PARTHENOGENESIS IN HUMANS
Parthenogenesis is define as, to give a birth to offspring without fertilization. It’s a general form of asexual reproduction. It can be seen in lower organisms and plants. It implicates the birth of an individual without fertilization. It’s a continuous evolutionary strategy that some organisms have hired to maintain a colony. There are benefits to organisms that exploit parthenogenesis, like reproduction without the involvement of direct fertilization.
In 18th century, the naturalist and philosopher, Charles Bonnet discovered the process of parthenogenesis. The word parthenogenesis derived from the Greek words “Parthenos” means “virgin” and “genesis” means “creation”.
Where is it found?
Lower vertebrates, invertebrates, some plants etc. Most animal species that shows parthenogenesis can also shows a phase of sexual reproduction. When the environmental condition are good and there is abundant food and water available that can sustain a large number of offsprings, the animal may prefer to go for parthenogenesis. When the optimal condition is not met, they shift to sexual reproduction and that results in variation of genetic material and hence the traits of the upcoming generation of the organisms.
Parthenogenesis can be seen in insects like aphids, generally during the spring season, when the temperature, humidity is favourable and plenty of nectar is available. In some species of ants, bees, and wasps, the ability to reproduce by both sexually and asexually is a part of establishing sexual differences, which proves to be a defining factor in their position and importance in their community. The ones which can reproduce sexually are placed a class higher in their ecosystem, which the ones which cannot reproduce sexually (as they are haploid due to them being born via parthenogenesis) are usually the working class in their society.
In honeybees, four unfertilized eggs are present. When the copulate, one egg fertilizes and germinates normally, while the other three germinate in the haploid form itself, without the need of them being fertilized. The key thing here is, without the fertilization of the first egg, the other three will not germinate. Hence the fertilization is crucial, but not necessarily of all the eggs. This is important to regulate population from increasing too dramatically, which will affect the hive diversity. The haploid bees are generally the workers of the hive. As this is not a true parthenogenesis as at least one egg needs to be fertilized for the other three to germinate by parthenogenesis.
Parthenogenesis in humans
Is parthenogenesis possible in humans? Theoretically YES, but practically the chances of parthenogenesis in humans is very very less. It can occur only in the female gender of the species, but there are no records or evidence of it actually happening. If a woman only give birth to daughters, who resembles the mother and the other child(ren) genotypically, there may be a negligible, but a chance indeed that it is a result of probable parthenogenesis as compared to an absolute chance of it being luck or a product of natural selection.
In the mid-1950’s, the British medical journal Lancet published an editorial pointing out that it could be difficult to establish suitable criteria for recognition of parthenogenesis in humans. There are two complications which may lead to a female human giving birth to a child by the method of parthenogenesis:
(1) budding from somatic cells of the mother
In the first complication, the mother and the daughter would be perfect clones, i.e. are genetically identical.
In the second complication, the mother would have to produce a sperm which would inseminate her own egg. Mother and daughter would not be genetically identical although the daughter would possess a subset of the mother’s genes.
Parthenogenetic Activation Methodologies
Before knowing about the parthenogenetic activation methods, we need to know about the biochemistry of Oocyte activation.
Oocyte activation process
Nonfertilized Egg/Oocyte stays remain at the metaphase2 until a stimulus, which may come from the fertilizing sperm. When the sperm and egg fused together, it triggers the secretion of Ca+2 and meiosis reactivation. Ca+2 will suppress the action of the metaphase promoting factor and it lead to metaphase to anaphase development (segregation of sister chromids). If the Ca+2 doesn’t secret, the activation and subsequent embryo development failed to occur. In mammals, this intracellular Ca+2 triggered by secretion of phospholipase C-zeta by sperm. Regardless of the identical nature of the sperm factor liable for bringing out the Ca2+oscillations, its presence is preeminent for successful oocyte activation and embryonic development.The activating agents tries to mimic like sperm, used for parthenogenesis, which triggers the Ca+2 secretion.
There are plenty of artificial stimuli have been found as agents to trigger the activation process in Oocytes. And that can be divided into the two groups, depending upon the nature of the activating stimulus. From the employed agents only Sr+2 can produce repetitive Ca+2 transients. We can activate the oocytes through intracytoplasmic Ca+2 elevations, but it’s not a single method that activates the oocytes from every species. Thus, parthenogenetic activation actions have conferred changing degrees of success, regarding activation rates and subsequent embryonic development according to the protocol employed and the species.
As we know that how embryos are developed after fertilization of oocyte and the three possible parthenotes that may result according to the activation protocol employed to stimulate exit from oocyte, in the absence of a fertilizing sperm.
For the parthenogenesis, treat the oocyte with activating agents like SrCl2 , ethanol, Ca+2 is followed by another chemical 6-DMAP or cytochalasin B, which blocks the second body extrusion. The appearing parthenote is a “pseudodiploid” heterozygous embryo, which has two sister chromatids of each maternal chromosome present in the M2 oocyte.
Parthenogenesis is often used in lower vertebrates but no evidence has been found for mammals exhibiting the same. Although it is possible in humans too, but the chances are so slim that the possibility of it happening is almost non-existent.