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The Resurrected Species Of Extinct Rat Demonstrates The Limitations Of CRISPR In Terms Of Reviving Species

An extinct rat shows CRISPR’s limits for resurrecting species

A Christmas Island rat was most likely to be mistaken for another Christmas Island rat before the early 1900s, as long as it moved and spoke like a Christmas Island rat. However, if one of these now-extinct rats ever makes its way back to the surface of the planet, it will be a genetically modified Norway brown rat. In addition, the rodent will not be as similar to the Christmas Island rat as some had hoped.

With the introduction of gene-editing technologies such as CRISPR, scientists have moved their focus away from cloning and toward genetic engineering as the most promising option for “de-extinction,” or the resuscitation of extinct species (SN: 10/7/20). However, in contrast to cloning, genetic engineering would not result in the creation of an identical clone of an extinct species. Instead, the procedure would modify the genome of an extant species to make it more similar to the genome of the intended extinct animal. The objective is to create a proxy as close to the extinct species as feasible in appearance.

Researchers tried to recover the genome of the Christmas Island rat to test the method’s boundaries. They were unsuccessful. The researchers retrieved around 95 percent of the extinct rat’s genetic instruction book by comparing pieces of the extinct rat’s genetic instruction book with the genome of a surviving cousin, the Norway brown rat. That may seem like a lot, but it implies that 5 percent of the genes were still missing, including several critical for the sense of smell and the immune system, according to a paper published in the journal Current Biology on April 11.

“It’s only possible to bring back what you can discover.” Furthermore, Tom Gilbert, an evolutionary scientist at the University of Copenhagen, points out that “we can’t discover everything.”
Gilbert and his colleagues extracted ancient DNA from two preserved skin samples of the Christmas Island rat to sequence the extinct rat’s genome. SN: 5/19/08: Ancient DNA, recovered from specimens that died ranging from a few decades to thousands of years ago, is far from flawless. As Gilbert explains it, the genome of an extinct species is like a book torn apart. To piece together this torn book, scan the shards and compare them to a reference image, for example.

Someone might rebuild the book properly if they had a copy of the original text in excellent condition. When searching for a reference genome, scientists hunt for a species that has split evolutionarily from the ancient species just recently — in other words, a species with a very similar book to the old species. The genomes will be quite near to one other, but they will not be identical.

For this reason, the scientists decided on the Christmas Island rat (Rattus clear) as the subject of their investigation. There was just a 2.6 million-year gap between it and its closest living cousin, the Norway brown rat (Rattus norvegicus).

The researchers discovered that the genome of the Christmas Island rat corresponded to around 95 percent of the genome of the Norway brown rat. According to further investigations, the approximately 5 percent that was missing could not be explained only by a defect in the technology or an insufficient reference genome. Instead, as a result of evolutionary divergence between the two species, the majority of that genetic information was lost in the process of evolution.

Furthermore, the genes that were lacking were not chosen at random. In general, they tended to fall into one of two primary brain areas in charge of the rat’s immunological responses and sense of smell. For example, if the genome of a Norway brown rat were modified to match the genome of a Christmas Island rat, the new creature would smell differently from its precursor. This might make it more difficult for a proxy Christmas Island rat to survive if released back into its natural environment.

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