Osmotic stress tolerance and transcriptomic response of Ramazzottius varieornatus (Eutardigrada: Ramazzottiidae) following tun formation

Abstract

To thrive in harsh environments, tardigrades have evolved the ability to enter the quiescent state of cryptobiosis, often characterized by transition into a so-called ‘tun’. Here, we investigate osmobiosis, a substate of cryptobiosis induced by rising osmolyte concentrations. We follow the behaviour and morphology of Ramazzottius varieornatus during transfer from freshwater conditions into 3 Osmol/kg sucrose solution. The tardigrades easily survive the extreme change in external osmolality. During gradual exposure, they initiate tun formation at 0.2 Osmol/kg, with most specimens fully contracted into a tun at 0.5 Osmol/kg. The first transcriptomic profiling of osmobiotic tuns in comparison to active tardigrades reveals a modest shift, with 16% of the 3322 differentially expressed transcripts having a |log2 fold change| > 1. A gene ontology enrichment analysis shows enrichment within protein homeostasis and neurohormonal signalling, with a ubiquitin-conjugating enzyme and neurotransmitter receptor transcripts being down- and upregulated, respectively. A putative Hsp70 is upregulated, whereas transcripts related to eutardigrade-specific proteins, antioxidant defence and DNA repair show minor fold changes. Among putative membrane transporters, a monocarboxylate and two amino acid transporters are downregulated. Our findings are in agreement with previous studies in Ramazzottius indicating that cryptobiosis and tun formation involve no change or modest change in transcription.