The clock gene period of the housefly, Musca domestica, rescues behavioral rhythmicity in Drosophila melanogaster. Evidence for intermolecular coevolution?

Extended disordered regions of ribosome-associated NAC proteins paralogs belong only to the germline in Drosophila melanogaster

The nascent polypeptide-associated complex (NAC) consisting of α- and β-subunits is an essential ribosome-associated protein conserved in eukaryotes. NAC is a ubiquitously expressed co-translational regulator of nascent protein folding and sorting providing for homeostasis of cellular proteins. Here we report on discovering the germline-specific NACαβ paralogs (gNACs), whose β-subunits, non-distinguishable by ordinary immunodetection, are encoded by five highly homologous gene copies, while the α-subunit is encoded by a single αNAC gene. The gNAC expression is detected in the primordial embryonic and adult gonads via immunostaining. The germline-specific α and β subunits differ from the ubiquitously expressed paralogs by the extended intrinsically disordered regions (IDRs) acquired at the N- and C-termini of the coding regions, predicted to be phosphorylated. The presence of distinct phosphorylated isoforms of gNAC-β subunits is confirmed by comparing of their profiles by 2D-isoeletrofocusing resolution before and after phosphatase treatment of testis ribosomes. We revealed that the predicted S/T sites of phosphorylation in the individual orthologous IDRs of gNAC-β sequences of Drosophila species are positionally conserved despite these disordered regions are drastically different. We propose the IDR-dependent molecular crowding and specific coordination of NAC and other proteostasis regulatory factors at the ribosomes of germinal cells. Our findings imply that there may be a functional crosstalk between the germinal and ubiquitous α- and β-subunits based on assessing their depletion effects on the fly viability and gonad development.

Interspecific studies of circadian genes period and timeless in Drosophila

The level of rescue of clock function in genetically arrhythmic Drosophila melanogaster hosts using interspecific clock gene transformation was used to study the putative intermolecular coevolution between interacting clock proteins. Among them PER and TIM are the two important negative regulators of the circadian clock feedback loop. We transformed either the D. pseudoobscura per or tim transgenes into the corresponding arrhythmic D. melanogaster mutant (per01 or tim01) and observed >50% rhythmicity but the period of activity rhythm was either longer (D. pseudoobscura-per) or shorter than 24 h (D. pseudoobscura-tim) compared to controls. By introducing both transgenes simultaneously into double mutants, we observed that the period of the activity rhythm was rescued by the pair of hemizygous transgenes (~24 h). These flies also showed a more optimal level of temperature compensation for the period. Under LD 12:12 these flies have a D. pseudoobscura like activity profile with the absence of morning anticipation as well as a very prominent earlier evening peak of activity rhythm. These observation are consistent with the view that TIM and PER form a heterospecific coevolved module at least for the circadian period of activity rhythms. However the strength of rhythmicity was reduced by having both transgenes present, so while evidence for a coevolution between PER and TIM is observed for some characters it is not for others.

Daily Activity of the Housefly, Musca domestica, Is Influenced by Temperature Independent of 3' UTR period Gene Splicing

Circadian clocks orchestrate daily activity patterns and free running periods of locomotor activity under constant conditions. While the first often depends on temperature, the latter is temperature-compensated over a physiologically relevant range. Here, we explored the locomotor activity of the temperate housefly Musca domestica Under low temperatures, activity was centered round a major and broad afternoon peak, while high temperatures resulted in activity throughout the photophase with a mild midday depression, which was especially pronounced in males exposed to long photoperiods. While period (per) mRNA peaked earlier under low temperatures, no temperature-dependent splicing of the last per 3' end intron was identified. The expression of timeless, vrille, and Par domain protein 1 was also influenced by temperature, each in a different manner. Our data indicated that comparable behavioral trends in daily activity distribution have evolved in Drosophila melanogaster and M. domestica, yet the behaviors of these two species are orchestrated by different molecular mechanisms.

