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How genes can leap from snakes to frogs in Madagascar

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https://www.quantamagazine.org/how-genes...-20221027/

EXCERPTS: After poring over genomes from frog and snake species around the world, the scientists reported in April in a paper in Molecular Biology and Evolution that this gene has somehow traveled from snakes to frogs at least 50 times all over the planet. But in Madagascar it has inserted itself into frogs with startling promiscuity: 91% of the frog species sampled there have it. Something seems to make Madagascar an exceptionally conducive place for the gene to get mobile.

[...] Unlike bacteria, viruses have a real knack for picking up genes from their eukaryotic hosts. Viruses, particularly the ones called retroviruses, have the tools for getting into a host’s cells and nuclei, and they are masters of inserting genetic material into host genomes. Up to 8% of the human genome is made up of the leftovers of retroviruses, fragments of long-ago infections in our species’ history.

Sometimes the transfer goes the other way, too. In a paper published in Nature Microbiology last December, Keeling, his collaborator Nicholas Irwin of the University of Oxford and their colleagues performed the first comprehensive analysis of horizontal gene transfers between 201 eukaryotes and 108,842 viruses. They found evidence for more than 6,700 gene transfers, with host-to-virus transfers about twice as common as virus-to-host transfers. They concluded that horizontal gene transfers had been major drivers of evolution on both sides: Viruses often used the eukaryotic genes they acquired to become more effective at infecting their hosts, while eukaryotes sometimes used elements of the viral genes to create novel features or to regulate their metabolisms in new ways.

Findings like these have persuaded some biologists that at least some horizontal gene transfers may be facilitated by viruses. If viruses can pick up genes from their hosts, and if they can leave behind pieces of their genomes, it seems possible that they could also sometimes ferry over genes from the last host they infected, or even one from generations ago, and give them to a new host.

The involvement of viruses could also help to solve another puzzle about horizontal transfers in eukaryotes. For the transfers to occur, the traveling genes need to clear an entire series of hurdles. First they must get from the donor species to the new host species. Then they must get into the nucleus and ensconce themselves in the host genome. But getting into the genome of just any cell won’t do: In multicellular creatures like frogs and herrings, a gene won’t be passed down to the animal’s offspring unless it can sneak into a germline cell — a sperm or an egg.

Viruses might make that series of events more likely. In small organisms like the nematode, Danchin said, the reproductive tract and its germ cells are not far from the intestinal tract, where viruses ingested on food can settle. Because frogs release their eggs and sperm into the open water, those cells are potentially vulnerable to viruses in the environment that could slip in genes.

Even with larger creatures, it might be easier than you think. At this point, it’s still a speculative idea, but “the reproductive tract is full of microbes and viruses,” Danchin said. “We know some viruses infect specifically germ cells.”

Keeling suggests that to understand the mystery of horizontal gene transfer, perhaps we should think of them as ecological consequences of an organism’s behaviors, its neighbors and its environment. If a horizontally transferred gene confers any survival benefit, it’s likely to be highly contingent on the specific scenario in which the recipient of the gene finds itself — an icy sea, a hot spring, an appetizing host plant with tough defenses. “They’re so tied to the ecology where that thing is, but it changes,” he speculated. With the wrong shift in the environment, the transferred gene “is no longer advantageous, and it’s lost.” (MORE - missing details)
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