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parthenogenesis

parthenogenesis is a type of asexual reproduction in which the offspring develops from unfertilized eggs. It is particularly common amongst arthropods and rotifers, can also be found in some species of fish, amphibians, birds, and reptiles, but not in mammals. Parthenogenetic development also occurs in some plants species, such as roses and orange trees.

Most animal species that reproduce parthenogenetically also display a phase of sexual behaviour and sexual reproduction. In most cases, parthenogenetic reproduction occurs when environmental conditions are favourable and there is plenty of food that can sustain the generation of large numbers of individuals in a short period of time. When external conditions change and food supplies become less abundant, or when the environment becomes unpredictable, these species shift to a sexual mode of reproduction. Although sexual reproduction is considerably slower and generates fewer organisms, it gives rise to individuals containing variations in their genetic material. Some of these individuals might be at an advantage over their predecessors, because they might be more able to adapt to new conditions.

In some species of insects, such as the aphids, parthenogenetic reproduction occurs in the spring and summer, when conditions are favourable for rapid population growth. As time goes by and conditions become less favourable, the parthenogenetically born individuals mate and lay fertilized eggs. These eggs hatch the following spring, when conditions are again favourable for another cycle of parthenogenetic reproduction.

In some species of ants, bees, and wasps, the ability to reproduce both sexually and asexually is part of the mechanism establishing sexual differences. Usually, females develop from unfertilized eggs, containing only half of the genetic material of the mother, whereas males develop from fertilized eggs, containing the genetic contributions of both mother and father.

In other species of insects, such as the rotifers, females produce unfertilized eggs that develop into females during the spring and summer. This process goes on for several generations. During the autumn, smaller eggs are laid, which develop into individuals lacking a digestive system, but capable of secreting sperm. These individuals mate with females, who then produce highly resistant, fertilized eggs that remain viable during long periods of unfavourable conditions. These eggs hatch in the following spring, giving rise only to females, who then engage in a new period of parthenogenetic reproduction.

Silvia Frenk

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parthenogenesis

parthenogenesis (pär´thənōjĕn´əsĬs) [Gr.,=virgin birth], in biology, a form of reproduction in which the ovum develops into a new individual without fertilization. Natural parthenogenesis has been observed in many lower animals (it is characteristic of the rotifers), especially insects, e.g., the aphid. In many social insects, such as the honeybee and the ant, the unfertilized eggs give rise to the male drones and the fertilized eggs to the female workers and queens. Parthenogenesis has also been observed in some snakes, fish, and monitor lizards. The phenomenon is rarer among plants (where it is called parthenocarpy) than among animals. Unusual patterns of heredity can occur in parthenogenetic organisms. For example, offspring produced by some types are identical in all inherited respects to the mother.

The phenomenon of parthenogenesis was discovered in the 18th cent. by Charles Bonnet. In 1900, Jacques Loeb accomplished the first clear case of artificial parthenogenesis when he pricked unfertilized frog eggs with a needle and found that in some cases normal embryonic development ensued. Artificial parthenogenesis has since been achieved in almost all major groups of animals, although it usually results in incomplete and abnormal development. Numerous mechanical and chemical agents have been used to stimulate unfertilized eggs. In 1936, Gregory Pincus induced parthenogenesis in mammalian (rabbit) eggs by temperature change and chemical agents. No successful experiments with human parthenogenesis have been reported.

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parthenogenesis

parthenogenesis The development of an individual from an egg without that egg undergoing fertilization. It occurs in some groups of animals (e.g. flatworms, leeches, aphids, rotifers), in which males may be absent, and in some plants (e.g. dandelion). The eggs (ova) that develop in this way are usually diploid, so all offspring are genetically identical with the parent. Commonly, parthenogenesis with only females in the population alternates with ordinary sexual reproduction, which allows the recombination of genetic material and presents a need for males. This alternation is called heterogamy.