The Effects of Sex-Ratio and Density on Locomotor Activity in the House Fly, Musca domestica

Although locomotor activity is involved in almost all behavioral traits, there is a lack of knowledge on what factors affect it. This study examined the effects of sex—ratio and density on the circadian rhythm of locomotor activity of adult Musca domestica L. (Diptera: Muscidae) using an infra—red light system. Sex—ratio significantly affected locomotor activity, increasing with the percentage of males in the vials. In accordance with other studies, males were more active than females, but the circadian rhythm of the two sexes was not constant over time and changed during the light period. There was also an effect of density on locomotor activity, where males at intermediate densities showed higher activity. Further, the predictability of the locomotor activity, estimated as the degree of autocorrelation of the activity data, increased with the number of males present in the vials both with and without the presence of females. Overall, this study demonstrates that locomotor activity in M. domestica is affected by sex—ratio and density. Furthermore, the predictability of locomotor activity is affected by both sex—ratio, density, and circadian rhythm. These results add to our understanding of the behavioral interactions between houseflies and highlight the importance of these factors when designing behavioral experiments using M. domestica.

Insect photoperiodic calendar and circadian clock: independence, cooperation, or unity?

The photoperiodic calendar is a seasonal time measurement system which allows insects to cope with annual cycles of environmental conditions. Seasonal timing of entry into diapause is the most often studied photoperiodic response of insects. Research on insect photoperiodism has an approximately 80-year-old tradition. Despite that long history, the physiological mechanisms underlying functionality of the photoperiodic calendar remain poorly understood. Thus far, a consensus has not been reached on the role of another time measurement system, the biological circadian clock, in the photoperiodic calendar. Are the two systems physically separated and functionally independent, or do they cooperate, or is it a single system with dual output? The relationship between calendar and clock functions are the focus of this review, with particular emphasis on the potential roles of circadian clock genes, and the circadian clock system as a whole, in the transduction pathway for photoperiodic token stimulus to the overt expression of facultative diapause.

Identification and characterization of circadian clock genes in the pea aphid Acyrthosiphon pisum

The molecular basis of circadian clocks is highly evolutionarily conserved and has been best characterized in Drosophila and mouse. Analysis of the Acyrthosiphon pisum genome revealed the presence of orthologs of the following genes constituting the core of the circadian clock in Drosophila: period (per), timeless (tim), Clock, cycle, vrille, and Pdp1. However, the presence in A. pisum of orthologs of a mammal-type in addition to a Drosophila-type cryptochrome places the putative aphid clockwork closer to the ancestral insect system than to the Drosophila one. Most notably, five of these putative aphid core clock genes are highly divergent and exhibit accelerated rates of change (especially per and tim orthologs) suggesting that the aphid circadian clock has evolved to adapt to (unknown) aphid-specific needs. Additionally, with the exception of jetlag (absent in the aphid) other genes included in the Drosophila circadian clock repertoire were found to be conserved in A. pisum. Expression analysis revealed circadian rhythmicity for some core genes as well as a significant effect of photoperiod in the amplitude of oscillations.

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.

Length variation in a specific region of the period gene correlates with differences in pupal diapause incidence in the flesh fly, Sarcophaga bullata

We report differences in the length of a specific region of the circadian clock gene period (per) that correlate with different capacities for pupal diapause in the flesh fly, Sarcophaga bullata. The conspicuous difference is located in a region we refer to as the putative C-terminal photoperiodic (CP) region. The length of the CP region correlates inversely with the incidence of diapause. A deletion of 33 amino acids in this region correlates with a significant increase in the incidence of diapause, from 78.1% to 93.0%, and an insertion of 9 amino acids in the same area correlates with a drop in the diapause incidence to 4.0%. This correlation suggests a possible functional role for this region of per in photoperiodism.

Comparative analysis of circadian clock genes in insects

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.

Circadian rhythm gene regulation in the housefly Musca domestica

The circadian mechanism appears remarkably conserved between Drosophila and mammals, with basic underlying negative and positive feedback loops, cycling gene products, and temporally regulated nuclear transport involving a few key proteins. One of these negative regulators is PERIOD, which in Drosophila shows very similar temporal and spatial regulation to TIMELESS. Surprisingly, we observe that in the housefly, Musca domestica, PER does not cycle in Western blots of head extracts, in contrast to the TIM protein. Furthermore, immunocytochemical (ICC) localization using enzymatic staining procedures reveals that PER is not localized to the nucleus of any neurons within the brain at any circadian time, as recently observed for several nondipteran insects. However, with confocal analysis, immunofluorescence reveals a very different picture and provides an initial comparison of PER/TIM-containing cells in Musca and Drosophila, which shows some significant differences, but many similarities. Thus, even in closely related Diptera, there is considerable evolutionary flexibility in the number and spatial organization of clock cells and, indeed, in the expression patterns of clock products in these cells, although the underlying framework is similar.

