No father can have children: parthenogenesis mystery
January 29, 2015 Source: Sina Technology
Window._bd_share_config={ "common":{ "bdSnsKey":{ },"bdText":"","bdMini":"2","bdMiniList":false,"bdPic":"","bdStyle":" 0","bdSize":"16"},"share":{ }};with(document)0[(getElementsByTagName('head')[0]||body).appendChild(createElement('script')) .src='http://bdimg.share.baidu.com/static/api/js/share.js?v=89860593.js?cdnversion='+~(-new Date()/36e5)];According to foreign media reports, this is a female python named Thelma. It is 6 meters long and has lived alone for four years at the Louisville Zoo in the United States. It has never been in contact with any male counterpart. However, it produced 61 eggs and hatched 6 healthy offspring. Did it mate with a male python many years ago and then store its sperm?
Genetic testing quickly gave the answer: Thelma is the world's first asexual reproduction of reticulated mites. The eggs it produces have all the genetic information that grows into female offspring, without the sperm or DNA of male pythons. Substituting sperm is a by-product of germ cell division, the body of Thelma, which binds to the polar body and eventually develops into an embryo. Therefore, each of its offspring has two copies of its half chromosome, in other words, they are semi-clone of Thelma.
How to breed in extreme environments?
Although very strange, we now know that Thelma is not unique. Scientists have found that parthenogenesis occurs in many species, including amphibians, reptiles, cartilage fish and bony fish, and even birds. As for the reasons behind this phenomenon, we are still not very clear.
Parthenogenesis, also known as parthenogenesis, was initially thought to be a phenomenon caused by extreme conditions. In the past records, this phenomenon only occurred in artificially farmed animals, so it is speculated that it is caused by pressure, isolation and the like. When other methods of continuation of the bloodstream are not feasible, perhaps only the path of parthenogenesis is left.
However, the reality is not necessarily the case. Scientists have found that some females use parthenogenesis to produce offspring even in the presence of males. Moreover, parthenogenesis can also occur in wild conditions, and even this phenomenon may have existed for hundreds of millions of years. This type of reproduction has certain advantages, especially on today's earth, the population of many species is rapidly declining. Of course, this also led to a discussion about the importance of sexual reproduction.
In addition, another important question is why vertebrate animals, including fish, reptiles and birds, have parthenogenesis, but mammals, including humans, have not found this ability. Things don't seem so simple...
Parthenogenesis turkey
Perhaps through the common domestic turkey, we can best understand "sexual reproduction." At the beginning of the 19th century, there were reports of chickens having parthenogenesis, and some people began to study similar phenomena in turkeys. They found that these large poultry were able to produce unfertilized eggs and successfully hatched offspring.
However, these hatched turkey chicks are male. Unlike mammals, male chromosomes dominate in birds. Soon, a single genital turkey family was selected and its male offspring developed normally and successfully reproduce.
Over the past 15 years, there have been a series of reports on the parthenogenesis of domesticated fish, snakes and lizards, and it seems that many species of sharks also have this behavior. On December 14, 2001, one of the three female narrow-headed hammerhead sharks raised together produced a healthy female offspring. The three female sharks were captured from the Florida Keys when they were underage. None of them have ever been exposed to male sharks.
According to Demian Chapman, a team at the State University of New York at Stony Brook, one of the three female narrow-headed hammerhead sharks was born. Subsequent genetic testing confirmed that no male sharks were involved. Later, scientists found the same phenomenon in the other four sharks. Chapman said: "It seems that this is a phenomenon that exists in many sharks."
Komodo dragon
In 2006, scientists reported that two different Komodo dragons, the world's largest lizards, also had parthenogenesis. The two giant lizards are housed in different institutions, one at the Chester Zoo and the other at the London Zoo. Both zoos are located in the UK.
At the time, scientists speculated that this monitor lizard could switch between sexual reproduction and asexual reproduction, that is, in the extreme case of no male existence, it can produce its own cloned offspring. In recent years, scientists have also found that a variety of snakes, including ticks and ticks, also undergo parthenogenesis without males.
Then the question is coming, why are they doing this?
No male life
One possible explanation comes from the raceweed in wild conditions. In fact, the racetail lizard contains many species, many of which are hybrid progeny from other species. These peculiar hybrid species are females, and males have been completely kicked off from their breeding process. Each female racetail lizard produces offspring through asexual reproduction.
This gender-specific strategy has its evolutionary advantage: no matter which one of the whip-tailed lizards is single, it can continue its own genes, and those race-tailed lizards that depend on males for reproduction will gradually die out. This is a very special parthenogenesis, only in the absence of males. Female weeping lizards trapped on isolated islands gradually transformed their biological characteristics for parthenogenesis.
Scientists believe that the nettle 蟒Thelma may also be parthenogenetic for similar reasons. It lives in a spacious cage, with plenty of food and temperature, and has good conditions for biological transformation into parthenogenesis. For other snakes, Komodo dragons and sharks, the situation is likely to be the same.
However, this view also has problems. In general, asexual reproduction comes at a price – essentially the ultimate form of inbreeding, which does not create genetic diversity. Therefore, autologously cloned animals are very vulnerable to disease and other threats, and they lack genetic diversity to overcome threats. For this reason, after the emergence of the parthenogenetic Komodo dragon, scientists suggested that this endangered lizard should not be isolated. They fear that if the Komodo dragons begin to have parthenogenesis, their genetic diversity will gradually decrease.
