Tardigrades are tough little creatures. The teensy little creepy-crawlies, which average less than a millimetre in length and are also known as water bears or moss piglets, are able to go into a dehydrated state and survive some of the harshest environments, including the vacuum of space. Yup.
Quite how these micro-animals manage to tolerate conditions that would be deadly to other creatures remains largely a mystery, though it's been a surprisingly controversial question of late as various groups of scientists attempt to find the answers by sequencing the tardigrade genome.
A new sequencing attempt, detailed in Nature Communications, identifies a protein the researchers say helps protect the tardigrade's DNA from radiation. They even suggest proteins like it could one day be applied to develop new protective mechanisms for other animals, and showed that it was possible to transfer its protective effect to human cells.
Takekazu Kunieda, a biologist at the University of Tokyo and lead author on the paper, told me in a phone call that, if this kind of tolerability could be transferred to other animals—something he admitted is "almost science fiction"—then "it may extend the frontier of the animal's habitat, even to outer space."
In the study, the researchers analysed the genome of the tardigrade species Ramazzottius varieornatus, one of the hardiest tardigrade varieties. They were particularly interested in finding genes unique to tardigrades which could explain their ability to tolerate extreme conditions. They found many tardigrade-unique proteins were "abundantly expressed," as they write, including one associated with DNA they refer to as Dsup, or "Damage suppressor".
"We hypothesized that the association of Dsup proteins with nuclear DNA might help to protect DNA from irradiation stress," the researchers explain in their paper. To test this, they made cultured human cells express the Dsup protein and exposed them to X-ray irradiation, which causes DNA damage. They found that the cells suffered fewer DNA breaks than control cells.
"When Dsup is expressed in the human cultured cells, the cells exhibit only half of the DNA damage after irradiation," explained Kunieda.
The researchers conclude that unique proteins in the tardigrades, such as Dsup, could account for their hardiness in extreme environments.
Tardigrade genomes have been sequenced before, but not this species, which is particularly hardcore. The new analysis also sheds light on a recent debate in the tardigrade research community about what could make the beasties so resilient.
The new analysis did not find extensive horizontal gene transfer in the tardigrade
Last year, US researchers sequenced the genome of tardigrade species Hypsibius dujardini and found an extraordinary amount of foreign DNA, which they suggested showed unprecedentedly high levels of horizontal gene transfer (when genes are incorporated into one organism from another). But another group of researchers sequenced the same species and didn't find the same results; they concluded the first team's findings could be due to contamination.
The new analysis also did not find extensive horizontal gene transfer in the tardigrade. Though it was a different species, Kunieda pointed out that, as the R. varieornatus species is particularly hardy, their findings suggested that horizontal gene transfer couldn't be a major cause of tardigrades' overall resilience.
The tardigrade gene associated specifically with Dsup has not been found in other species so far. "Our species is one of the most radiotolerant species in tardigrades, so it may be the cause of the difference in tolerance among tardigrades," Kunieda posited.
Next, he said he will look for other interesting genes in the tardigrade genome that might contribute to the tardigrade's protective arsenal.
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