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This article comes from the official account of Wechat: ID:fanpu2019, author: gu Shuchen

There are genes of other species in the genome of one animal? This phenomenon is now found to be widespread in nature, and even humans carry 145 foreign genes. In recent years, biologists have also found that there are snake genes in frogs, and different species "share" the same gene to enjoy its function. This may be a gift left by our ancestors. Recently, a team discovered a "just happened" gene transfer event. Why can two different species share a gene? how does this gene "jump"? New research may give us the answer.

Written by Gu Shuchen (Institute of Life Sciences, Zhejiang University)

No species is an island. When human ancestors wandered the ancient continent, they passed by many species, and they also left a mark on human genes, and eventually became the ghosts of human genes, quietly describing that period of history. After many years, scientists suddenly discovered that humans have carried 145 foreign genes. About 8% of these genes come from viruses, about 2% from ancient humans (such as Neanderthals or Denisovans), and some from bacteria, fungi, animals and plants.

What is horizontal gene transfer (HGT)? Compared with humans, the "reciprocity" in microbiology is more frequent. Before the earth is occupied by human beings and other higher creatures, microbes are the masters of the earth. At that time, the living environment was mostly poor, and many microbes lived in craters, hot springs and other places, so survival was a top priority. As a result, different microbes will also survive by exchanging more "useful" genes to better adapt to different environments. Although these microbes are unrelated to each other, some of their genes do cross the species barrier and are successfully transferred from one microbe to another. This is the phenomenon of "Horizontal Gene Transfer" (HGT), which is opposed to the concept of "vertical transmission of genes" (genetic material brought from parents to offspring) in genetics. In other words, even among species with reproductive isolation, they have a chance to get this "little genetic gift" from their next-door neighbor.

With the in-depth study of genes, scientists have found many examples of horizontal gene transfer, both in animals and plants. For example, two different species of fish, herring and cucumber, living in the polar regions, "share" exactly the same antifreeze protein-preventing their blood and tissues from freezing in polar waters [3]. In addition, in the tropical rainforest of Madagascar, biologists were surprised to find that a gene called "BovB" in the genome of local frogs seems to be derived from snakes (see "transgenic factories in Madagascar": why do frogs have snake genes? ) [4]. With more and more examples of horizontal gene transfer, scientists are more confused about the mechanism behind this phenomenon: how do these genes "jump" between different species?

The Great Discovery of small nematodes recently, researchers from the Alejandro Burga Laboratory of the Institute of Molecular Biotechnology of the Austrian Academy of Sciences caught a HGT event on the spot. They observed a gene that had just undergone a "jump" in the nematode and found a vector "Maverick" to help the gene jump. Mavericks has been detected in a variety of animals, including invertebrates and vertebrates, which have the characteristics of many viral genes. Considering these properties, biologists suspect that Mavericks and similar genetic factors may contribute to horizontal gene transfer in the evolution of life. The findings were published in the June 30, 2023 issue of the journal Science, entitled "Virus-like transposons cross the species barrier and drive the evolution of genetic incompatibilities" [5].

Scientists discovered that this phenomenon was "purely accidental". In 2021, while studying the nematode C.briggsae, Israel Campo Bes, a graduate student of the team, stumbled upon that this nematode has an almost identical gene with another nematode, C.plicata, with a striking similarity of nearly 97 per cent of nucleotides. It looks like one nematode copied its genes and somehow pasted them into the genome of another nematode. This finding is surprising because C.briggsae and C.plicata are two species with reproductive isolation. The huge difference between their genomes is equivalent to the difference between the genomes of humans and fish. However, they actually share almost the same gene, which clearly shows that this is a recent HGT event.

The discovery also shocked Alejandro Burga, a molecular geneticist in the lab. In order to find out the origin of the "shared" gene, Burga and his team decided to examine the DNA around these "shared" genes. Eventually they found a repetitive sequence around the gene, which is characteristic of transposons. Transposons are genetic factors that can move within the genome and can insert copies of themselves into different locations of the gene by copying themselves. In addition, they found residues of viral genes: a gene that expresses a viral coat protein, a gene that promotes viral replication, and a "glue" gene used to integrate viral DNA into the host genome. These findings suggest that the transferred genes are embedded in a set of virus-like genes and a transposon, which are thought to be part of Mavericks.

