Expression plasticity biases the propensity for and direction of
genetic expression divergence
Evolutionary theory and empirical studies suggest that environmentally
induced plasticity may facilitate adaptation to novel environments. To
test this prediction, we first asked whether genes with significant
ancestral expression plasticity were more likely to show genetic
divergence in expression. To compare expression changes upon
colonization of low-predation environments by high-predation fish to
those present after generations of adaptation to low-predation
environments, we examined the extent of overlap in gene sets
differentially expressed between high-predation fish reared with and
without predators and high- versus low-predation fish reared in a
shared, predator-free environment. In both drainages, the DE genes with
significant ancestral plasticity were more likely to show significant
population divergence than expected by chance (Aripo: 264 transcripts,
95% CI for differences in conditional probability of significant
divergence for plastic genes compared to conditional probability of
divergence for non-plastic genes: [0.125, 0.150]; Quare: 409
transcripts, 95% CI: [0.211,0.239]), supporting an association
between plastic responses and genetic divergence.
We next asked whether the directions of expression plasticity and
genetic divergence were associated. We did so for those genes with both
significant ancestral plasticity and population differences, as well as
the larger set of all transcripts with significant population
differences (i.e. including those without significant expression
plasticity). We performed the second, more conservative comparison
because we reasoned that even subthreshold expression plasticity could
influence the direction of adaptive divergence. Within both drainages,
we found a signature of non-adaptive plasticity associated with genetic
expression divergence. Approximately 84% of DE genes exhibited
population and rearing expression changes in non-concordant directions
in the Aripo drainage (95% CI for difference in probability of same
versus opposite expression direction: [-0.564,-0.499]), and 68% of
DE genes exhibiting non-concordant expression in the Quare drainage
(95% CI: [-0.520,-0.456]) (Fig. 3). Patterns were consistent in
both drainages when we considered the larger set of all genes with
significant population expression differences (Aripo: 80% of genes with
non-concordant expression, 95% CI: [-0.811,-0.752]; Quare: 66% of
genes with non-concordant expression, 95% CI: [-0.803,-0.733]).