Isogenic locusts and genetic variability in the effects of temperature on neuronal threshold

Abstract

Physiological variability was previously encountered while studying the effects of temperature on identified motorneurons. Of 22 locusts examined from within the breeding colony, 3 deviant animals were found in which the current threshold of the fast extensor tibiae (FETi) motorneuron did not decrease as was expected with increasing temperature (Fig. 1). We have used clones of isogenic locusts to study the genetic basis, physiological extent, and behavioral effects of a similar abnormal threshold response of FETi to increases in temperature. Over 30 clones, raised by parthenogenetic breeding, were used to isolate naturally-occurring genotypic variability. In this study we have focused our attention on the behaviorally abnormal clone 7 animals that have a low probability of jumping which is not altered by heating; and the behaviorally normal clone 8 animals that have a higher probability of jumping which increases with increasing temperature (Table 1). In clone 8 animals, the physiological properties of FETi show the normal steady-state responses to increases in temperature: the current threshold decreases (Fig. 1), the voltage threshold decreases, and the membrane resistance remains relatively unchanged. In clone 7 animals, the physiological properties of FETi show abnormal steady-state responses to increases in temperature: the current threshold increases (Fig. 1), the voltage threshold remains relatively unchanged (Fig. 2), and the membrane resistance decreases (Fig. 3), all of which are similar to the abnormal responses observed occasionally from the heterogenic breeding colony. These abnormalities in FETi of clone 7 animals are regarded as a neurophysiological correlate of the absence of an increase in jumpiness with increasing temperature. An abnormal response to heating in clone 7 animals is also found in AAdC (Fig. 6), ASFlTi, and the first basalar motorneuron. A normal response to heating in clone 7 animals is found in the spiracle closer motorneurons (Fig. 7), the DUM neurons, and the CI neuron.