Parthenogenesis

Parthenogenesis

Discussion
Ask a question about 'Parthenogenesis'
Start a new discussion about 'Parthenogenesis'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
Parthenogenesis is a form of asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 found in female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

s, where growth and development of embryos occur without fertilization by a male. In plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis
Apomixis
In botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. This definition notably does not mention meiosis...

.

The word "parthenogenesis" comes from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 παρθένος, parthenos, meaning "virgin", and γένεσις, genesis, meaning "birth". The term is sometimes used inaccurately to describe reproduction modes in hermaphroditic
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

 species that can reproduce by themselves because they contain reproductive organs of both sexes in a single individual's body.

Parthenogenesis occurs naturally in some invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

 animal species (e.g., water fleas, aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...

s, nematodes, some bee
Bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, some Phasmida, some scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

 species, and parasitic wasp
Parasitic wasp
The term parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. They are primarily parasitoids of other animals, mostly other arthropods...

s) and some vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s (e.g., some reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s,
fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

,
and very rarely bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s and shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s). This type of reproduction has been induced artificially in fish and amphibians.

Normal egg cells form after meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 and are haploid, with half as many chromosomes as their mother's body cells. Haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the diploid chromosome number. If the chromosome number of the haploid egg cell is doubled during development, the offspring is "half a clone" of its mother. If the egg cell was formed without meiosis, it is a full clone
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...

of its mother.

The offspring produced by parthenogenesis in species that use the XY sex-determination system
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...

 have two X chromosomes and are female. In species that use the ZW sex-determination system
ZW sex-determination system
The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects , and some reptiles, including Komodo dragons...

 they have either two Z chromosomes (male) or two W chromosomes (non-viable or female), or (theoretically) if clonal parthenogenesis was involved (also called apomixis), they could have one Z and one W chromosome (female).

Occurrence and types


Parthenogenesis is seen to occur naturally in aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...

s, Daphnia
Daphnia
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style...

, rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s, nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s and some other invertebrates, as well as in many plants and certain lizards. Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...

s and the hammerhead-
Hammerhead shark
The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna while the...

 and blacktip shark
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...

s have recently been added to the list of vertebrates—along with several genera of fish, amphibians, and reptiles—that exhibit differing forms of asexual reproduction, including true parthenogenesis, gynogenesis, and hybridogenesis (an incomplete form of parthenogenesis). As with all types of asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

, there are both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis.

The offspring of parthenogenesis will be all female if two like chromosomes determine the female sex (such as the XY sex-determination system
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...

), but they will be male if two like chromosomes determine the male sex (such as the ZW sex-determination system
ZW sex-determination system
The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects , and some reptiles, including Komodo dragons...

), because the process involves the inheritance and subsequent duplication of only a single sex chromosome. The offspring may be capable of sexual reproduction, if this mode exists in the species. In many cases, parthenogenesis occurs when one gender (typically the male) is unavailable in the general vicinity. Once males are again available, the parthenogenesis-created females would be capable of mating with the males and creating normal offspring.

Parthenogenesis is distinct from artificial animal cloning, a process where the new organism is necessarily genetically identical to the cell donor. In cloning, the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 of a diploid cell from a donor organism is inserted into an enucleated egg cell and the cell is then stimulated to undergo continued mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

, resulting in an organism that is genetically identical to the donor. Parthenogenesis is different, in that it originates from the genetic material contained within an egg cell. Egg cells may be produced via meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 or mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 oogenesis. If produced by mitosis, the egg that undergoes parthenogenesis can be either haploid or diploid, leading to a number of possible outcomes in terms of the genetic fingerprint of the parthenogen. A diploid parent organism that undergoes parthenogenesis via meiosis will create haploid offspring with a new genetic fingerprint due to crossing over
Crossing Over
Crossing Over may refer to:* Chromosomal crossover, a cellular process* Crossing Over , a 1998 album by Hesperus* Crossing Over, a book by John Edward* Crossing Over , a 2009 film...

 of the chromosomes in the parent. Because there are so many variables in parthenogenesis, there is little that can be said for sure unless the specific methods of the particular parthenogenetic tendencies of an organism are known.

