Beyond cyanogenesis: Temperature gradients drive environmental
adaptation in North American white clover (Trifolium repens L.)
Abstract
Species that repeatedly evolve phenotypic clines across environmental
gradients have been highlighted as ideal systems for characterizing the
genomic basis of local environmental adaptation. However, few studies
have assessed the importance of observed phenotypic clines for local
adaptation: conspicuous traits that vary clinally may not necessarily be
the most critical in determining local fitness. The present study was
designed to fill this gap, using a plant species characterized by
repeatedly-evolved adaptive phenotypic clines. White clover is naturally
polymorphic for its chemical defense cyanogenesis; climate-associated
cyanogenesis clines have evolved throughout its native and introduced
range worldwide. We performed landscape genomic analyses on 415 wild
genotypes from 43 locations spanning much of the North American species
range to assess the relative importance of cyanogenesis loci vs. other
genomic factors in local climatic adaptation. We find clear evidence of
local adaptation, with temperature-related climatic variables best
describing genome-wide differentiation between sampling locations.
However, landscape genomic analyses indicate no significant contribution
of cyanogenesis loci to local adaptation. Instead, several genomic
regions containing promising candidate genes for plant response to
seasonal cues are identified — some of which are shared with
previously-identified QTLs for locally-adaptive fitness traits in North
American white clover. Our findings suggest that local adaptation in
white clover is likely determined primarily by genes controlling the
timing of growth and flowering in response to local seasonal cues. More
generally, this work suggests that caution is warranted when considering
the importance of conspicuous phenotypic clines as primary determinants
of local adaptation.