Critical thermal maxima in knockdown-selected Drosophila: are thermal endpoints correlated?

Abstract

SUMMARY To explore the correlation of traits linked to thermotolerance, we compared three thermal endpoints (knockdown temperature and two critical thermal maxima) among replicate populations of Drosophila melanogasterselected for high, or low, knockdown temperature. The high knockdown flies maintain normal posture and locomotor ability within a knockdown column at temperatures ⩾40°C, whereas the low knockdown flies fall out of the column at much cooler temperatures (∼35°C, on average). The critical thermal maximum (CTmax) for respiratory control in the selected knockdown populations was determined by analyzing CO2output of individuals during exposure to a temperature ramp (from 30°C to>45°C) and was indicated by an abrupt alteration in the pattern of CO2 release. The CTmax for locomotor function was determined by monitoring activity (concurrent with CO2analysis) during the temperature ramp and was marked by the abrupt cessation of activity. We hypothesized that selection for high knockdown temperature may cause an upward shift in CTmax, whereas selection for low knockdown may lower CTmax. Correlations among the three thermal endpoints varied between the high and low knockdown flies. Finally, we compared metabolic profiles, as well as Q10 values, among the high and low knockdown males and females during the temperature ramp.