Not only that, Mavericks in another nematode has an extra gene that expresses a protein called fusogen, which fuses the virus with the cell's membrane structure and transfers the viral genome into the cell. Without fusin, viruses (especially enveloped viruses) cannot transfer its genes. The existence of this protein strongly indicates that Mavericks can produce virus-like particles and invade different types of cells.

Burga's team then quickly searched the nematode gene database, and they soon discovered that this was not an isolated horizontal gene transfer, and they found many examples of other genes embedded in Mavericks. Among the more than 100 nematode genomes spanning more than 10 genera, two genes are often taken away by Mavericks as "goods" and widely transmitted between different species. In gold deposits a kilometer deep from North America to India to South Africa, complete and incomplete genetic factors have infiltrated nematode populations in different parts of the world.

Although this circumstantial evidence strongly suggests that Mavericks promotes horizontal gene transfer between nematode species, biologists have not yet witnessed their actions. Therefore, the focus of the next step of research is to find a way to observe the process of Mavericks producing virus-like particles under the microscope, and to observe every frame of the gene "jump".

The virus-like transposon Maverick can be used as a horizontal gene transfer (HGT) vector. Image source: IMBA-IMP Graphics's discovery of the genetic "jump" mechanism inspired by "jumping" has reshaped our understanding of biological evolution. Mavericks is an ancient and fragmented jumping gene, which is ubiquitous in the genomes of protozoa, fungi and animals (including humans). At first, these huge moving elements were thought to be ineffective, outdated remnants of genetic mutations. But later studies have shown that Mavericks can be reactivated and they can mediate horizontal gene transfer between some native species [6]. However, so far, no complete Mavericks has been studied in multicellular animals. This time, nematodes provide us with a rare opportunity, and nematodes are not just model animals for biological experiments, many nematodes are parasites that can infect crops and livestock. If we have a clearer understanding of how Mavericks works, we may be able to use it to introduce specific disease-causing genes into parasites to prevent and control parasites.

In addition, the discovery also suggests that Mavericks can carry certain antiviral and antibacterial genes, suggesting that it may be used to develop new drugs or vaccines, or to turn it into a tool for gene transfer for applications such as gene therapy and gene editing.

There are many phenomena of horizontal gene transfer through Mavericks or other genetic factors in nature. The horizontal transfer of these genes will affect the genetic diversity and adaptability of species, resulting in the formation of new species or extinction of species. In order to protect species diversity and really understand the laws of biological evolution, we also need to know more about these vectors that can help genes jump between species.

reference

[1] Crisp A, Boschetti C, Perry M, Tunnacliffe A, Micklem G. Expression of multiple horizontally acquired genes is a hallmark of both vertebrate and invertebrate genomes. Genome Biol. 2015 Mar 13: 16 (1): 50.

[2] Nakamura Y, Itoh T, Matsuda H, Gojobori T. Biased biological functions of horizontally transferred genes in prokaryotic genomes. Nat Genet. 2004 Jul;36 (7): 7606.

[3] Graham LA, Davies PL. Horizontal Gene Transfer in Vertebrates: A Fishy Tale. Trends Genet. 2021 Jun;37 (6): 501503.

Kambayashi C, Kakehashi R, Sato Y, Mizuno H, Tanabe H, Rakotoarison A, K ü nzel S, Furuno N, Ohshima K, Kumazawa Y, Nagy ZT, Mori A, Allison A, Donnellan SC, Ota H, Hoso M, Yanagida T, Sato H, Vences M, Kurabayashi A. Geography-Dependent Horizontal Gene Transfer from Vertebrate Predators to Their Prey. Mol Biol Evol. 2022 Apr 10: 39 (4): msac052.

[5] Widen SA, Bes IC, Koreshova A, Pliota P, Krogull D, Burga A. Virus-like transposons cross the species barrier and drive the evolution of genetic incompatibilities. Science. 2023 Jun 30th 380 (6652): eade0705.

[6] Barreat JGN, Katzourakis A. Phylogenomics of the Maverick Virus-Like Mobile Genetic Elements of Vertebrates. Mol Biol Evol. 2021 May 4: 38 (5): 1731-1743.

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