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This article comes from the official account of Wechat: ID:fanpu2019, the original title: "Madagascar's genetically modified factory": why do frogs have snake genes? Author: gu Shuchen

What happened when the snake gene was found in the frog? In fact, it is not surprising that this is the horizontal transfer of genes. In recent years, researchers have found many phenomena of horizontal gene transfer, not only in prokaryotes such as bacteria, but also in eukaryotes, even between plants and insects. This may be the wisdom of survival.

You may have seen "shared XX" a lot, but have you ever heard of "shared genes"? In fact, some genes in our bodies may be similar to lizards, frogs, crabs, and even sea urchins, and even genes in some organisms are transferred from other organisms! Although this kind of gene transfer seems incredible, in fact, gene transfer between species is a frequent occurrence in the process of biological evolution. In nature, the mode of transmission of this genetic material is called horizontal gene transfer (Horizontal Gene Transfer, HGT).

Compared with horizontal gene transfer, we are more familiar with another mode of gene transmission: Vertical Gene Transfer (VGT). Parents pass on their genetic material to you and then from you to your children and grandchildren. This kind of genetic material is accompanied by the process of reproduction and handed down from generation to generation, which is the "vertical transmission" of genes, and it is also the most extensive way of genetic material transmission in nature.

Horizontal gene transfer refers to the transfer of genetic information between distant or near organisms across interspecific isolation, which is different from the conventional vertical gene transfer from parent to offspring. Once upon a time, horizontal gene transfer across species was considered to be a rare event only among microorganisms, but thanks to the development of sequencing technology, there is growing evidence that this process has been going on throughout the Tree of Life. More horizontal gene transfer has been confirmed in the past decade. And this phenomenon occurs not only in prokaryotes (the contribution of horizontal gene transfer in some prokaryotes is as high as 25% [1]) [2, 3], but also in eukaryotes [4] and between the two [5]. And the genetic material of horizontal gene transfer is not only DNA, but also mRNA [6], and even the whole organelle [7].

Recently, researchers have found that a gene Bovine-B (BovB), which originated from snakes, has repeatedly jumped the barrier between species and entered frogs around the world, and this phenomenon seems to occur much more frequently in Madagascar than elsewhere. After carefully studying the genomes of frogs and snakes around the world, scientists published a paper in Molecular Biology and Evolution (Molecular Biology and Evolution) in April 2022 that the gene somehow spread from snake to frog at least 50 times on Earth [8]. But in Madagascar, the gene is transferred at an alarming rate: 91% of frog species there have the gene. Madagascar seems to have a special motivation to promote the flow of the gene, and scientists believe that horizontal transmission of the gene may occur more frequently in some places, in which the ecological environment may play an important role.

The horizontal transfer of genetically modified art genes in nature is very common in bacteria. In prokaryotes such as bacteria, the DNA is exposed, showing different sizes of rings, which is more prone to gene transfer. Especially in the process of replication, bacteria replicate the genes of other species to adapt to changes in the environment. This is one of the reasons why bacteria are easy to develop resistance to antibiotics: resistance genes are easily transmitted horizontally, and bacteria with antibiotic resistance genes will reproduce under the selective pressure of antibiotics. Bacteria without genetic mutations will die, and drug-resistant bacteria gradually replace the original flora as the main flora. The gene exchange of bacteria is so frequent that some scientists even propose that bacteria form a network related to life, rather than branching genealogies of bacteria [9].

Unlike prokaryotes such as bacteria, eukaryotes use fortress-like nuclei to protect their genomes. DNA coils carefully in the nucleus of eukaryotes and invokes one of the genes with the corresponding enzyme when necessary. However, in recent years, examples of horizontal gene transfer involving eukaryotes have also been found. For example, a paper published in Trends in Genetics in March 2021 confirmed that in icy waters of the Arctic, North Pacific and North Atlantic, cucumber fish (Osmerus mordax) obtained genes from herring (Clupea harengus) in the same waters, which kept its blood from freezing [10], making it better able to survive in cold water.

Herring (top) and cucumber (bottom) are cold-water fish, and they carry the same genes so that they can survive in cold water. Photo Source: Four Oaks & Jack Perks what is even more amazing is that sometimes horizontal transmission of genes can also be transferred from animal to plant across different species. This amazing discovery made by Chinese scientists after 20 years of follow-up research was reported in the March 2021 issue of Cell. They found that Bemisia Tabaci (Bemisia tabaci), the only "super pest" identified by the United Nations Food and Agriculture Organization (FAO), had obtained a new gene from plants by horizontal gene transfer, and used this new gene to bypass the plant's defense system, allowing Bemisia Tabaci to successfully parasitize on many plants. The reason why Bemisia Tabaci can cause waves all over the world, causing great damage to many plants, such as tomatoes, cucumbers, legumes, cotton and so on, may also be the reason behind [11].

