First, we collected different Forest ant species within the genus Aphaenogaster that live hot to cool places and open (flat woods) vs closed (deciduous forests) habitats. Then, we lab-acclimated them at 25 degrees C, and measured their heat tolerance and stress response with a slow ramp (0.1 C/ min) protocol.
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The stress response (fold induction of Hsp) was analyzed as a reaction norm, or performance curve, or function-valued trait. We fitted a boltzmann function and extracted 3 parameters: slope, inflection point (Tm), and max. We used these parameters as traits themselves to determine how they related with heat tolerance (CTmax).
CTmax itself, measured as loss of righting response, was shaped by both shared ancestry and local environment (panel C). These two seemingly separate forces actually overlap in variance. Without considering phylogeny, we saw a significant effect of habitat type and Tmax (panel B). Including phylogeny masks these effects (Panel A).
In fact, the evolution of CTmax tracks the habitats on the phylogeny of forest ants (Aphaenogaster).
Next, we asked in what way does the stress response (using Hsp expression as a proxy) relate to CTmax? Here are a set of predictions:
Patterns of the stress response reveal greater tolerance and greater defense in more heat tolerant colonies.
And differences in the reaction norms of the stress response correspond to colonies/species living in open vs closed habitats.