Organization of endogenous clocks in insects

Insect and mammalian circadian clocks show striking similarities. They utilize homologous clock genes, generating self-sustained circadian oscillations in distinct master clocks of the brain, which then control rhythmic behaviour. The molecular mechanisms of rhythm generation were first uncovered in the fruit fly Drosophila melanogaster, whereas cockroaches were among the first animals where the brain master clock was localized. Despite many similarities, there exist obvious differences in the organization and functioning of insect master clocks. These similarities and differences are reviewed on a molecular and anatomical level.

How similar are daily and seasonal biological clocks?

Daily and seasonal timing systems in insects have usually been supposed to share similar mechanisms, because both rely in large measure on information from the daily light-dark cycle: daily clocks can ensure that activity coincides with the appropriate time of day, and seasonal time is indicated most reliably by daylength. However, several lines of evidence suggest that the systems are different. For example, receptor features, photosensitive pigments, clocks, and the effectors that mediate responses to information derived from the clock may have different daily, seasonal and general functions and properties, and several different systems are known. There are many different additional elements in the seasonal response. Therefore, these responses may not rely on similar timing mechanisms, despite the long-standing belief that the seasonal clock has circadian components. Such a difference would be consistent with the fact that temporal responses serve a very wide range of purposes, meeting many different ecological needs on different time frames. Consequently, understanding the seasonal relevance of the photoperiodic responses is more important than revealing any possible involvement with circadian systems.

Evolution of temporal order in living organisms

Circadian clocks are believed to have evolved in parallel with the geological history of the earth, and have since been fine-tuned under selection pressures imposed by cyclic factors in the environment. These clocks regulate a wide variety of behavioral and metabolic processes in many life forms. They enhance the fitness of organisms by improving their ability to efficiently anticipate periodic events in their external environments, especially periodic changes in light, temperature and humidity. Circadian clocks provide fitness advantage even to organisms living under constant conditions, such as those prevailing in the depth of oceans or in subterranean caves, perhaps by coordinating several metabolic processes in the internal milieu. Although the issue of adaptive significance of circadian rhythms has always remained central to circadian biology research, it has never been subjected to systematic and rigorous empirical validation. A few studies carried out on free-living animals under field conditions and simulated periodic and aperiodic conditions of the laboratory suggest that circadian rhythms are of adaptive value to their owners. However, most of these studies suffer from a number of drawbacks such as lack of population-level replication, lack of true controls and lack of adequate control on the genetic composition of the populations, which in many ways limits the potential insights gained from the studies. The present review is an effort to critically discuss studies that directly or indirectly touch upon the issue of adaptive significance of circadian rhythms and highlight some shortcomings that should be avoided while designing future experiments.

Identification of putative noncoding polyadenylated transcripts in Drosophila melanogaster

Analysis of EST and cDNA collections from a number of metazoan species has identified genes encoding long polyadenylated transcripts that do not contain ORFs of lengths typical for protein-encoding mRNAs. Noncoding functions of such polyadenylated transcripts have been elucidated in only a few examples. The corresponding genes neither contain hallmark sequence motifs nor appear to have been conserved across phyla. Thus, it is impossible to systematically identify new members of this class of gene by using sequence homology and traditional gene-finding algorithms that depend on protein-coding potential. Consequently, even their approximate number has not been established for any metazoan genome. We curated polyadenylated transcripts with limited protein-coding capacity from intergenic regions of the Drosophila melanogaster genome. We used RT-PCR assays, hybridization to RNA blots and whole-mount embryos, and computational analyses to characterize candidate transcripts. We verify the structures and expression of 17 distinct, likely non-protein-coding polyadenylated transcripts. We show that the expression of many of these transcripts is conserved in other Drosophila species, indicating that they have important biological functions.

Molecular Evolution and Population Genetics of Circadian Clock Genes

This article discusses a number of common methodologies used in the field of population genetics and evolution and reviews their application within circadian rhythm research. We examine the basic principles behind phylogenetic analysis and how these can be used to illuminate clock gene evolution. We then discuss genetic variation between and within species and show how neutrality tests can reveal the signatures of selection or drift on clock genes. These tests are particularly important for moving beyond "just so" stories when discussing the evolution of clock phenotypes, and we provide relevant circadian examples. We also focus on methods that can be used to study genetic variation, such as quantitative trait loci analysis. We discuss the various bootstrapping or resampling techniques that can be applied to generate confidence intervals in the various methodologies and then examine the use of interspecific transformation studies, which can, and have, provide some useful insights, not only into clock gene evolution in particular, but "behavioral" gene evolution in general. Finally, we assess gene/protein alignments and protein structure predictions and their implicit evolutionary bases.

