• Parthenogenesis

From the ancient Greek παρθένος γένεσις - virgin birth.

Parthenogenesis is a type of sexual reproduction in which the female sex cell develops without fertilization - virgin development of unfertilized eggs. Parthenogenesis is widespread in planktonic crustaceans. In most fishes, normal formation of larvae does not occur during parthenogenesis. Thus, in salmonid fish parthenogenetic development of the egg goes only until the embryo hatches from the egg, in herring - until the stage of free-swimming larva. In some cases, a distinction is made between gynogenesis, when the development of the egg cell proceeds in the absence of the male nucleus, and androgenesis, when the female nucleus is absent.

A form of sexual reproduction in organisms in which female sex cells (ova) develop into an adult organism without fertilization. Although parthenogenetic reproduction is not accompanied by the fusion of male and female gametes, parthenogenesis is still considered sexual reproduction because the organism develops from a sex cell. Parthenogenesis is thought to have arisen during the evolution of separate-sex forms.

When parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species, since all individuals of such species are able to leave offspring. This mode of reproduction is found among relatively primitive organisms. When females develop from fertilized eggs and males from unfertilized eggs, parthenogenesis helps to regulate the sex ratio. Parthenogenetic species and races are often polyploid and arise as a result of distant hybridization, thus revealing heterosis and high viability.

Parthenogenesis should be referred to sexual reproduction and should be distinguished from sexless reproduction, which is carried out with the help of somatic organs and cells (reproduction by division, budding, etc.).

The ability to parthenogenesis is noted in turbellarians, trematodes, snails, snails, nematodes, tachypods, onychophorans, and other groups of animals. Cyclic parthenogenesis is characteristic of rotifers.

Crustaceans are capable of parthenogenesis among arthropods. Among them, parthenogenesis is observed in barnacles, shields, and naked gills (Artemia). Cyclic or obligate parthenogenesis is characteristic of all Cladocera. Daphnia, for example, reproduce by ameiotic parthenogenesis. Under favorable conditions, only females appear in daphnia. If conditions begin to change (drying up of the water body), males hatch from the same eggs and fertilize females. The females lay eggs. Fertilized eggs rest at the bottom of the pond and are able to survive the drying up of the pond.

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