Developmental and environmental regulation of antifreeze proteins in the mealworm beetle Tenebrio molitor

The yellow mealworm beetle, Tenebrio molitor, contains a family of small Cy s-rich and Thr-rich thermal hysteresis proteins that depress the hemolymph freezing point below the melting point by as much as 5.5 degrees C (Delta T = thermal hysteresis). Thermal hysteresis protein expression was evaluated throughout development and after exposure to altered environmental conditi ons. Under favorable growth conditions, small larvae (11-13 mg) had only lo w levels of thermal hysteresis proteins or thermal hysteresis protein messa ge, but these levels increased 10-fold and 18-fold, respectively, by the fi nal larval instar (> 190 mg), resulting in thermal hysteresis > 3 degrees C . Exposure of small larvae (11-13 mg) to 4 weeks of cold (4 degrees C) caus ed an approximate to 20-fold increase in thermal hysteresis protein concent ration, well in excess of the less than threefold developmental increase se en after 4 weeks at 22 degrees C. Exposure of large larvae (100-120 mg) to cold caused 12-fold and sixfold increases in thermal hysteresis protein mes sage and protein levels, respectively, approximately double the maximum lev els they would have attained in the final larval instar at 22 degrees C. Th us, thermal hysteresis increased to similar levels (> 4 degrees C) in the c old, irrespective of the size of the larvae (the overwintering stage). At p upation, thermal hysteresis protein message levels decreased > 20-fold and remained low thereafter, but thermal hysteresis activity decreased much mor e slowly. Exposure to cold did not reverse this decline. Desiccation or sta rvation of larvae had comparable effects to cold exposure, but surprisingly , short daylength photoperiod or total darkness had no effect on either the rmal hysteresis or message levels. As all environmental conditions that cau sed increased thermal hysteresis also inhibited growth, we postulate that d evelopmental arrest is a primary factor in the regulation of T. molitor the rmal hysteresis proteins.