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.