Molecular mechanisms of recovery from freezing and supercooling in the antarctica midge, Belgica antarctica

Freeze-tolerance, or the ability to survive internal ice formation, is a relatively rare cold tolerance strategy that facilitates survival in environments with a high risk of freezing. The ability to survive internal ice formation presents many challenges, including physical damage from ice crystals and cellular dehydration, in addition to the general consequences of low temperature. However, the molecular mechanisms that underscore freeze-tolerance are largely uncharacterized. Here, we compared changes in gene expression during recovery from freezing and supercooling at the same temperature in larvae of Belgica antarctica. We found that, despite challenges associated with ice formation, freezing did not result in greater overall levels of differential expression. During recovery from freezing, we observed upregulation of genes involved in cell differentiation, water transport, and response to stress and downregulation of genes involved in ATP production, spermine synthesis, and protein folding. Because low temperatures are associated with the accumulation of harmful metabolites, we hypothesized that recovery from freezing and supercooling would elicit upregulation of antioxidant and detoxification enzymes. Further, because freezing is more stressful, we predicted there would be stronger expression of these genes during recovery in frozen larvae. However, the antioxidant gene catalase was downregulated in both frozen and supercooled larvae during recovery, while numerous cytochrome P450 transcripts were both up- and downregulated, primarily during the later stages of recovery. Taken together, our results indicate that recovery from freezing involves dynamic activation and suppression of specific biological processes, and that the majority of gene expression changes are driven by changes in temperature rather than ice formation per se.