Additive and mostly adaptive plastic responses of gene expression to multiple stress in Tribolium castaneum

Gene expression is known to be highly responsive to the environment and
important for adjustment of metabolism but there is also growing evidence
that differences in gene regulation contribute to species divergence and
differences among locally adapted populations. However, most studies so
far investigated populations when divergence had already occurred.
Selection acting on expression levels at the onset of adaptation to an
environmental change has not been characterized. Understanding the
mechanisms is further complicated by the fact that environmental change is
often multivariate, meaning that organisms are exposed to multiple
stressors simultaneously with potentially interactive effects. Here we use
a novel approach by combining fitness and whole-transcriptome data in a
large-scale experiment to investigate responses to drought, heat and their
combination in Tribolium castaneum . We found that fitness was reduced by
both stressors and their combined effect was almost additive. Expression
data showed that stressor responses were acting independently and did not
interfere physiologically.  Since we measured expression and
fitness within the same individuals, we were able to estimate selection on
gene expression levels. We found that variation in fitness can be
attributed to gene expression variation and that selection pressures were
environment dependent and opposite between control and stress conditions.
We could further show that plastic responses of expression were largely
adaptive, i.e.  in the direction that should increase fitness.