A litter of offspring resulting from parthenogenesis may contain genetically identical siblings. In organisms possessing an XY chromosome system, parthenogenic offspring are always female, but they are not necessarily genetically identical to one another or to their mother (some chromosome segments may differ because of meiosis).

Parthenogenesis may be achieved through an artificial process as described below under the discussion of mammals.

Alternation between parthenogenesis and sexual reproduction
Sexual 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...

 is called cyclical parthenogenesis or heterogamy
Heterogamy
Heterogamy has a number of biological definitions:*In reproductive biology, heterogamy is the alternation of differently organized generations, applied to the alternation between parthenogenetic and a sexual generation...

. A form of reproduction related to parthenogenesis, but that requires the presence of only sperm that do not fertilize an egg, is known as gynogenesis. In hybridogenesis the sperm fertilizes the egg, but its chromosomes are not carried to subsequent generations.

Insects


Parthenogenesis in insects can cover a wide range of mechanisms.

The different forms include:
  1. Thelytoky - parthenogenesis in which only female offspring are produced and no mating is observed
  2. Pseudogamy
    Pseudogamy
    Pseudogamy refers to aspects of reproduction. It has come to have different meanings in zoology and in botany.- In zoology :In zoology it is used for a type of parthenogenesis in which the sperm stimulates the egg cell to develop into an embryo, but no male inheritance is transmitted...

    (or gynogenesis or sperm-dependent parthenogenesis) - here mating occurs and the eggs require activation by entry of sperm but only the maternal chromosomes are expressed
  3. Automixis - parthenogenesis in which the eggs underwent meiosis
    Meiosis
    Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

  4. Apomixis - parthenogenesis in which the eggs did not undergo meiosis


Polyembryony is another process that produces multiple clonal offspring from a single egg. This is known in some hymenopteran parasitoids and in Strepsiptera
Strepsiptera
The Strepsiptera are an order of insects with ten families making up about 600 species...

.

In automictic species the offspring can be haploid or diploid. Diploids are produced by doubling or fusion of gametes after meiosis. Fusion is seen in the Phasmatodea, Hemiptera (Aleurodids and Coccidae), Diptera, and some Hymenoptera.

In addition to these forms is hermaphroditism, where both the eggs and sperm are produced by the same individual, but is not a type of parthenogenesis. This is seen in three species of Icerya scale insects.

Parasitic bacteria like Wolbachia
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...

have been noted to induce automictic thelytoky in many insect species with haplodiploid systems. They also cause gamete duplication in unfertilized eggs causing them to develop into female offspring.

An example of non-viable parthenogenesis is common among domesticated honey bee
Honey bee
Honey bees are a subset of bees in the genus Apis, primarily distinguished by the production and storage of honey and the construction of perennial, colonial nests out of wax. Honey bees are the only extant members of the tribe Apini, all in the genus Apis...

s. The queen bee is the only fertile female in the hive; if she dies without the possibility for a viable replacement queen, it is not uncommon for the worker bees to lay eggs. Worker bees are unable to mate, and the unfertilized eggs produce only drones (males), which can mate only with a queen. Thus, in a relatively short period, all the worker bees die off, and the new drones follow.

In one subspecies from South Africa, Apis mellifera capensis
Apis mellifera capensis
Apis mellifera capensis, the Cape honey bee or Cape bee is a southern South African subspecies of the Western honey bee.They are unique among honey bee subspecies because workers can lay diploid, female eggs, by means of thelytoky, while workers of other subspecies can only lay haploid, male eggs...

, workers are capable of producing diploid eggs parthenogenetically, and thus the queen can be replaced if she dies.

It is believed that a few other bee
Bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s may be truly parthenogenetic, for example, at least one species of small carpenter bee, in the genus Ceratina
Ceratina
The cosmopolitan bee genus Ceratina, often referred to as small carpenter bees, is the sole lineage of the tribe Ceratinini, and closely related to the more familiar carpenter bees...