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parthenogenesis

parthenogenesis The development of an individual from an egg without that egg undergoing fertilization. It occurs in some groups of animals (e.g. flatworms, leeches, aphids, rotifers), in which males may be absent. The eggs (ova) that develop in this way are usually diploid, so all offspring are genetically identical with the parent. Commonly, parthenogenesis with only females in the population alternates with ordinary sexual reproduction, which allows the recombination of genetic material and presents a need for males. This alternation is called heterogamy.

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parthenogenesis

parthenogenesis The development of an organism from an unfertilized egg. This occurs sporadically in many plants (e.g. dandelions and hawkweeds) and in a few animals, but in some species it is the main and sometimes only method of reproduction. For example, in some species of aphid, males are absent or very rare. The eggs formed by the females contain the full (diploid) number of chromosomes and are genetically identical. Variation is consequently very limited in species that reproduce parthenogenetically. See arrhenotoky; thelytoky.

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parthenogenesis

par·the·no·gen·e·sis / ˌpär[unvoicedth]ənōˈjenəsis/ • n. Biol. reproduction from an ovum without fertilization, esp. as a normal process in some invertebrates and lower plants. DERIVATIVES: par·the·no·ge·net·ic / -jəˈnetik/ adj. par·the·no·ge·net·i·cal·ly / -jəˈnetik(ə)lē/ adv.

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parthenogenesis

parthenogenesis Development of a female sex cell or gamete without fertilization. Since there is no involvement of a male gamete, it leads to the production of offspring that are genetically identical to the mother. This process occurs naturally among some plants and invertebrates, such as aphids.

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parthenogenesis

parthenogenesis (par-thin-oh-jen-i-sis) n. reproduction in which an organism develops from an unfertilized ovum. It is common in plants and occurs in some lower animals (e.g. aphids).

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parthenogenesis

parthenogenesis The development of an individual from an egg without fertilization; it occurs in some plants (e.g. dandelion).

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parthenogenesis

parthenogenesis reproduction without sexual union. XIX. f. Gr. parthénos virgin + génesis birth, GENESIS.

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parthenogenesis

parthenogenesisglacis, Onassis •abscess •anaphylaxis, axis, praxis, taxis •Chalcis • Jancis • synapsis • catharsis •Frances, Francis •thesis • Alexis • amanuensis •prolepsis, sepsis, syllepsis •basis, oasis, stasis •amniocentesis, anamnesis, ascesis, catechesis, exegesis, mimesis, prosthesis, psychokinesis, telekinesis •ellipsis, paralipsis •Lachesis •analysis, catalysis, dialysis, paralysis, psychoanalysis •electrolysis • nemesis •genesis, parthenogenesis, pathogenesis •diaeresis (US dieresis) • metathesis •parenthesis •photosynthesis, synthesis •hypothesis, prothesis •crisis, Isis •proboscis • synopsis •apotheosis, chlorosis, cirrhosis, diagnosis, halitosis, hypnosis, kenosis, meiosis, metempsychosis, misdiagnosis, mononucleosis, myxomatosis, necrosis, neurosis, osmosis, osteoporosis, prognosis, psittacosis, psychosis, sclerosis, symbiosis, thrombosis, toxoplasmosis, trichinosis, tuberculosis •archdiocese, diocese, elephantiasis, psoriasis •anabasis • apodosis •emphasis, underemphasis •anamorphosis, metamorphosis •periphrasis • entasis • protasis •hypostasis, iconostasis

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Parthenogenesis

Parthenogenesis

Types of parthenogenic organisms

Cellular mechanisms

Sexual vs. non-sexual reproduction

Resources

Parthenogenesis in animals refers to reproduction in which a new individual genetically identical to the parent develops from an unfertilized egg. The analogous event in plants, which results in seed formation without fertilization, is called agamospermy. Parthenogenesis is viewed as an aberration of sexual reproduction because animals that reproduce by parthenogenesis evolved from organisms that once reproduced sexually. In sexual reproduction female sex cells (ova) must be fertilized by male sex cells (typically sperm), for development to occur.

