Natural Populations of Drosophila melanogaster Reveal Features of an Uncharacterized Circadian Property: The Lower Temperature Limit of Rhythmicity

Molecular mechanisms and trade-offs underlying fluctuating thermal regimes during low-temperature storage

Insects exposed to constant low temperatures (CLT) exhibit high rates of mortality as well as a variety of sublethal effects. In many species, interruptions of CLT with brief pulses of warm temperatures (fluctuating thermal regimes, FTR) lead to increases in survival and fewer sublethal effects. However, we still lack a complete understanding of the physiological mechanisms activated during FTR. In this review, we discuss recent advances in understanding FTR's underlying molecular mechanisms. We discuss knowledge gaps related to potential trade-offs between FTR's beneficial effects and the costs of these repairs to overwintering reserves and reproduction. We present the hypothesis that the warm pulse of FTR helps to maintain daily rhythmicity.

Principles underlying the complex dynamics of temperature entrainment by a circadian clock

Autonomously oscillating circadian clocks resonate with daily environmental (zeitgeber) rhythms to organize physiology around the solar day. Although entrainment properties and mechanisms have been studied widely and in great detail for light-dark cycles, entrainment to daily temperature rhythms remains poorly understood despite that they are potent zeitgebers. Here we investigate the entrainment of the chronobiological model organism Neurospora crassa, subject to thermocycles of different periods and fractions of warm versus cold phases, mimicking seasonal variations. Depending on the properties of these thermocycles, regularly entrained rhythms, period-doubling (frequency demultiplication) but also irregular aperiodic behavior occurs. We demonstrate that the complex nonlinear phenomena of experimentally observed entrainment dynamics can be understood by molecular mathematical modeling.

Flies as models for circadian clock adaptation to environmental challenges

Life on earth is assumed to have developed in tropical regions that are characterized by regular 24 hr cycles in irradiance and temperature that remain the same throughout the seasons. All organisms developed circadian clocks that predict these environmental cycles and prepare the organisms in advance for them. A central question in chronobiology is how endogenous clocks changed in order to anticipate very different cyclical environmental conditions such as extremely short and long photoperiods existing close to the poles. Flies of the family Drosophilidae can be found all over the world-from the tropics to subarctic regions-making them unprecedented models for studying the evolutionary processes that underlie the adaptation of circadian clocks to different latitudes. This review summarizes our current understanding of these processes. We discuss evolutionary changes in the clock genes and in the clock network in the brain of different Drosophilids that may have caused behavioural adaptations to high latitudes.

Multiple feedback loops of the Arabidopsis circadian clock provide rhythmic robustness across environmental conditions

Although circadian oscillators in diverse eukaryotes all depend on interlinked transcriptional feedback loops, specific components are not conserved across higher taxa. Moreover, the circadian network in the model plant Arabidopsis thaliana is notably more complex than those found in animals and fungi. Here, we combine mathematical modeling and experimental approaches to investigate the functions of two classes of Myb-like transcription factors that antagonistically regulate common target genes. Both CCA1/LHY- and RVE8-clade factors bind directly to the same cis-element, but the former proteins act primarily as repressors, while the latter act primarily as activators of gene expression. We find that simulation of either type of loss-of-function mutant recapitulates clock phenotypes previously reported in mutant plants, while simulated simultaneous loss of both type of factors largely rescues circadian phase at the expense of rhythmic amplitude. In accord with this prediction, we find that plants mutant for both activator- and repressor-type Mybs have near-normal circadian phase and period but reduced rhythmic amplitude. Although these mutants exhibit robust rhythms when grown at mild temperatures, they are largely arrhythmic at physiologically relevant but nonoptimal temperatures. LHY- and RVE8-type Mybs are found in separate clades across the land plant lineage and even in some unicellular green algae, suggesting that they both may have functioned in even the earliest arising plant circadian oscillators. Our data suggest that the complexity of the plant circadian network may have arisen to provide rhythmic robustness across the range of environmental extremes to which plants, as sessile organisms, are regularly subjected.

