Circadian modulation of ecdysteroid titer in Galleria mellonella larvae

How clocks and hormones act in concert to control the timing of insect development

During the last century, insect model systems have provided fascinating insights into the endocrinology and developmental biology of all animals. During the insect life cycle, molts and metamorphosis delineate transitions from one developmental stage to the next. In most insects, pulses of the steroid hormone ecdysone drive these developmental transitions by activating signaling cascades in target tissues. In holometabolous insects, ecdysone triggers metamorphosis, the remarkable remodeling of an immature larva into a sexually mature adult. The input from another developmental hormone, juvenile hormone (JH), is required to repress metamorphosis by promoting juvenile fates until the larva has acquired sufficient nutrients to survive metamorphosis. Ecdysone and JH act together as key endocrine timers to precisely control the onset of developmental transitions such as the molts, pupation, or eclosion. In this review, we will focus on the role of the endocrine system and the circadian clock, both individually and together, in temporally regulating insect development. Since this is not a coherent field, we will review recent developments that serve as examples to illuminate this complex topic. First, we will consider studies conducted in Rhodnius that revealed how circadian pathways exert temporal control over the production and release of ecdysone. We will then take a look at molecular and genetic data that revealed the presence of two circadian clocks, located in the brain and the prothoracic gland, that regulate eclosion rhythms in Drosophila. In this context, we will also review recent developments that examined how the ecdysone hierarchy delays the differentiation of the crustacean cardioactive peptide (CCAP) neurons, an event that is critical for the timing of ecdysis and eclosion. Finally, we will discuss some recent findings that transformed our understanding of JH function.

The Circadian Control of Eclosion

Eclosion is the stage in development when the adult insect emerges from the shell of its old cuticle. The sequence of behaviors necessary for eclosion is coordinated by an integrated system of hormones and is activated by hormones that relay developmental readiness. The circadian clock, which controls the timing of behaviors such as the rest: activity rhythm of adult insects, also controls eclosion timing. A number of groups are actively investigating the mechanisms by which the circadian clock restricts or gates eclosion to a particular time of day. Data from these studies are beginning to reveal details of the molecular and physiological basis of the eclosion rhythm.

Morph-associated JH titer diel rhythm in Gryllus firmus: Experimental verification of its circadian basis and cycle characterization in artificially selected lines raised in the field

Previous studies demonstrated a high-amplitude, diel cycle for the hemolymph JH titer in the wing-polymorphic cricket, Gryllus firmus. The JH titer rose and fell in the flight-capable morph (long-winged, LW(f)) above and below the relatively temporally invariant JH titer in the flightless (short-winged, SW) morph. The morph-specific JH titer cycle appeared to be primarily driven by a morph-specific diel cycle in the rate of JH biosynthesis. In the present study, cycles of the JH titer and rate of JH biosynthesis in the LW(f) morph persisted in the laboratory under constant darkness with an approximate 24h periodicity. The JH titer cycle also shifted in concert with a shift in the onset of the scotophase, was temperature compensated in constant darkness, and became arrhythmic under constant light. These results provide strong support for the circadian basis of the morph-specific diel rhythm of the JH titer and JH biosynthetic rate. Persistence of the JH titer cycle under constant darkness in multiple LW-selected and SW-selected stocks also provides support for the genetic basis of the morph-associated circadian rhythm. The morph-specific JH titer cycle was observed in these stocks raised in the field, in both males and females, in each of 3 years studied. The onset of the cycle in the LW(f) morph, a few hours before sunset, correlated well with the onset of the cycle, a few hours before lights-off, in the laboratory. The morph-specific JH titer cycle is a general feature of G. firmus, under a variety of environmental conditions, and is not an artifact of specific laboratory conditions or specific genetic stocks. It is a powerful experimental model to investigate the mechanisms underlying endocrine circadian rhythms, their evolution, and their impact on life history evolution.

Molecular mechanism and cellular distribution of insect circadian clocks

Circadian clocks are endogenous timing mechanisms that control molecular, cellular, physiological, and behavioral rhythms in all organisms from unicellulars to humans. Circadian rhythms influence many aspects of insect biology, finetuning life functions to the light and temperature cycles associated with the solar day. Genetic studies in the fruit fly Drosophila melanogaster have led to the cloning and characterization of several genes involved in the mechanism of the circadian clock. Periodic transcription and translation of these clock genes form the basis of a molecular feedback loop that has a "circa" 24-hour period. Rhythmic expression of clock genes in specific brain neurons appears to control behavioral rhythms in adult flies. However, clock genes are also expressed in other tissues, both within and outside of the nervous system. These observations prompted chronobiologists to investigate whether nonneural tissues possess intrinsic circadian clocks, what role they may be playing, and what the relationships are between clocks in the nervous system and those in peripheral tissues. Answers to those questions are providing important insights into the overall organization of the circadian system in insects.

Insect circadian clocks: is it all in their heads?

Circadian rhythms are ubiquitous in living organisms, synchronizing life functions at the biochemical, physiological, and behavioral levels. The rhythm-generating mechanisms, collectively known as circadian clocks, are not fully understood in any organism. Research in the fruit fly Drosophila has led to the identification of several clock genes that are involved in the function of the brain-centered clock, which controls behavioral rhythms of adult flies. With the use of clock genes as markers, putative circadian clocks were mapped in the fly peripheral organs and shown to be independent from clocks located in the brain. A homologue of fruit fly period gene has been identified in moths and other insects, allowing investigations of this gene's role in known insect rhythms. This approach may increase our understanding of how circadian clocks are organized into the circadian system that orchestrates temporal integration of life processess in insects.

Rhythms of Drosophila period gene expression in culture

The Drosophila clock genes period (per) and timeless (tim) have been studied behaviorally and biochemically, but to date there has been no viable culture system for studying the cell biology of the Drosophila clock. We have cultured pupal ring glands attached to the central nervous system and observed rhythms of period gene expression in the prothoracic gland for 4-7 days. A daily rhythm of Per protein can be entrained by light in culture, even when neural activity is blocked by tetrodotoxin. In cultures maintained for a week in constant darkness, a per-luciferase reporter gene revealed circadian rhythms of bioluminescence. As the first circadian culture system from Drosophila, the prothoracic gland provides unique advantages for investigating the interactions between clock genes and cellular physiology.