Fig. 2

Illustrations of the expected thermal response patterns in the two species under alternative mechanistic hypotheses of temperature adaptation. Although both temperature extremes were investigated in a similar way, for simplicity only the response to low temperatures is illustrated here. Each column indicates the distribution across all response categories in A. picea, which has a lower CT_min and is therefore better adapted to low temperatures, for the set of transcripts identified as cold-induced (either High or Bimodal categories) in the species with higher CT_min , A. carolinensis, relative to the null hypothesis of equal marginal frequencies. The dashed boxes highlight cells that would indicate matched responses in the two species, and the color of each cell (blue = excess, orange = deficit) represents the deviation of the observed from expected number of transcripts. The Enhanced Response hypothesis (left) proposes that the increase in cold tolerance in A. picea is achieved by amplifying existing molecular mechanisms, and thus there should be an excess of shared response types between species. In contrast, the Tolerance Hypothesis (right) predicts that A. picea is less reliant on induced responses to confer cold-tolerance than A. carolinensis, leading to an excess of shifts from induction in A. carolinensis to the Not Responsive or down-regulation categories in A. picea