Parthenogenesis (; from the Greek + q.v..) is a natural form of asexual reproduction in which the embryo develops directly without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertilized egg cell. Parthenogenesis occurs naturally in some plants and animals, and in a few other eukaryotes such as oomycetes. It has been induced artificially in some mammals.
Normal egg cells form in the process of meiosis 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. Depending on the mechanism involved in restoring the diploid number of chromosomes, parthenogenetic offspring may have anything between all and half of the mother's alleles. In some types of parthenogenesis, the offspring that have all of the mother's genetic material are called full clones and those having only half are called half clones. Full clones are usually formed without meiosis. If meiosis occurs, the offspring get only a fraction of the mother's alleles, since crossing over of DNA takes place during meiosis, creating variation.
Parthenogenetic offspring in species that use either the XY or the XO sex-determination system have two X chromosomes and are female. In species that use the ZW sex-determination system, they have either two Z chromosomes (male) or two W chromosomes (mostly non-viable but rarely a female), or they could have one Z and one W chromosome (female).
Life history types
Some species reproduce exclusively by parthenogenesis (such as the bdelloid rotifers), while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis (other terms are cyclical parthenogenesis, heterogamy and heterogony). Some species can reproduce both sexually and through parthenogenesis: offspring in the same clutch of a species of tropical lizard can be a mix of sexually produced offspring and parthenogenically produced offspring. In California condors, parthenogenesis is facultative, and can occur even when a male is present and available for a female to breed with. In times of stress, offspring produced by sexual reproduction may be fitter as they have new, possibly beneficial gene combinations. In addition, sexual reproduction provides the benefit of meiotic recombination between non-sister chromosomes, a process associated with repair of DNA double-strand breaks and other DNA damages that may be induced by stressful conditions.
Many taxa with heterogony have within them species that have lost the sexual phase and are now completely asexual. Many other cases of obligate parthenogenesis (or gynogenesis) are found among polyploids and hybrids where the chromosomes cannot pair for meiosis.
In animals, the production of female offspring by parthenogenesis is called thelytoky (found among aphids) while the production of males by parthenogenesis is called arrhenotoky (found among bees). When unfertilized eggs develop into both males and females, the phenomenon is called deuterotoky.
Mechanisms
Parthenogenesis can occur without meiosis. This occurs when mature egg cells are produced by ordinary cell division (mitosis); these cells directly develop into embryos in a process called apomixis. In plants, this can happen in cells of the gametophyte, which is haploid. Parthenogenesis occurs in some 10% of fern species and around 1% of flowering plants. The offspring produced by apomixis are full clones of their mother, as in aphids.
Parthenogenesis involving meiosis, the reduction division that normally produces haploid eggs and sperms from diploid cells, is more complicated. In some cases, the offspring are haploid (e.g., male honey bees). In other species, where haploid individuals are unable to breed, the cells are restored to diploidy by various means in a process called automixis. Offspring produced by automixis differ from one another and from their mother, as they are randomised half clones of their mother.
Automixis
Automixis covers several ways that offspring can be produced neither fully sexually nor by cloning; some of these are parthenogenetic. Meiosis begins normally with a halving of the number of chromosomes to create haploid cells. Diploidy is however then restored by the doubling of the chromosomes without cell division, either before meiosis begins or after meiosis is completed. This is called an endomitotic cycle. Diploidy can be restored by fusion of the first two blastomeres, or by fusion of the meiotic products. The chromosomes may not separate at one of the two anaphases (restitutional meiosis) or the nuclei produced may fuse, or one of the polar bodies may fuse with the egg cell at some stage during its maturation.
Some authors consider all forms of automixis sexual, since genetic recombination is involved. Others classify the endomitotic variants as asexual, and consider the resulting embryos parthenogenetic, but vary in the threshold for classifying automixis as a sexual process according to when the products of anaphase I or of anaphase II are joined. The criterion for sexuality ranges from restitutional meiosis, to those where the nuclei fuse, or to only those where gametes are mature at the time of fusion.
The genetic composition of the offspring depends on what type of automixis takes place. When endomitosis occurs before meiosis, or when central fusion occurs (restitutional meiosis of anaphase I or the fusion of its products), the offspring get all to more than half of the mother's genetic material and heterozygosity is mostly preserved; if the mother has two alleles for a locus, the offspring will likely get both. This is because in anaphase I the homologous chromosomes are separated. Heterozygosity is not completely preserved when crossing over occurs in central fusion. In the case of pre-meiotic doubling, recombination, if it happens, occurs between identical sister chromatids (half chromosomes).