From the above studies, it can be found that horizontal gene transfer can help different species skip the slow process of obtaining new genes by mutation or recombination, and accelerate the innovation and evolution of the genome. But not all newly transferred genes play a role in organisms. One example is BovB, which was first found in cows and later proved to jump between bizarre animal combinations, and it's not a traditionally functional gene-it just has the ability to copy its own DNA loci to other locations at will. Although it can not be ruled out that BovB may also play some functions to benefit frogs, it is more likely that BovB keeps itself alive just because of its own strong ability to replicate itself. As depicted in the book selfish genes, what BovB genes do is to maximize their chances of survival. This helps explain why jumping genes like BovB are always involved when eukaryotes are with the genetic material of other organisms.

When the virus helps spread and read here, you must be curious about how horizontal gene transfer occurs in eukaryotes. Unfortunately, at present, we are not very clear about the specific level of transmission mechanism. But after all, horizontal transfer really happened, and there must be something that helped the gene transfer. Scientists have found that viruses are indeed a tool that can help horizontal gene transfer. Viruses, especially retroviruses, have the ability to enter host cells and even nuclei, and they can insert genetic material into the host genome. In fact, about 8% of the DNA in our human genome comes from the virus, which is also a fragment left by ancient humans infected by the virus.

In addition, the horizontal gene transfer through the virus is also mutual, that is, not only the host will obtain new genes from the virus gene transfer, the virus will also obtain the host gene. A paper published in Nature Microbiology (Nature Microbiology) in December 2021 presented for the first time a comprehensive analysis of horizontal gene transfer between 201 eukaryotes and 108842 viruses [12]. They found evidence of more than 6700 gene transfers, of which host-to-virus transfer was about twice as frequent as virus-to-host transfer. As a result, the researchers believe that horizontal gene transfer is the main driving force of evolution on both sides. Viruses infect their hosts more effectively through acquired eukaryotic genes, while eukaryotes use viral genes to acquire new characteristics.

The involvement of viruses also provides a possible explanation for horizontal gene transfer between eukaryotes. In eukaryotes, transferred genes need to pass through a series of obstacles. First, they must move from the donor species to the new host species. They must then enter the nucleus of the host species and settle in the genome. It is more difficult to pass transferred genes to offspring, which means that horizontally transferred genes must be inserted into the germ cells of the host species, such as sperm or eggs. The involvement of the virus will make this process more likely. Small organisms, such as nematodes (Nematode), whose reproductive tract and germ cells are very close to the intestines, viruses that settle in the intestines are likely to bring genes into the germ cells. Frogs release eggs and sperm into the water when they breed, and these cells can acquire some new genes from viruses in their surroundings. In some large creatures, such horizontal transfer may also be easier than we think. Some viruses specifically infect germ cells and they facilitate gene transfer to germ cells at the level.

Scientists believe that we can think of horizontal gene transfer as the result of the interaction of an organism and other organisms associated with it or parasitic, as well as the natural ecological environment. Horizontal transfer of genes often gives the host organism a survival advantage, so the occurrence of this phenomenon is closely related to the specific environment in which the organism lives. But the acquired genes will also change with the change of the environment, and when the transferred genes no longer provide favorable living conditions, they will also be lost [13].

The impact of the ecological environment in order to understand the frequency of horizontal gene transmission, scientists took frog samples from around the world for DNA sequencing. They found that of the 149 frog species outside Madagascar, 50 had the BovB gene. Of the 32 frogs from Madagascar, 29 carry the gene. The proportion of frogs in Madagascar getting horizontal gene transfer is significantly higher than that in other parts of the world.

The scenery of the African island nation of Madagascar, the picture is the famous baobab tree. Photo Source: what are the special factors in the environment of Bernard Gagnon Madagascar that make horizontal gene transfer occur so frequently? At present, scientists do not have an accurate answer to this question. The authors speculate that snakes living in Madagascar may have a different BovB gene from snakes in other parts of the world, making them more likely to enter new hosts. In addition, Madagascar is rich in parasites, such as leeches that feed on the blood of other animals, which may bring snake blood containing the BovB gene into frogs. Or the gene is transferred into the genome of the leech and then into the frog with the help of viruses or other microbes.

Snake BovBs is transmitted by parasites. Image source: reference [8] in addition, they also found that at least two frog strains had BovB genes acquired after their ancestors migrated to Madagascar from Africa. From this point of view, the speed of horizontal gene transfer is not average, and there are great differences in different geographical environments. Scientists believe that if more research can be done to observe the phenomenon of gene transfer around the world and the speed of gene transfer between different regions, we may be surprised to find that the impact of geographical factors on horizontal gene transfer may be more important than we thought.

However, it is not easy to capture and confirm the process of horizontal gene transfer-the DNA sequence may disguise its identity and erase the trace of its transfer through mutation in the process of transmission. At present, a research team is studying red algae in Yellowstone Park in the United States and found that some red algae in the hot springs here have obtained genes from bacteria in the same region, and the differences in these genes are very small. If algae without this gene can be found in other hot springs in Yellowstone, it is likely that the process of horizontal gene transfer has not yet begun, and they will have a chance to capture the process of gene transfer.

Perhaps biological evolution is not as slow as Darwin said. When the ecological environment blows mischievous wind, it will blow DNA everywhere, wandering among the branches of the tree of life, promoting the direction of different branches. The evolutionary model of modern science and the development of genetics will help us to trace the older phenomenon of gene transfer and better understand where we come from and how to better coexist with the environment.

reference

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