Types of parthenogenic organisms

The term parthenogenesis was first used in 1849 by the biologist Richard Owens. Although most animals reproduce sexually, some species of vertebrates and invertebrates reproduce by parthenogenesis. Of these species the most frequently studied are fish, reptiles, and insects. Parthenogenetic animals are classified as either facultative or obligate. Facultative parthenogens (usually invertebrates) can reproduce either parthenogenetically or sexually at all times, whereas obligate parthenogens are animals in which individuals of at least one generation reproduce by parthenogenesis. Obligate parthenogens may be further subdivided into either constant parthenogens or cyclical parthenogens. All generations of species showing constant parthenogenesis reproduce by parthenogenetic methods, and are typically composed of only females. Examples of these organisms include species of lizards, minnows, and brine shrimp. Cyclical parthenogens, such as aphids, alternate parthenogenetic generations with a sexual generation. For example, in the summer months aphids reproduce by parthenogenesis, but the onset of the fall acts as a signal for new offspring to develop into males which then mate with available females ( sexual reproduction) producing fertilized eggs that hatch in the spring.

Cellular mechanisms

Unlike sexually reproducing animals, parthenogens are faced with the unique problem of how to maintain a complete set of chromosomes (the cellular structures composed of DNA and protein that contain the genetic information cells need to function properly). In animals that reproduce sexually, meiosis occurs in cells destined to become eggs or sperm. Meiosis is the process where the chromosome content of a dividing cell is divided and reduced, producing egg or sperm cells with only half the normal number of chromosomes. When an egg is fertilized the chromosomes from the sperm are injected into the egg so restoring the fertilized eggs chromosome number to that of the parents body cells. Fertilization does not occur in parthenogenetic animals, which have developed special mechanisms to insure that a full set of chromosomes are passed on to the next generation.

Sexual vs. non-sexual reproduction

Most organisms reproduce sexually because there is a competitive advantage in producing offspring with genetic contributions from two individuals rather than one. The genetic recombination which occurs during meiosis and on fertilization allows new gene combinations to come together in the next generation. Organisms with new gene combinations are more variable and offer more options for selection pressures to select the best adaptations for the environmental conditions, for example making use of different food resources or being more resistant to pathogens.

Parthenogenetic animals receive all of their genes from one parent and therefore no new gene combinations are created. It may seem that this method of reproduction would put species that use it at a competitive disadvantage to sexually reproducing animals, but it may be advantageous in some cases. To reproduce, a sexually reproducing organism must first find a mate and then combine gametes with this mate. This process requires a great deal of time and energy, and it may well result in no offspring. Parthenogenic organisms do not experience this cost of reproduction and therefore usually can reproduce sooner after birth and produce more offspring. Animals which live in environments that are hospitable for only a short time period are often parthenogenic because mating would take time that these organisms do not have; these animals need to produce large numbers of offspring to compensate for the low survival rate of the offspring. Minnows found in the southwestern United States living in rivers that dry to the point where only puddles remain demonstrate parthenogenetic reproduction. Using this mode of reproduction eliminates the need for a suitable mate to be present in a given puddle.

Another advantage of parthenogenetic reproduction is that most offspring are unlikely to survive the dry months, regardless of whether or not sexual recombination occurs. Therefore, organisms that produce a greater quantity of offspring are more likely to have one survive to the next generation.

KEY TERMS

Agamospermy Seed development that occurs from an egg cell of a plant without it first being fertilized.

Apomixis Egg production without meiosis that results in the egg retaining a complete set of chromosomes.

Automixis Egg production in which meiosis is altered so that the egg retains a complete set of chromosomes.

Chromosomes he structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.

Constant parthenogens Animals that always reproduce parthenogenetically.

Cyclical parthenogens Obligate parthenogens that alternate sexually reproductive generations with parthenogenic generations.

