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Subject: Virus triggered evolution
Replies: 19 Views: 496

shadow27 28.02.21 - 09:07am
The rise of the mammals may be feel like a familiar tale, but there's a twist you likely don't know about: If it wasnt for a virus, it might not have happened at all.

One of the few survivors of the asteroid impact 65 million years ago was a small, furry, shrew-like creature that lived in underground burrows and only ventured out at night, when predators werent active. The critter - already the product of some 100 million years of evolution - looked like a modern mammal, with body hair and mammary glands, except for one tiny detail: according to a recent genetic study, it didn't have a placenta. And its kind might never have evolved one if not for a chance encounter with a retrovirus. * +

shadow27 28.02.21 - 09:09am
Unlike most viruses, which infect, replicate, and then leave their host, retroviruses elbow their way into their host's genome where they are copied and passed on to daughter cells for the life of the host. This retrovirus, however, managed to sneak its way into one of our ancestor's sperm or egg cells, able to be passed on to every cell in every subsequent generation. Virus and host had become one.

The viral DNA used its own genes to copy itself, inserting those copies elsewhere in the hosts genome. These copies could be expressed in different parts of the body at different points in time, a symbiotic relationship that gave the shrew some extra raw materials with which to develop new functions. * +

shadow27 28.02.21 - 09:10am
''Viral proteins already have functions. Its much easier to borrow these than to evolve them from scratch,'' says Aris Katzourakis, an evolutionary biologist at the University of Oxford.

In that would-be mammal living 160 million years ago, a symbiotic retrovirus enabled it to evolve a placenta over many generations. In order to let a fetus mature inside a mother's uterus, an animal needed a way to provide oxygen and nutrients while removing waste and keeping both blood supplies separate. * +

shadow27 28.02.21 - 09:12am
Early mammals used the spare viral parts left in the junk drawers of the genome to use a viral gene to help create the placenta, and other symbiotic viruses help turn us from a ball of cells into a fully-formed squalling infant and protect us from pathogens.

Scientists are discovering that the so-called ''junk DNA'' - a significant portion of which is from symbiotic viruses - is actually a potent force in the evolution of new species. Although the evolution of pregnancy via the placenta might be some of the most persuasive evidence that viruses stashed deep within the genome can help give rise to new species, it's not the only proof. New studies revealing the role of endogenous retroviruses in the more recent evolution of humans show that these snippets of DNA are helping to blur the boundary between human and virus. Humans are, in a very real sense, part virus. * +

shadow27 28.02.21 - 09:13am
''The boundaries between organisms are a bit more merged now, a bit more shadowy. We need to break down those boundaries,'' says University of Queensland virologist Paul Young. ''The more we look, the more we find overlap,''

Viruses can infect all organisms, from the largest blue whales to the tiniest bacteria. To a host, infections can range from unnoticed to deadly. To the virus, however, infection is an opportunity to unleash its incredible genome copying abilities. Within hours, it can make millions or even billions of copies of itself. * +

shadow27 28.02.21 - 09:14am
Retroviruses, however, use a slower, stealthier approach. After entering the cell, the retrovirus uses an enzyme called reverse transcriptase to turn its RNA into DNA before making its way to the nucleus. Once in the nucleus, it inserts its DNA into the host's genome.

Most of the time, when a virus integrates its genome with the host's, the new hybrid genome dies when the cell and its descendants do. Sometimes, however, a virus will infect a sperm or egg cell. If fertilization occurs, the offspring will have a copy of the viral genome in every single one of its cells. It can pass the hybrid genome on to its offspring, creating what scientists call a fully endogenous retrovirus - a fancy term for a virus that comes from within. * +

shadow27 28.02.21 - 09:16am
The process requires an astonishingly rare set of circumstances be met, Katzourakis says. ''Although endogenous retroviruses make up a pretty large proportion of our genome, in terms of the number of times they've infiltrated our genome over the past sixty or so million years, it only comes down to about 30 or 40 distinct occasions,'' he says.

In humans, even the most recent of these infiltrations happened tens of thousands of years ago. In domestic sheep and koalas, however, retroviruses are currently establishing themselves, which gives researchers the opportunity to watch the process in action. In the 1980s, Queensland veterinarian Jon Hanger noticed something odd in a breeding colony of about 70-80 koalas.. * +

shadow27 28.02.21 - 09:18am
Every year, the colony lost around 10 percent of its members to immunosuppression or cancer, especially leukemias, a cancer of white blood cells.

About 60 to 70 percent of the deaths were from cancer. ''That's an unusually high number of cancer deaths in any animal, including ours,'' Young says.

Although Hanger was working full-time as a veterinarian, he couldnt shake his curiosity. The mystery drove him back to school to get a PhD on the topic. By 2000, Hanger and Young had identified the full-length retroviral genome that was causing disease in koalas. The virus, however, wasnt actively transmitted from koala to koala. Instead, a retrovirus had embedded itself in the koalas' germline and passed from parent to child. * +

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