ALTITUDINAL DIFFERENTIATION IN THE DIAPAUSE RESPONSE OF 2 SPECIES OF DUNG FLIES

1. Seasonality is a prime selective factor promoting genetic different iation of populations. Local adaptation in diapause response was inves tigated in the two geographically and altitudinally widespread dung fl ies Scathophaga stercoraria (Diptera: Scathophagidae) and Sepsis cynip sea (Diptera: Sepsidae). 2. Replicate sibships from three sites in Swi tzerland with low and high altitude dung fly populations were raised i n a common laboratory environment simulating the natural decreasing ph otoperiod and temperature regimen before winter. From field phenologie s, the critical photoperiod inducing diapause was predicted to be long er for high than for low altitude populations (12 vs. 10 h for Sc. ste rcoraria, and 12.25 vs. 11 h for Se. cynipsea) if they are locally ada pted. 3. Contrary to expectation and to Sc. stercoraria, which diapaus es in the pupal stage, Se. cynipsea diapauses in the adult stage. 4. L ow but significant levels of genetic differentiation in pre-winter adu lt emergence were evident between low and high altitude populations of both species, but they were far from the differences predicted. Scath ophaga stercoraria also showed geographical differentiation independen t of altitude. 5. Sepsis cynipsea females stopped reproducing at some point before winter, but altitude did not affect the timing of adult d iapause. High altitude females and females that did not initiate repro duction before winter survived the simulated winter better. 6. Both sp ecies largely used temperature rather than photoperiod as a cue induci ng winter diapause, an untypical case of phenotypic plasticity. The hy pothesis that Sc. stercoraria, whose generation times are much longer than those of Se. cynipsea, responds to a greater extent to temperatur e rather than photoperiod only was rejected.