The cycling and distribution of PER-like antigen in relation to neurons recognized by the antisera to PTTH and EH in Thermobia domestica

The cephalic nervous system of the firebrat contains antigens recognized by antisera to the clock protein period (PER), the prothoracicotropic hormone (PTTH) and the eclosion hormone (EH). The content of the 115 kDa PER-like antigen visualized on the western blots fluctuates in diurnal rhythm with a maximum in the night. The oscillations entrained in a 12:12 h light/dark (LD) cycle persist in the darkness and disappear in continuous light. They are detected by immunostaining in 14 pairs of the protocerebral neurons and are extreme in four suboesophageal neurons and two cells in each corpus cardiacum that contain PER only during the night phase. No circadian fluctuations occur in three lightly stained perikarya of the optic lobe. Five cell bodies located in each brain hemisphere between the deuto-and the tritocerebrum retain weak immunoreactivity under constant illumination. In all cells, the staining is confined to the cytoplasm and never occurs in the cell nuclei. The cells containing PER-like material do not react with the anti-PTTH and anti-EH antisera, which recognize antigens of about 50 and 20 kDa, respectively. The anti-PTTH antiserum stains in each brain hemisphere seven neurons in the protocerebrum, eight in the optic lobe, and 3-5 in the posterior region of the deutocerebrum. The antiserum to EH reacts in each hemisphere with just two cells located medially to the mushroom bodies. No cycling of the PTTH-like and EH-like antigens was detected.

Development of PDF-immunoreactive cells, possible clock neurons, in the housefly Musca domestica

Even though the housefly Musca domestica shows clear circadian rhythms in its behavioural and physiological processes, a circadian pacemaker system controlling these rhythms has not yet been described morphologically in this species. In M. domestica, neurons immunoreactive to pigment-dispersing factor (PDF), a neurotransmitter/neuromodulator of circadian information arising from a circadian clock and transmitted to target cells, are similar in their number and distribution to the PDF neurons of Drosophila melanogaster. In D. melanogaster these neurons co-localize PER protein and have been identified as clock neurons in that species. Here we report PDF-immunoreactive cells in the housefly's brain during postembryonic development in the larval and pupal stages, as well as in the adult fly soon after eclosion. In the housefly's brain, there are three groups of PDF-immunoreactive neurons: two groups with small (sPDFMe) and large (lPDFMe) cell bodies in the proximal medulla of the optic lobe; and one group in the dorsal protocerebrum (PDFD). Three out of four sPDFMe can be detected during the first hour of larval development, but the fourth sPDFMe is observed in the larva only from 48 hours after hatching, along with five lPDFMe neurons, seen first as two subgroups, and three out of four PDFD neurons. During postembryonic development these neurons show changes in their structure and immunoreactivity. New PDF neurons are observed during pupal development but these neurons mostly do not survive into adulthood. In the adult fly's brain, the PDF neurons have also been examined in double-labelled preparations made with a second antibody directed against the product of one of several clock genes: period (per), timeless (tim), or cryptochrome (cry). Among them, only immunoreactivity to CRY-like protein has been detected in the brain of M. domestica and has shown a daily rhythm in its concentration, as examined immunocytochemically. CRY was co-localized with PDF in the sPDFMe of the housefly's brain fixed during the day. The possibility that the sPDFMe neurons are the housefly's clock neurons is discussed.

