Polyspermy
Encyclopedia
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
.
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 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 charge at the egg surface.
Electrical polyspermy blocks operate in many animal species, including frog
s, clam
s, and marine worm
s, but not in the several mammal
s 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, with the fast, but transient, electrical block superseded after the first minute or so by a more slowly developing permanent mechanical block.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.
. 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
), and additional mechanisms that are not well understood modify the egg's plasma membrane.
. 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.
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
that has been fertilized
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
by more than one sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
. Diploid
Ploidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
organisms normally contain two copies of each chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
, 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
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition
Sperm competition
Sperm competition is a term used to refer to the competitive process between spermatozoa of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable offspring...
. Such a situation is often deleterious to the female: in other words, the male-male competition among sperm spills over to create sexual conflict
Sexual conflict
Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
.
Fast block of polyspermy
The eggs of sexually reproducingSexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
organisms are adapted
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to avoid this situation. The defenses are particularly well characterized in the sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
, 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
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
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 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 charge at the egg surface.
Electrical polyspermy blocks operate in many animal species, including frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s, clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...
s, and marine worm
Marine worm
Any worm that lives in a marine environment is considered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida , Chaetognatha, Hemichordata, and Phoronida....
s, but not in the several mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s that have been studied (hamster
Hamster
Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....
, rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
, mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
). 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
Evolutionary game theory
Evolutionary game theory is the application of Game Theory to evolving populations of lifeforms in biology. EGT is useful in this context by defining a framework of contests, strategies and analytics into which Darwinian competition can be modelled. It originated in 1973 with John Maynard Smith...
, 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 oviductOviduct
In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...
. 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
Cortical reaction
The cortical reaction occurs when a sperm cell unites with the egg's plasma membrane, modification of the zona pellucida that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.The cortical reaction...
" modifies the extracellular coat of the egg (the zona pellucida
Zona pellucida
The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
), and additional mechanisms that are not well understood modify the egg's plasma membrane.
Evolutionary advantage
Female defenses select for ever more aggressive male sperm however, leading to an evolutionary arms raceEvolutionary arms race
In evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...
. 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
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
rates seen in mammalian species.
Further reading
- Ginzberg, A. S. 1972. Fertilization in Fishes and the Problem of Polyspermy, Israel Program for Scientific Translations, Jerusalem.
- 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.