DISJUNCTIVE CLINE OF CRITICAL PHOTOPERIOD IN THE REPRODUCTIVE DIAPAUSE OF DROSOPHILA LACERTOSA

Diapause characteristics (e.g., critical photoperiod) show a latitudinal cline and altitudinal variation, both of which are adaptations to the geographical change of seasonal environment (Danilevsky, 1961; Bradshaw, 1976; Saunders, 1976; Beck, 1980). Consequently we are apt to assume that diapause characteristics in each local population are well adapted to the local seasonal environment. However adaptation of local populations may be disrupted by gene flow from neighboring populations which are adapted to different environments. Camin and Ehrlich (1958) maintained that maladapted banding patterns of water snake in the islands of Lake Erie are caused by gene flow from mainland populations. Stearns and Sage (1980) considered that maladaptation in a freshwater population of mosquito fish is due to gene flow from brackish-water populations. To study the effect ofgene flow on diapause characteristics, I examined the clinal variation of critical photoperiod extending across a geographical barrier to gene flow in Drosophila lacertosa. This species is common in Japan (Wakahama, 1963; Beppu, 1980), and has a reproductive diapause induced by short photoperiods (Ichij6 et al., 1980). The Japanese northern island (Hokkaido) and the Japanese main island (Honshu) are separated from each other by Tsugaru Strait, which is more than 20 km wide. This strait is assumed to be an effective barrier to gene flow for drosophilid flies.