Genomic signatures of adaptation across a precipitation gradient from niche center to niche edge

Evaluating the potential for species to adapt to changing climates relies
on understanding current patterns of adaptive variation and selection,
which might vary in intensity across a species’ niche, hence affecting our
inference of where adaptation might be most important in the future. Here,
we investigate the genetic basis of adaptation in Lactuca serriola along a
steep precipitation gradient in Israel approaching the species’ arid niche
limit and use candidate loci to inform predictions of its past and future
adaptive evolution. Environmental association analyses combined with
generalized dissimilarity models revealed 108 candidate genes showing
non-linear shifts in allele frequencies across the gradient, with 66% of
these genes under strong selection near the dry niche edge. We detected
selection acting on genes with separate suites of biological functions,
specifically related to phenology and responses to environmental
stressors, including osmotic stress, at the dry niche edge, and related to
biotic interactions and defense closer to the niche center. The adaptive
genetic composition of populations, as inferred through polygenic risk
scores, point to intensified selection operating towards the dry niche
edge. However, inference of past and future evolutionary change predicts
larger adaptive shifts occurring in the mesic part of the range, which is
most affected by climate change. Our study reveals that adaptive shifts in
response to climate change can be heterogeneous across a species'
range and not necessarily strongest near its niche edge.