Is the apomixis and parthenogenesis the same
Reproduction? - I'll do that myself
Every litter or egg-laying of animals in the zoo is a happy event. But the pregnancy of the ball python (Python regius) from the Saint Louis zoo in the USA came as a big surprise to her keepers, as her last encounter with a male was 15 years earlier. The 62-year-old snake, who is also breaking records because of her age, was found in September entangling seven eggs that she had laid in her body.
Two of the eggs that the snake laid were taken for genetic testing to determine whether the snake had reproduced sexually, that is, whether it was fertilized by a sperm cell or in a non-sexual way. It is known that snakes can retain sperm cells in the body from a previous encounter with a male. Even so, it was difficult to assume that this had happened in the present case, because the longest known period of time that a snake could hold semen in its body was seven years and here it was twice as long. Three of the eggs hatched in the hatchery and two eggs did not survive.
If the tests found that no sperm cell was involved in fertilization, then that event would be considered a virgin birth or parthenogenesis: an event in which a female produces a fetus without a male involved in fertilization, and therefore none obtained genetic material from a male.
It is rare for pythons to reproduce asexually, but it is not impossible, and in fact, it is quite common in other reptile species. There are species of snakes and some lizards in which there are no males and all females reproduce by parthenogenesis. In other cases where females normally reproduce sexually, that is, mating a female with a male, a process known as facultative parthenogenesis may produce offspring without a sperm cell.
Virgin births occur in many animal species: amphibians, reptiles, insects, fish, and birds. This is what happened, for example, with the zebra shark (Stegostoma fasciatum) named Leonie, who shared an aquarium with other females in Australia and laid three eggs in 2016, from which live sharks hatched. Two years earlier, at Louisville Zoo in the USA, a female python named Thelma, who had never met a male, laid six eggs, from which healthy young snakes developed. In 2006, a Komodo dragon named Flora arrived at the Chester Zoo in England. In 2006, a Komodo dragon named Flora achieved a similar feat at Chester Zoo, England.
A zebra shark in Australia laid three eggs from which live sharks hatched, although she only shared the aquarium with other females. Zebra shark (Stegostoma fasciatum) Image: Andrea Izzotti, Shutterstock
How does this work?
Sexual fertilization requires the union of two cells, the female's egg and a male's sperm. Each cell provides half of the genetic material needed to create a living being. In parthenogenesis, the female's body succeeds in providing all of the genetic material that is necessary for the creation of a living being without a sperm cell.
Normally, eggs are formed in the ovaries of the female's body. This happens in a complex process called reduction division (meiosis), in which the cells replicate, reorganize and divide. At the end of this process, each egg cell contains only half the mother's chromosome set, which means that there is only one copy of the respective chromosome instead of two.
The process of meiosis in the female also creates a by-product: cells that are smaller than the egg cell, the so-called polar bodies, which in most cases are not involved in fertilization. Under certain circumstances, a polar body can combine with an egg and form a fetus. This process is known as automixis and it leads to some mixing of the mother's genes, because during meiotic division the chromosomes are divided between the cells and the genes in the egg are not necessarily the same genes that are in the polar body. The offspring created in this way are very similar to the mother, but are not perfect replicas of her.
In another type of parthenogenesis called apomixis, the cell that makes up the egg is replicated in a process of division called mitosis. As a result, the mother's entire genetic material is retained and is available as a double and not a single set of chromosomes, as in the case of the reduction division. The cell that replicates and divides eventually becomes the fetus. The cells formed from their replication do not go through a process of gene mixing as in the case of reduction division. Therefore, the resulting offspring are clones of the mother, i. H. they are genetically completely identical to her. This process is fairly rare in the animal world, but it is common in the plant world.
In species in which the sex of the offspring is determined by the father, only females are produced by the females that reproduce by automixis, because the offspring receive two X chromosomes from the mother. In the species in which the mother determines the sex of the offspring, such as certain birds or reptiles, females or males can be born. In very rare cases, animals such as aphids can produce male offspring that are genetically identical to the mother, but then they only have one X chromosome. Usually these males are fertile, but since they can only pass on one X chromosome, all of their offspring are female.
The six-lane Rennechsee (Aspidoscelis uniparens), an animal from the deserts of North America, are only females. They reproduce by virgin generation, without mating. All offspring are also female. Lizard of this species in Arizona. Source: Suzanne L. Collins, Science Photo Library
Not just in captivity
For many years, scientists believed that virgin generation would only occur in captivity, in extreme situations associated with stress or isolation. They saw it as a way for animals to continue their dynasty when no other options were available. This thesis was refuted in 2012 when scientists from the University of Tulsa in the USA discovered the virgin generation of pit vipers (Crotalinae) in the wild.
