Genetics: Water bear genome reveals adaptations to the extreme

The genome of the water bear (Ramazzottius variornatus), a small aquatic animal also known as tardigrade, has been sequenced in a new study published in Nature Communications this week. The paper identifies a gene in tardigrades whose protein confers resistance against DNA damage in human cultured cells. The findings suggest that proteins unique to tardigrades are relevant to helping cells tolerate sources of DNA damage.

Tardigrades can survive in extremely stressful environments, including the vacuum of space, though how exactly they do so is unknown. This ability has prompted investigation of their genomes, with the first genome sequence of a tardigrade (Hypsibius dujardini) suggesting that a significant amount of acquisition of genes from other species through horizontal gene transfer (transmission of DNA between genomes of different species) took place in tardigrade evolution. However, the source of tardigrades’ tolerance of extreme environments has been a mystery.

Takekazu Kunieda and colleagues present a high-quality genome from the tardigrade species R. variornatus, known for its tolerance of high-stress environments. They see no evidence of extensive horizontal gene transfer but find that, compared to flies and worms, there is an increase in genes responsible for the tolerance of stressful environments. Furthermore, they identify a protein, which they believe to be unique to tardigrades, that binds to DNA and can help protect cultured human cells against X-ray radiation. Based on a detailed comparison between tardigrade genes and those of other species, the authors find that the gene that codes for this protective protein is likely to be unique to the tardigrade lineage. These results argue against this tolerance deriving from horizontal gene transfer.

Although it is still unclear how tardigrades’ unique adaptations function at either the molecular or organism level, these results suggest that tardigrades have evolved unique strategies to cope with stressful conditions.