Biogeographic variation in Mytilus galloprovincialis heat shock gene expression across the eastern Pacific range

As modern environments are altered by a range of perturbations and abiotic changes, little is known about the ability of organisms to adapt or acclimatize to these variables on evolutionarily short time scales and across a variety of habitats. To examine acclimatization across environmental conditions, the plasticity of transcriptional activity related to the heat shock response (HSR) was examined in subtidal populations of the mussel Mytilus galloprovincialis at multiple sites along the coast of California, USA. Transcript levels of the constitutive heat shock cognate hsc71 and the inducible isoform hsp70 were observed for field populations at multiple sites along the biogeographic distribution. Field levels of both hsc71 and hsp70 exhibited an inverse relationship with latitude, with the highest transcript abundances in the southern, warm-acclimatized population and lower levels at more northern sites. Up-regulation of the HSR was also characterized for each population via acute temperature exposures across a thermal gradient. In response to these acute thermal challenges, constitutive hsc71 transcripts did not significantly up-regulate, while inducible hsp70 transcripts did increase. The temperature of initial hsp70 induction ( T on) varied with latitude, up-regulating at 17 °C at the northern-most site, 22.5 °C in central populations, and 25.5 °C for the southern population. However the temperature and abundance of maximal transcription ( T max) did not vary between sites. Results demonstrate latitudinal acclimatization in M. galloprovincialis, reveal the capacity of the HSR to exhibit different acclimatization patterns, and support the suggestion that elevated levels of hsp70 and hsc71 heat shock proteins may provide a buffer to low level acute thermal stresses.