How Daylight Length and Altitude Affect Egg-Laying Patterns in Himalayan Drosophila ananassae Flies

The high-altitude strain of Drosophila ananassae from Badrinath exhibited a phase angle difference (Psi) increase from approximately 2 hours in an 8-hour photophase to approximately 11 hours in an 18-hour photophase.

• The high-altitude strain was entrained by all light-dark cycles except for the 6-hour photophase, where it showed weak rhythmicity.
• The low-altitude strain was entrained by only three light-dark cycles with photophases of 10, 12, and 14 hours, becoming arrhythmic in cycles with 6, 8, 16, and 18 hours.
• Lights-off was identified as the key signal for phase determination, with the high-altitude strain ovipositing approximately 6 hours before lights-off and the low-altitude strain approximately 1 hour after.
• The tau of the high-altitude strain increased from approximately 19 hours to 25 hours when transferred to constant darkness after different light-dark cycles.
• In contrast, the tau of the low-altitude strain increased from approximately 26 hours to 28 hours under similar conditions.

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