Phenotypic and genomic insights into population differentiation,
introgression, and selection in Quercus rubra across a narrow but
steep environmental gradient
Abstract
Adaptive differentiation in functional traits and their underlying loci
can occur across a small geographic area if natural selection is
stronger than the countervailing effects of gene flow and drift. We
investigated this hypothesis in a long-lived, wind-pollinated species,
Quercus rubra, across a fine spatial scale with a steep climate
gradient. We examined phenotypic differentiation in a common garden
study with eight populations sampled 0-160 km from the coast of Lake
Superior. We estimated genomic differentiation for these and 22
additional populations from the same region, along with two populations
of a congener, Quercus ellipsoidalis, using RAD-seq. We found a
strong signal of population differentiation associated with climate in
the common garden study, and differentiation was significantly
associated with at least one climate factor for nine of ten measured
traits. At the genomic level, we discovered widespread introgression
from Q. ellipsoidalis into Q. rubra that increased with
distance from the lake. Pairwise FST among Q.
rubra populations was low, but both distance-based and environmental
association analyses identified loci under selection, with one locus in
common across all analyses (CalS10/GSL8). This locus was
associated with the precipitation of the driest month, a climate factor
that was also significant in the common garden analyses. In sum, this
study reveals signatures of selection at the phenotypic and genomic
level consistent with climate adaptation, a pattern that is usually seen
across a much broader geographic scale.