Effect of Light Intensity on the Oviposition Rhythm of the Altitudinal Strains ofDrosophila Ananassae

Abstract

The sensitivity of the circadian photoreceptors mediating entrainment of the eclosion rhythm and phase shifts of oviposition rhythm of the high altitude (HA) strain of Drosophila ananassae originating from Badrinath (5123 m above sea level) in the Himalayas was compared with the low altitude (LA) strain from Firozpur (179 m above sea level). Reduced photic sensitivity of the HA strain is regarded as the result of natural selection, which led to the weakening of the coupling mechanism between the circadian pacemaker and light at the high altitude of origin. The present study was designed to determine whether or not the photic entrainment of the oviposition rhythm of the HA strain of D. ananassae is also altered by the high altitude of its origin, and the results are compared with those of the LA strain. The effects of light intensity on the phase angle difference (Psi), degree of rhythmicity (R), the percent oviposition in photophase, the threshold light intensity (i.e., the intensity at which stable entrainment occurred), and the saturation light intensity (i.e., the intensity beyond which the values of Psi or amplitude of rhythm remained unaltered) were determined. Entrainment was studied in light-dark cycles in which the light intensity of 12 h of photophase varied from 1 to 1000 lux, and complete darkness prevailed in all scotophases. The oviposition rhythm of the HA strain was arrhythmic from 1 to 90 lux, weakly rhythmic at 95 lux, but rhythmic at or above 100 lux, while that of the LA strain was weakly rhythmic at 1 lux but rhythmic at or above 2 lux. Oviposition of the HA strain occurred mostly in the photophase, while that of the LA strain occurred in the scotophase; as a result, the oviposition medians of the HA strain were around the subjective forenoons while those of the LA strain were around the subjective evenings. The percent of oviposition in photophase increased from 68 to 98 in the HA strain and from 5 to 33 in the LA strain as light intensity increased from 1 to 1000 lux. In the HA strain, the Psi values were significantly less and values of R and percent oviposition in photophase were significantly more than those of the LA strain at each level of light intensity. Threshold and saturation intensities for Psi were 100 and 700 lux, respectively, for the HA strain, but just 2 and 45 lux, respectively, for the LA strain. The saturation intensity for R was 650 and 700 lux for the HA and LA strains, respectively. These results extend the confirmation that the reduced photic sensitivity of the HA strain might have been acquired through natural selection in response to environmental conditions at the high altitude of its origin.

Effects of Photophase and Altitude on Oviposition Rhythm of the Himalayan Strains ofDrosophila Ananassae

“…For example, the POP of the HA strain increased from 60 to 100 as the photophase increased from 6 to 14 h. It was not surprising to see such a noteworthy increment in POP of this strain because it predominantly oviposits in the forenoon in the field or in the subjective midday of LD cycles in the laboratory (Khare et al, 2005). The POP of the HA strain also increased as the light intensity of the 12 h photophase was increased (Satralkar et al, 2007a). An increment in duration or intensity of the photophase might be interpreted by females of this strain as long and sunny summer days at Badrinath, which would be conducible to enhance oviposition events during daylight hours.…”

“…Contrary to this, the photophase of 16 h (see Figure 3B) or 18 h unambiguously dampened the oviposition rhythm of the LA strain. Credible evidence for the diminished photic sensitivity in the HA strain emerges from earlier studies on the effects of light intensity and photophase on the eclosion and oviposition rhythms of this strain (Khare et al, 2004(Khare et al, , 2005Satralkar et al, 2007a). Strains of D. auraria (Pittendrigh et al, 1991) and D. littoralis (Lankinen, 1986) from higher latitudes, like the HA strain of D. ananassae, showed a progressive reduction in the photic sensitivity of the circadian pacemaker that mediated the eclosion rhythm.…”

“…Thus, such arrhythmicity in LD cycles should be attributed to the primary arrhythmicity (Zimmerman, 1969) in the oviposition rhythm of these strains, as the constituent parts of the pacemaker controlling the oviposition rhythm might not be oscillating at all due to the failure of the rhythm to develop in non-entraining LD cycles in which they were reared for two successive generations. Primary arrhythmicity was also observed in the oviposition rhythm of these strains of D. ananassae when they were reared in LD 12:12 cycles in which the intensity of the photophase was below the threshold light intensity for stable entrainment (Satralkar et al, 2007a), and these strains continued to be arrhythmic even when transferred to DD (Satralkar et al, in preparation) …”

“…The increment in its POP was attributed to the arrhythmicity in the 16 and 18 h photophases, when its oviposition events were distributed over the 24 h of the cycle (see Figure 3B). However, the increment in light intensity resulted in drastic decrement in POP of the LA strain (Satralkar et al, 2007a). The increment in light intensity was apparently regarded by this strain as blazing hot summer days at Firozpur when it never oviposits in the daytime hours.…”

“…The objectives of the present study were to determine the effects of varying photophase of LD cycles and altitude of origin on the parameters of entrainment and t of the oviposition rhythm of two strains of D. ananassae originating from different altitudes. Data pertaining to the eclosion and oviposition rhythms of these altitudinal strains of D. ananassae are already available (Khare et al, 2002(Khare et al, , 2004(Khare et al, , 2005Satralkar et al, 2007a).…”