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Before the early 1900s, if it walked like a Christmas Island rat and talked like a Christmas Island rat, it probably was a Christmas Island rat. But if one of these now-extinct rats ever walks the Earth again, it will most likely be a genetically modified Norway brown rat. And the rodent won’t be as similar to the Christmas Island rat as some would hope, a new study finds. Instead of focusing on iconic species like the woolly mammoth or the Tasmanian tiger, a team of paleogeneticists have studied how, using gene editing, they could resurrect the humble Christmas Island rat, which died out around 120 years ago. Flannery, Tim & Schouten, Peter (2001). A Gap in Nature: Discovering the World's Extinct Animals. Atlantic Monthly Press, New York. ISBN 978-0-87113-797-5. a b Thomas, Oldfield (1887). "Report on a Zoological Collection made by the Officers of HMS Flying-Fish at Christmas Island, Indian Ocean. Communicated by Dr. A. Gunther, VPZS, Keeper of the Zoological Department, British Museum". Proceedings of the Zoological Society of London: 513–514.
As a science, it’s awesome,” Gilbert says. But “is this the best use of the money in a world where we can’t keep our rhinos alive?” Lamoreux, J.; Woinarski, J.; Burbidge, A.A. (2016). " Rattus macleari". IUCN Red List of Threatened Species. 2016: e.T19344A22440729. doi: 10.2305/IUCN.UK.2016-1.RLTS.T19344A22440729.en . Retrieved 12 November 2021.To get the extinct rat’s genome, Gilbert and his colleagues took ancient DNA from two preserved skin samples of the Christmas Island rat. Ancient DNA, extracted from specimens that died anywhere from a few decades to thousands of years ago, is far from perfect ( SN: 5/19/08). Gilbert describes an extinct species’ genome as a book that has been shredded. One way to reconstruct this shredded book is to scan the fragments and compare them with a reference. Mammoths have roughly the same evolutionary distance from modern elephants as brown rats and Christmas Island rats. Wyatt KB, Campos PF, Gilbert MT, Kolokotronis SO, Hynes WH, etal. (2008). "Historical mammal extinction on Christmas Island (Indian Ocean) correlates with introduced infectious disease". PLOS ONE. 3 (11): e3602. Bibcode: 2008PLoSO...3.3602W. doi: 10.1371/journal.pone.0003602. PMC 2572834. PMID 18985148. With the advent of gene-editing technology such as CRISPR, scientists have shifted from cloning to genetic engineering as the most promising method for “de-extinction,” or the resurrection of species that have died out ( SN: 10/7/20). But unlike cloning, genetic engineering wouldn’t create an exact replica of an extinct species. Instead, the technique would edit an existing animal’s genome so that it resembles that of the desired extinct animal. The challenge is making that proxy as similar to the extinct species as possible. By doing these kinds of analyses, which is not hard to do, you can at least come up with the what will you get, what will you not get, and you can use that to decide is it worth doing,” Gilbert says.
For that reason, the Christmas Island rat ( Rattus macleari) was an obvious choice for analysis for the scientists. It diverged from a close relative, the Norway brown rat ( Rattus norvegicus), only around 2.6 million years ago. The rat is named after Captain John Maclear (1838–1907) of the British survey-ship HMS Flying-Fish, who collected the specimen from Christmas Island in 1886. It was described as a new species by Oldfield Thomas the next year, although it was originally described under the genus Mus. [2] [7] Maclear was earlier commander on HMS Challenger for the Challenger Expedition of 1872–1876 under its commission captain, Sir George Nares. Even if gene-editing were perfected, replica animals created with the technique would thus have certain critical deficiencies. a b Harper, Francis (1945). Extinct and vanishing mammals of the Old World. New York: American Committee for International Wild Life Protection. pp.206–208.With an intact copy of the original book, someone could theoretically reconstruct the book perfectly. When identifying a reference genome, researchers look for a species that diverged evolutionarily from the extinct species fairly recently —in other words, a very similar book. The genomes will match closely, but not perfectly. CRX019583 (Mali), CRX019522 (Cambodia5), CRX019633 (China1), CRX019515 (AH1), CRX019516 (AH2), CRX019517 (BJ1) and CRX019639 (SD1) The team used brown rats, commonly used in lab experiments, as the modern reference species, and found they could reconstruct 95 percent of the Christmas Island rat genome.
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