Threadworms revived after tens of thousands of years

In 2018, scientists discovered two species of microscopic threadworms in the Siberian permafrost,which turned out to be true survivors. They could be revived: after at least 46 000 years in the ice. And one of these nematode species is even a new species called Panagrolaimus kolymaensis, named after the river Kolyma where they were found. In a study, published in »PLOS Genetics«, the team led by Teymuras Kurzchalia from the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden compares the survival mechanism of the Siberian sibmaensis;survival mechanism of the Siberian worm with the famous nematode worm Caenorhabditis elegans – a model organism used in laboratories around the world.

»The radiocarbon dating is absolutely precise, and we now know that they really have survived 46 000 years«, says Kurzchalia.Panagrolaimus species are found all over the world and are known to survive in environments where they are regularly exposed to desiccation or freezing, says Ann Burnell, an emeritus professor of biology at Maynooth University in Ireland, who was not involved in the new study.

If the worms really are as old as the study suggests, they would be by far the most impressive example of what scientists call cryptobiosis: the ability of an organism to suspend its own metabolism under poor conditions. However, some scientists are sceptical about the results of the study, which was already the case in 2018 when the specimens were first reported. At the time, external researchers raised concerns that the nematodes studied could be modern contaminants. Byron Adams, a biologist at Brigham Young University, was one of these sceptics and is also not convinced by the new work of Kurzchalia and his colleagues. »I would like to believe that the animals described have survived 40,000 years in permafrost«, says Adams. »And if I were a betting man, I would put money on the fact that it could be true and that these things really are that old 

Founded scepticism

Adams, however, is of the opinion that the analysis in the study does not prove the age of the worms, but only that of the plant material found in the vicinity. »I do not doubt the age of the organic material in the permafrost«, he says. »These values are probably accurate.« Adams adds, however, that »the authors have not yet proven that the animals found are not surface contaminants«. One way to verify the age would be to take soil samples in the area and confirm that the nematodes they contain are different species from those currently living in the permafrost.

Dresden biologist Kurzchalia was also not involved in the original collection, which was carried out in 2002 as part of a year-long series of field trips. However, he says he trusts the sterility procedures the scientists used to avoid modern contamination. Kurzchalia only came across the worms much later, after he became interested in the first reports of these »resurrected« nematodes and invited a Russian co-author to bring some specimens to his lab for analysis.

In addition to radiocarbon dating, the authors of the new study also confirmed that they were able to successfully induce the nematodes to enter and exit the sleep-like state of cryptobiosis when conditions changed in their favour. However, University of Otago biologist David Wharton, who was not involved in the study, notes that the freezing mechanism tested by the researchers is not realistic, as it allowed the nematodes to dry out before abrupt freezing. It is more likely that in nature, temperatures gradually drop while the water remains present, he adds. »This is hardly a natural situation,« says Wharton. »Since the nematodes need water to be active and reproduce, it is more likely that they are frozen in contact with water.«

Certainly a new species

In contrast, the genetic analyses, which according to Kurzchalia presented a challenge, are indisputable because P. kolymaensis is parthenogenic: the females of the species can reproduce without a male partner. The type of genetic analysis used by the team also required 2000 to 4000 worms, which first had to be counted. For the usual laboratory species C. elegans this is trivial, but difficult to achieve when working with P. kolymaensis. But the effort to produce enough heat has paid off, says Adams.

During the analyses, the researchers also looked for genes that are known to be used by C. elegans when it enters the dormant state of cryptobiosis as a dauer larva. Kurzchalia's lab had previously shown that these dauer larvae need to process a sugar called trehalose to survive freezing. In the new study, the team found that the genes required for this process are also present in P. kolymaensis . »This ¨survival package is the same as 46 000 years ago«, says Kurzchalia.

News of the "Spectrum" from 28.07.2023