Thermotolerance and hsp70 heat shock response in the cold-stenothermal chironomid Pseudodiamesa branickii (NE Italy)

To better understand the physiological capability of cold-stenothermal organisms to survive high temperature stress, we analyzed the thermotolerance limits and the expression level of HSP70 gene under temperature stress of Pseudodiamesa branickii, an alpine stream’s species of Chironomids (Diptera).The effect of short-term high temperature shock on survival was examined in larvae of this species. The lethal temperature (LT100) and the lethal temperature 50% (LT50) for short-term exposure were 36.0 and 32.2°C, respectively. In addition, we tested the effects on survival of long-term shock and the hypothesis that a short pre-exposition to less severe high temperature would generate higher thermotolerance. The duration of the exposure influenced negatively the survival, whereas a prior exposure to a less severe high temperature generated an increase of survival, with a lethal temperature 2°C higher. To point out the molecular basis of this high thermal tolerance, the expression of the HSP70 gene family, including the constitutive (HSC70) and the inducible (HSP70) forms, was surveyed via quantitative RT-PCR after short term and long term exposure to high temperature. Larvae of P. branickii showed significant up-regulation of both members of the HSP70 family expression in response to increased temperatures, whereas in response to increased time of exposure a decrease of the expression level was underlined. Whereas previous studies have shown that cold-stenothermal Antarctic organisms lack the classical heat shock response, our results indicate that this cold-stenothermal alpine chironomid maintained an inducible heat shock response at a level potentially set during its temperate evolutionary past.