Thermal tolerance of larvae of seven Chironomus species and up-regulation of heat shock protein-coding genes in Chironomus sulfurosus

Abstract Global warming directly and indirectly affects the biology and ecology of aquatic insects. Chironomids are ubiquitous on the earth and are suitable for systematic analysis of the warming effect on insects. Here, we measured the survival rate for the fourth-instar larvae of seven Chironomus species at different temperatures. The mid-temperature for survival after 1-h temperature exposure to Chironomus sulfurosus larvae was the highest (43 °C) among seven Chironomus species. In addition, 14.5% of C. sulfurosus larvae survived at 40 °C from the first to the fourth instar for 14 days, whereas larvae of the other three species with relatively high thermal tolerance in the 1-h exposure test could not. These results indicate that only C. sulfurosus larvae are tolerant of 40 °C. We examined the expression levels of specific genes encoding four heat shock proteins and one heat shock protein cognate in C. sulfurosus larvae at various temperatures using quantitative real-time PCR. The expression levels of genes for hsc70 , hsp67, hsp60, hsp27, and hsp23 genes were higher at 40 °C than those at 35 °C, at which 97% of larvae survived for 14 days. Therefore, in conjunction with the present phylogenetic analysis, the thermal tolerance of C. sulfurosus is associated with the up-regulated hsp and hsc genes, which may underlie a mechanism for the organism’s response to high temperatures.