how do viruses mutate and evolve

Microbiology and Molecular Biology Reviews. [21], Many viruses (for example, influenza A virus) can "shuffle" their genes with other viruses when two similar strains infect the same cell. “There’s no question that once the population is largely immune, the virus will die down,” Parrish says. Viruses, though not technically alive, also mutate and evolve as they infect a hosts’ cells and replicate. A recent study based on comparisons of viral protein folding structures, however, is offering some new evidence. A news brief on the emergence of another new virus, avian flu. But these mechanisms do not work for RNA and when an RNA virus replicates in its host cell, changes in their genes are occasionally introduced in error, some of which are lethal. Once inside the cell, the virus has to evade the cell’s immune defenses and then commandeer the appropriate parts of the host’s biochemistry to churn out new viruses. It also reproduces at a lightning-fast rate a single virus can spawn billions of copies in just one day. After all, many of the viruses we know of that trigger severe problems in a new host species cause mild or no disease in the host they originally came from. RNA viruses tend to have higher mutation rates, probably because of their genetic make-up. That was never going to be as easy to do for Covid-19 because people who don’t show symptoms can spread the virus. About 220 to 250 viruses are known to infect people, but only about half are transmissible — many only weakly — from one person to another, says Jemma Geoghegan, an evolutionary virologist at the University of Otago, New Zealand. The bacteria that cause cholera spread through diarrhea, so severe disease is good for them. By March, the infection began to spread to other Asian countries and overseas. Half is a generous estimate, she adds, since many other spillover events probably fizzle out before they can even be counted. Evolution always favors increased transmissibility, because viruses that spread more easily are evolutionarily fitter — that is, they leave more descendants. This indicates that some viruses emerged early in the evolution of life, and that they have probably arisen multiple times. As coronavirus cases continue to rise across the globe, scientists and policymakers are fervently working to mitigate this public health threat. But SARS-CoV-2 carries other changes in the spike protein that appear to have arisen after it jumped to people, since they don’t occur in the bat or pangolin viruses. Fold Super Families (FSFs) are proteins that show similar folding structures independent of the actual sequence of amino acids, and have been found to show evidence of viral phylogeny. Sign up for the newsletter. From an astrobiological point of view it has therefore already been proposed that on celestial bodies such as Mars not only cells but also traces of former virions or viroids should be actively searched for. And when Belgian researchers sequenced OC43’s genome in 2005 and compared it to other known coronaviruses, they concluded that it likely originated as a cattle virus and may have jumped to people right around 1890. First, the virus has to be able to invade the new host’s cells: That’s a minimum requirement for making the host sick. The big question now is: What happens next? Some germs do just fine even if they make you very sick. Keep up-to-date on: © 2021 Smithsonian Magazine. (Holmes is blunter: “Trying to predict virulence evolution is a mug’s game,” he says.). For example, the genomes of most vertebrate species contain hundreds to thousands of sequences derived from ancient retroviruses. “One thing you learn about evolution is never to generalize,” says Edward Holmes, an evolutionary virologist at the University of Sydney, Australia, and author of an article on the evolution of emerging viruses in the Annual Review of Ecology, Evolution, and Systematics. Just as natural selection has shaped the evolution of humans, plants, and all living things on the planet, natural selection shapes viruses, too. But transmissibility and virulence aren’t linked in any dependable way, Read says. Amy is the first player, and her word is “CAT.” She whispers her word to Ben, who accidentally hears “MAT.” Ben whispers his word to Carlos, who hears “MAD.” As the game of telephone goes on, the word will transform further and further away from its original form. The next time the same virus comes to a host cell, it may find that it is no longer able to attach to the cell's surface membrane. SARS-CoV-2 shows these two stages clearly. But it could also remain a serious threat or perhaps even evolve to become more lethal. The new coronavirus, like all other viruses, mutates, or undergoes small changes in its genome. And from the virus’s perspective, this theory asserts, hosts that are less sick are more likely to be moving around, meeting others and spreading the infection onward. He and his colleagues looked at decades’ worth of genetic sequence data for rabies viruses that had undergone such host shifts. Why did SARS go away while today’s coronavirus just keeps on spreading? How viruses evolve, and in some cases, become deadly. (It has since ticked up again.). This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Viruses have been able to continue their infectious existence due to evolution. Vote for the Readers' Choice winner of the 18th Annual Smithsonian Magazine Photo Contest, Newly Restored Pompeiian Frescoes Capture Hunting Scenes in Vivid Detail, 'Miraculously' Well-Preserved Ceremonial Chariot Found at Villa Outside of Pompeii, Oldest Known Mummification Manual Reveals How Egyptians Embalmed the Face, Most People Don't Know When to Stop Talking, According