Several terrestrial tardigrades enter a state called anhydrobiosis when the surrounding environment becomes desiccated. In this state, the tardigrade Ramazzottius varieornatus is known to tolerate various extreme environments (high and low temperatures/pressures, high dosage of ultraviolet and gamma rays). Recent studies have reported several novel protection proteins that may play important roles in the mechanism of R. varieornatus anhydrobiosis. Nevertheless, a single cell gel electrophoresis conducted on R. varieornatus showed double strand breaks in the DNA when irradiated with 4000 Gy Co60 gamma rays, a dosage that does not strongly affect the life span, implying that there is also a prominent repair system. In order to determine the main factors of this repair system, we conducted a transcriptomic analysis for the identification of genes responding to severe stress. We irradiated R. varieornatus with 500 Gy of Co60 gamma ray, a dosage below the LD50 of eggs, and conducted a time-course low-input RNA-Seq analysis for every three hours. We have found a quick response group, which contains the gene Ku 70/80. These genes are known as the first responder in the non-homologous end joining pathway, which works as the main pathway for double strand repair in eukaryotes. In addition, a novel gene family with no similar proteins in public databases was found to have the same expression profiles with Ku80. Homologs of this gene family in Hypsibius dujardini showed increase in expression during the rehydration phase, implying that this putative gene family is related to cellular repair. This is one of the first reports of a multi-replicate time-course RNA-Seq analysis of tardigrades, and these results would shed new light into the response to excessive stress of extremophiles.
|
