Photoperiodic induction of diapause: opening the black box

We have used high-density oligonucleotide arrays to study global circadian gene expression in Drosophila melanogaster. Coupled with an analysis of clock mutant (Clk) flies, a cell line designed to identify direct targets of the CLOCK (CLK) transcription factor and differential display, we uncovered several striking features of circadian gene networks. These include the identification of 134 cycling genes, which contribute to a wide range of diverse processes. Many of these clock or clock-regulated genes are located in gene clusters, which appear subject to transcriptional coregulation. All oscillating gene expression is under clk control, indicating that Drosophila has no clk-independent circadian systems. An even larger number of genes is affected in Clk flies, suggesting that clk affects other genetic networks. As we identified a small number of direct target genes, the data suggest that most of the circadian gene network is indirectly regulated by clk.

Photoperiodism is a process whereby organisms are able to use both absolute measures of day length and the direction of day length change as a basis for regulating seasonal changes in physiology and behavior. The use of day length cues allows organisms to essentially track time-of-year and to "anticipate" relatively predictable annual variations in important environmental parameters. Thus, adaptive types of seasonal biological changes can be molded through evolution to fit annual environmental cycles. Studies of the formal properties of photoperiodic mechanisms have revealed that most organisms use circadian oscillators to measure day length. Two types of paradigms, designated as the external and internal coincidence models, have been proposed to account for photoperiodic time measurement by a circadian mechanism. Both models postulate that the timing of light exposure, rather than the total amount of light, is critical to the organism's perception of day length. In mammals, a circadian oscillator(s) in the suprachiasmatic nucleus of the hypothalamus receives photic stimuli via the retinohypothalamic tract. The circadian system regulates the rhythmic secretion of the pineal hormone, melatonin. Melatonin is secreted at night, and the duration of secretion varies in inverse relation to day length; thus, photoperiod information is "encoded" in the melatonin signal. The melatonin signal is presumably "decoded" in melatonin target tissues that are involved in the regulation of a variety of seasonal responses. Variations in photoperiodic response are seen not only between species but also between breeding populations within a species and between individuals within single breeding populations. Sometimes these variations appear to be the result of differences in responsiveness to melatonin; in other cases, variations in photoperiod responsiveness may depend on differences in patterns of melatonin secretion related to circadian variation. Sites of action for melatonin in mammals are not yet well characterized, but potential targets of particular interest include the pars tuberalis of the pituitary gland and the suprachiasmatic nuclei. Both these sites exhibit uptake of radiolabeled melatonin in various species, and there is some evidence for direct action of melatonin at these sites. However, it appears that there are species differences with respect to the importance and specific functions of various melatonin target sites.

Mechanisms composing Drosophila's clock are conserved within the animal kingdom. To learn how such clocks influence behavioral and physiological rhythms, we determined the complement of circadian transcripts in adult Drosophila heads. High-density oligonucleotide arrays were used to collect data in the form of three 12-point time course experiments spanning a total of 6 days. Analyses of 24 hr Fourier components of the expression patterns revealed significant oscillations for approximately 400 transcripts. Based on secondary filters and experimental verifications, a subset of 158 genes showed particularly robust cycling and many oscillatory phases. Circadian expression was associated with genes involved in diverse biological processes, including learning and memory/synapse function, vision, olfaction, locomotion, detoxification, and areas of metabolism. Data collected from three different clock mutants (per(0), tim(01), and Clk(Jrk)), are consistent with both known and novel regulatory mechanisms controlling circadian transcription.