Multiomics study of a heterotardigrade, Echinisicus testudo, suggests convergent evolution of anhydrobiosis-related proteins in Tardigrada

Many limno-terrestrial tardigrades can enter an ametabolic state, known as anhydrobiosis, upon desiccation, in which the animals can withstand extreme environments. The molecular components of anhydrobiosis are beginning to be elucidated, such as the expansion of oxidative stress response genes and gain of tardigrade-specific heat-soluble protein families designated CAHS and SAHS. However, to date, studies have predominantly investigated the class Eutardigrada, and the mechanisms in the remaining class, Heterotardigrada, remains elusive. To address this gap in the research, we report a multiomics study of the heterotardigrade Echiniscus testudo, which is one of the most desiccation-tolerant species. Surprisingly, none of the previously identified tardigrade-specific anhydrobiosis-related genes was conserved, while the loss and expansion of existing pathways were partly shared. Furthermore, we identified two families novel abundant heat-soluble proteins, which we named E. testudo Abundant Heat Soluble (EtAHS), and the proteins exhibited structural changes from random coil to alpha helix as the water content was decreased in vitro. These characteristics of EtAHS proteins are analogous to those of CAHS in eutardigrades, while there is no conservation at the sequence level. Our results suggest that Heterotardigrada have partly shared but distinct anhydrobiosis machinery compared with Eutardigrada, possibly due to convergent evolution within Tardigrada.