Facultative parthenogens Animals with the potential to reproduce parthenogenetically or sexually at all times.

Fertilization Union of male and female sex cells to form a diploid cell.

Meiosis Cell division which produces sex cells with only half the chromosome number as the parent.

Obligate parthenogens Animals in which individuals of at least one generation reproduce partheno-genetically.

Recombination Process where genes from two individuals are contributed to an offspring.

Sex cells Cells which contribute genes to new offspring.

Parthenogenesis may also be advantageous in stable environments with ample food resources. These environments favor organisms with the ability to reproduce quickly allowing their offspring to consume the food resources before others do. This is the reason why certain cyclical parthenogens are so successful. For example, aphids reproduce parthenogenetically in the summer to exploit the abundant leaves which they feed upon. In the fall aphids produce fertilized eggs which may endure fluctuating environmental conditions when dormant during the winter or limited food supplies when they hatch in the spring.

Resources

BOOKS

Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Dennis Bray, Karen Hopkin, Keith Roberts, and Peter Walter. Essential Cell Biology. 2nd ed. New York: Garland Science/Taylor & Francis Group,.

Gilbert, Scott F. Developmental Biology. Sunderland, MA: Sinauer Associates, 2006.

Lodish, Harvey F. Molecular Cell Biology. New York: W.H. Freeman & Company, 2003.

Steven MacKenzie

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Parthenogenesis

Parthenogenesis

Parthenogenesis in animals refers to reproduction in which a new individual genetically identical to the parent develops from an unfertilized egg. The analogous event in plants, which results in seed formation without fertilization , is called agamospermy. Parthenogenesis is viewed as an aberration of sexual reproduction because animals that reproduce by parthenogenesis evolved from organisms that once reproduced sexually. In sexual reproduction female sex cells (ova) must be fertilized by male sex cells (typically sperm), for development to occur.

Types of parthenogenic organisms

The term parthenogenesis was first used in 1849 by the biologist Richard Owens. Although most animals reproduce sexually, some species of vertebrates and invertebrates reproduce by parthenogenesis. Of these species the most frequently studied are fish , reptiles, and insects . Parthenogenetic animals are classified as either facultative or obligate. Facultative parthenogens (usually invertebrates) can reproduce either parthenogenetically or sexually at all times, whereas obligate parthenogens are animals in which individuals of at least one generation reproduce by parthenogenesis. Obligate parthenogens may be further subdivided into either constant parthenogens or cyclical parthenogens. All generations of species showing constant parthenogenesis reproduce by parthenogenetic methods, and are typically composed of only females. Examples of these organisms include species of lizards, minnows , and brine shrimp . Cyclical parthenogens, such as aphids , alternate parthenogenetic generations with a sexual generation. In the summer months aphids reproduce by parthenogenesis, but the onset of the fall acts as a signal for new offspring to develop into males which then mate with available females (sexual reproduction) producing fertilized eggs that hatch in the spring.


Cellular mechanisms

Parthenogens, unlike sexually reproducing animals, are faced with the unique problem of how to maintain a complete set of chromosomes. Chromosomes are cellular structures composed of DNA and protein that contain the genetic information cells need to function properly. In animals that reproduce sexually, reduction division (meiosis ) occurs in cells destined to become eggs or sperm. Meiosis is the process where the chromosome content of a dividing cell is divided and reduced, producing egg or sperm cells with only half the normal number of chromosomes. When an egg is fertilized the chromosomes from the sperm are injected into the egg so restoring the fertilized egg's chromosome number to that of the parents' body cells. Fertilization does not occur in parthenogenetic animals, which have developed special mechanisms to insure that a full set of chromosomes are passed on to the next generation.

The cellular mechanisms by which parthenogenetic animals maintain a full set of chromosomes are known as apomixis and automixis. Each mechanism either alters or suppresses meiosis. Apomictic parthenogens are those in which meiosis is completely suppressed whereas automictic parthenogens are those in which the early stages of meiosis occur but the event is altered so that no chromosome division results.