to Science, Long-Lost Babbler Bird Documented in Borneo for the First Time in Over 170 Years, Lost Languages Discovered in One of the World's Oldest Continuously Run Libraries, The True History Behind Netflix's 'The Dig' and Sutton Hoo, Polar Bears Live on the Edge of the Climate Change Crisis, Inside Naples' World-Famous Pizza Culture, How Navajo Physicians Are Battling the Covid-19 Pandemic. In the case of SARS, the virus made people sick enough that health workers were able to contain the disease before it got out of hand. RNA viruses, like the flu and measles, are more prone to changes and mutations compared with DNA viruses, such as herpes, smallpox, and human papillomavirus (HPV). This suggests that the mutation first arose either in pangolins or an as yet unidentified species and happened to allow the virus to jump over to people, too. It's normal for viruses, including SARS-CoV-2, to mutate. Viruses and bacteria change quickly in part because they replicate like mad. or This evolutionary two-step — first spillover, then adaptation to the new host — is probably characteristic of most viruses as they shift hosts, says Daniel Streicker, a viral ecologist at the University of Glasgow. They enter the cell, take over its genetic information replication mechanism, replicate themselves and then the copies leave the cell to continue infecting. Since larger populations contain more genetic variants than smaller populations do, measuring genetic diversity in their samples enabled the scientists to estimate how widespread the virus was at any given time. Viral evolution is a subfield of evolutionary biology and virology that is specifically concerned with the evolution of viruses. It’s possible the virus could lose its lethal character and settle into an evolutionary détente with humanity. [27] Although it is not possible to prove which of these rapidly evolving viruses is the earliest, for such a closely related group of viruses to be found in such diverse hosts suggests the possibility that their common ancestor is ancient.[28]. Phage T4 is an obligate intracellular parasite that reproduces within the host bacterial cell and its progeny are released when the host is destroyed by lysis. So, too, for coronaviruses: The ones behind SARS (severe acute respiratory syndrome), MERS (Middle East respiratory syndrome) and Covid-19 all probably originated in bats and arrived in people via another, stepping-stone species, likely palm civets, camels and possibly pangolins, respectively. Date: January 27, 2012. These could be changes to the virus particles that disguise them so they are not identified by the cells of the immune system or changes that make antiviral drugs less effective. Other evolutionary biologists disagree. How viruses evolve. The group includes measles virus, which infects humans and primates; canine distemper virus, which infects many animals including dogs, cats, bears, weasels and hyaenas; rinderpest, which infected cattle and buffalo; and other viruses of seals, porpoises and dolphins. While the human system’s job is to kill the foreign body that enters the system, it is the virus’ job to evade the immune system and create more copies of itself. And many such findings have reinforced just how quickly pathogens mutate and evolve in response to environmental cues. In addition, most viruses provide many offspring, so any mutated genes can be passed on to many offspring quickly. The virus can now spread from one host to another, like humans. Viruses are ancient. Review some background information on natural selection. [24][25] Most species of viruses are now known to have common ancestors, and although the "virus first" hypothesis has yet to gain full acceptance, there is little doubt that the thousands of species of modern viruses have evolved from less numerous ancient ones. ), viruses overall have high chances for mutations. This phenomenon is called genetic shift, and is often the cause of new and more virulent strains appearing. Continue "Bacteriophage T4 genome". Viruses have very short generational cycles. Many of the scariest viruses that have caused past or current epidemics originated in other animals and then jumped to people: HIV from other primates, influenza from birds and pigs, and Ebola probably from bats. With the new coronavirus, Parrish says, “we’re sort of in that 1918 period where the virus is spreading fast in a naive population.” But that will change as more people either catch Covid-19 or are vaccinated (if and when that becomes possible) and develop some level of immunity. [23], Through these mechanisms new viruses are constantly emerging and present a continuing challenge in attempts to control the diseases they cause. Why, for that matter, did both these coronaviruses spill over into people at all, from their original bat hosts? That is, there are changes in their genetic code over time. [16], The evolutionary history of viruses can to some extent be inferred from analysis of contemporary viral genomes. Mutation rates are evolvable and can respond to selection. That’s what elevates a virus from an occasional nuisance to one capable of causing widespread harm. These are small changes (or mutations) in the genes of influenza viruses that can lead to changes in the surface proteins of the virus: HA (hemagglutinin) and NA (neuraminidase). These factors also have an effect on how the level of virulence will change over time. At an extreme, an organism that’s “perfectly” adapted to its constant environment would do best to reduce its mutation rate to zero—there are no more beneficial mutations, so all mutations are likely worse than the current genotype (see C in Fig 2).

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