Evolutionary and acclimation-induced variation in the heat-shock responsesof congeneric marine snails (genus Tegula) from different thermal habitats: Implications for limits of thermotolerance and biogeography

Heat stress sufficient to cause cellular damage triggers the heat-shock res ponse, the enhanced expression of a group of molecular chaperones called he at-shock proteins (hsps), We compared the heat-shock responses of four spec ies of marine snails of the genus Tegula that occupy thermal niches differi ng in absolute temperature and range of temperature, We examined the effect s of short-term heat stress and thermal acclimation on the synthesis of hsp s of size classes 90, 77, 70 and 38kDa by measuring incorporation of S-35-l abeled methionine and cysteine into newly synthesized proteins in gill tiss ue, Temperatures at which enhanced synthesis of hsps first occurred (T-on), temperatures of maximal induction of hsp synthesis (T-peak) and temperatur es at which hsp synthesis was heat-inactivated (T-off) were lowest in two l ow-intertidal to subtidal species from the temperate zone, T. brunnea and T . montereyi, intermediate in a mid- to low-intertidal species of the temper ate zone, T. funebralis, and highest in a subtropical intertidal species fr om the Gulf of California, T. rugosa. Synthesis of hsps and other classes o f protein by T. brunnea and T. montereyi was heat-inactivated at temperatur es commonly encountered by T. funebralis during low tides on warm days, In turn, protein synthesis by T.funebralis was blocked at the upper temperatur es of the habitat of T. rugosa. Acclimation of snails to 13 degrees C, 18 d egrees C and 23 degrees C shifted T-on and T-peak for certain hsps, but did not affect T-off, The heat-shock responses of field-acclimatized snails we re generally reduced in comparison with those of laboratory-acclimated snai ls. Overall, despite the occurrence of acclimatory plasticity in their heat -shock responses, genetically fixed differences in T-on, T-peak and Toff ap pear to exist that reflect the separate evolutionary histories of these spe cies and may play important roles in setting their thermal tolerance limits and, thereby, their biogeographic distribution patterns.