title: Thermal reactionomes reveal divergent responses to thermal extremes in warm and cool-climate ant species.
abstract: BACKGROUND
  The distributions of species and their responses to climate change are in part determined by their thermal tolerances. However, little is known about how thermal tolerance evolves. To test whether evolutionary extension of thermal limits is accomplished through enhanced cellular stress response (enhanced response), constitutively elevated expression of protective genes (genetic assimilation) or a shift from damage resistance to passive mechanisms of thermal stability (tolerance), we conducted an analysis of the reactionome: the reaction norm for all genes in an organism's transcriptome measured across an experimental gradient. We characterized thermal reactionomes of two common ant species in the eastern U.S, the northern cool-climate Aphaenogaster picea and the southern warm-climate Aphaenogaster carolinensis, across 12 temperatures that spanned their entire thermal breadth.
  
  
  RESULTS
  We found that at least 2 % of all genes changed expression with temperature. The majority of upregulation was specific to exposure to low temperatures. The cool-adapted A. picea induced expression of more genes in response to extreme temperatures than did A. carolinensis, consistent with the enhanced response hypothesis. In contrast, under high temperatures the warm-adapted A. carolinensis downregulated many of the genes upregulated in A. picea, and required more extreme temperatures to induce down-regulation in gene expression, consistent with the tolerance hypothesis. We found no evidence for a trade-off between constitutive and inducible gene expression as predicted by the genetic assimilation hypothesis.
  
  
  CONCLUSIONS
  These results suggest that increases in upper thermal limits may require an evolutionary shift in response mechanism away from damage repair toward tolerance and prevention.
authors.affiliation: Department of Biology, University of Vermont, Burlington, VT, 05405, USA. john.stantongeddes.research@gmail.com.|Data Scientist, Dealer.com, 1 Howard St, Burlington, VT, 05401, USA. john.stantongeddes.research@gmail.com.
authors.forename: John
authors.initials: J
authors.lastname: Stanton-Geddes
authors.affiliation: Department of Biology, University of Vermont, Burlington, VT, 05405, USA.
authors.forename: Andrew
authors.initials: A
authors.lastname: Nguyen
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN, 37996, USA.
authors.forename: Lacy
authors.initials: L
authors.lastname: Chick
authors.affiliation: Vermont Genetics Network, University of Vermont, Burlington, VT, 05405, USA.
authors.forename: James
authors.initials: J
authors.lastname: Vincent
authors.affiliation: Vermont Genetics Network, University of Vermont, Burlington, VT, 05405, USA.
authors.forename: Mahesh
authors.initials: M
authors.lastname: Vangala
authors.affiliation: Department of Biological Sciences, North Carolina State University, Raleigh, NC, 27695, USA.
authors.forename: Robert R
authors.initials: RR
authors.lastname: Dunn
authors.affiliation: Harvard Forest, Harvard University, Petersham, MA, 01336, USA.
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Ellison
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN, 37996, USA.|Center for Macroecology, Evolution and Climate, University of Copenhagen, Universitetsparken 15, DK-2100, Copenhagen, Denmark.
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: Sanders
authors.affiliation: Department of Biology, University of Vermont, Burlington, VT, 05405, USA.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Gotelli
authors.affiliation: Department of Biology, University of Vermont, Burlington, VT, 05405, USA.
authors.forename: Sara Helms
authors.initials: SH
authors.lastname: Cahan
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics*; D000818:Animals; D001000:Ants / Q000235:genetics*; D005075:Biological Evolution; D002980:Climate; D003080:Cold Temperature; D005786:Gene Expression Regulation; D017344:Genes, Insect; D006358:Hot Temperature; D013045:Species Specificity; D059467:Transcriptome; D014481:United States