HAMDOUN, A. M.; CHERR, G. N.: Phenotypic plasticity of HSP70 and HSP70 gene expression in the Pacific oyster (Crassostrea gigas): Implications for Resting Thermal Limits and Induction of Thermal Tolerance

Heat shock responses are thought to play a major role in determining maximal thermal limits and distribution of sessile intertidal bivalves. We have found that Pacific oysters, Crassostrea gigas, living at a range of tidal heights encounter temperatures ranging from 1 to 45�C, and daily fluctuations in excess of 20�C. Thus, we hypothesized that the thermal limits of oysters are relatively plastic, and that these limits are correlated with changes in HSP70 family expression. We outplanted sibling oysters along a tidal height gradient and monitored the changes in cognate and inducible (HSC/HSP) expression during the progression from spring through winter. We found that the resting levels of HSC77 and HSC72 protein are positively correlated with increases in ambient temperature. The elevated levels of HSC70 family members were also associated with moderate increases (2-3 degrees) in upper thermal limits. More dramatic changes were observed in the threshold temperatures (Ton) for HSP70 family induction. Adaptation to high tidal height, and higher ambient temperatures, resulted in increases of Ton by as much as 9�C. Northern blots of HSC72 mRNA corroborate these findings, and suggest that these changes are a consequence of transcriptional regulation of HSP expression. A potential cost of these changes in Ton is that there was no induction of thermal tolerance after sub-lethal heat shock at temperatures that result in induced thermal tolerance during the winter months. Thus an important deleterious consequence associated with this acclimatory response is a narrowing of the range of sub-lethal temperatures that result in induced thermal tolerance