Host plant domestication facilitates rapid population growth and regional adaptation in a oligolectic bee pollinator

The domestication of plant species for agricultural purposes has resulted in some of the clearest examples of how strong selective pressures can dramatically alter genomic architecture over short time scales. However, cultivated plants do not exist in a vacuum: they interact with a multitude of insect associates, with which they may share evolutionary histories that predate domestication. For these insect associates, the transition from wild to domesticated hosts can represent a radical alteration of ecological conditions, especially when agriculture facilitates rapid geographic expansion outside of their original range. This is the case for the hoary squash bee Eucera pruinosa, part of a clade that has specialized to feed on the pollen of Cucurbita species (squashes/pumpkins), and thus has had an evolutionary history that is uniquely dependent upon the progress of human agriculture in North America. We sequenced genomes from across E. pruinosa's range, to look for evidence of demographic and adaptive transitions that may have occurred as a result of increasing intensity of Cucurbita cultivation over the past several thousand years. We find that the diversification of E. pruinosa resembles that of other organisms that moved northward in the wake of climatic shifts at the end of the Pliestocene; but is followed by a range-wide, superexponential increase in population size that coincides with the domestication of C. pepo spp ovifera (c. 4-5 kya). Further, we find evidence of adaptive evolution only in a lineage that has expanded into the Eastern USA and subsists solely on cultivated Cucurbita.