A comparative study of circadian behaviours and the underlying neuronal correlates of five drosophilid species

Abstract

Many organisms show rhythmicity in physiological processes, metabolism, behaviours and reproduction. Depending upon the period, rhythms are classified as - ultradian rhythms (milliseconds to hr), circadian rhythms (24 hr), tidal rhythms, annual rhythms etc. The rotation of the earth on its axis, its revolution around the sun and the revolution of the moon around the earth subject organisms to geophysical cycles such as daily cycles (24 hr) of light, temperature and to seasonal changes. Organisms appear either to cope with the daily changes in the external environment or even to make use of these periodic changes. Many organisms from simple unicellular beings to complex mammals exhibit daily rhythms (DeCoursey, 2004) and this 24 hr rhythmic pattern is not merely a response to the external environment and is in fact produced endogenously as was first demonstrated by Jacques de Mairan, in 1729 in the heliotrope plant (de Mairan, 1729). These daily rhythms are characterised by several features: 1) they are endogenously generated (produced within the organism) self-sustaining (continue to oscillate in the absence of any external time cues) with a free running periodicity of ~ 24 hr, hence the name circadian (from Latin ‘circa’ – approximately and ‘diem’ – a day) 2) they are entrainable (can be synchronized to periodic external conditions like light, temperature etc) and 3) they are temperature compensated (circadian period is not significantly altered by physiologically tolerable changes in temperature). The circadian system can be conceptualised as consisting of input pathways, which synchronize the clock to the environment; central oscillator, which maintains time; and output pathways, which convey information from central oscillator to temporally organize physiology and behaviour.