Variation in hsp70 levels and thermotolerance among terrestrial salamanders

The Plethodon glutinosus complex is a geographically widespread salamander group inhabiting forests throughout the eastern United States and is locally abundant throughout the coastal plain. As such it is exposed to a variety of climatic conditions and makes the perfect candidate group for examining large-scale variation in thermal tolerance limits and heat shock protein expression. Thirteen populations of the P. glutinosus complex representing three physiographic regions were examined for variation in thermotolerant capabilities along environmental gradients. Measures of thermotolerance included the preferred temperature (PT), critical thermal maximum (CTM), and levels of heat shock protein 70 (hsp70) as detected by ELISA. Within the P. glutinosus complex there was an unexpected inverse relationship between the stress response and climate. While salamanders exhibited a significant amount of geographic variation in PT and CTM, there was a significant negative relationship between CTM and both geographic location and climate - salamanders from warm, southern climates had lower CTMs then salamanders from northern, cool climates. Hsp70 levels varied significantly among populations and physiographic regions but not among heat shocked or control treatment groups. In addition, piedmont populations contained significantly higher levels of hsp70 than either coastal plain or montane populations. Patterns observed in the stress response of the P. glutinosus complex reflect a "constitutive" based heat shock strategy. This strategy acknowledges that hsp70 induction incurs a fitness cost and that organisms from both relatively stable and highly disturbed habitats should trade-off inducible thermotolerance for high levels of constitutive protein. This strategy predicts that organisms from habitats with intermediate levels of disturbance would have the highest levels of inducible protein. However, because this confers a potentially high energetic cost, selection should ultimately push the population towards one side of the model or the other. Salamanders occupy relatively stable habitats on a global scale of disturbance, but even within these types of habitats trade-offs are evident. Piedmont salamanders which occupy the intermediately disturbed habitats had hsp70 levels that exceeded levels observed in both montane and coastal plain populations. Coastal plain populations behaviorally avoid stressful conditions and hence have stress responses similar to the lower stress montane populations.