Nematodes – the most widespread and resilient animal phylum on our planet – make up 80% of all animal life by numbers and are found on every continent and in all ecosystems, from polar regions and ocean floors to tundra and deserts. These roundworms are often deemed ‘extremophiles’, or organisms that thrive under ‘extreme’ environments that are too harsh for human existence, such as high pressure, temperature or salinity. In the summer of 2002, Stanislav Gubin, from the Russian Academy of Sciences (RAS), collected frozen soil from undisturbed, never-before-thawed permafrost deposits located in the Kolyma Lowland of northeast Siberia, Russia. The soils from this region of the northeastern Arctic have been frozen for thousands of years at temperatures between –3 and –11°C. When they scrutinized the samples, Anastasia Shatilovich (Institute of Physicochemical and Biological Problems in Soil Science, RAS, Russia), Vamshidhar Gade (Zoological Institute, RAS, Russia) and an international team of colleagues found apparently frozen nematodes in two of more than 300 samples from the site. The team decided to try to resuscitate the animals, to figure out what species they were.
Shatilovich and colleagues relied on radiocarbon dating of the plants found in the vicinity of the nematodes to determine their age, because the nematodes themselves were too small for this sort of analysis. It turned out that the plants were between 45,839 and 47,769 years old. But could the researchers resuscitate the ancient worms? They mixed a sugar cube sized sample of the permafrost soil into a solution of some basic salts and checked every few days for signs of moving worms. Amazingly, some of the tiny animals emerged from their ∼46,000 year slumber, and luckily for the researchers, the revived worms were parthenogenetic – which is Greek for ‘virgin birth’ – meaning these female animals could reproduce without fertilization by a male. After cultivating the revived animals in the lab for over 100 generations, the team next tried to find out what kinds of nematodes they were.
Based on the nematodes’ appearance and small snippets of DNA in their genomes, the scientists suspected that the animals belonged to a genus of nematodes known as Panagrolaimus. But because many Panagrolaimus worms look the same by eye, they still weren't sure which species they had rescuscited from the deep freeze. So next, the team assembled the entire genome of the animals, compared it with the DNA of other nematodes and discovered it to be a completely novel species, which they named Panagrolaimus kolymaensis. But how had these miraculous creatures survived for so long in the frozen soil?
Caenorhabditis elegans, a nematode species commonly studied in the laboratory, is known to go into a form of suspended animation known as ‘dauer diapause’ to survive difficult situations. Might this newly discovered species from the ancient past have used the same molecular mechanisms for survival? The researchers compared the P. kolymaensis genome with that of C. elegans and found several genes that have the same functions in the two species which could be involved in their remarkable ability to survive the most extreme conditions. Many of the shared genes between P. kolymaensis and C. elegans were related to the synthesis of the sugar trehalose, which protects cell membranes when drying out – which often happens when an animal freezes – and could possibly help the two species to survive harsh freezing conditions. Having discovered the extraordinary ability of P. kolymaensis to survive almost 50,000 years in the Siberian permafrost, Shatilovich, Gade and their colleagues wondered, is there an upper limit to the protection offered by the nematodes’ genes? Perhaps this line of research will someday reveal whether it is possible to stay frozen in time forever.