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Microbe With Bizarrely Tiny Genome May Be Evolving Into a Virus

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1 month 1 week ago
sciencehabit shares a report from Science.org: The newly discovered microbe provisionally known as Sukunaarchaeum isn't a virus. But like viruses, it seemingly has one purpose: to make more of itself. As far as scientists can tell from its genome -- the only evidence of its existence so far -- it's a parasite that provides nothing to the single-celled creature it calls home. Most of Sukunaarchaeum's mere 189 protein-coding genes are focused on replicating its own genome; it must steal everything else it needs from its host Citharistes regius, a dinoflagellate that lives in ocean waters all over the world. Adding to the mystery of the microbe, some of its sequences identify it as archaeon, a lineage of simple cellular organisms more closely related to complex organisms like us than to bacteria like Escherichia coli. The discovery of Sukunaarchaeum's bizarrely viruslike way of living, reported last month in a bioRxiv preprint, "challenges the boundaries between cellular life and viruses," says Kate Adamala, a synthetic biologist at the University of Minnesota Twin Cities who was not involved in the work. "This organism might be a fascinating living fossil -- an evolutionary waypoint that managed to hang on." Adamala adds that if Sukunaarchaeum really does represent a microbe on its way to becoming a virus, it could teach scientists about how viruses evolved in the first place. "Most of the greatest transitions in evolution didn't leave a fossil record, making it very difficult to figure out what were the exact steps," she says. "We can poke at existing biochemistry to try to reconstitute the ancestral forms -- or sometimes we get a gift from nature, in the form of a surviving evolutionary intermediate." What's already clear: Sukunaarchaeum is not alone. When team leader Takuro Nakayama, an evolutionary microbiologist at Tsukuba, and his colleagues sifted through publicly available DNA sequences extracted from seawater all over the world, they found many sequences similar to those of Sukunaarchaeum. "That's when we realized that we had not just found a single strange organism, but had uncovered the first complete genome of a large, previously unknown archaeal lineage," Nakayama says.

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