Will living in complete darkness or being in light for 24 hours for the rest of our lives affect our circadian rhythm (sleep–wake cycles)? Probably not, at least in the case of fruit flies.
Experiments carried out on 330 generations of drosophila (fruit flies) confirmed that circadian rhythm was persistent in flies that were kept in complete darkness or complete light 24 hours a day for over 19 years. This may be due to the intrinsic value of the body’s time-keeping system in coordinating our internal physiological functions.
This was the finding of a study by a group of scientists led by Prof. Vijay Kumar Sharma at the Chronobiology Lab (where study of the biological clock is carried out) at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru; Prof. Sharma passed away last year after the completion of the work.
Sleep-wake assay
Using drosophila activity monitors, locomotor activity patterns of flies kept in the three different conditions (total darkness, total light, and normal day–night conditions) were monitored. Interestingly, the flies maintained in complete darkness exhibited a relatively better sleep–wake cycle than the ones in complete light. “The control group had cues of day and night in the form of 12 hours of light and 12 hours of darkness, while the flies in the other two groups which were not exposed to such cues also maintained circadian rhythms over several hundred generations,” explains Dr. Radhika Shindey from JNCASR and first author of the paper.
The results of the study help dispel the notion that continuous darkness may regress the body’s biological clock and show that absence of light may have caused the evolution of a more robust clock in flies. The results were recently published in Chronobiology International.
Clock and reproduction
The time at which flies emerge as adults from pupae and the time when adult females laid eggs were examined, since appropriate timing of adult emergence and oviposition (egg-laying) behaviours are thought to be important for survival and reproduction.
For the experiment, a few flies taken from the group kept in complete darkness and complete light were exposed to normal cycle of 12 hours of light and 12 hours of darkness. They found that flies which were taken from the group exposed to complete light tend to lay eggs at the time of dusk when exposed to normal light and darkness cycle. On the other hand, flies taken from the group exposed to complete darkness tend to lay eggs at about noon.
In nature, a high temperature during the day may increase the risk of drying of eggs, which may be the reason why egg-laying mostly takes place in the evenings. Since the flies in complete darkness have not been exposed to light for several generations they may have slowly lost this ability of restricting egg-laying to the evenings.
“We have to conduct more oviposition experiments for longer time duration to get a proper understanding of this rhythm,” she adds.
Not just time-keeping
“The most interesting find of the study was that the circadian rhythm did not degenerate over 19 years,” says Vishwanath Varma, PhD scholar at JNCASR and co-author of the paper. Complete regression of the circadian clock is very unlikely because of the several other functions of the core circadian genes. “It has been shown that circadian clock genes are also involved in other physiological functions (DNA repair, reproductive fitness) apart from time-keeping,” he adds. This could be one of the main reasons why the circadian rhythm will continue to be preserved even in absence of light-dark cues for hundreds of generations.
