RNA editing generates mRNA isoforms with distinct stabilities that may expand the thermal tolerance of mRNA and proteins in Mytilus species.

Ectothermic organisms may expand their thermal tolerance by producing multiple protein isoforms with differing thermal sensitivities. While such isoforms commonly originate from allelic variation at a single locus (allozymes) or from gene duplication that gives rise to paralogs with distinct thermal responses, this study investigated mRNA editing as an alternative, post-transcriptional mechanism for generating mRNA variants. Cytosolic malate dehydrogenase (cMDH) was examined in foot tissue of two congeners of the marine mussel genus Mytilus, which occupy different thermal environments. Multiple editing events were detected within the mRNA coding region in both species. Editing sites were species-specific, with no shared positions identified. In M. coruscus, editing occurred at 117, 123, 135, 190, 195, 204, 279, and 444, while in M. galloprovincialis, editing was detected at 216 and 597. Each species exhibited multiple edited mRNA variants, and these isoforms were associated with differential protein expression. These findings suggest that mRNA editing may contribute an additional layer of molecular variation. The generation of diverse mRNA isoforms from a single DNA coding sequence may enhance enzymatic flexibility across temperature ranges, supporting eurythermal physiological performance and mitigating thermal stress. Moreover, the presence of multiple edited transcripts within individual organisms raises important caveats about the limitations of approaches that deduce amino acid sequences or estimate adaptive variation solely from genomic data.