[Show abstract][Hide abstract] ABSTRACT: Molecular genetic analysis of the fruit fly Drosophila melanogaster has revolutionized our understanding of the transcription/translation loop mechanisms underlying the circadian molecular oscillator. More recently, Drosophila has been used to understand how different neuronal groups within the circadian pacemaker circuit interact to regulate the overall behavior of the fly in response to daily cyclic environmental cues as well as seasonal changes. Our present understanding of circadian timekeeping at the molecular and circuit level is discussed with a critical evaluation of the strengths and weaknesses of present models. Two models for circadian neural circuits are compared: one that posits that two anatomically distinct oscillators control the synchronization to the two major daily morning and evening transitions, versus a distributed network model that posits that many cell-autonomous oscillators are coordinated in a complex fashion and respond via plastic mechanisms to changes in environmental cues.
Critical Reviews in Biochemistry and Molecular Biology 01/2008; 43(1):37-61. DOI:10.1080/10409230701829128 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: After a slow start, the comparative analysis of clock genes in insects has developed into a mature area of study in recent years. Brain transplant or surgical interventions in larger insects defined much of the early work in this area, before the cloning of clock genes became possible. We discuss the evolution of clock genes, their key sequence differences, and their likely modes of regulation in several different insect orders. We also present their expression patterns in the brain, focusing particularly on Diptera, Lepidoptera, and Orthoptera, the most common non-genetic model insects studied. We also highlight the adaptive involvement of clock molecules in other complex phenotypes which require biological timing, such as social behaviour, diapause and migration.
[Show abstract][Hide abstract] ABSTRACT: While roles of the clock genes period (per) and timeless (tim) are relatively well understood in relation to circadian clocks, their potential roles in insect photoperiodism remain enigmatic. In this study, the expression of per and tim genes under two contrasting photoperiods is described in the central nervous system of photoperiodically sensitive, newly hatched first instar larvae of the flesh fly, Sarcophaga crassipalpis. Using qPCR, diel oscillations were observed in the mRNA levels of both genes under long-day (15 h light:9h dark, promotes direct development) and short-day conditions (11 h light:13 h dark, induces pupal diapause). Peak per and tim mRNA oscillations were closely associated with the light/dark transition. The conspicuous difference between the two photoperiodic conditions was that the sharp increase in per and tim mRNA abundance occurred during the light phase under long days but during the dark phase under short days. The diel oscillations were, at least in part, driven by an endogenous component, as demonstrated by transferring larvae to continuous darkness. The cells displaying Tim- and Per-like immunoreactivities (Tim- and Per-LIRs) were localized using anti-Drosophila-Per and anti-Chymomyza-Tim antibodies. Per-LIR and Tim-LIR co-localized in three groups of cells in each brain hemisphere. Two other groups, one in the brain hemispheres and the other in the fused ventral nerve ganglion, expressed only the Per-LIR.
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