. Many parasitic wasp
Parasitic wasp
The term parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. They are primarily parasitoids of other animals, mostly other arthropods...

s are known to be parthenogenetic, sometimes due to infections by Wolbachia
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...

.

In Cataglyphis cursor, a European formicine ant
Formicinae
Formicinae is a subfamily within the Formicidae containing ants of moderate evolutionary development.Formicines retain some primitive features such as the presence of cocoons around pupae, the presence of ocelli in workers, and little tendency toward reduction of palp or antennal segmentation in...

, the queen can reproduce by parthenogenesis. The workers are fertile and can mate with the males in the colony.

In Central and South American electric ants, Wasmannia auropunctata, queens produce more queens through parthenogenesis. Sterile workers usually are produced from eggs fertilized by males. In some of the eggs fertilized by males, however, the fertilization can cause the female genetic material to be ablated from the zygote, in a process called ameiotic parthenogenesis. In this way, males pass on only their genes to become fertile male offspring. This is the first recognized example of an animal species where both females and males can reproduce clonally resulting in a complete separation of male and female gene pools.

Crustaceans


Crustacean reproduction varies both across and within species. The water flea Daphnia pulex
Daphnia pulex
Daphnia pulex is the most common species of water flea. It has a cosmopolitan distribution: the species is found throughout the Americas, Europe and Australia. It is a model species, and was the first crustacean to have its genome sequenced.-Description:...

alternates between sexual and parthenogenetic reproduction. Among the better-known large decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 crustaceans, some crayfish reproduce by parthenogensis. "Marmorkrebs
Marmorkrebs
', or marbled crayfish, are parthenogenetic crayfish that were discovered in the pet trade in Germany in the 1990s. are closely related to the "slough crayfish", Procambarus fallax. Procambarus fallax is widely distributed across Florida, but there are no known natural populations of Marmorkrebs...

" are parthenogenetic crayfish
Crayfish
Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related...

 that were discovered in the pet trade
Fishkeeping
Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...

 in the 1990s. Offspring are genetically identical to the parent, indicating it reproduces by apomixis, i.e. parthenogenesis in which the eggs did not undergo meiosis. Spinycheek crayfish (Orconectes limosus
Orconectes limosus
Orconectes limosus is a species of crayfish in the family Cambaridae. It is native to the east coast of North America, from Maine to the lower James River, Virginia, but has also been introduced to Europe. It is unusual in that it lives in silty streams, rather than the clear water usually...

) can reproduce both sexually and by parthenogenesis. The Louisiana red swamp crayfish (Procambarus clarkii
Procambarus clarkii
Procambarus clarkii is a freshwater crayfish species, native to the Southeastern United States, but found also on other continents, where it is often an invasive pest. It is known variously as the red swamp crawfish, red swamp crayfish, Louisiana crawfish, Louisiana crayfish or mudbug.-Range and...

), which normally reproduces sexually, has also been suggested to reproduce by parthenogenesis, although no individuals of this species have been reared this way in the lab.

Rotifers


In bdelloid rotifers, females reproduce exclusively by parthenogenesis, while in monogonont rotifers, females can alternate between sexual and asexual reproduction (cyclical parthenogenesis). At least in one normally cyclical parthenogenetic species obligate parthenogenesis can be inherited: a recessive allele leads to loss of sexual reproduction in homozygous offspring.

Flatworms


At least two species in the genus Dugesia
Dugesia
Dugesia is a genus of dugesiid flatworms containing some common representatives of the class Turbellaria. These common flatworms are found in freshwater habitats. Their digestion tract consists of a central tubular pharynx. Each branch consists of ceca, which delivers the nutrients to the body...

, flatworms in the Turbellaria
Turbellaria
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...

 sub-division of the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Platyhelminthes, include polyploid individuals that reproduce by parthenogenesis. This type of parthenogenesis requires mating, but the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 does not contribute to the genetics of the offspring (the parthenogenesis is pseudogamous, alternatively referred to as gynogenetic). A complex cycle of matings between diploid sexual and polyploid parthenogenetic individuals produces new parthenogenetic lines.

