Diapause induction as an interplay between seasonal token stimuli, and modifying and directly limiting factors: hibernation in Chymomyza costata

Larvae of wild type (WT) strain of Chymomyza costata Zetterstedt (Diptera: Drosophilidae) enter diapause (stop developing) in response to short-day signal at a constant 18 degrees C, whereas larvae of a non-photoperiodic-diapause (NPD) strain do not respond to photoperiodic signalling and continue in larval development irrespective of daylength. The present study shows that WT larvae also respond reliably to thermoperiodic signalling (daily cycles of temperature) under constant darkness, whereas the NPD larvae do not, suggesting that the pathways transducing the environmental token stimuli (photoperiod and thermoperiod) onto the diapause developmental programme might merge functionally in the central biological clock system known to be mutated in NPD strain. Temperature and larval population density modify the output of token stimuli signalling. High temperatures (>24 degrees C) tend to avert, whereas low temperatures (<18 degrees C), especially in combination with constant darkness, stimulate diapause induction in WT strain. Overcrowding (>200 larvae per 5 g of larval diet) lengthens the duration of larval development and induces a diapause-like' developmental arrest of relatively weak intensity in up to 60% of larvae of both strains. At high temperatures (>30 degrees C), all WT larvae continue direct development but subsequently die during the pupal stage. Low temperature exposure (<12 degrees C) causes quiescence in the majority of the larvae of both strains. Starvation blocks development and causes mortality when applied in larvae younger than day 3 of the third instar. Older larvae survive starvation and their photoperiodically-induced developmental pre-programming is not affected. Collectively, the results show that diapause induction in C. costata is a result of various interacting effects of multiple environmental factors.