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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. 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.^[1]

Fast block of polyspermy

The eggs of sexually reproducing organisms are adapted to avoid this situation.^[2] 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.^[3] 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.^[4]

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).^[5] 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such as a sea urchins have a two-step polyspermy prevention strategy, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block.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.

Slow block of polyspermy

In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg.^[6] Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the "cortical reaction" modifies the extracellular coat of the egg (the zona pellucida), and additional mechanisms that are not well understood modify the egg's plasma membrane.^[7] The zona pellucida is modified by serine proteases that are released from the cortical granules. The proteases destroy the protein link between the cell membrane and the vitelline envelope, remove any receptors that other sperm have bound to, and help to form the fertilization envelope from the cortical granules.^[8]

Evolutionary advantage

Female defenses select for ever more aggressive male sperm however, leading to an evolutionary arms race. On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether. This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species.^[9]

References

^ Arnqvist, G. & Rowe, L. (2005) Sexual conflict. Princeton University Press, Princeton ISBN 0-691-12217-2
^ Eberhard, W. G. 1996. Female Control: Sexual Selection by Cryptic Female Choice. Princeton University Press, Princeton, New Jersey.
^ Jaffe, LA.. 1976. Fast block to polyspermy in sea urchin eggs is electrically mediated. Nature" 261:68-71.
^ Iwao, Y., Jaffe, L.A. 1989. Evidence that the voltage-dependent component in the fertilization process is contributed by the sperm. Developmental Biology. 134:446-51.
^ Jaffe, L. A., M. Gould. 1985. Polyspermy-preventing mechanisms. In C. B. Metz & A. Monroy (editors) Biology of Fertilization. Academic, New York. pp. 223-250.
^ Braden, A. W. H. 1953. Distribution of sperms in the genital tract of the female rabbit after coitus. Australian Journal of Biological Sciences 6:693-705.
^ (Gardner, A.J, Evans, J.P. 2006. Mammalian membrane block to polyspermy: new insights into how mammalian eggs prevent fertilisation by multiple sperm. "Reprod Fertil Dev." 18:53-61.
^ Gilbert, Scott F. (2014). Developmental Biology, 10th ed. Sunderland, MA USA: Sinauer Associates, Inc. Publishers. p. 130.
^ Morrow, E. H., G. Arnqvist, T. E. Pitcher. 2002. The evolution of infertility: Does hatching rate in birds coevolve with female polyandry? Journal of Evolutionary Biology. 15:702-709.

External links

Animation of polyspermy

UpToDate Contents

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1. 出産年齢の上昇に伴う受胎能保持の選択肢 fertility preserving options for women of advancing age

English Journal

Inhibitors of serine proteases decrease sperm penetration during porcine fertilization in vitro by inhibiting sperm binding to the zona pellucida and acrosome reaction.

Beek J1, Nauwynck H2, Appeltant R3, Maes D3, Van Soom A3.
Theriogenology.Theriogenology.2015 Jul 26. pii: S0093-691X(15)00380-5. doi: 10.1016/j.theriogenology.2015.07.022. [Epub ahead of print]
Serine proteases are involved in mammalian fertilization. Inhibitors of serine proteases can be applied to investigate at which point these enzymes exert their action. We selected two serine protease inhibitors, 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF, 100 μM) and soybean t
PMID 26303413

In vitro fertilization in pigs: New molecules and protocols to consider in the forthcoming years.

Romar R1, Funahashi H2, Coy P3.
Theriogenology.Theriogenology.2015 Jul 23. pii: S0093-691X(15)00375-1. doi: 10.1016/j.theriogenology.2015.07.017. [Epub ahead of print]
Assisted reproduction technology (ART) protocols are used in livestock for the improvement and preservation of their genetics and to enhance reproductive efficiency. In the case of pigs, the potential use of embryos for biomedicine is being followed with great interest by the scientific community. O
PMID 26271164

Outstanding questions concerning sperm-epithelial binding in the mammalian oviduct.

Hunter RH1, Gadea J2.
Zygote (Cambridge, England).Zygote.2015 Jul 14:1-7. [Epub ahead of print]
Preovulatory binding of viable spermatozoa in the oviduct isthmus is widely accepted as a preliminary to fertilization, but details of physiological events associated with epithelial binding and release from binding are themselves little understood. Important questions include the potential number,
PMID 26168848

Japanese Journal

Sericin Accelerates the Production of Hyaluronan and Decreases the Incidence of Polyspermy Fertilization in Bovine Oocytes During In Vitro Maturation

HOSOE Misa,YOSHIDA Nao,HASHIYADA Yutaka [他]
The journal of reproduction and development 60(4), 268-273, 2014-08
NAID 40020147088

Sericin Accelerates the Production of Hyaluronan and Decreases the Incidence of Polyspermy Fertilization in Bovine Oocytes During In Vitro Maturation

, , , , ,
Journal of Reproduction and Development 60(4), 268-273, 2014
… The sizes of the perivitelline space that might relate to polyspermy, the expressions of Has2 and CD44 mRNA, the amount of hyaluronan (hyaluronic acid: HA) contained in the oocytes and the rates of blastocyst formation following insemination were then compared between the oocytes cultured with 0.05% sericin and 5% FBS, because the polyspermy rates in oocytes cultured with 0.05% sericin were significantly lower than in those cultured with 5% FBS. …
NAID 130003390838

Evaluation of Zona Pellucida Function for Sperm Penetration During In Vitro Fertilization in Pigs

TANIHARA Fuminori,NAKAI Michiko,KANEKO Hiroyuki,NOGUCHI Junko,OTOI Takeshige,KIKUCHI Kazuhiro
The Journal of reproduction and development 59(4), 385-392, 2013-08-01
NAID 10031195876

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