A landscape genomic approach on the Cork oak and Holm oak in the Western Mediterranean: insights into the species’ capacity to respond to climate change.

The Mediterranean region has been described as a ‘climate change hotspot’, with increased temperatures and decreased precipitation expected to affect the region in the coming decades. Cork Oak (Quercus suber), and Holm Oak (Quercus ilex and Quercus rotundifolia) are considered keystone tree species of the Western Mediterranean basin. This work uses Genotyping by Sequencing to obtain thousands of SNPs from samples collected throughout the species ranges, which were used (i) to investigate population structure, (ii) to detect evidence of local adaptation and iii) to project the gene-environment relationship across space through Gradient Forest to identify areas more sensitive to climate change. Our results reveal a contrasting population structure and differentiation pattern in Holm Oak and Cork Oak. While the former shows marked structure and differentiation the latter shows much less differentiation. Furthermore, a considerable degree of differentiation is observed between Iberian and Moroccan populations of Q. rotundifolia, but not in Q. suber. For the Iberian populations, both Q. rotundifolia and Q. suber species show a relatively unstructured pattern. Our results support the status of Q. ilex and Q. rotundifolia as two genetically distinct species. Regarding the detection of local adaptation, we identified a considerable number of putative SNPs under selection in both species. Most of these SNPs showed an association with multiple temperature and precipitation-related bioclimatic variables. Annotation of the genomic areas in which the putative SNPs under selection were located, revealed several genes associated with heat and water stress. Finally, Gradient Forest allowed for the identification of geographic areas within the Western Mediterranean region most sensitive to climate change: southwestern Iberia and northern Morocco. Our findings provide a preliminary assessment of the capacity of these species to respond to future climate change and contribute towards a potential management strategy for their conservation in the Mediterranean Basin.