Evolving Gene Expression Plasticity Under Climate Change: A Case Study of Thermal Adaptation in the Invasive Tunicate <scp> <i>Herdmania momus</i> </scp>

Phenotypic plasticity provides organisms with immediate resilience to environmental variability, yet its evolutionary trajectories and long-term role in adaptation under climate change remain unresolved. The invasive ascidian Herdmania momus, originating from the Red Sea and expanding into the rapidly warming and environmentally variable Mediterranean Sea, provides an ideal natural model for examining how gene expression plasticity evolves under accelerating climate change. By comparing gene expression plasticity of H. momus derived from native and invasive populations under temperature stress, we investigated the evolutionary trajectories of gene expression plasticity during the early stages of biological invasion. Our results reveal widespread transcriptional shifts and pronounced regional differences in plastic responses, indicating that gene expression plasticity can evolve rapidly following recent colonisation. Invasive Mediterranean populations exhibited reduced plasticity under both heat and cold stress. Genes associated with energy metabolism displayed consistent upregulation in both native and invasive ranges, underscoring their conserved role in thermal adaptation. Reaction norm analyses revealed that front-loading, characterised by elevated baseline expression but reduced plasticity, was the predominant pattern in Mediterranean populations, followed by high plasticity, dampening and amplifying responses. Notably, front-loading was enriched in genes involved in cellular stress responses, Sterol Regulatory Element-Binding Protein (SREBP) signalling and protein ubiquitination, suggesting that the evolution of plasticity should be function-dependent during rapid colonisation of changing climates. These findings shed light on the role of phenotypic plasticity in shaping adaptive evolution during biological invasions and in the broader context of climate change.