The scientists collected eggs from 59 snakes from two pit viper species and analyzed their DNA in a kind of paternity test. They found that two of the litters were virgin, or, to be precise, facultative parthenogenesis. This discovery refuted the thesis that the stress caused by captivity was the only cause of this extreme type of reproduction, because in nature there are an abundance of male pit vipers and the females do not have to "manage on their own".
Although virgin generation has been demonstrated in 80 species of vertebrate animals and many other species of invertebrates, to this day it is not fully understood why it occurs and what causes it. Some believe that virgin generation is a holdover from an ancient form of reproduction used by certain vertebrates. Boas and pythons are known to appear relatively early in evolution and therefore they are more prone to virgin conceptions than other snakes. When these snakes evolved millions of years ago, there were very few of them and therefore it was difficult to find a partner to mate with. Hence the ability to replicate itself was developed.
Other scientists believe that females reproduce through virgin conception because sexual reproduction requires a lot of energy: they have to get along with males who fight among themselves and find the male who is best suited to mate. Yet another hypothesis is that hormonal imbalance caused by an external stimulus triggers virgin birth. There is even evidence that wasps reproduced asexually after being exposed to a particular bacterium.
Virgin generation also occurs in the wild, e.g. B. in different pit viper species. The snake named North American copper head (Agkistrodon contortrix) is also virgin. Source: Jim Merli, Science Photo Library
Carrot and stick
The virgin generation gave rise to many species during evolution, but it also came at a price. On the positive side, the ability to have a virgin generation allowed the animals to pass on their genes without having to spend energy looking for a partner. In this way, the species can also be preserved under adverse living conditions, such as For example, a female Komodo dragon that gets to a deserted island could build a population of Komodo dragons without the help of a male.
However, since all of their offspring would be genetically identical or at least very similar, the population would be very susceptible to diseases and environmental influences that they would encounter on the island. A population that reproduces sexually has greater genetic diversity, which leads to a greater chance of survival - thanks to the greater genetic diversity within the population, there will be some individuals who have developed traits that enable them to assign the changed environmental conditions to survive.
But sexual reproduction also has disadvantages. First and foremost, two partners are needed, males and females, who need to be in the same place at the same time and want to mate. This requirement can be challenging with a small population. From an evolutionary point of view, sexual reproduction has remained the predominant and more successful method to date, but undoubtedly there are advantages in virgin generation.
The Komodo dragon (Varanus komodoensis), which lands on a desert island, could build a population of Komodo dragons without the help of a male. Komodo dragon on an Indonesian island. Cyril Ruoso, Nature Picture Library, Science Photo Library
And what about mammals?
To date, no mammal is known that can reproduce via virgin birth, either in captivity or in the wild - unless scientific evidence is found of the birth of Jesus from his mother, the Virgin Mary. However, scientists managed to create this type of reproduction artificially.
In the 1930s Gregory Pincus, a Harvard University physiologist, did research on mammalian reproduction. His work later led to the invention of the birth control pill. Pincas claimed that he was able to maiden hares, but other scientists were unsuccessful in repeating the experiment.
Only decades later, in 2004, scientists at the University of Tokyo succeeded in using genetic engineering to induce virgin generation, without sperm cells, in mice. The researchers managed to unite two egg cells, each containing a set of chromosomes, and get them to divide and develop into a fetus. Then 371 such embryos were implanted in 26 mice, of which 24 were initially able to implant in the uterus. In the end, only two fetuses survived the pregnancy and were born. One of the baby mice, named Kaguya, grew up and was able to mate with other males and give birth to offspring. This process is very similar to cloning and could be viewed as an artificial form of apomixis - like in virgin generation by apomixis, where the genetic material of the offspring is the same as that of the mother.
In order for the fertilization process with two egg cells to be possible, the researchers had to switch off the genes for genomic imprinting - a process in which some of the genes from one parent, which are passed on to the offspring, are inactivated and are not expressed, so that only the genes from the other parent end up being expressed. The researchers were able to show that the genomic imprinting processes mean that virgin generation is not possible in mammals. They argue that for this reason it is very unlikely, and probably impossible, that mammals could naturally produce offspring by virgin conception.
In spite of all of this, it is evident that nature has produced this mode of reproduction in other animals. There is no doubt that this mode of reproduction, in addition to sexual reproduction, offers a sure route to long-term survival.
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