Evolution Of Circadian Rhythms Research Articles

Drosophila Populations Reared Under Tropical Semi-natural Conditions Evolve Season-dependent Differences in Timing of Eclosion.

Circadian clocks are considered an evolutionary adaptation to environmental cycles, helping organisms to adapt to daily and seasonal changes. However, most studies on the evolution of circadian rhythms have been carried out in controlled laboratory conditions; hence evolution of circadian clocks and rhythms in organisms reared under the influence of naturally varying time cues is not well understood. To address this, we reared large outbred fly populations in an outdoor enclosure on our institutional grounds in Bengaluru, southern India for about 150 generations, at the same time maintaining their ancestral control populations under standard laboratory conditions. Studying their rhythms in eclosion, a vital behavior for Drosophila, in the laboratory and semi-natural environments revealed that flies reared under semi-natural conditions differed in the timing of eclosion under semi-natural conditions in a season-dependent manner from their laboratory-reared counterparts. These differences were manifested under harsh semi-natural environments but not under mild ones or in standard laboratory conditions. Further analysis revealed that this phenotype might be responsive to seasonal changes in temperature cycles which was confirmed in the laboratory with simulated light and temperature cycles that approximated semi-natural conditions. Our results highlight key intricacies on the relative impact of intensity and timing of environmental cues for predicting the timing of Drosophila eclosion under tropical naturalistic conditions. Overall, our research uncovers previously unexplored aspects of adaptive circadian timekeeping in complex natural conditions, offering valuable insight into the evolution of clocks.

Evolution of circadian rhythms in Drosophila melanogaster populations reared in constant light and dark regimes for over 330 generations

ABSTRACTOrganisms are believed to have evolved circadian clocks as adaptations to deal with cyclic environmental changes, and therefore it has been hypothesized that evolution in constant environments would lead to regression of such clocks. However, previous studies have yielded mixed results, and evolution of circadian clocks under constant conditions has remained an unsettled topic of debate in circadian biology. In continuation of our previous studies, which reported persistence of circadian rhythms in Drosophila melanogaster populations evolving under constant light, here we intended to examine whether circadian clocks and the associated properties evolve differently under constant light and constant darkness. In this regard, we assayed activity-rest, adult emergence and oviposition rhythms of D. melanogaster populations which have been maintained for over 19 years (~330 generations) under three different light regimes – constant light (LL), light–dark cycles of 12:12 h (LD) and constant darkness (DD). We observed that while circadian rhythms in all the three behaviors persist in both LL and DD stocks with no differences in circadian period, they differed in certain aspects of the entrained rhythms when compared to controls reared in rhythmic environment (LD). Interestingly, we also observed that DD stocks have evolved significantly higher robustness or power of free-running activity-rest and adult emergence rhythms compared to LL stocks. Thus, our study, in addition to corroborating previous results of circadian clock evolution in constant light, also highlights that, contrary to the expected regression of circadian clocks, rearing in constant darkness leads to the evolution of more robust circadian clocks which may be attributed to an intrinsic adaptive advantage of circadian clocks and/or pleiotropic functions of clock genes in other traits.

The Sleep-Wake Cycle and Circadian Rhythm Disruption: Shift Work and Cardiovascular Disease Risk

Sleep is a crucial and complex behavioral state that occupies one-third of an individual’s life. A couple of theories have been proposed in relation to sleep. The first is the theory of dynamic stabilization, which explains the evolution of sleep. The second is the circadian resonance hypothesis, which discusses the evolution of circadian rhythms. Shift work involves hours of work that are irregular from normal working hours and conflicts the rhythmic processes of the circadian system. Shift work has been associated with negative health outcomes, especially cardiovascular disease and obesity. These health outcomes are important to consider since 20% of the population in developed countries performs shift work. This paper addresses certain processes responsible for these health outcomes and proposes a pathway that explains high rates of cardiovascular disease among shift workers.

Rhythmic gene expression in a purple photosynthetic bacterium, Rhodobacter sphaeroides

Circadian rhythms are known to exist in all groups of eukaryotic organisms as well as oxygenic photosynthetic bacteria, cyanobacteria. However, little information is available regarding the existence of rhythmic behaviors in prokaryotes other than cyanobacteria. Here we report biological rhythms of gene expression in a purple bacterium Rhodobacter sphaeroides by using a luciferase reporter gene system. Self-bioluminescent strains of Rb. sphaeroides were constructed, which produced a bacterial luciferase and its substrate, a long chain fatty aldehyde, to sustain the luminescence reaction. After being subjected to a temperature or light entrainment regime, the reporter strains with the luciferase genes driven by an upstream endogenous promoter expressed self-sustained rhythmicity in the constant free-running period. The rhythms were controlled by oxygen and exhibited a circadian period of 20.5 h under aerobic conditions and an ultradian period of 10.6–12.7 h under anaerobic conditions. The data suggest a novel endogenous oscillation mechanism in purple photosynthetic bacteria. Elucidation of the clock-like behavior in purple bacteria has implications in understanding the origin and evolution of circadian rhythms.

CIRCADIAN PACEMAKER FUNCTION AND ENTRAINMENT DURING MATURATION OF TRANSGENIC HYPERTENSIVE TGR(mREN2)27 AND SPRAGUE-DAWLEY RATS

TGR(mREN2)27 (TGR) transgenic rats develop hypertension due to the mouse mRen-2 gene inserted in their genome. At 5 weeks of age, the blood pressure of TGR rats starts rising, until a maximum is reached at 10 weeks of age. Adult TGR rats show peak values of blood pressure (BP) during the light phase, while heart rate (HR) and motor activity (MA) peak at night. In the present experiment, we evaluated the evolution of circadian rhythms in motor activity, heart rate, and blood pressure of TGR and Sprague-Dawley (SD) rats under 12h light-dark cycles (LD 12:12). Results confirmed that the blood pressure of TGR rats starts to increase at 5 weeks of age, reaching a plateau by the 11th week. Parallel to the increase in blood pressure levels, there was a decrease in the period length of the blood pressure rhythm, a delay in the onset of the alpha phase of the blood pressure rhythm with respect to that of motor activity and heart rate, and a decrease in heart rate levels. In all of the variables studied, the alpha phase of SD rats always started before darkness, whereas that of TGR rats started after lights off. In general, heart rate and motor activity levels of TGR rats were higher than those of SD rats. The amplitude of the circadian rhythms studied was greater in TGR rats than in SD rats. The present results suggest that the different evolution of circadian rhythms in TGR and SD rats might be due to differences in the functioning of the entrainment pathway or the circadian clock itself, which can be detected in young rats and that are probably caused by the expression of the mouse transgene. (Chronobiology International, 18(4), 627–640, 2001)