Snails


Parthenogenetic Thiarid snails have slender cone shaped shells. They live in muddy stream bottoms and feed on detritus and algae.

Reptiles



Most reptiles reproduce sexually, but parthenogenesis has been observed to occur naturally in certain species of whiptail
Cnemidophorus
Cnemidophorus is a genus of lizards which belong to the family of Teiidae, which are commonly referred to as whiptail lizards or racerunners. Note that the nomenclature for this genus has recently been re-examined by T. W. Reeder et al. in 2002...

s, gecko
Lepidodactylus lugubris
The Mourning Gecko or Common Smooth-Scaled Gecko is a species of gecko. This small , nocturnal gecko feeds on small insects and flower nectar. This species is notable because it is parthenogenic, and there are no remaining males that have been found...

s, rock lizard
Darevskia
Darevskia is a genus of wall lizards of the family Lacertidae.-Species:*Darevskia alpina*Darevskia armeniaca*Darevskia bendimahiensis*Darevskia brauneri*Darevskia caucasica*Darevskia clarkorum*Darevskia daghestanica...

s, blindsnake
Ramphotyphlops braminus
Ramphotyphlops braminus is a harmless blind snake species found mostly in Africa and Asia, but has been introduced in many other parts of the world. Completely fossorial, they are often mistaken for earthworms, except that they are not segmented. The specific name is a Latinized form of the word...

s, Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...

s and boa constrictor
Boa constrictor
The Boa constrictor is a large, heavy-bodied species of snake. It is a member of the family Boidae found in North, Central, and South America, as well as some islands in the Caribbean. A staple of private collections and public displays, its color pattern is highly variable yet distinctive...

s.

Reptiles use the ZW chromosome system, which produces either males (ZZ) or females (ZW or WW). Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes.

Parthenogenesis has been studied extensively in the New Mexico whiptail
Cnemidophorus neomexicanus
The New Mexico whiptail is a species of lizard found in the southern United States in New Mexico and Arizona, and in northern Mexico in Chihuahua. It is the official state reptile of New Mexico...

 (genus Cnemidophorus
Cnemidophorus
Cnemidophorus is a genus of lizards which belong to the family of Teiidae, which are commonly referred to as whiptail lizards or racerunners. Note that the nomenclature for this genus has recently been re-examined by T. W. Reeder et al. in 2002...

), of which 15 species reproduce exclusively by parthenogenesis. These lizards live in the dry and sometimes harsh climate of the southwestern United States and northern Mexico. All these asexual species appear to have arisen through the hybridization of two or three of the sexual species in the genus leading to polyploid individuals. The mechanism by which the mixing of chromosomes from two or three species can lead to parthenogenetic reproduction is unknown. Because multiple hybridization events can occur, individual parthenogenetic whiptail species can consist of multiple independent asexual lineages. Within lineages, there is very little genetic diversity, but different lineages may have quite different genotypes.

An interesting aspect to reproduction in these asexual lizards is that mating behaviors are still seen, although the populations are all female. One female plays the role played by the male in closely related species, and mounts the female that is about to lay eggs. This behaviour is due to the hormonal cycles of the females, which cause them to behave like males shortly after laying eggs, when levels of progesterone are high, and to take the female role in mating before laying eggs, when estrogen dominates. Lizards who act out the courtship ritual have greater fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

 than those kept in isolation, due to the increase in hormones that accompanies the mounting. So, although the populations lack males, they still require sexual behavioral stimuli for maximum reproductive success.

The Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...

, which normally reproduces sexually, has also been found able to reproduce asexually by parthenogenesis. A case has been documented of a Komodo Dragon switching back to sexual reproduction after a known parthenogenetic event. It has been postulated that this gives an advantage to colonization of islands, where a single female could theoretically have male offspring asexually, then switch to sexual reproduction with them to maintain a higher level of genetic diversity than asexual reproduction alone can generate.

