Résumé
1- Climate influences population genetic variation of marine
species. Capturing those impacts remains challenging for marine
fishes dispersing over a large geographic scale
spanning steep environmental gradients. This requires an
extensive spatial sampling of individuals or populations, representative
of seascape heterogeneity, combined with a set
of highly informative molecular markers able to
reveal climatic-associated genomic variations. 2- We explored how
space, dispersal and environment shape the genomic patterns of two marine
fish species. We hypothesized that population structure
and climate-associated genomic signatures of
selection will be stronger in the less mobile species, as
restricted gene flow tends to facilitate the fixation of locally adapted
alleles. 3- To investigate our hypothesis, we genotyped two
species that share a common environment but have contrasting dispersal
abilities, the white seabream (Diplodus sargus) and striped red mullet
(Mullus surmuletus). We collected 823 samples across the Mediterranean
Sea, which ranks among the oceanic basins the most affected by climate
change and human pressures. We used genotyping by sequencing
(GBS) to detect 8 206 Single Nucleotides Polymorphisms (SNPs) for seabream
and 2 794 for mullet. For each species, we identified
highly differentiated genomic regions and disentangled the relative
contribution of space, dispersal
and environmental (climate, productivity) variables on
genetic structure to test the prevalence of gene flow and local
adaptation. 4- We observed contrasted patterns of gene flow
and adaptive genetic variation between the two species.
The seabream showed a separate Alboran sea population but further
panmictia across the Mediterranean Sea, while the mullet
revealed additional differentiation. The
within-Mediterranean differentiation of mullet was significantly
correlated to summer – and winter temperatures as well as
productivity. Functional annotation
of climate-associated
outlier SNPs then identified candidate
genes involved in heat tolerance that could be examined to
further predict species’ responses to climate change. 5- Our results
illustrate the key steps of a comparative seascape genomics study that
aims to unravel the evolutionary processes at play in marine species, in
order to better anticipate their response to climate change. Defining
population adaptation capacities and climatic niches can then serve
to include complementary evolutionary processes
in species conservation planning.