GEOGRAPHICAL VARIATION IN THE REPRODUCTIVE CYCLE AND PHOTOPERIODIC DIAPAUSE OF DROSOPHILA PHALERATA AND D. TRANSVERSA (DROSOPHILIDAE:DIPTERA).

Abstract

Adaptation to seasonal changes is of great importance to insects. The energyconsuming developmental processes are limited in the north by winter and often in the south by summer drought. Many insects spend the adverse seasons in a state of dormancy controlled by photoperiod (Beck, 1968). Geographical variation in photoperiodic reactions is well known in insects (see reviews by Danilevskii, 1965; Beck, 1968; Saunders, 1976; Hoy, 1978). In general, a clear south-north cline is observed, with northern populations entering the hibernation stage earlier than southern. Such geographical variation in Drosophila littoralis has been studied by Lumme and Oikarinen (1977) and Oikarinen and Lumme (1979) (see also Lumme, 1978). However, field studies corresponding to laboratory results have been lacking. We undertook to study the adaptation to seasonal environment by relating field observations on population dynamics to the laboratory determined photoperiodic reactions. We studied populations at different latitudes to find out how different climatic conditions are reflected in the population dynamics in the field, and, on the other hand, in photoperiodic reaction curves. Photoperiodic timing of adult reproductive diapause in the genus Drosophila was first demonstrated in D. phalerata Meigen and D. transversa Fallen (Geyspits and Simonenko, 1970). From photoperiodic reaction curves obtained in laboratory studies, Geyspits et al. (1976) inferred that D. phalerata must have a summer diapause as well as a winter one. Geyspits and Simonenko (1970) studied populations of D. phalerata from two different latitudes and found diapause differences between them. Both species are quite common and easily collected with fermenting baits. Both are known to be fungivorous (Burla and Bachli, 1968). Their distributions overlap widely, but D. phalerata reaches its ecological margin in Central Finland. It is more common in central and southern parts of Europe (Shorrocks, 1977), but its southern limit is not well known. Drosophila transversa is abundant up to the coast of the Arctic Ocean, but rather rare in Central Europe, where it is mostly found at high altitudes (Burla, 1951; Shorrocks, 1977). These species thus obviously have different adaptations to climate. Can this be seen in their phenology or photoperiodic reactions?