Parthenogenesis may also occur naturally when males and females are both present, which might explain why the wild Komodo dragon population is approximately 75 percent male.

Sharks


A bonnethead
Bonnethead
The bonnethead shark or shovelhead, Sphyrna tiburo, is a member of the hammerhead shark genus Sphyrna. The Greek word sphyrna translates as hammer, referring to the shape of this shark's head - tiburo is the Taino word for shark....

, a type of small hammerhead shark
Hammerhead shark
The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna while the...

, was found to have produced a pup, born live on 14 December 2001 at Henry Doorly Zoo
Henry Doorly Zoo
The Omaha's Henry Doorly Zoo is a zoo in Omaha, Nebraska, located at 3701 South 10th Street.It is accredited by the Association of Zoos and Aquariums and a member of the World Association of Zoos and Aquariums. Its mission is conservation, research, recreation, and education.Omaha's Henry Doorly...

 in Nebraska, in a tank containing three female hammerheads, but no males. The pup was thought to have been conceived through parthenogenic means. The shark pup was apparently killed by a stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...

 within days of birth. The investigation of the birth was conducted by the research team from Queen's University Belfast, Southeastern University in Florida, and Henry Doorly Zoo itself, and it was concluded after DNA testing that the reproduction was parthenogenic. The testing showed the female pup's DNA matched only one female who lived in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of her mother, but rather, contained only half of her mother's DNA ("automictic parthenogenesis"). This type of reproduction had been seen before in bony fish, but never in cartilaginous fish such as sharks, until this documentation.

In 2002, two white-spotted bamboo shark
White-spotted bamboo shark
The whitespotted bamboo shark, Chiloscyllium plagiosum, is a carpet shark with an adult size that approaches one metre in length This small, mostly nocturnal species is harmless to humans. The whitespotted bamboo shark is occasionally kept as a pet in larger home aquariums. It can grow up to ...

s were born at the Belle Isle Aquarium
Belle Isle Aquarium
The Belle Isle Aquarium, located on Belle Isle Park in Detroit, Michigan, was the oldest continually-operating aquarium in North America. Designed by noted architect Albert Kahn, it opened on August 18, 1904 and closed April 3, 2005 [...

 in Detroit. They hatched 15 weeks after being laid. The births baffled experts as the mother shared an aquarium with only one other shark, which was female. The female bamboo sharks had laid eggs in the past. This is not unexpected, as many animals will lay eggs even if there is not a male to fertilize them. Normally, the eggs are assumed to be inviable and are discarded. This batch of eggs was left undisturbed by the curator as he had heard about the previous birth in 2001 in Nebraska and wanted to observe whether they would hatch.

Other possibilities had been considered for the birth of the Detroit bamboo sharks including thoughts that the sharks had been fertilized by a male and stored the sperm for a period of time, as well as the possibility that the Belle Isle bamboo shark is a hermaphrodite, harboring both male and female sex organs, and capable of fertilizing its own eggs, but that is not confirmed.

In 2008, a Hungarian aquarium had another case of parthenogenesis after its lone female shark produced a pup without ever having come into contact with a male shark. In the same year, a female Atlantic blacktip shark
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...

 in Virginia reproduced via parthenogenesis.

On 10 October 2008 scientists confirmed the second case of a virgin birth in a shark. The Journal of Fish Biology reported a study in which scientists said DNA testing proved that a pup carried by a female Atlantic blacktip shark in the Virginia Aquarium & Marine Science Center contained no genetic material from a male.

The repercussions of parthenogenesis in sharks, which fails to increase the genetic diversity of the offspring, is a matter of concern for shark experts, taking into consideration conservation management strategies for this species, particularly in areas where there may be a shortage of males due to fishing or environmental pressures. Although parthenogenesis may help females who cannot find mates, it does reduce genetic diversity.

Sharks have an XY sex-determination system
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...

, so they produce only female (XX) offspring by parthenogenesis. As a result, sharks cannot restore a depleted male population through parthenogenesis, so an all-female population must come in contact with an outside male before sexual reproduction resulting in males can occur.

