Cold adaptive potential of chironomids overwintering in a glacial stream

Insects inhabiting cold streams must either tolerate or avoid freezing to survive. The present study reports the strategy adopted by fourth-instar larvae of two chironomid species [Pseudodiamesa branickii (Nowicki) and Diamesa cinerella (Meigen)] overwintering in a glacial stream (in the Italian Alps). The cold adaptive potential of both species under acute cold stress is investigated down to -30°C. Supercooling points, lower lethal temperatures (LLTs), haemolymph thermal hysteresis, whole body content of sugars and polyols, and the expression of heat shock protein (HSP) genes (hsc70 and hsp70) expression are estimated. Comparable thermal hysteresis (>2°C) is measured in the two species, both of which accumulate glucose and sucrose as the main cryoprotectants. According to the supercooling points (=-6.37 and -6.85°C, respectively) and LLT100 (=-16.2 and -14.7°C, respectively), P. branickii and D. cinerella can both be considered as freeze tolerant. However, the cumulative proportion of individual freezing values and the LLT50 (-9.14 and -6.13°C, respectively) suggest that P. branickii is more cold hardy than D. cinerella, whereas the gene expression data (i.e. an absence of up-regulation of hsp70 in D. cinerella) suggest that D. cinerella is more cold hardy than P. branickii. These findings are discussed in relation to the validity of the different metabolic indicators for defining the level of cold hardiness of a species, even in relation to its cold stenothermy. The results are also discussed in relation to climate warming, which represents a serious threat for species from glacier-fed streams.