Adaptive abilities of the Mediterranean red coral Corallium rubrum in a heterogeneous and changing environment: from population to functional genetics

Mediterranean benthic ecosystems have been deeply impacted by thermal anomalies during the last decades. Adaptive capacities for marine species facing climate change can include individual acclimatization (during the individual life-span) and genetic selection (considered at the population level). The Mediterranean red coral, Corallium rubrum, is well suited to study adaptive evolution in heterogeneous environment. This is a sessile species, which inhabits contrasted environments with a strong genetic structure, and differential responses to thermal stress between populations. This study proposes an integrative study of the adaptive response of C. rubrum to thermal stress. To understand the underlying mechanisms of thermal adaptation in this species, we studied three populations of C. rubrum from different depths (5 m, 20 m and 40 m depths) and therefore different thermal regimes in the same area. We first surveyed in situ the thermal environment and the corresponding stress levels at the different depths studied here. Then we submitted red coral colonies to different heat shocks in aquaria that mimicked in situ stresses (common garden conditions). We measured the expression levels of several candidate genes. Heat shock protein 70 (HSP 70) showed significant differences of expression depending on the depth of origin of the individuals and of their thermal history. Based on a complementary analysis of genetic structure, our study shows the evolution of a differential response at a local scale which might be explained by local adaptation or acclimatization. Our results also underline the trade-off between fitness and potential deleterious consequences linked to heat stress response. It also strongly emphasizes the conservation value of populations living at the edge of the species' range, as they represent an irreplaceable genetic pool for evolutionary rescue.