Genetics and molecular biology of rhythms in Drosophila and other insects

Application of generic variants (Sections II-IV, VI, and IX) and molecular manipulations of rhythm-related genes (Sections V-X) have been used extensively to investigate features of insect chronobiology that might not have been experimentally accessible otherwise. Most such tests of mutants and molecular-genetic xperiments have been performed in Drosophila melanogaster. Results from applying visual-system variants have revealed that environmental inputs to the circadian clock in adult flies are mediated by external photoreceptive structures (Section II) and also by direct light reception chat occurs in certain brain neurons (Section IX). The relevant light-absorbing molecuLes are rhodopsins and "blue-receptive" cryptochrome (Sections II and IX). Variations in temperature are another clock input (Section IV), as has been analyzed in part by use of molecular techniques and transgenes involving factors functioning near the heart of the circadian clock (Section VIII). At that location within the fly's chronobiological system, approximately a half-dozen-perhaps up to as many as 10-clock genes encode functions that act and interact to form the circadian pacemaker (Sections III and V). This entity functions in part by transcriptional control of certain clock genes' expressions, which result in the production of key proteins that feed back negatively to regulate their own mRNA production. This occurs in part by interactions of such proteins with others that function as transcriptional activators (Section V). The implied feedback loop operates such that there are daily variations in the abundances of products put out by about one-half of the core clock genes. Thus, the normal expression of these genes defines circadian rhythms of their own, paralleling the effects of mutations at the corresponding genetic loci (Section III), which are to disrupt or apparently eliminate clock functioning. The fluctuations in the abundance of gene products are controlled transciptionally and posttranscriptionally. These clock mechanisms are being analyzed in ways that are increasingly complex and occasionally obscure; not all panels of this picture are comprehensive or clear, including problems revolving round the biological meaning or a given features of all this molecular cycling (Section V). Among the complexities and puzzles that have recently arisen, phenomena that stand out are posttranslational modifications of certain proteins that are circadianly regulated and regulating; these biochemical events form an ancillary component of the clock mechanism, as revealed in part by genetic identification of Factors (Section III) that turned out to encode protein kinases whose substrates include other pacemaking polypeptides (Section V). Outputs from insect circadian clocks have been long defined on formalistic and in some cases concrete criteria, related to revealed rhythms such as periodic eclosion and daily fluctuations of locomotion (Sections II and III). Based on the reasoning that if clock genes can regulate circadian cyclings of their own products, they can do the same for genes that function along output pathways; thus clock-regulated genes have been identified in part by virtue of their products' oscillations (Section X). Those studied most intensively have their expression influenced by circadian-pacemaker mutations. The clock-regulated genes discovered on molecular criteria have in some instances been analyzed further in their mutant forms and found to affect certain features of overt whole-organismal rhythmicity (Sections IV and X). Insect chronogenetics touches in part on naturally occurring gene variations that affect biological rhythmicity or (in some cases) have otherwise informed investigators about certain features of the organism's rhythm system (Section VII). Such animals include at least a dozen insect species other than D. melanogaster in which rhythm variants have been encountered (although usually not looked for systematically). The chronobiological "system" in the fruit fly might better be graced with a plural appellation because there is a myriad of temporally related phenomena that have come under the sway of one kind of putative rhythm variant or the other (Section IV). These phenotypes, which range well beyond the bedrock eclosion and locomotor circadian rhythms, unfortunately lead to the creation of a laundry list of underanalyzed or occult phenomena that may or may not be inherently real, whether or not they might be meaningfully defective under the influence of a given chronogenetic variant. However, such mutants seem to lend themselves to the interrogation of a wide variety of time-based attributes-those that fall within the experimental confines of conventionally appreciated circadian rhythms (Sections II, III, VI, and X); and others that consist of 24-hr or nondaily cycles defined by many kinds of biological, physiological, or biochemical parameters (Section IV).

A novel C-terminal domain of drosophila PERIOD inhibits dCLOCK:CYCLE-mediated transcription

The essence of the Drosophila circadian clock involves an autoregulatory feedback loop in which PERIOD (PER) and TIMELESS (TIM) inhibit their own transcription by association with the transcriptional activators dCLOCK (dCLK) and CYCLE (CYC). Because PER, dCLK, and CYC each contain a PAS domain, it has been assumed that these interaction domains are important for negative feedback. However, a critical role for PAS-PAS interactions in Drosophila clock function has not been shown. Nuclear transport of PER is also believed to be an essential regulatory step for negative feedback, but this has not been directly tested, and the relevant nuclear localization sequence (NLS) has not been functionally mapped. We evaluated these critical aspects of PER-mediated transcriptional inhibition in Drosophila Schneider 2 (S2) cells. We mapped the dCLK:CYC inhibition domain (CCID) of PER and discovered that it lies in the C terminus, downstream of the PAS domain. Using deletion mutants and site-directed mutagenesis, we identified a novel NLS in the CCID of PER that is a potent regulator of PER's nuclear transport in S2 cells. We further found that nuclear transport, primarily through this novel NLS, is essential for the inhibitory activity of PER. The data indicate that nuclear PER inhibits dCLK:CYC-mediated transcription through a novel domain that additionally contains a potent NLS.