If terminal fusion (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half of the mother's genetic material is present in the offspring, and the offspring are mostly homozygous.
Sex of the offspring
In plants, apomixis is asexual development of seeds or embryos without fertilisation. In animals, offspring produced by apomixis are clones of the mother and hence are usually female. In the case of XO animals such as aphids, parthenogenetically produced males and females are clones of their mother except that the males lack one of the X chromosomes (XO).
When meiosis is involved, the sex of the offspring depends on the type of sex determination system and the type of apomixis. In species that use the XY sex-determination system, parthenogenetic offspring have two X chromosomes and are female. In species that use the ZW sex-determination system the offspring genotype may be one of ZW (female), ZZ (male), or WW (non-viable). In the ZW system, the offspring are produced by a variety of forms of endoreduplication (genome copying without mitotic cell division) within the egg cell, or by fusion of nuclei during meiosis.
In polyploid obligate parthenogens, like the whiptail lizard, all the offspring are female.
Many hymenopteran insects, such as honey bees, are haplodiploid. This means that female eggs are produced sexually, using sperm from a drone father, while the production of further drones (males) depends on the queen (and occasionally workers) producing unfertilized eggs. This means that females (workers and queens) are always diploid, while males (drones) are always haploid, produced parthenogenetically.
Facultative
Facultative parthenogenesis occurs when a female can produce offspring either sexually or via asexual reproduction. Facultative parthenogenesis is extremely rare in nature; it is widespread in mayflies and may be the default reproductive mode of all mayfly species. It has been believed to be a response to a lack of a viable male. However, California condors and the tropical lizard Lepidophyma smithii can produce parthenogenic offspring in the presence of males, indicating that facultative parthenogenesis may occur more widely.
In aphids, a generation sexually conceived by a male and a female produces only females. This is because of non-random segregation of the sex chromosomes 'X' and 'O' during spermatogenesis.
The term facultative parthenogenesis is sometimes used to describe cases of spontaneous parthenogenesis in animals that usually reproduce sexually. Cases in sharks, snakes, Komodo dragons, and some domesticated birds have been described. The occurrence of such asexually produced eggs in sexual animals can be explained by a meiotic error, leading to eggs produced via automixis.
Obligate
Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means. Many species have transitioned to obligate parthenogenesis over evolutionary time. Well-documented transitions to obligate parthenogenesis have been found in numerous metazoan taxa, albeit through highly diverse mechanisms. These transitions often occur as a result of inbreeding or mutation within large populations. web. Some salamanders and geckos reproduce entirely by parthenogenesis, as do more than 80 female-only species of reptiles (mostly lizards but including a snake species, Indotyphlops braminus), amphibians, and fishes. Such females produce an unfertilised diploid ovum, its two sets of genes provided solely by the mother. Thus, a male is not needed to provide sperm to fertilize the egg. This form of asexual reproduction could be a serious threat to biodiversity due to the subsequent lack of gene variation and potentially decreased fitness of the offspring.
Some invertebrate species that feature (partial) sexual reproduction in their native range reproduce solely by parthenogenesis in areas to which they have been introduced. Relying solely on parthenogenetic reproduction has several advantages for an invasive species: it obviates the need for individuals in a very sparse initial population to search for mates; and an exclusively female sex distribution allows a population to multiply and invade more rapidly (potentially twice as fast). Examples include several aphid species and the willow sawfly, Nematus oligospilus, which is sexual in its native Holarctic habitat but parthenogenetic where it has been introduced into the Southern Hemisphere.
Evolution
Taxonomic distribution
Taxa that use parthenogenesis include those shown on the phylogenetic tree.
Parthenogenesis does not apply to isogamous species such as protists.
Evolutionary mechanisms
It has been asserted that "obligate parthenogenesis is an evolutionary dead end." However, as in sexually-reproducing species, a species reproducing only parthenogenetically reaches an equilibrium state of genetic variability when the rate of creation of variants (alleles) by mutation equals the rate of destruction by natural selection. Michael Lynch and Wilfried Gabriel demonstrated in 1983 that such a balance can be attained in "a few hundred generations". They suggested that evolution might proceed as fast in such parthenogens as in species that always reproduce sexually. They noted that sexual reproduction can sometimes occur in parthenogenetic species, including via backcrossing with a sexually-reproducing parent species.