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Environmental correlates of SNP variation across Quercus robur populations from the species’ southern range margin

Increasing drought stress driven by ongoing climate change has been identified as a major threat for the performance and persistence of Pedunculate oak (Quercus robur) forests in the southern and southeastern parts of the species’ distribution range. The potential of oak populations to cope with a rapidly changing environment has important ecological and management implications. It is assumed to depend largely on their standing genetic variation. We investigated variation at 160 SNPs located in candidate genes broadly related to abiotic stresses and bud phenology in Quercus robur populations residing near the species’ southern range margin in Croatia, to explore possible effects of local adaptation and their intrinsic capacity to cope with predicted climatic trends at the molecular level. We used complementary methods in order to test for possible natural selection effects: model-based FST outlier approaches and a landscape genomic approach testing environmental associations of SNPallele frequencies. FST-based tests revealed 8 outlier candidate SNPs showing a divergence on average 6-7 times stronger than the rest of the markers, while 37 SNPs (of which 6 outliers) showed significant environmental associations. Among them, 34 SNPs (including 3 outliers) were significantly related to one or several environmental predictors using a latent factor mixed model, while BayeScEnv detected 3 SNPs among the previous outliers that were associated with two or more environmental variables. According to identified associations, important environmental drivers of potentially adaptive population divergence are suggested to be precipitation seasonality, degree-days below 0 °C and Hargreaves reference evaporation (Eref). The overall genetic differentiation among populations being very low (overall FST ~ 0.013) and the genetic clustering being weak or not significant (depending on the method used), we discuss the biological significance of markers linked to environmental variables representative of regional-scale gradients of precipitation, temperature and aridity. Possible benefits of developing genetically informed guidelines for a climate-change integrated conservation and management of these marginal Pedunculate oak forests are also outlined.

genome–environment association; landscape genomics; local adaptation; Pedunculate oak

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