Would you believe it if you were told that there are organisms that can exist and function perfectly well in the near absence of water? Perhaps not. All our lives, we are taught how water is the basis of all life forms. Not only does it drive many chemical reactions, it also acts as an important solvent in cells.
However, a group of organisms called tardigrades can indeed survive in the near absence of water. Tardigrades are small (between 0.2-1mm in length) invertebrates that can be marine, freshwater or terrestrial species. Terrestrial tardigrades are curious organisms. They are cryptobiotic creatures, meaning that they have the potential to survive extreme environmental conditions like high and freezing temperatures, organic solvents and high pressure. A particular kind of environmental stress they can battle is the near absence of water. Anhydrobiosis, derived from the Greek for ‘life without water’, is a dormant state in which these organisms are able to almost completely exclude the bulk of body water and shut down their metabolism to imperceptible levels.
While many studies have been directed at understanding cryptobiosis, we still understand little about the molecular basis of coping with these stresses. To address this, a group led by Prof. Mark Blaxter from University of Edinburgh’s School of Biological Sciences, in collaboration with Dr Kazuharu Arakawa’s group in Japan, took a comparative genetics approach. They compared the genomes of two tardigrades, Hypsibius dujardini and Ramazzottius varieornatus. While H. dujardini needs about 48 hours of preconditioning before it can enter a state of anhydrobiosis with high survival rate, R. varieornatus can do this with rapidity. This suggests that there are at least two distinct mechanisms by which these organisms are able to prepare for the anhydrobiotic state of life. By comparing the genomes of these two tardigrades, the authors hope to elucidate the differences in these two mechanisms.
Many genes with likely roles as anhydroprotectants (proteins or sugars that protect the organism when they get into a state of anhydrobiosis) were conserved between the two tardigrades. However, the response of these two to anhydrobiosis was driven by the differential expression of genes. They compared the gene expression at fully hydrated states and post desiccated states to see which genes changed in expression. H. dujardini showed a differential expression of several proteins involved in protection against anhydrobiosis more readily than R. varieornatus. However, upon slow desiccation, R. varieornatus showed differential expression of genes similar to H. dujardini. This can be justified by the assumption that in spite of its ability to survive rapid anhydrobiosis, R. varieornatus is more likely to face a situation where the changes in environment towards the stress are more gradual. Therefore, when they are slowly desiccated, they respond in much the same way as other tardigrades. The comparison of the tardigrade genomes with other organisms also revealed that these have higher copy numbers of genes that encode proteins involved in protective functions. This observation could explain how they are able to deal with the stress more effectively.
Understanding how tardigrades cope with environmental stresses is an important step towards understanding the process of evolution itself.
This article was written by Aishwarya Sivakumar and edited by Bonnie Nicholson.