Selection for phase angle difference of the adult locomotor activity in Drosophila rajasekari affects the activity pattern, free-running period, phase shifts and sensitivity to light

Laboratory selection for the phase angle difference (Psi, the time from lights-off in a 24 h light-dark cycle to an activity onset) of the adult locomotor activity in Drosophila rajasekari reared in LD (light:dark cycles) of 12:12 h for 59 generations resulted in the early and late strains which differed in Psi value by about 8 h. The selection affected the activity pattern in LD 12:12; in contrast to the wildtype, which had a broad plateau of activity pattern, the early strain exhibited a biphasic activity pattern with morning and evening peaks, whereas the late strain had a single evening peak which extended for 6 h in the dark. The selection significantly decreased and increased the activity level per cycle in LD 12:12, continuous darkness (DD) and continuous light (LL) in the early and late strains respectively when compared to that of the wild-type (P < 0.01). The free running period (tau) in DD was shortened in the early strain and lengthened in the late strain by the shortening and lengthening of the activity phases respectively, the rest phases remained unchanged in these strains from that of the wild-type. Phase response curves (PRCs) were measured for light pulses in all strains, the PRC for the early strain was characterized by larger phase shifts when compared to the PRC for the late or for the wild type flies. The ability of tau to be progressively lengthened by increasing intensity of LL was increased and decreased in the early and late strains respectively. Moreover, the threshold light intensity of LL to generate arrhythmicity was apparently decreased in the early strain and increased in the late strain, suggesting that the selection for Psi had differently affected the subjective light sensitivity in these strains.