Highly conserved Drosophila ananassae timeless gene functions as a clock component in Drosophila melanogaster

The behavior and physiology of Drosophila are subject to rhythms that are controlled by the circadian clock genes, period, timeless, clock and cycle, all of which are thought to participate in central pacemaker control. The molecular mechanism of rhythm in Drosophila has been studied in detail. However, rhythm and clock genes have mostly been analyzed in Drosophila melanogaster. To confirm whether the tim gene exists and works as a clock component in other Drosophila species, we cloned a tim homolog from Drosophila ananassae that shared 85.9% similarity with Drosophila melanogaster tim at the amino acid level. In addition, the PER interaction domains and NLS were highly conserved. Introduction of the D. ananassae tim homolog rescued the rhythm of the locomotor activity of about 44% of a population of D. melanogaster tim(01) flies. At the molecular level, hs-tim introduced not only TIM but PER oscillation in transgenic flies. These results indicate that the tim gene in D. ananassae functions as a component of the circadian clock in D. melanogaster.

maT--a clade of transposons intermediate between mariner and Tc1

A group of transposons, named maT, with characteristics intermediate between mariner and Tc1 transposons, is described. Two defective genomic copies of MdmaT from the housefly Musca domestica, with 85% identity, were found flanking and imbedded in the MdalphaE7 esterase gene involved in organophosphate insecticide resistance. Two cDNA clones, with 99% identity to each other and 72%-89% identity to the genomic copies were also obtained, but both represented truncated versions of the putative open reading frame. A third incomplete genomic copy of MdmaT was also identified upstream of the putative M. domestica period gene. The MdmaT sequences showed high identity to the transposable element Bmmar1 from the silkworm moth, Bombyx mori, and to previously unidentified sequences in the genome of Caenorhabditis elegans. A total of 16 copies of full-length maT sequences were identified in the C. elegans genome, representing three variants of the transposon, with 34%-100% identity amongst them. Twelve of the copies, named CemaT1, were virtually identical, with eight of them encoding a putative full length, intact transposase. Secondary structure predictions and phylogenetic analyses confirm that maT elements belong to the mariner-Tc1 superfamily of transposons, but their intermediate sequence and predicted structural characteristics suggest that they belong to a unique clade, distinct from either mariner-like or Tc1-like elements.

The period gene of the German cockroach and its novel linking power between vertebrate and invertebrate

Circadian clock protein PERIOD (PER) is essential for the endogenous clockworks in diverse lineages within Metazoa, but the protein sequences, the clock protein interactions, and the photic responses are variant and different between vertebrate and invertebrate PER homologs. Here we identified the German cockroach PER homologs and found it could bridge the huge phylogenetic gap and make possible a more precise protein sequence comparison between vertebrate and invertebrate PER homologs. Seven blocks of conserved regions (c1-c7) interspersed within PER proteins were defined, and two new significant homologies were found in the upstream portion of c3 region and in the c7 region, respectively. In addition, we found all dipteran insects PER homologs lack the c7 region and its following amino acid residues. Our results not only reveal the homology and divergence, but also imply the constraint and plasticity of divergent PER proteins during the course of evolution. These findings lay a solid foundation for understanding the general and divergent properties of circadian clockworks in diverse lineages within Metazoa.

Evolutionary behavioral genetics in Drosophila

Behavioral genes have a special evolutionary interest because they are potentially involved in speciation and in many forms of adaptation. Dozens of loci affecting different aspects of behavior have been already identified and cloned in Drosophila. Some of these genes determine variation in such ethologically complex phenotypes as the male "love song" that is produced during courtship and the locomotor "sleep-wake" activity cycles that are controlled by the circadian clock. Although the evolutionary analysis of most behavioral genes in Drosophila is relatively new, it has already given important insights into the forces shaping the molecular variation at these loci and their functional consequences.

The period gene in two species of tephritid fruit fly differentiated by mating behaviour

The period gene is important for the generation and maintenance of biological rhythms. It served as an ideal candidate for the investigation of the mating time isolation between two sibling Queensland fruit fly species, Bactrocera tryoni and Bactrocera neohumeralis. We have isolated the homologues of the period gene in the two species, and show that their putative amino acid sequences are identical. No length polymorphism was detected in the Thr-Gly repeat region. per mRNA expression, assayed in light-dark diurnal conditions, displayed circadian oscillation in both the head and abdomen of B. tryoni and B. neohumeralis, with the same cycling phase. An alternatively spliced intron was identified in the 3' untranslated region. The effect of temperature on the splicing and mRNA expression was examined.

