Polyspermy

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In biology, polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the egg and one each from multiple sperm. Usually, the result is an inviable zygote.

[edit] Evolved defenses against polyspermy

The eggs of sexually reproducing organisms are adapted to avoid this situation. The defenses are particularly well characterized in the sea urchin, which responds to the acceptance of one sperm by inhibiting the successful penetration of the egg by subsequent sperm. Similar defenses exist in other eukaryotes.

The prevention of polyspermy in sea urchins depends on a change in the electrical voltage across the surface of the egg, which is caused by the fusion of the first sperm with the egg. Unfertilized sea urchin eggs have a negative voltage inside, but the voltage becomes positive upon fertilization. When sea urchin sperm encounter an egg with a positive voltage, sperm-egg fusion is blocked. Thus, after the first sperm contacts the egg and causes the voltage change, subsequent sperm are prevented from fusing. This "electrical polyspermy block" is thought to result because a positively charged molecule in the sperm surface membrane is repelled by the positive voltage at the egg surface.

Electrical polyspermy blocks operate in many animal species, including frogs, clams, and marine worms, but not in the several mammals that have been studied (hamster, rabbit, mouse). In species without an electrical block, polyspermy is usually prevented by secretion of materials that establish a mechanical barrier to polyspermy. Animals such as a sea urchins have a two-step polyspermy prevention strategy, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block. It is thought that electrical blocks evolved in those species where a very fast block to polyspermy is needed, due to the presence of many sperm arriving simultaneously at the egg surface, as occurs in animals such as sea urchins. In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds. In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct.

A further block to polyspermy also exists which appears to degrade additional nuclei within the newly-formed zygote after the first nuclear fusion. However, the mechanics of this block are not yet known.