Abstract
Organisms possess an endogenous molecular clock which enables them to adapt to environmental rhythms and to synchronize their metabolism and behavior accordingly. Circadian rhythms govern daily oscillations in numerous physiological processes, and the underlying molecular components have been extensively described from fruit flies to mammals. Drosophila larvae have relatively simple nervous system compared to their adult counterparts, yet they both share a homologous molecular clock with mammals, governed by interlocking transcriptional feedback loops with highly conserved constituents. Larvae exhibit a robust light avoidance behavior, presumably enabling them to avoid predators and desiccation, and DNA-damage by exposure to ultraviolet light, hence are crucial for survival. Circadian rhythm has been shown to alter light-dark preference, however it remains unclear how distinct behavioral strategies are modulated by circadian time. To address this question, we investigate the larval visual navigation at different time-points of the day employing a computer-based tracking system, which allows detailed evaluation of distinct navigation strategies. Our results show that due to circadian modulation specific to light information processing, larvae avoid light most efficiently at dawn, and a functioning clock mechanism at both molecular and neuro-signaling level is necessary to conduct this modulation.
Highlights
Circadian rhythms are ≅ 24 h oscillations displayed by various organisms[1,2,3,4], maintained by an endogenous timekeeping mechanism that can be entrained to the environment by external cues called ‘zeitgebers’ (German for “time-givers”) such as light, temperature and even social interactions in mammals[5,6,7]
Owing to the experimental advantages and considerable homologies with mammals, Drosophila has been extensively used as a model system to study the genetic and cellular mechanisms as well as the fundamental neural circuits of circadian rhythmicity and entrainment of the molecular clock[49]
We further find that each one of pdf, per and clk mutants show severe deficits in circadian modulated behaviors supporting that a functional molecular clock as well as proper neural signaling of pacemaker neurons is essential for maintaining circadian-modulated visually-guided navigational decision making
Summary
Circadian rhythms are ≅ 24 h oscillations displayed by various organisms[1,2,3,4], maintained by an endogenous timekeeping mechanism that can be entrained to the environment by external cues called ‘zeitgebers’ (German for “time-givers”) such as light, temperature and even social interactions in mammals[5,6,7]. Navigational decision-making based on temporal cues can be assessed by using a temporal light gradient of recurring one-minute cycles, each encompassing a phase of linear light intensity increase and a corresponding phase of intensity decrease (Fig. 1B) Under this condition, larvae compare the light intensity change over time by sampling the environment through head-sweeps. All temporal navigation strategies are measured distinctly for the two phases and consist of turn size (the degree of turn angle), turn rate (the average number of turns an animal makes per minute) and head-sweep acceptance rate (the percentage of accepted head-sweeps) These strategies are presented by a delta between the two light intensity phases. The overall navigation of larvae is demonstrated by navigation index, summarized by both spatial and temporal navigation strategies, and indicates directionality with respect to the light source where negative values represent navigation biased away
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