Birds



There were early claims of parthenogenesis in birds but this can be attributed to non-scientific reasoning and motivation. According to Giraldus Cambrensis
Giraldus Cambrensis
Gerald of Wales , also known as Gerallt Gymro in Welsh or Giraldus Cambrensis in Latin, archdeacon of Brecon, was a medieval clergyman and chronicler of his times...

 and Conor O'Cathfai who wrote an account of the history and topography of Ireland in the latter part of the 12th century, the Barnacle Goose
Barnacle Goose
The Barnacle Goose belongs to the genus Branta of black geese, which contains species with largely black plumage, distinguishing them from the grey Anser species...

 reproduces without mating. It was claimed that since this makes it not the progeny of flesh, it could not be considered flesh itself, and this was used as a justification for eating it during periods of fasting. This appears to be the only basis for the claim that the goose is parthenogenetic, and no scientific evidence has been discovered in the modern era.

Parthenogenesis occurs in turkeys through doubling of haploid cells to diploid, and the rate at which this occurs could be increased by selective breeding. The offspring produced by parthenogenesis were healthy, and as doubled haploids they were homogametic, and consequently all were male (in turkeys, like in many birds and in contradiction to mammals, the males are homogametic).

Mammals


There are no known cases of naturally occurring mammalian parthenogenesis in the wild. Parthenogenetic progeny of mammals would have two X chromosomes, and would therefore be female.

In 1936, Gregory Goodwin Pincus reported successfully inducing parthenogenesis in a rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

. In April 2004, scientists at Tokyo University of Agriculture
Tokyo University of Agriculture
The , abbreviated as Nodai or Tokyo nodai , is a private university which treats agriculture in Japan.The campus is in three places, Setagaya, Atsugi, and Okhotsk .- Outline :...

 used parthenogenesis successfully to create a fatherless mouse. Using gene targeting, they were able to manipulate two imprinted loci H19/IGF2 and DLK1/MEG3
MEG3
MEG3 is a maternally expressed, imprinted long non-coding RNA gene. At least 12 different isoforms of MEG3 are generated by alternative splicing. Expression of MEG3 is lost in cancer cells. It acts as a growth suppressor in tumour cells, and activates p53. A pituitary transcript variant has been...

 to produce bi-maternal mice at high frequency and subsequently show that fatherless mice have enhanced longevity
Longevity
The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....

.

Induced parthenogenesis in mice
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...

 and monkey
Monkey
A monkey is a primate, either an Old World monkey or a New World monkey. There are about 260 known living species of monkey. Many are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys...

s often results in abnormal development. This is because mammals have imprinted genetic regions, where either the maternal or the paternal chromosome is inactivated in the offspring in order for development to proceed normally. A mammal created by parthenogenesis would have double doses of maternally imprinted genes and lack paternally imprinted genes, leading to developmental abnormalities. It has been suggested that defects in placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...

l folding or interdigitation are one cause of swine parthenote abortive development. As a consequence, research on human parthenogenesis is focused on the production of embryonic stem cells for use in medical treatment, not as a reproductive strategy.

Use of an electrical or chemical stimulus can produce the beginning of the process of parthenogenesis in the asexual development of viable offspring.
During oocyte development, high metaphase promoting factor (MPF) activity causes mammalian oocytes to arrest at the metaphase II stage until fertilization by a sperm. The fertilization event causes intracellular calcium oscillations, and targeted degradation of cyclin B, a regulatory subunit of MPF, thus permitting the MII-arrested oocyte to proceed through meiosis.

To initiate parthenogenesis of swine oocytes, various methods exist to induce an artificial activation that mimics sperm entry, such as calcium ionophore treatment, microinjection of calcium ions, or electrical stimulation. Treatment with cycloheximide, a non-specific protein synthesis inhibitor, enhances parthenote development in swine presumably by continual inhibition of MPF/cyclin B. As meiosis proceeds, extrusion of the second polar is blocked by exposure to cytochalasin B. This treatment results in a diploid (2 maternal genomes) parthenote [(Bischoff et al., 2009), PMID 19571260] Parthenotes can be surgically transferred to a recipient oviduct for further development, but will succumb by developmental failure after ~30 days of gestation. The swine parthenote placentae often appears hypo-vascular: see free image (Figure 1) in linked reference.

