In biology, polyspermy describes the

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

of an egg by more than one

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...

Diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

organisms normally contain two copies of each

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

, 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 unviable

zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...

. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of

sperm competition Sperm competition is the competitive process between Spermatozoon, spermatozoa of two or more different males to fertility, fertilize the same Egg cell, egg during sexual reproduction. Competition can occur when females have multiple potential m ...

. Such a situation is often deleterious to the female: in other words, the male–male competition among sperm spills over to create sexual conflict.

Pathological polyspermy and the blocks

In the classical case (at least from the historical human perspective), monospermic fertilization is the norm. For monospermic organisms, polyspermy is a detrimental process where eggs are incorrectly fertilized. It causes the formation of multiple microtubule-organizing centers from multiple, leading to disruption of mitosis, disarrayment in cleavage furrow formation, and ultimately, cell death. As a result, a number of "blocks" have evolved to prevent polyspermy in such organisms. 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. Only two cases of human polyspermy leading to birth of children have been reported.

Fast block of polyspermy

The eggs of sexually-reproducing organisms are adapted to avoid this situation. The defenses are particularly well characterized in the

sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

, which respond to the acceptance of one sperm by inhibiting the successful penetration of the egg by subsequent sperm. Similar defenses exist in other

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

. 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 A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

is repelled by the positive charge at the egg surface. Electrical polyspermy blocks operate in many animal species, including

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

s, and marine worms, but not in the several

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s that have been studied ( hamster,

rabbit Rabbits are small mammals in the family Leporidae (which also includes the hares), which is in the order Lagomorpha (which also includes pikas). They are familiar throughout the world as a small herbivore, a prey animal, a domesticated ...

mouse A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...

). 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 Strategy (from Greek στρατηγία ''stratēgia'', "troop leadership; office of general, command, generalship") is a general plan to achieve one or more long-term or overall goals under conditions of uncertainty. In the sense of the " a ...

, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block. Electrical blocks are helpful in 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, it occurs 2-3 seconds after the first sperm enters the egg. It is a chemical process which involves changing the potential of egg from a resting potential of -70 mv to 10 mv. It involves an influx of sodium ion into the egg. The membrane of the egg changes from negatively to positively charged. Sperm cannot enter a positively charged egg. The positive charge only lasts for 60 seconds.

Slow block of polyspermy

In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the

oviduct The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...

. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg. Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the " cortical reaction" modifies the extracellular coat of the egg (the

zona pellucida The ''zona pellucida'' (Latin meaning "transparent zone") is the specialized area surrounding mammalian oocytes (eggs). It is also known as an egg coat. The ''zona pellucida'' is essential for oocyte growth and fertilization. The ''zona pelluc ...

), and additional mechanisms that are not well understood modify the egg's plasma membrane. 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. The cortical reaction occurs due to calcium oscillations inside the oocyte. What triggers such oscillations is PLC-zeta, a phospholipase unique to sperm that is very sensitive to calcium concentrations. When the first spermatozoa get inside the oocyte, it brings in PLC-zeta, that is activated by oocyte's basal calcium concentrations, initiates the formation of IP3 and causes calcium release from endoplasmic reticulum stores, generating the oscillations in calcium concentration that will activate the oocyte and block polyspermy.

Physiological polyspermy

Physiological polyspermy happens when the egg normally accepts more than one sperm but only one of the multiple sperm will 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 Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are ...

, reptiles and amphibians. Some vertebrates that are both

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

or anamniote, including urodele amphibians,

cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...

, 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 (described above) 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 polyploidy-preventing mechanisms that act inside the egg. This is quite different from the normal polyspermy block on the outside of the egg. Even though multiple sperm enter the cell and each form an aster (see: ), only one male pronucleus is chosen to merge with the female pronucleus. In amphibians, the sperm pronucleus needs to be in the animal hemisphere to be chosen. It's also known that the chosen pronucleus also has the biggest aster, though details on how this is achieved (and how precisely it affects choice) remains unknown. The addition pronuclei are degraded during cleavage. In birds, the chosen pronucleus moves to the center of the germinal disk, while the others move to the periphery.

Chicken and zebra finch eggs require multiple sperm

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 the 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.

Compensable polyspermy

Yet another case involves normally monospermic organisms that "tolerate" polyspermy without ill effects. In other words, they too can choose one sperm pronucleus to fuse with, discarding the rest. A number of species is known to exhibit this unusual behavior, but this case is very poorly studied.

Evolutionary view

Pathological polyspermy is costly for females as it decreases their chances to produce offspring. This would make investing into defenses that block polyspermy viable. 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 In biology, infertility is the inability of a male and female organism to Sexual reproduction, reproduce. It is usually not the natural state of a healthy organism that has reached sexual maturity, so children who have not undergone puberty, whi ...

rates seen in mammalian species., Morrow, E. H., G. Arnqvist, T. E. Pitcher. 2002. The evolution of infertility: Does the hatching rate in birds coevolve with female polyandry? ''Journal of Evolutionary Biology''. 15:702-709. The existence of compensable polyspermy suggests an alternative solution: females could also evolve tolerance of polyspermy to reduce its costs.

References

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External links

Animation of polyspermy
Reproduction