On a leaf in the tropical rainforest of Madagascar, the tiny golden mantera frog hides a secret, a secret shared with countless other frogs (fork tongue frog, reed frog, etc.) and their predator ( python and other kinds of snakes) .
(Picture source network)
In this small island country, many species are endemic, and recently, geneticists' research on these species has come to an amazing discovery: One gene in many frog genomes in this region seems to be from snakes.
After carefully studying the genomes of frogs and snakes around the world, scientists published a paper in Molecular Biology and Evolution in April this year that this gene has been transferred from snakes to frogs in some way at least 50 times.
But in Madagascar, the frequency of this gene transfer is amazing: at least 91% of frog species sampled in the region have it. It seems that the existence of certain factors has led to Madagascar eventually becoming a breeding ground for this gene transfer.
When he first saw the snake version of the gene for the first time on a frog, he was very confused.
He consulted this question with a colleague who specializes in genomics, and the colleague immediately shouted: "This must have happened horizontal gene transfer " - that is, transfer genes from one species to another, and non-classical children can only obtain gene from their parents (as opposed to horizontal genes, this pattern is called vertical gene transfer) . ”
” Despite the application of more advanced genome sequencing technology that has allowed biologists to reevaluate this view, Kurabayashi eventually embarked on the path of research to track this rare horizontal gene transfer. The latest paper from
The problem is complicated by more frequent horizontal gene transfers in some regions, and it also implies that researchers should probably go beyond simple genetic rules when seeking mechanisms for horizontal gene transfer, and more Focus on the ecological environment in which species are located.
Currently, researchers who perform genomic analysis are still working to explore the common or rare degree of horizontal gene transfer in complex organisms, but there is no doubt that Madagascar should be the focus of their research.
(image source)
researchers suspect that pythons in Madagascar (middle)ht The BovB gene version found in ml4 and other snakes may be particularly prone to horizontal gene transfer. Painted Frog (left) and Jinmantera Frog (right) are two of the many frogs that obtained BovB.
When genes linger
Horizontal gene transfer is very common in bacteria, and there are a large number of living in almost any environment on the earth. Single-celled organisms, they get genes from the environment as easily as picking up cat hair from the ground with a cotton brush. This is one of the reasons why many bacteria can obtain resistance to antibiotic : protective genes (resistance genes) are easily transmitted, and natural selection allows resistant bacteria to gain survival advantages and pass their genes to the next generation.
However, cells of eukaryotes such as humans, frogs and snakes are different, and their nucleus is like a bastion to protect the genome.DNA is carefully curled up and stored in the "Castle Library", while cell also has a failsafe device to prevent its DNA from being damaged or repairing already damaged DNA.
However, examples involving eukaryotic horizontal gene transfer are constantly entering the field of scientists. herring and cucumber are two fish that survive in the cold waters of the Arctic, North Pacific and Atlantic (evolutionary) completely unrelated. However, they have an exact same gene. The protein encoded by this gene can prevent their blood from freezing. Studies have shown that this gene is likely transferred from herring to cucumber buzz.
plus Molecular biologist Laurie Graham and her colleagues reported this phenomenon last year. This discovery may seem a bit counterintuitive to most people, so Graham is also having a hard time publishing this paper.
(Picture source network)
herring (Part 1) and guaroon (Part 2) are two unrelated cold water fish , but due to horizontal gene transfer that occurred a long time ago, they both carry the same genes, preventing their blood from freezing in the cold sea water.
Similarly, Etienne G.J. Danchin, an evolutionary biologist at the National Institute of Agriculture, Food and Environment in France, and his colleagues are studying a set of enzymes obtained from bacteria in nematode . In addition, according to the study of Jinling Huang of East Carolina University and colleagues, it seems that now has gene family that exist in more than 100 plants was transferred from microorganism a long time ago.
Why are these (seemingly impossible) horizontal gene transfer favored by evolution? The blood of the cucumber fish with herring protein gene does not freeze, while the digestive enzyme obtained from bacteria allows them to get more energy from the plant cells they eat. The hot spring red algae studied by Debashish Bhattacharya, an evolutionary biologist at Rutgers University and his student Julia Van Etten, can obtain genes from bacteria to resist extreme environments.
All these examples show that if a gene improves the survival rate of a species, it will not take long before individuals with this gene in the population and their offspring will dominate.
However, not all transfer genes are bound to bring advantages, BovB is a well-known transposon that undergo self-replication and are able to move randomly around the genome. In addition, although transposons may have profound impacts on the genome, BovB is not a gene with traditional functions. To be more appropriate, it is just a piece of DNA fragment that can be replicated (does not have any actual functions) .
Kurabayashi pointed out that although the possibility of the BovB gene benefiting frogs cannot be ruled out, the more likely reason is that the advantages of BovB in self-replication lead to its continued existence. This also explains to a certain extent why when eukaryotes obtain genetic material from other organisms, they often carry many transposons like BovB.
It sounds strange that eukaryotes obtain genes from bacteria, but what is even more strange is that there are much fewer examples of bacteria obtaining eukaryotes genes. For some reason, bacteria don't want our genes. Eukaryotic genes have structural characteristics, so they are not the perfect material for bacterial evolution, and there may be other factors we do not know about.
biologist Patrick Keeling, a British Columbia University biologist, said, "Maybe eukaryotes don't have the genes that interest bacteria."