Humans


On June 26, 2007, International Stem Cell Corporation (ISCC), a California-based stem cell research company, announced that their lead scientist, Dr. Elena Revazova, and her research team were the first to intentionally create human stem cells from unfertilized human eggs using parthenogenesis. The process may offer a way for creating stem cells that are genetically matched to a particular woman for the treatment of degenerative diseases that might affect her. In December 2007, Dr. Revazova and ISCC published an article illustrating a breakthrough in the use of parthenogenesis to produce human stem cells that are homozygous in the HLA
Human leukocyte antigen
The human leukocyte antigen system is the name of the major histocompatibility complex in humans. The super locus contains a large number of genes related to immune system function in humans. This group of genes resides on chromosome 6, and encodes cell-surface antigen-presenting proteins and...

 region of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

. These stem cells are called HLA homozygous parthenogenetic human stem cells (hpSC-Hhom) and have unique characteristics that would allow derivatives of these cells to be implanted into millions of people without immune rejection. With proper selection of oocyte donors according to HLA haplotype
Haplotype
A haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...

, it is possible to generate a bank of cell lines whose tissue derivatives, collectively, could be MHC-matched
Major histocompatibility complex
Major histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...

 with a significant number of individuals within the human population.

On August 2, 2007, after much independent investigation, it was revealed that discredited South Korean scientist Hwang Woo-Suk
Hwang Woo-Suk
Hwang Woo-suk is a South Korean veterinarian and researcher. He was a professor of theriogenology and biotechnology at Seoul National University who became infamous for fabricating a series of experiments, which appeared in high-profile journals, in the field of stem cell research...

 unknowingly produced the first human embryos resulting from parthenogenesis. Initially, Hwang claimed he and his team had extracted stem cells from cloned human embryos, a result later found to be fabricated. Further examination of the chromosomes of these cells show indicators of parthenogenesis in those extracted stem cells, similar to those found in the mice created by Tokyo scientists in 2004. Although Hwang deceived the world about being the first to create artificially cloned human embryos, he did contribute a major breakthrough to stem cell research by creating human embryos using parthenogenesis. Although the truth about the results of Hwang's work was just discovered, those embryos were created by him and his team before February 2004, making Hwang the first, unknowingly, to perform the process of parthenogenesis to create a human embryo and ultimately a human parthenogenetic stem cell line successfully.

Oomycetes


Apomixis can apparently occur in Phytophthora
Phytophthora
Phytophthora is a genus of plant-damaging Oomycetes , whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The genus was first described by Heinrich Anton de Bary in 1875...

, an Oomycete. Oospore
Oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi. Also the result of plasmogamy/karyogamy in oomycetes, which in turn leads to the development of hyphae, then mycelium....

s derived after an experimental cross were germinated, and some of the progeny were genetically identical to one or other parent, which would imply that meiosis did not occur and the oospores developed by parthenogenesis.

Gynogenesis


A form of asexual reproduction related to parthenogenesis is gynogenesis. Here, offspring are produced by the same mechanism as in parthenogenesis, but with the requirement that the egg merely be stimulated by the presence of sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 in order to develop. However, the sperm cell does not contribute any genetic material to the offspring. Since gynogenetic species are all female, activation of their eggs requires mating with males of a closely related species for the needed stimulus. Some salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s of the genus Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed that the success of those salamanders may be due to rare fertilization of eggs by males, introducing new material to the gene pool, which may result from perhaps only one mating out of a million. In addition, the amazon molly
Amazon molly
The Amazon molly, Poecilia formosa, is a freshwater fish which reproduces through gynogenesis. This means although females must mate with a male, genetic material from the male is not incorporated into the already diploid egg cells the mother is carrying , resulting in clones of the mother being...

 is known to reproduce by gynogenesis.

