermy and its lethal consequences. While the identification of mammalian zona binding proteins on sperm remains controversial (Snell and White, 1996), the zona pellucida surrounding eutherian eggs has a well-documented role in mediating specific sperm- egg interactions (e.g., Roldan and Yanagimachi, 1989). The mammalian zona pellucida is an extracellular matrix that surrounds growing oocytes, ovulated eggs and pre-implantation embryos. Although critical for normal folliculogenesis and for passage of the embryo down the oviduct, it has been most intensely investigated for its role in mediating fertility. In mammals, fertilization takes place in the ampulla of the oviduct where individual gametes fuse to form a one-cell zygote. Although tens of millions of sperm are deposited in the lower female genital track, few progress as far as the oviduct. For successful fertilization, a motile sperm must penetrate three investments of the ovulated egg: the cumulus oophorus composed of cumulus cells interspersed in a matrix of hyaluronic acid; the extracellular zona pellucida surrounding the egg; and the egg plasma membrane, with which the sperm head fuses. The acrosome, a lysosomal-like structure at the anterior head of the sperm, must be intact for penetration through the cumulus oophorus and initial binding to the zona pellucida. The binding of sperm to the zona then causes the outer acrosomal and plasma membranes to fuse, creating fenestrations and releasing lytic enzymes that may modify the sperm surface, the zona pellucida, or both. Subsequently, the sperm penetrates the 2415 Development 125, 2415-2424 (1998) Printed in Great Britain The Company of Biologists Limited 1998 DEV3837 The mammalian zona pellucida surrounding ovulated eggs mediates sperm binding at fertilization, provides a postfertilization block to polyspermy, and facilitates passage of pre-implantation embryos down the oviduct.passage of pre-implantation embryos down the oviduct....
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