Parthenogenesis


Parthenogenesis 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. In plants, parthenogenesis is a component process of apomixis. In algae, parthenogenesis can mean the development of an embryo from either an individual sperm or an individual egg.
Parthenogenesis occurs naturally in some invertebrate animal species, a few vertebrates, such as some fish, amphibians, reptiles, and birds, and some plants and algae. It has been induced artificially in several animal species that naturally reproduce through sex, including fish, amphibians, and mice.
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 anywhere 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 X0 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 or two W chromosomes, or they could have one Z and one W chromosome.

Life history types

Some species reproduce exclusively by parthenogenesis, while others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis. The switch between sexuality and parthenogenesis in such species may be triggered by the season, or by a lack of males or by conditions that favour rapid population growth. In these species, asexual reproduction occurs either in summer or as long as conditions are favourable. This is because in asexual reproduction, a successful genotype can spread quickly without being modified by sex or wasting resources on male offspring who will not give birth. Some species can produce both sexually and through parthenogenesis, and 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, facultative parthenogenesis 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 are found among polyploids and hybrids where the chromosomes cannot pair for meiosis.
The production of female offspring by parthenogenesis is referred to as thelytoky while the production of males by parthenogenesis is referred to as arrhenotoky. When unfertilized eggs develop into both males and females, the phenomenon is called deuterotoky.

Types and mechanisms

Parthenogenesis can occur without meiosis through mitotic oogenesis. This is called apomictic parthenogenesis. Mature egg cells are produced by mitotic divisions, and these cells directly develop into embryos. In flowering plants, cells of the gametophyte can undergo this process. The offspring produced by apomictic parthenogenesis are full clones of their mother, as in aphids.
Parthenogenesis involving meiosis is more complicated. In some cases, the offspring are haploid. In other cases, collectively called automictic parthenogenesis, the ploidy is restored to diploidy by various means. This is because haploid individuals are not viable in most species. In automictic parthenogenesis, the offspring differ from one another and their mother. They are called half clones of their mother.

Automixis

Automixis includes several reproductive mechanisms, some of which are parthenogenetic.
Diploidy can be restored by the doubling of the chromosomes without cell division before meiosis begins or after meiosis is completed. This is an endomitotic cycle. Diploidy can also 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 l 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 as they involve recombination. Many others classify the endomitotic variants as asexual and consider the resulting embryos parthenogenetic. Among these authors, the threshold for classifying automixis as a sexual process depends on when the products of anaphase I or of anaphase II are joined. The criterion for sexuality varies from all cases of restitutional meiosis, to those where the nuclei fuse or to only those where gametes are mature at the time of fusion. Those cases of automixis that are classified as sexual reproduction are compared to self-fertilization in their mechanism and consequences.
The genetic composition of the offspring depends on what type of automixis takes place. When endomitosis occurs before meiosis or when central fusion occurs, the offspring get all to more than half of the mother's genetic material and heterozygosity is mostly preserved. 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.
If terminal fusion occurs, a little over half of the mother's genetic material is present in the offspring, and the offspring are mostly homozygous. This is because at anaphase II the sister chromatids are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis after meiosis, the offspring is completely homozygous and has only half the mother's genetic material. This can result in parthenogenetic offspring being unique from each other and from their mother.

Sex of the offspring

In apomictic parthenogenesis, the offspring are clones of the mother and hence are usually female. In the case of aphids, parthenogenetically produced males and females are clones of their mother except that the males lack one of the X chromosomes.
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, ZZ, or WW. ZW offspring are produced by endoreplication before meiosis or by central fusion. ZZ and WW offspring occur either by terminal fusion or by endomitosis in the egg cell.
In polyploid obligate parthenogens, like the whiptail lizard, all the offspring are female.
In many hymenopteran insects, such as honeybees, female eggs are produced sexually, using sperm from a drone father, while the production of further drones depends on the queen producing unfertilized eggs. This means that females are always diploid, while males are always haploid and are 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, with only a few examples of animal taxa capable of facultative parthenogenesis. One of the best-known examples of taxa exhibiting facultative parthenogenesis are mayflies; presumably, this is the default reproductive mode of all species in this insect order. Facultative parthenogenesis has generally been believed to be a response to a lack of a viable male. A female may undergo facultative parthenogenesis if a male is absent from the habitat or if it is unable to produce viable offspring. However, California condors and the tropical lizard Lepidophyma smithii both can produce parthenogenic offspring in the presence of males, indicating that facultative parthenogenesis may be more common than previously thought and is not simply a response to a lack of males.
In aphids, a generation sexually conceived by a male and a female produces only females. The reason for this is the non-random segregation of the sex chromosomes 'X' and 'O' during spermatogenesis.
Facultative parthenogenesis is often used to describe cases of spontaneous parthenogenesis in normally sexual animals. For example, many cases of spontaneous parthenogenesis in sharks, some snakes, Komodo dragons, and a variety of domesticated birds were widely attributed to facultative parthenogenesis. These cases are examples of spontaneous parthenogenesis. 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. Some documented species, specifically salamanders and geckos, rely on obligate parthenogenesis as their major method of reproduction. As such, there are over 80 species of unisex reptiles, amphibians, and fishes in nature for which males are no longer a part of the reproductive process. A female produces an ovum with a full set provided solely by the mother. Thus, a male is not needed to provide sperm to fertilize the egg. This form of asexual reproduction is thought in some cases to 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 sexual reproduction in their native range are found to 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. 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.