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March 10, 2022 03:30 am

Bringing Back Extinct Creatures May Be Impossible

An anonymous reader quotes a report from Science.org: An extinct rat that once lived on an island in the Indian Ocean may have put the kibosh on scientists' dreams of resurrecting more famous extinct animals like the woolly mammoth. The Christmas Island rat disappeared just over 100 years ago, but researchers now say even its detailed genome isn't complete enough to bring it back to life. The work "shows both how wonderfully close -- and yet -- how devastatingly far" scientists are from being able to bring back extinct species by genetically transforming a close relative in what's called "de-extinction," says Douglas McCauley, an ecologist at the University of California, Santa Barbara, who was not involved with the study. [...] To bring back an extinct species, scientists would first need to sequence its genome, then edit the DNA of a close living relative to match it. Next comes the challenge of making embryos with the revised genome and bringing them to term in a living surrogate mother. So far, scientists have sequenced the genomes of about 20 extinct species, including a cave bear, passenger pigeon, and several types of mammoths and moas. But no one has yet reported re-creating the extinct genome in a living relative. In the new study, Tom Gilbert, an evolutionary geneticist at the University of Copenhagen, thought it best to start small. "If we want to try something so crazy, why not start with a simple model," he reasoned. So, he, Jian-Qing Lin, a molecular biologist at Shantou University, and their colleagues, focused on the Christmas Island rat (Rattus macleari), which disappeared by 1908 from that island, located about 1200 kilometers west of Australia. This species "should be a dreamy candidate for de-extinction," McCauley says, given its close relationship with the Norway rat, a well-studied lab animal with a complete genome sequence that scientists already know how to modify. Gilbert and Lin extracted DNA from the skins of two preserved Christmas Island rats and sequenced it many times over to get as much of the genome as possible. They achieved more than 60 times' coverage of it. Old DNA only survives in small fragments, so the team used the genome of the Norway rat as a reference to piece together as much as possible of the vanished rat's genome. Comparing the two genomes revealed almost 5% of the Christmas Island rat's genome was still missing, Lin, Gilbert, and their colleagues report today in Current Biology. The lost sequences included bits of about 2500 of the rat's estimated 34,000 genes. "I was surprised," Gilbert says. The recovered DNA included the genes for the Christmas Island rat's characteristic rounded ears, for example, but important immune system and olfaction genes were either missing or incomplete. The work "really highlights the difficulties, maybe even the ridiculousness, of [de-extinction] efforts," says Victoria Herridge, an evolutionary biologist at the Natural History Museum in London. Herridge says many of the missing genes make each species unique. It's also worth noting that the human genome differs by just 1% from those of chimps and bonobos. Others researchers like Andrew Pask, a developmental biologist at the University of Melbourne, Parkville, says that the missing 5% of an extinct animal's genome likely won't affect how the transformed animal looks or behaves.

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