Perception of Daily Time: Insights from the Fruit Flies

SIMPLE SUMMARY: The ability to perform time-dependent functions has been linked to survival in animals. For a prey, to forage when many predators are active, can be deadly. Performing the right task at the right time is dependent on the animal’s ability to track daily time. Perception of time in the animal kingdom dates long back on the evolutionary time-scale. From the unicellular prokaryotes to humans; we all tune our physiology and behaviours according to the time of the day. To do this in the face of unreliable environmental parameters, we use an internal timekeeper. The state of the timekeeper is readjusted daily, by feeding weighted updates about the external factors that oscillate in nature, e.g., light and temperature. In this review, using the example of the fruit flies, we discuss how animals perceive the external and internal times and connect them in the brain. ABSTRACT: We create mental maps of the space that surrounds us; our brains also compute time—in particular, the time of day. Visual, thermal, social, and other cues tune the clock-like timekeeper. Consequently, the internal clock synchronizes with the external day-night cycles. In fact, daylength itself varies, causing the change of seasons and forcing our brain clock to accommodate layers of plasticity. However, the core of the clock, i.e., its molecular underpinnings, are highly resistant to perturbations, while the way animals adapt to the daily and annual time shows tremendous biological diversity. How can this be achieved? In this review, we will focus on 75 pairs of clock neurons in the Drosophila brain to understand how a small neural network perceives and responds to the time of the day, and the time of the year.