CIURA, S.M.; BIBEAU, M.R.; SCHULTE, P.M.*: Intraspecific variation in heat shock proteins and thermal tolerance
Populations of Fundulus heteroclitus are distributed through a thermal cline along the Atlantic Coast of North America, from Newfoundland to Florida. At the northern end of the species range, fish experience average annual temperatures ~12 degrees cooler than their southern counterparts. To determine the maximum critical temperature (CT max) of fish from the northern and southern populations, fish were acclimated to common conditions of 20 degrees celcius in the laboratory. These fish were exposed to increasing water temperature at a rate of 0.3 degrees per minute until they were unable to maintain their orientation. CT maxwas significantly higher in the southern population (p<0.01). To address the molecular basis of this difference, we have cloned and sequenced the 70 kD heat shock protein and heat shock cognate from both populations. There are several amino acid differences between the two genotypes, which could be functionally important. We have also examined circadian variation in thermal tolerance. In fish held at short-day photoperiod, but at constant temperature, there is a diurnal rhythm in CTmax, with thermal tolerance at noon being significantly higher than early morning and evening values. In contrast, fish held at a long-day photoperiod do not exhibit this diurnal rhythm. Values of CT max for these fish are uniformly high. The diurnal rhythm at short-day photoperiod could represent a strategy for coping with daily variation in temperature during the spring and fall in the natural habitat.