Polyspermy

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. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often deleterious to the female: in other words, the male-male competition among sperm spills over to create sexual conflict. 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. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often deleterious to the female: in other words, the male-male competition among sperm spills over to create sexual conflict. Physiological polyspermy happens when the egg normally accepts more than one sperm but only one of the multiple sperm will actually fuse its nucleus with the nucleus of the egg. Physiological polyspermy is present in some species of vertebrates and invertebrates. Some species utilize physiological polyspermy as the proper mechanism for developing their offspring. Some of these animals include birds, ctenophora, reptiles and amphibians, just to name a few. Some vertebrates that are both amniote or anamniote. For example, urodele amphibians, cartilaginous fish, birds and reptiles, undergo physiological polyspermy because of the internal fertilization of their yolky eggs. Sperm triggers egg activation by the induction of free calcium ion concentration in the cytoplasm of the egg. This induction plays a very critical role in both physiological polyspermy and monomeric polyspermy species. The rise in calcium causes activation of the egg. The egg will then be altered on both a biochemical and morphological level. In mammals as well as sea urchins, the sudden rise in calcium concentration occurs because of the influx of calcium ions within the egg. These calcium ions are responsible for the cortical granule reaction, and are also stored in the egg's endoplasmic reticulum. Unlike physiological polyspermy, monospermic fertilization deals with the analysis of the egg calcium waves, as this is the typical reproduction process in all species. Species that undergo physiological polyspermy have polypoidy-preventing mechanisms that act inside the egg. This is quite different from the normal polyspermy block on the outside of the egg. In the journal Proceedings of the Royal Society B, as reported in the New York Times, Dr. Nicola Hemmings, an evolutionary biologist at the University of Sheffield, and one of the study’s authors, reported that eggs of zebra finches and chickens require multiple sperm, from 10 to hundreds of sperm, to penetrate the egg to ensure successful fertilization and growth of the bird embryo. Polyspermy is very rare in human reproduction. The decline in the numbers of sperm that swim to the oviduct is one of two ways that prevents polyspermy in humans. The other mechanism is the blocking of sperm in the fertilized egg. According to Developmental Biology Interactive, if an egg becomes fertilized by multiple sperm, the embryo will then gain various paternal centrioles. When this happens, there is a struggle for extra chromosomes. This competition causes disarrayment in cleavage furrow formation and the normal consequence is death of the zygote.. Only two cases of human polyspermy leading to birth of children have been reported. 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 charge 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 charge inside, but the charge becomes positive upon fertilization. When sea urchin sperm encounter an egg with a positive charge, sperm-egg fusion is blocked. Thus, after the first sperm contacts the egg and causes the change, subsequent sperms 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 charge 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 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.