The properties of the pacemaker controlling the adult locomotor activity rhythm of the high-altitude Himalayan (haH) strain (Hemkund Sahib, 4121 m above sea level) of Drosophila helvetica are strikingly different from those of the low-altitude Himalayan (laH) strain (Birahi, 1132 m above sea level) of the same species. The haH strain has a unimodal activity pattern with a delayed peak occurring about 4.5 h after lights-on of the entraining light-dark (LD) cycle, while the laH strain has a bimodal activity pattern with the morning and evening peaks. It is rather unusual for a wild type strain of any Drosophila species to have a unimodal activity pattern during entrainment as observed in the haH strain. The single activity peak of the haH strain is regarded as a consequence of delayed morning peak merging with the evening one. Three experiments were performed to test this hypothesis. The first experiment examined whether the single activity peak could be dissociated into two components by LD cycles in which photoperiods varied from 10 to 16 h per 24 h. The haH strain again exhibited a unimodal activity pattern with a delayed peak in 10, 12, and 14 h photoperiods but a bimodal activity pattern in 16 h photoperiod. The laH strain had bimodality in 10 and 12 h photoperiods, unimodality in a 14 h photoperiod, but complete arrhythmicity in a 16 h photoperiod. In the second experiment, the haH flies were transferred from LD 16:8 to LL at 5 lux to confirm whether the bimodality of this strain in LD 16:8 cycles was not the result of masking by the long photoperiod of 16 h. Bimodality of the haH strain persisted in LL too; moreover, the morning component free-ran with period (tau) <24 h, while the evening component free-ran with tau>24 h. The third experiment examined the LL-induced splitting of activity peak of the haH strain. Flies were transferred from LD 12:12 cycles to LL at 0, 1, 5, and 15 lux. The haH strain was rhythmic in LL at 0 and 1 lux with a unimodal activity pattern. It was also rhythmic in LL at 5 lux, but the single activity peak was split into two discrete components; the morning component free-ran with tau<24 h, while the evening component free-ran with tau>24 h. This strain, however, was completely arrhythmic in LL at 15 lux. The laH strain was uniformly arrhythmic in LL at all levels of light intensity. These results suggest that the single but late activity component of the haH strain during entrainment appears to be the consequence of merging the delayed morning peak with the evening one as an adaptation to the environmental conditions at the altitude of origin of this strain, where these flies begin activity in the forenoon owing to non-permissible low temperature in the morning.
