Cell-cycle genes in the mussel Mytilus galloprovincialis are cued by aerial heat stress in a fluctuating tidal environment.

Mytilus galloprovincialis is a sessile, facultative anaerobic mussel inhabiting intertidal zones, where it experiences daily fluctuations in oxygen, temperature, and food availability. These environmental fluctuations require coordinated shifts in metabolism and cellular processes such as the cell cycle. As growth in mussels is environmentally driven, understanding how external perturbations modulate cell-cycle regulation is essential. Despite conserved cell-cycle machinery across eukaryotes, its dynamics in facultative anaerobic marine invertebrates remain largely unexplored. Mussels switch between aerobic (high-tide submergence) and anaerobic (low-tide emergence) respiration over tidal fluctuations. We hypothesized that cell-cycle gene expression (cyclins) is synchronized with tidal cues, peaking during anaerobic metabolism (low tide) in control-acclimated mussels to avoid DNA damage from oxidative stress during aerobic metabolism. Alternatively, peak expression would occur during submergence after experiencing acute heat stress in air to avoid higher risk of DNA damage by aerial heat compared to damage by oxidative stress in cool water. We also predicted higher cyclin transcript abundance in heat-stressed mussels compared to control mussels. To test these inferences, we exposed mussels to simulated tidal conditions and measured gene expression of cyclins and stress markers. We performed an RNA/DNA ratio assay as a proxy for short-term growth. As predicted, acute heat-stressed mussels exhibited significantly elevated cyclin transcript abundance compared to control mussels. Unexpectedly, cyclin D and E transcripts were upregulated during low tide in the heat-stressed mussels, while peak cyclin B expression was delayed until post-stress resubmergence. Furthermore, we did not observe significant changes in RNA/DNA ratio across the treatments. Our findings demonstrate that external environmental cues can modulate the timing and magnitude of cell-cycle gene expression in M. galloprovincialis, advancing our understanding of growth regulation in facultative anaerobes. We also assessed the potential of cyclin gene expression regulation through RNA secondary structure (RNA thermometer).