Abstract
As climate change progresses, reef-building corals face multiple
environmental stressors that they must adapt to. Can corals adapt to all
these stressors at the same time, or would some adaptations exclude
others (i.e., a genetic tradeoff)? Here, we reanalyzed RNA-seq and
Tag-seq data from four studies that compared gene expression in corals
from tidal pools of different daily heat stress levels, with varying
proximity to a CO2 seep, containing different kinds of
algal symbionts, and separated by 15 degrees of latitude. We computed
both the genetic divergence (FST) and log-fold
expression change per gene for each contrast, and then compared these
results to see whether the same genes were involved in adaptation and/or
acclimatization to different environmental gradients. We find that
genetic divergence patterns are entirely independent from each other
(with the exception of just two shared FST
outliers between the two cases of CO2 adaptation), while
at the gene expression level there is significant overlap in up- and
down-regulated genes among multiple environmental contrasts. These
results suggest that coral populations maximize their local fitness
predominantly via gene expression plasticity (involving similar
pathways) rather than through selection-driven genetic divergence.
Still, there remains the possibility of highly polygenic adaptation with
extensive redundancy (many alternative genetic ways to achieve the same
phenotype). Regardless of the mechanism, the overall lack of tradeoffs
that we observed here suggests that corals have substantial capacity to
withstand multiple simultaneous stressors.