Geographical variation in the larval diapause of Chymomyza costata (Diptera; Drosophilidae)

Abstract

Induction of the larval diapause in Chymomyza costata is controlled by two mechanisms, temperature-dependent induction and a photoperiodic response. Geographical, clinal variation was observed in both mechanisms. Critical photoperiods increased northwards 1 h 9 min per 5 degrees of latitude. The cline relaxes north of 65°N, however. The critical temperature measured under long-day conditions varied little in areas south of 63°N, but increased from 13°C at that latitude to 17°C at 65°N and to 24°C at 68°N. Selection experiments revealed extensive intrapopulation variation in both diapause characteristics. The mean temperature in the middle of the breeding season for C. costata is above the long-day critical temperature in areas south of 63°N, the same or slightly lower than the critical temperature in areas between 63°N and 66°N and well below the critical temperature in areas north of 67°N. The relative importance of temperature and photoperiod in diapause induction changes from south to north, the photoperiod being the main determinant in the south and temperature assuming the main role in the north. The forms and positions of the two dines can be explained when data on the estimated number of generations per year are combined with data on weather and illumination conditions at each locality.