Sexual vs. non-sexual reproduction

Most organisms reproduce sexually because there is a competitive advantage in producing offspring with genetic contributions from two individuals rather than one. The genetic recombination which occurs during meiosis and on fertilization allows new gene combinations to come together in the next generation. Organisms with new gene combinations are more variable and offer more options for selection pressures to select the best adaptations for the environmental conditions, for example making use of different food resources or being more resistant to pathogens .

Parthenogenetic animals receive all of their genes from one parent and therefore no new gene combinations are created. It may seem that this method of reproduction would put species that use it at a competitive disadvantage to sexually reproducing animals but it may be advantageous in some cases. To reproduce, a sexually reproducing organism must first find a mate and then combine gametes with this mate. This process requires a great deal of time and energy, and it may well result in no offspring. Parthenogenic organisms do not experience this cost of reproduction and therefore usually can reproduce sooner after birth and produce more offspring. Animals which live in environments that are hospitable for only a short time period are often parthenogenic because mating would take time that these organisms do not have; these animals need to produce large numbers of offspring to compensate for the low survival rate of the offspring. Minnows found in the southwestern United States living in rivers that dry to the point where only puddles remain, demonstrate parthenogenetic reproduction so eliminating the need for a suitable mate to be present in a given puddle. Another advantage of parthenogenetic reproduction is that most offspring are unlikely to survive the dry months, regardless of whether or not sexual recombination occurs. Therefore organisms which produce a greater quantity of offspring are more likely to have one survive to the next generation.

Parthenogenesis may also be advantageous in stable environments with ample food resources. These environments favor organisms with the ability to reproduce quickly allowing their offspring to consume the food resources before others do. This is the reason why certain cyclical parthenogens are so successful. For example, aphids reproduce parthenogenetically in the summer to exploit the abundant leaves which they feed upon. In the fall aphids produce fertilized eggs which may endure fluctuating environmental conditions when dormant during the winter or limited food supplies when they hatch in the spring.

See also Asexual reproduction.

Resources

books

Catton, Chris, and James Gray. Sex In Nature. New York: Facts on File, 1985.

Colinvaux, Paul. Ecology. New York: John Wiley & Sons, 1986.

Hughes, Roger. A Functional Biology of Clonal Animals. London: Chapman and Hall, 1989.

Suomalainen, Esko, Anssi Suara, and Juhani Lokki. Cytology and Evolution in Parthenogenesis. Boca Raton, FL: CRC Press, 1987.


Steven MacKenzie

KEY TERMS


Agamospermy

—Seed development that occurs from an egg cell of a plant without it first being fertilized.

Apomixis

—Egg production without meiosis that results in the egg retaining a complete set of chromosomes.

Automixis

—Egg production in which meiosis is altered so that the egg retains a complete set of chromosomes.

Chromosomes

—he structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.

Constant parthenogens

—Animals that always reproduce parthenogenetically.

Cyclical parthenogens

—Obligate parthenogens that alternate sexually reproductive generations with parthenogenic generations.

Facultative parthenogens

—Animals with the potential to reproduce parthenogenetically or sexually at all times.

Fertilization

—Union of male and female sex cells to form a diploid cell.

Meiosis

—Cell division which produces sex cells with only half the chromosome number as the parent.

Obligate parthenogens

—Animals in which individuals of at least one generation reproduce parthenogenetically.

Recombination

—Process where genes from two individuals are contributed to an offspring.

Sex cells

—Cells which contribute genes to new offspring.

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"Parthenogenesis." The Gale Encyclopedia of Science. . Encyclopedia.com. 10 Sep. 2018 <http://www.encyclopedia.com>.

"Parthenogenesis." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 10, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/parthenogenesis

"Parthenogenesis." The Gale Encyclopedia of Science. . Retrieved September 10, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/parthenogenesis

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Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

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