Most of us know that sexual reproduction requires an exchange of gametes from two genetically distinct individuals. Well, this is only applicable in a type of sexual reproduction called bisexual or biparental, being the most known and common reproductive mechanism in vertebrate animals, but not the only one, we can also find as sexual reproduction mechanisms hermaphroditism and parthenogenesis.
In this case we are going to try to explain what parthenogenesis consists of.
It is a mechanism of sexual reproduction in which an embryo develops from an unfertilized egg. Although it may seem a strange and difficult mechanism to occur in nature, it is a very frequent mechanism in plants, invertebrate animals and sometimes, in some vertebrate species.
Parthenogenesis can be divided into two types:
Ameiotic or diploid parthenogenesis: there is no meiosis, the zygote being formed by mitosis. As there are no haploid cells, it is considered an "asexual" form of reproduction. The descendants are clones of the progenitor since the complementary chromosomes pass intact to the descendants (diploids). Only females are involved.
Meiotic or haploid parthenogenesis: haploid ova are formed by meiosis, which may or may not be activated by male influence. The offspring will carry half of the genetic information of its progenitor (haploid). In this type of parthenogenesis we can find three cases:
Amphitoky: type of parthenogenesis in which males and females originate from unfertilized eggs.
Arrhenotoky: type of parthenogenesis in which only males originate from unfertilized eggs. Female individuals will be given by sexual reproduction.
Telytoky: type of parthenogenesis in which only females originate from unfertilized eggs.
In the case of reptiles, parthenogenesis can take two forms, obligatory or optional. Whereas in forced parthenogenesis the eggs always develop through parthenogenesis without the opportunity to be fertilized, in facultative parthenogenesis the eggs can develop with or without fertilization.
The obligatory parthenogenesis in reptiles has occurred in nature in more than 40 species, finding only one species of snake Indotyphlops braminus. There are always females. This parthenogenesis has an origin by hybridization of sexual species, except in two species of Lepidophyma, in which meiosis is avoided, leading to a loss of sexuality.
In facultative parthenogenesis, sexual reproduction is combined with asexual reproduction, ova can develop with or without fertilization. All cases that have occurred in reptiles have been found in captivity. It has been observed in several species of varans and snakes that have remained in captivity in the absence of males. Male offspring usually occur. The origin of this parthenogenesis is based on automixis.
Parthenogenesis could have evolved to solve the problem of the encounter between females and males. This method generates advantages such as a larger increase in population than species with biparental reproduction, and a loss in the fatal alleles within that population, as they would not pass to the next generation.
On the other hand, its disadvantage is the loss of genetic variability, so if the environment in which they find themselves changes abruptly they will not be able to adapt optimally to the new conditions, due to the limited capacity of genetic recombination presented by parthenogenesis.
Comentários