Temperature Tolerance and Acclimatization in Drosophila Subobscura

INTRODUCTION In the course of a study of hybrid vigour in Drosophila subobscura it was desired to investigate the effects of inbreeding and of outcrossing on a character which can undergo adaptive modification during development. Since it was known from the work of Mellanby (1939, 1954) that some insects can become acclimatized to changes in temperature, it was decided to study this capacity in D. subobscura. It was found that acclimatization does take place, and that its extent is greater in outbred than in inbred flies (Maynard Smith, 1956). In the course of this investigation some results were obtained of interest to physiologists rather than to geneticists, and these are reported here. Drosophila has certain advantages as a subject for research in insect physiology. First, a number of inbred lines exist, and by crossing such lines vigorous and phenotypically uniform first generation hybrids are obtained. Using such hybrids, variations due to genetic differences between individuals can be reduced, with a corresponding gain in the accuracy with which the effects of changed environmental conditions can be measured. Secondly, the generation time is short (about one month in subobscura), and the methods of laboratory culture have been improved over many years, so that it is relatively easy to determine the influences of changed conditions at various stages of the life cycle. It has therefore been possible to obtain rather accurate estimates of the time for which flies can survive at various temperatures in dry and in saturated air, and to study the reversibility of the changes occurring at high temperatures. The most striking feature of these results has been the very high temperature coefficients obtained for the processes causing death both in dry and in wet air. Acclimatization has been found to take two forms. The survival time at high temperatures depends not only on the temperature at which adults have been kept immediately before testing, but also on the temperature at which they were raised during pre-adult life. Thus the temperature during pre-adult life has a long-lasting effect on the capacity of adults to withstand exposure to high temperatures.