In animalsparthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.
Parthenogenesis occurs naturally in some plants, some invertebrate animal species including nematodeswater fleassome scorpionsaphidssome mites, some beessome Phasmida and parasitic wasps and a few vertebrates such as some fish amphibiansreptiles   and very rarely birds .
This type of reproduction has been induced artificially in a few species including fish and amphibians. Normal egg cells form after meiosis and are haploidwith 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.
The offspring having 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 will 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 systemthey have either two Z chromosomes male or two W chromosomes mostly non-viable but rarely a femaleor they could have one Z and one W chromosome female. Some species reproduce exclusively by parthenogenesis such as the Bdelloid rotiferswhile others can switch between sexual reproduction and parthenogenesis. This is called facultative parthenogenesis other terms are cyclical parthenogenesis, heterogamy   or heterogony  .
The switch between sexuality and parthenogenesis in such species may be triggered by the season aphidsome gall waspsor by a lack of males or conditions that favour rapid population growth rotifers and cladocerans like daphnia. In these species asexual reproduction occurs either in summer aphids 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 won't give birth.
In times of stress, offspring produced by sexual reproduction may be fitter as they have Parthenogenesis and asexual reproduction example, 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.
Origin and function of meiosis.
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. The production of female offspring by parthenogenesis is referred to as thelytoky e.
When unfertilized eggs into both males and females, the phenomenon is called deuterotoky. Parthenogenesis can occur without meiosis through mitotic oogenesis. This is called Parthenogenesis and asexual reproduction example 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. Parthenogenesis involving meiosis is more complicated. In some cases, the offspring are haploid e. In other cases, collectively called automictic parthenogenesisthe 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 from their mother.
They are called half clones of their mother. Automixis  is a term that covers several reproductive mechanisms, some of which are parthenogenetic. Diploidy might be restored by the doubling of the chromosomes without cell division before meiosis begins or after meiosis is completed.