Signatures of natural selection in a foundation tree along Mediterranean climatic gradients

Abstract

Temperature and precipitation regimes are rapidly changing, resulting in forest dieback and local extinction events, particularly in Mediterranean-type climates. Strategic forest management approaches that enhance forests’ resilience to future climates are urgently required, however adaptation to climates in heterogeneous landscapes with multiple selection pressures may be complex. For widespread trees in Mediterranean-type climates we hypothesized that patterns of local adaptation are associated with climate; precipitation is a stronger factor of adaptation than temperature; functionally related genes show similar signatures of adaptation; and adaptive variants are independently sorting across the landscape. To test our hypotheses, we sampled 28 populations across the geographic and climatic distribution of Eucalyptus marginata (jarrah), in south-west Western Australia, and obtained 13,534 independent single nucleotide polymorphic (SNP) markers across the genome. While overall levels of population differentiation were low (FST=0.04), environmental association analyses found a total of 2,336 unique SNPs potentially associated with five climate variables of temperature and precipitation. Allelic turnover was identified for SNPs associated with temperate seasonality and mean precipitation of the warmest quarter (39.2% and 36.9% deviance explained, respectively), suggesting that both temperature and precipitation are important factors in adaptation. SNPs within similarly function genes, according to gene ontology enrichment analysis, had analogous allelic turnover along climate gradients, while SNPs among temperature and precipitation variables had orthogonal patterns of adaptation. These contrasting patterns of adaptation provide evidence that there may be standing genomic variation adapted to changing climates, providing the substrate needed to promote adaptive management strategies to bolster forest resilience in the future.

Genetic offset and vulnerability modelling: misinterpretations of results and violations of evolutionary principles

“…temperature, precipitation, altitude); however, other factors correlated with these variables (abiotic and/or biotic) may be the actual driving force of selection (Rellstab et al, 2015). This is pertinent in complex landscapes where selection processes can be multidimensional (White & Butlin, 2021;Filipe et al, 2022;Lotterhos, 2023). Consequently, estimating GOs and predicting risks of maladaptation are problematic when the true driver(s) of selection are concealed.…”