Effect of Clonal Selection on Daphnia Tolerance to Dark Experimental Conditions

Recent studies have demonstrated substantial effects of environmental stress that vary among clones. Exposure to ultraviolet radiation (UV) is an important abiotic stressor that is highly variable in aquatic ecosystems due to diel and seasonal variations in incident sunlight as well as to differences in the UV transparency of water among water bodies, the depth distribution of organisms, and the ability of organisms to detect and respond to UV. In contrast to the convention that all UV is damaging, evidence is accumulating for the beneficial effects of exposure to low levels of UV radiation. Whereas UV has been frequently observed as the primary light-related stressor, herein we present evidence that dark conditions may be similarly “stressful” (reduction of overall fitness), and stress responses vary among clones of the freshwater crustacean Daphnia parvula. We have identified a significant relationship between survivorship and reduced fecundity of clones maintained in dark conditions, but no correlation between tolerance of the clones to dark and UV radiation. Low tolerance to dark conditions can have negative effects not only on accumulated stresses in organisms (e.g. the repair of UV-induced damage in organisms with photolyase), but potentially on the overall physiology and fitness of organisms. Our results support recent evidence of the beneficial effects of low-level UV exposure for some organisms.

period and timeless mRNA Splicing Profiles under Natural Conditions in Drosophila melanogaster

Previous analysis of Drosophila circadian behavior under natural conditions has revealed a number of novel and unexpected features. Here we focus on the oscillations of per and tim mRNAs and their posttranscriptional regulation and observe significant differences in molecular cycling under laboratory and natural conditions. In particular, robust per mRNA cycling from fly heads is limited to the summers, whereas tim RNA cycling is observed throughout the year. When both transcripts do cycle, their phases are similar, except for the very warmest summer months. We also study the natural splicing profiles of per and tim transcripts and observe a clear relationship between temperature and splicing. In natural conditions, we confirm the relationship between accumulation of the per(spliced) variant, low temperature, and the onset of the evening component of locomotor activity, first described in laboratory conditions. Intriguingly, in the case of tim splicing, we detect the opposite relationship, with tim(spliced) expression increasing at higher temperatures. A first characterization of the 4 different TIM protein isoforms (resulting from the combination of the natural N-terminus length polymorphism and the C-terminus alternative splicing) using the 2-hybrid assay showed that the TIM(unspliced) isoforms have a stronger affinity for CRY, but not for PER, suggesting that the tim 3' splicing could have physiological significance, possibly in temperature entrainment and/or adaptation to seasonal environments.

Warming Up Your Tick-Tock: Temperature-Dependent Regulation of Circadian Clocks

Circadian clocks are endogenous time-keeping mechanisms to adaptively coordinate animal behaviors and physiology with daily environmental changes. So far many circadian studies in model organisms have identified evolutionarily conserved molecular frames of circadian clock genes in the context of transcription-translation feedback loops. The molecular clockwork drives cell-autonomously cycling gene expression with ~24-hour periodicity, which is fundamental to circadian rhythms. Light and temperature are two of the most potent external time cues to reset the circadian phase of the internal clocks, yet relatively little is known about temperature-relevant clock regulation. In this review, we describe recent findings on temperature-dependent clock mechanisms in homeothermic mammals as compared with poikilothermic Drosophila at molecular, neural, and organismal levels. We propose thermodynamic transitions in RNA secondary structures might have been potent substrates for the molecular evolution of temperature-relevant post-transcriptional mechanisms. Future works should thus validate the potential involvement of specific post-transcriptional steps in temperature-dependent plasticity of circadian clocks.

Heating and cooling the Drosophila melanogaster clock

Most biological phenomena are under control of a circuit known as the 'molecular circadian clock.' Over the past forty years of research in Drosophila melanogaster, studies have made significant advances in our understanding of the molecular timing mechanism of this circuit, which is determined by a core inhibitory feedback loop. While the timing mechanism of the molecular circadian clock is endogenous, it is well established that exogenous cues such as light and temperature modulate its timing. In the following article, we summarize our current understanding of how temperature interacts with the molecular circadian clock in adult Drosophila.