Of course, parthenogenesis is also a very reasonable choice in extreme conditions where there are no males in the environment.
Parthenogenesis in the wild
In the wild, some vertebrates also have parthenogenesis. In 2012, scientists discovered that some pythons often develop parthenogenesis in wild conditions.
Warren Booth of Tulsa University and colleagues collected eggs from 59 litters of pythons (both species) and analyzed the "DNA fingerprints" of these eggs. They found that two of the eggs were from the product of parthenogenesis, or more specifically facultative parthenogenesis.
Therefore, stress factors under artificial farming conditions may not be the cause of parthenogenesis. Moreover, the number of male pythons in the wild is large, and females do not perform parthenogenesis because they have no choice.
“We used to call sexual parthenogenesis a novelty in evolution, but it is not as novel as people think,†Warren Booth said. “The reproductive biology work we are doing now is amazing. From the surrounding animals, we can learn a lot of new knowledge about reproductive patterns."
Ancient breeding method
Warren Booth speculated that parthenogenesis may actually be a very old form of vertebrate reproduction. Animals that are best at this behavior, such as crickets and crickets, are also the oldest of some snakes. Later species, such as cobras, can produce only one or two offspring in parthenogenesis and often fail to survive.
Perhaps in the age of these ancient snakes, it may be hundreds of millions of years ago, their number is so rare, or it is difficult to find a heterosexual spouse, so only parthenogenesis can be used. However, fossil records cannot tell us about the information.
In fact, it is extremely difficult to figure out how many wild animals can breed in this way. It is almost impossible to understand whether wild fish can be parthenogenesis. For sharks, the only way to get the DNA of female sharks and their offspring is to check the percentage difference. Because many shark species are endangered, such research is difficult.
So the problem still exists. Why do parthenogenesis in the case of asexual reproduction? Especially in the wild environment where male individuals are abundant? Moreover, even though parthenogenesis is an ancient evolutionary product, why still exist today?
Healthy offspring
There is an answer that may exist among these issues. James Hanken, an evolutionary biologist at Harvard University, points out that if asexual reproduction is really bad, then it should not be so long. Therefore, although genetic diversity is important, it is not all. The offspring produced by parthenogenesis alone support this.
Young sharks from parthenogenesis have fewer genetic diversity than offspring, but they are equally healthy, "without any harmful recessive genes," Demian Chapman said.
Gordon Schuett of Georgia State University is the first scientist to record facultative parthenogenesis in snakes. According to him, the reason why females decide to breed their offspring alone may be because of the huge consumption of sexual reproduction. Females need to endure competition and fight between males, and finding ideal partners is sometimes difficult.
Another view is that something outside of evolution is at work. Parthenogenesis may be triggered by some external factors. Like hormones, or hormone imbalance? Or even the role of a pathogenic body such as a virus or a parasite? There is a kind of wasp that, when infected by a certain kind of bacteria, will have asexual reproduction. Warren Booth supports this view. He said: "We found this behavior in birds, snakes and sharks. They seem to be evolving, so there are other factors driving."
Gordon Sulet is not so sure, he can hardly believe that there is only one factor that causes so many species to perform the same behavior. However, Warren Booth is working on genetic testing of a variety of organisms, including the Komodo dragon, cockroach, cockroach and many animal tissues kept in the laboratory. He tried to find a definitive genetic trait that indicates the presence of a virus or other motivating factor.
If this stimuli are not found, it may mean that parthenogenetic ability remaining in different species can be used as an alternative mechanism to be activated if it is not conducive to sexual reproduction or excessive sexual reproduction.
Peter Bowman of the University of Kansas Medical Center said that if the above view is true, we may see more parthenogenetic behavior as the number of wild animals declines. Scientists are already waiting to see if Mori, the world's heaviest snake, will also join the list of vertebrates that are capable of parthenogenesis.
“The evolution of this way in nature is really magical. From an adaptive point of view, this does enhance the ability of species to survive for a long time because it has a backup strategy,†says Peter Bowman. “The most sexual reproduction. One shortcoming is that two individuals are required to appear in one place at the same time. When the population density is very low, this is a big problem. From the perspective of evolution, sexual reproduction is still the more successful breeding method. But Peter Bowman added: "Obviously the two mechanisms have their own advantages."
So what about mammals?
The final puzzle is that if parthenogenesis is more widespread than scientists estimate, why can't mammals, including humans, do it?
There are currently no examples of mammals performing natural parthenogenesis, either in captivity or in wild conditions. However, at Harvard University in the 1930s, a scientist named Gregory Pincus began researching the reproductive system of mammals. His work has made him one of the inventors of human contraceptives. Maybe mammals can do it too.
At the time, he claimed that it caused a parthenogenesis in the rabbit, which caused a lot of controversy because other scientists could not repeat the result. A few decades later, in 2004, scientists reported that a genetically engineered mouse had a parthenogenesis behavior. The offspring of this mouse not only survived, but also able to have their own offspring.
Scientists say that because of some fundamental biological characteristics, it is difficult, if not impossible, for mammals to spontaneously produce viable offspring. However, perhaps one day, a mammal will give us a surprise in a certain place. Like the nettle, Thelma, and other snakes, turkeys and sharks, it gives birth to a “miracle†offspring that poses a fundamental challenge to our understanding of reproductive biology. (任天)
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