Hybridogenesis


In hybridogenesis, reproduction is not completely asexual, but instead hemiclonal: Half the genome is passed intact to the next generation, while the other half is discarded. It occurs in some animals that are themselves hybrids between two different species.

Hybridogenetic females can mate with males of a "donor" species and both will contribute genetic material to the offspring. When each female offspring produces her own eggs, however, the eggs will contain no genetic material from her father (the donor), only the chromosomes from her own mother; the set of genes from the father is invariably discarded. This process continues, so that each generation is half (or hemi-) clonal on the mother's side and has half new genetic material from the father's side. This form of reproduction is seen in some live-bearing fish of the genus Poeciliopsis
Poeciliopsis
Poeciliopsis is a genus of fish in the Poeciliidae family.Species include:* Poeciliopsis infans – Lerma Livebearer* Poeciliopsis monacha* Poeciliopsis occidentalis – charalito...

as well as in the waterfrog Rana esculenta.

Automixis


Automixis is a term that covers several reproductive mechanisms, some of which are parthenogenetic.

Automixis is defined as reproduction involving the merger of two products (from the four possible products) of the same meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

. If this merger occurs before these meiotic products have matured into gametes, then the process is considered to be parthenogenesis. In automictic parthenogenesis the individual offspring differ from one another and from their mother.

Sometimes the meiotic products that combine are exact copies, which is sometimes described as "half a clone". The offspring still is unique and not a clone of its mother.

See also

  • Apomixis
    Apomixis
    In botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. This definition notably does not mention meiosis...

     for a similar process in plants
  • Arrhenotoky
    Arrhenotoky
    Arrhenotoky or arrhenotokous parthenogenesis is a form of parthenogenesis in which unfertilized eggs develop into haploid males.This form is observed in some marine invertebrates, beetles, scorpions, mites, bees, etc....

  • Kaguya, a mouse created in 2004 by induced parthogenesis
  • Komodo Dragon, virgin birth recorded in Komodo dragon
  • Miraculous births
    Miraculous births
    Miraculous births are a common motif in historical literature and religious texts. Stories of miracle births often include miraculous conceptions and features such as intervention by a deity, supernatural elements, astronomical signs, hardship or in the case of some mythologies complex plots...

  • Parthenocarpy
    Parthenocarpy
    In botany and horticulture, parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are actually aborted while still small...

     plants with seedless fruit
  • Thelytoky
    Thelytoky
    Thelytoky comes from the Greek thely, meaning "female", and tokos, meaning "birth". Thelytokous parthenogenesis is a type of parthenogenesis in which females are produced from unfertilized eggs. It is rare in the animal kingdom and has only been reported in about 1500 species...


People
  • Charles Bonnet
    Charles Bonnet
    Charles Bonnet , Swiss naturalist and philosophical writer, was born at Geneva, of a French family driven into Switzerland by the religious persecution in the 16th century.-Life and work:Bonnet's life was uneventful...

     conducted a series of experiments establishing what is now termed parthenogenesis in aphides or tree-lice
  • Jacques Loeb
    Jacques Loeb
    Jacques Loeb was a German-born American physiologist and biologist.-Biography:...

     was able to cause the eggs of 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...

    s to begin embryonic development without sperm
  • Gregory Goodwin Pincus experimented with parthenogenesis

Further reading

  • Dawley, Robert M. & Bogart, James P. (1989). Evolution and Ecology of Unisexual Vertebrates. Albany, New York: New York State Museum. ISBN 1-55557-179-4.
  • Futuyma, Douglas J. & Slatkin, Montgomery. (1983). Coevolution. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-228-3.
  • Maynard Smith, John. (1978). The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0-521-29302-2.
  • Michod, Richard E. & Levin, Bruce R. (1988). The Evolution of Sex. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-459-6.
  • Stearns, Stephan C. (1988). The Evolution of Sex and Its Consequences (Experientia Supplementum, Vol. 55). Boston: Birkhauser. ISBN 0-8176-1807-4.

External links