Chronobiology internationalFeb 14, 2012

Unexpected blocking effects of bright light and high temperature in the fruit fly Drosophila malerkotliana

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Updated Mar 28, 2026

Abstract

A 1000-lux photophase at 25°C resulted in nocturnal activity in Drosophila malerkotliana, contrasting with diurnal patterns observed at lower light levels.

Flies exposed to light-dark cycles at 10, 100, and 500 lux exhibited a diurnal activity pattern, while those at 1000 lux shifted to nocturnal behavior.
Transitioning from light-dark cycles to continuous darkness triggered free-running rhythms in all groups, indicating the influence of the circadian pacemaker.
Flies in the 1000-lux group displayed a free-running rhythm that began abruptly from the last lights-on phase, suggesting noninvolvement of the circadian pacemaker.
Nocturnal activity was consistently observed at 1000 lux in different light-dark cycle configurations, affirming the impact of high light intensity.
At a lower temperature of 20°C, flies maintained a diurnal activity pattern even under 1000-lux conditions, indicating temperature's role in the masking effect.

Simplified

Synergic contribution of light and temperature is known to cause a paradoxical masking effect (inhibition of activity by bright light and high temperature) on various rhythms of animals. The present study reports the paradoxical masking effects of 1000-lux photophase at 25°C on the locomotor activity rhythm of Drosophila malerkotliana. Flies were subjected to light (L)-dark (D) 12:12 cycles wherein the photophase was varied from 10 to 1000 lux, whereas the scotophase was set to 0 lux in these and subsequent LD cycles. At 10, 100, and 500 lux, the flies were diurnal; however, at 1000 lux they were nocturnal. Transfer from LD 12:12 cycles to continuous darkness (DD) initiated free-running rhythmicity in all flies. Free-running rhythms of the flies switched from the 10-lux to the 500-lux groups started from the last activity-onset phase of the rhythm following 3-5 transient cycles, suggesting involvement of the circadian pacemaker. In contrast, the free-running rhythm of the flies of the 1000-lux group began abruptly from the last lights-on phase of the LD cycle, indicating noninvolvement of the pacemaker. Furthermore, all flies showed nocturnal activity in the two types of LD 12:12 cycles when the photophase was 1000 lux. The first type of LD cycles had three succeeding photophases of 100, 1000, and again 100 lux, whereas the second type of LD cycles had only one photophase of 1000 lux, but the LD 12:12 cycles were reversed to DL 12:12 cycles. Apparently, the combined effects of light and temperature caused such paradoxical masking effects. This hypothesis was tested by repeating the above experiments at 20°C. Flies in all experiments exhibited a diurnal activity pattern, even when the photophase was 1000 lux. Thus, the present study demonstrates that the paradoxical masking effect in D. malerkotliana was caused by the additive influence of light intensity and temperature. This strategy appears to have physiological significance, i.e., to shun and thus protect against the bright photophase at high temperature in the field.

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Unexpected blocking effects of bright light and high temperature in the fruit fly Drosophila malerkotliana

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