Cold temperature resistance in Drosophila lutescens and D. takahashii.

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Abstract
Differentiation in low temperature resistance was investigated in 2 closely related species, D. lutescens and D. takahashii. Drosophila flies at various deveopmental phases from an egg to an adult fly stage were chilled by low temperature of -3.degree. C for 6 h and the percent of hatching or hatchability for eggs, the percent of emergence for nonadult individuals and the percent of flies remaining alive after the treatment or the survival rate for adult flies were measured. Cold resistance is in general higher in D. lutescens than in D. takahashii. For instance, adult flies 10 days after emergence of D. takahashii were all killed by the chilling, while in D. lutescens nearly all flies of the same age survived the low temperature treatment. The intraspecific variation of cold tolerance in both species was in the order of pupae > aged eggs > young eggs > larvae, leaving the larvae the most susceptible. Cold tolerance of adult flies in D. lutescens showed little variation in relation to aging of flies, but in D. takahashii older flies proved to be more susceptible than younger ones. Genetic investigation by means of the chromosome substitution has proved that the cold tolerance is closely associated with the ratio of D. lutescens chromosomes involved in hybrid flies, suggesting that the relevant genes may be distributed in either sex chromosomes and autosomes of D. lutescens. The successful propagation of D. lutescens in the northern parts of Japan islands in contrast to D. takahashii, distribution of which is strictly limited to the southern parts, may most probably be due to the genetic differences in cold tolerance, mainly of adult flies, during the winter time.