Thermal stress causes DNA damage and mortality in a tropical insect

Cold tolerance is considered an important factor determining the geographic distribution of insects. We have previously shown that despite its tropical origin, the cockroach Gromphadorinha coquereliana is capable of surviving exposures to cold. However, the freezing tolerance of this species had not yet been examined. Low temperature is known to alter membrane integrity in insects, but whether chilling or freezing compromises DNA integrity remains a matter of speculation. In the present study, we subjected the G. coquereliana adults to freezing to determine their supercooling point (SCP) and evaluated whether the cockroaches were capable of surviving partial and complete freezing. Next, we conducted single cell gel electrophoresis (SCGE) assays to determine whether heat, cold and freezing altered hemocyte DNA integrity. The SCP of this species was high and around -4.76°C, which is within the typical range of freezing-tolerant species. Most cockroaches survived to 1 day after partial ice formation (20% mortality), but died progressively in the next few days after cold stress (70% mortality after 4 days). One day after complete freezing, most insects died (70% mortality), and after 4 days, 90% of them had succumbed. The SCGE assays showed substantial levels of DNA damage in hemocytes. When cockroaches were heat-stressed, the level of DNA damage was similar to that observed in the freezing treatment, though all heat-stressed insects survived. The present study shows that G. coquereliana can be considered as moderately freeze-tolerant, and that extreme low temperature stress can affect DNA integrity, suggesting that this cockroach may possess an efficient DNA repair system.