Short-day and long-day expression patterns of genes involved in the flesh fly clock mechanism: period, timeless, cycle and cryptochrome

Though our knowledge of the molecular details of the circadian clock has advanced rapidly, the functional elements of the photoperiodic clock remain largely unknown. As a first step to approach this issue, we report here the sequences and expression patterns of period (per), timeless (tim), cycle (cyc) and cryptochrome (cry) mRNAs in the flesh fly Sarcophaga crassipalpis. Nucleotide and deduced amino acid sequences of the genes in S. crassipalpis show high similarity to homologous genes in other insects that have been investigated. S. crassipalpis TIM has a unique C-terminus that contains a poly Q region. A diel rhythmicity of per and tim mRNA abundance was detected in the adult heads (peak during scotophase), while cry and cyc mRNA abundance remained fairly constant throughout. The abundance of cyc mRNA was quite low when compared to per, tim and cry mRNA. Rearing temperature affected the amount of per and tim mRNAs: abundance of per mRNA increased at 20 degrees C when compared to 25 degrees C, but that of tim mRNA decreased. Photoperiod influenced the expression patterns of per and tim mRNA: the peak of per mRNA expression shifted in concert with onset of the scotophase, while a shift in tim mRNA expression was less pronounced. The amplitude of tim mRNA was severely dampened under long daylength, but that of per mRNA was not affected. These distinct patterns of expression suggest that this information could be used to determine photoperiodic responses such as diapause.

Flies, clocks and evolution

The negative feedback model for gene regulation of the circadian mechanism is described for the fruitfly, Drosophila melanogaster. The conservation of function of clock molecules is illustrated by comparison with the mammalian circadian system, and the apparent swapping of roles between various canonical clock gene components is highlighted. The role of clock gene duplications and divergence of function is introduced via the timeless gene. The impressive similarities in clock gene regulation between flies and mammals could suggest that variation between more closely related species within insects might be minimal. However, this is not borne out because the expression of clock molecules in the brain of the giant silk moth, Antheraea pernyi, is not easy to reconcile with the negative feedback roles of the period and timeless genes. Variation in clock gene sequences between and within fly species is examined and the role of co-evolution between and within clock molecules is described, particularly with reference to adaptive functions of the circadian phenotype.

Localization of the clock controlling circadian rhythms in the first neuropile of the optic lobe in the housefly

The visual system of a fly expresses several circadian rhythms that have been detected in the photoreceptors of the compound eye and in the first neuropile, the lamina, of the underlying optic lobe. In the lamina, axons of two classes of interneuron, L1 and L2, exhibit cyclical size changes, swelling by day and shrinking by night. These rhythmic size changes may be generated by circadian oscillators located inside and/or outside the optic lobe. To localize such oscillators, we have examined changes in the axonal cross-sectional areas of L1 and L2 within the lamina of the housefly (Musca domestica) under conditions of 12 h of light and 12 h of darkness (LD12:12), constant darkness (DD) or continuous light (LL) 24 h after the medulla was severed from the rest of the brain. After the lesion, the axon size changes of L1 and L2 were maintained only in LD conditions, but were weaker than in control flies. In DD and LL conditions, they were eliminated. This indicates that circadian rhythms in the lamina of a fly are generated central to the lamina and medulla neuropiles of the optic lobe. Cyclical changes of light and darkness in LD conditions are still able, however, to induce a weak daily rhythm in the axon sizes of L1 and L2.

Molecular components of the circadian system in Drosophila

Much of our current understanding of how circadian rhythms are generated is based on work done with Drosophila melanogaster. Molecular mechanisms used to assemble an endogenous clock in this organism are now known to underlie circadian rhythms in many other species, including mammals. The genetic amenability of Drosophila has led to the identification of some genes that encode components of the clock (so-called clock genes) and others that either link the clock to the environment or act downstream of it. The clock provides time-of-day cues by regulating levels of specific gene products such that they oscillate with a circadian rhythm. The mechanisms that synchronize these oscillations to light are understood to some extent. However, there are still large gaps in our knowledge, in particular with respect to the mechanisms used by the clock to control overt rhythms. It has, however, become clear that in addition to the brain clock, autonomous or semi-autonomous clocks occur in peripheral tissues where they confer circadian regulation on specific functions.