Viral transmission
Unlike bacteria, viruses have a real trick to obtain genes from eukaryotic hosts. In viruses, retroviruses have tools to enter host cells and nuclei, which are masters of inserting genetic material into the host genome. Up to 8% of the human genome is made up of residues of retroviruses that infect humans long ago.
(Picture source network)
Sometimes gene transfer can also be carried out in another way. , based on a paper published in Nature Microbiology last December, Keeling and his collaborator, Nicholas Irwin from , Oxford University, conducted the first comprehensive analysis of horizontal gene transfer between 201 eukaryotes and 108,842 viruses. They found evidence of more than 6,700 gene transfers, of which the metastatic event from the host to the virus was about twice as high as the transfer from the virus to the host.
From this they concluded that horizontal gene transfer is the main driving force for both evolution : viruses use the eukaryotic genes they obtain to better infect the host, and eukaryotes sometimes use part of the viral gene to form new features or regulate their own metabolism in new ways.
These findings inspired some biologists to understand that at least some horizontal gene transfer may be promoted by the virus. If viruses can obtain genes from both hosts and leave their own genomic fragments, then they can obtain genes from the last host they infect, or even from a host several generations ago, and then transfer these genes to a new host. The participation of
virus also helps to solve another problem regarding eukaryotic horizontal gene transfer. In order to perform transfer, these genes need to overcome a series of difficulties: first they need to move from the donor species to the new host species (i.e., the receptor species ), and then they must enter the nucleus of the receptor species and incorporate themselves into the host genome. But it is not possible to incorporate any cell genome: in multicellular organisms such as frogs and herring, genetic changes in somatic cells are not passed on to offspring, and only genetic changes in germline cells—sperm or egg cells—can be inherited.
And the help of the virus may make this series of events easier. Danchin said that small organisms like nematodes have their genital tracts and germ cells that are not far from the intestine, and viruses ingested through food can settle in the genital tract. As frogs release eggs and sperm into open waters, the possibility of these cells being infected by viruses will also increase greatly.
Even in larger organisms, this process is more likely to happen than imagined. "The genital tract is full of microorganisms and viruses," Danchin said. "We all know that some viruses can infect the genital tract."
Keeling believes that to understand the mystery of horizontal gene transfer, we should take into account the behavior of organisms, other organisms in habitats, and environmental factors. If a horizontally transferred gene brings survival advantages, it is largely related to the specific scenarios in which the gene receptor is located, such as cold oceans, hot springs, and indigestive food source (plant cells) . "They are closely related to the ecology" "As the environment changes, they may be lost again when these metastatic genes no longer have advantages."
Ecological clues
Horizontal gene transfer of eukaryotic clocks may have been happening: in the pond in the backyard, in the soil under the feet, in the animals, insects and plants that make up the ecosystem."I think the actual situation of this transfer may be much more than we know," Bhattacharya said. "We just can't see them because they were swept out of the house (which can be understood as being lost by the environment selection) "
(image source)
nematodes are very large in size Small, the viruses in their gut may be separated by just a few cells from the genital tract
Specious reason
To check how common frogs have BovB, the Kurabayashi team contacted their colleagues to obtain DNA sequencing results from frog samples from around the world. They found that of 149 species, 50 carry the BovB gene. The 32 Madagascar frogs they tested accounted for less than a quarter of all sampled species, but 29 carried the gene from the snake. Furthermore, at least two frog lineages are BovB genes that were obtained only after their ancestors migrated from Africa to Madagascar.
Graham said that the most interesting thing about this paper is that "the frequency of horizontal gene transfer occurs is not consistent, but varies by geographical region. If you shift your sight to a global scale and see if the frequency of gene transfer occurs in different places is the same, the results may be more surprising. Perhaps geographical factors are more important than we think."
At present, we still don’t know how the environment in Madagascar promotes horizontal gene transfer. The Kurabayashi team suspects that the BovB gene of Madagascar snakes is different from the versions elsewhere, and that they may have a stronger ability to enter new hosts.
The rich parasites on the island may also be a contributing factor. "There are many leeches in Madagascar," said Miguel Vences, a herpetologist at the Brunswick Institute of Technology in Germany. "The blood-sucking creatures use the blood of a variety of animals as their food source, including frogs and snakes." Vences speculated that leeches may introduce blood containing the snake BovB gene into the frog, or that the leeches themselves have obtained the BovB gene because they have been relieved from the snake before. Then the other work is handed over to some kind of virus that we don't know.
However, it is not easy to prove or refute the actual scenario where such horizontal gene transfer occurs. If there is no natural selection to protect these DNA sequences , traces of their existence will gradually be erased in the following period of time. If the viruses were involved in this transfer, they might leave little evidence. Therefore, researchers may be able to determine the specific mechanism of such metastasis only by directly observing the occurrence of metastasis events.
Bhattacharya is in the early stages of similar projects. In the hot springs of in Yellowstone National Park, he and his colleagues are looking for a transfer phenomenon that may still be in progress. They are studying the DNA of a red algae that is obtained from bacteria that also survive in hot springs, and that these genes differ slightly from their original versions. “We are not talking about what happened millions of years ago” “We are talking about highly similar DNA, which exists in two different lives simultaneously in the same environment.”
If scientists can find that some algae in the area near the hot springs lack any of these transfer genes, they may be witnessing the diffusion of these genes from the hot springs (referring to Lemonade Creek) to the surrounding (other hot springs) .
original link:
https://www.quantamagazine.org/how-genes-can-leap-from-snakes-to-frogs-20221027/
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author/Catching butterflies cat
review/Mo Shi Er
edit/Fruit Li Zhenzhen
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