Insect cold-tolerance and lipidome: Membrane lipid composition of two chironomid species differently adapted to cold.

Cell-membrane fluidity is a fundamental parameter in cold resistance. It is regulated by a fine tuning of lipid composition, usually involving a great chemical diversity among head-groups, chain lengths, and degree of unsaturation. To give new insights on Alpine chironomid cold adaptation, we analysed the lipid membrane composition of Diamesa tonsa and Pseudodiamesa branickii, two species known to have different cold-tolerance, stronger in the former. Membrane lipid composition was analysed by NMR and HPLC-MS in larvae under natural (4 °C) and laboratory conditions (30 min at - 4 °C). In both species the major class of membrane lipids were phosphatidylethanolamine (PE), reaching 93% in D. tonsa and 80% in P. branickii, followed by a minor relative amount of phosphatidylcholine (PC). Phospholipids (PL) acyl chains were highly unsaturated given the presence of a relevant amount of polyunsaturated fatty acid (PUFA), among which a high proportion of ω-3 chains. This study demonstrated that these species have a similar lipidome (e.g. relevant amount of PUFA and predominance of PE), but with relevant differences on which to base different membrane fluidity: (i) a higher unsaturation index and chain length of both PE and PC and a higher ratio PE/PC ratio in D. tonsa than in P. branickii; (ii) the absence of modifications in the lipid composition in D. tonsa under sub-zero temperature. These differences might support the different cold-tolerance of the two species. In fact, we suggest that the high PE/PC ratio and the low sterols content (as in D. tonsa) could be involved in the formation of highly deformable membranes increasing their capacity to survive freezing. Interestingly, LC-MS analysis of D. tonsa lipidome revealed a new class of lipids that we named 'PpC', absent in P. branickii, that is worth investigating.