Supplementary material from "Evolutionary trade-offs between baseline and plastic gene expression in two congeneric oyster species"

Organismal responses to environmental stresses are a determinant of the effect of climate change. These can occur through the regulation of gene expression, involving genetic adaptation and plastic changes as evolutionary strategy. Heat shock protein (hsp) family genes are extensively expanded and play important roles in thermal adaptation in oysters. We investigated expression of all heat-responsive hsps in two allopatric congeneric oyster species, Crassostrea gigas and C. angulata, which are respectively distributed along the northern and southern coasts of China, using common garden and reciprocal transplant experiments. Our results showed that hsps in C. gigas have evolved higher basal levels of expression under ambient conditions at each field site, with lower expression plasticity in response to heat stress in comparison to C. angulata, which exhibited lower baseline expression but higher expression plasticity. This pattern was fixed regardless of environmental disturbance, potentially implying genetic assimilation. Our findings indicate divergent adaptive strategies with underlying evolutionary trade-offs between genetic adaptation and plasticity at the molecular level in two oyster congeners in the face of rapid climate change.