Consequently we are apt to assume that diapause characteristics in each local population are well adapted to the local seasonal environment. However adaptation oflocal 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. Steams 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 of gene flow on diapause characteristics, I examined the dina! 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 eta!., 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.
The ground cricket Dianemobius nigrofasciatus overwinters as an egg in Japan, being univoltine in Hokkaido and northern Honshu and bivoltine farther south. In Hokkaido, however, this cricket is heard singing in winter in several fumarolic fields covered with moss and grasses locally known as “bokke”. In such warm “islets” the adult density was high in early summer and again in autumn, indicating that the cricket is bivoltine in contrast to the univoltine life cycle outside the bokke habitats in Hokkaido. Eggs laid by females collected at regular intervals from a bokke habitat showed a clear seasonal cycle of diapause incidence. At 26°C, the bokke strains produced non‐diapause eggs under long days and diapause eggs under short days as in the southern bivoltine populations, although the critical day‐length was longer than in the south. Several strains derived from non‐bokke habitats in Hokkaido and northern Honshu produced high percentages of diapause eggs under long days as well as short days as expected for the univoltine life cycle. Winter adults singing in bokke habitats could be either survivors of the autumn generation or individuals derived from eggs laid in autumn and then matured in response to the high soil temperature. In the laboratory, the proportion of egg diapause in short days was decreased by selection only for several generations. Phylogenetic trees of bokke and non‐bokke populations, based on both the nucleotide sequence of the mitochondrial COI gene and four allozyme loci, suggest that bokke populations have not been isolated from non‐bokke populations for an evolutionarily significant time.
Drosophila moriwakii Okada & Kurokawa (Diptera; Drosophilidae) generally has only one generation per year and enters aestivo‐hibernal reproductive diapause in Sapporo, northern Japan, but a small fraction of the population produced a second generation in summer at a place where breeding resources were abundant. In this species, diapause seems to be controlled by flight activity. When flies were cultured in cages in which they were able to fly freely, they entered diapause irrespective of photoperiod, but they did not do so at long daylengths when cultured in small vials in which they were prevented from flying. Furthermore, flies with wings removed did not enter diapause at long daylengths even if they were cultured in the cages.
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