Now that the genes controlling embryonic patterning have been identified in several model organisms, long-standing questions concerning the evolution of developmental systems are open to investigation. Examination of the expression of even-skipped in a variety of insects reveals that Insect germ-type designations apparently do not reflect the variations in the mechanisms of segmentation evident throughout insect phylogeny.

Serotonin is an important neuromodulator associated with a wide range of physiological effects in the central nervous system. The exact mechanisms for how serotonin influences brain development are not well understood, although studies in invertebrate and vertebrate model organisms are beginning to unravel a regulatory role for serotonin in neuronal morphology and circuit formation. Recent data suggests a developmental window during which altered serotonin levels permanently impact circuitry, however, the temporal constraints and molecular mechanisms responsible are still under investigation. Growing evidence suggests that alterations in early serotonin signaling contribute to a number of neurodevelopmental and neuropsychiatric disorders. Thus, understanding how altered serotonin signaling affects neuronal morphology and plasticity, and ultimately animal physiology and pathophysiology, will be of great significance.

Serotonin signaling plays a key role in the regulation of development, mood and behavior. Drosophila is well suited for the study of the basic mechanisms of serotonergic signaling, but the small size of its nervous system has previously precluded the direct measurements of neurotransmitters. This study demonstrates the first real-time measurements of changes in extracellular monoamine concentrations in a single larval Drosophila ventral nerve cord. Channelrhodopsin2-mediated, neuronal type-specific stimulation is used to elicit endogenous serotonin release, which is detected using fast-scan cyclic voltammetry at an implanted microelectrode. Release is decreased when serotonin synthesis or packaging are pharmacologically inhibited, confirming that the detected substance is serotonin. Similar to tetanus-evoked serotonin release in mammals, evoked serotonin concentrations are 280 -640 nM in the fly, depending on the stimulation length. Extracellular serotonin signaling is prolonged after administering cocaine or fluoxetine, showing that transport regulates the clearance of serotonin from the extracellular space. When ChR2 is targeted to dopaminergic neurons, dopamine release is measured demonstrating that this method is broadly applicable to other neurotransmitter systems. This study shows that the dynamics of serotonin release and reuptake in Drosophila are analogous to those in mammals, making this simple organism more useful for the study of the basic physiological mechanisms of serotonergic signaling.

Spectacular examples of cooperative behavior emerge among a variety of animals and may serve critical roles in fitness [1, 2]. However, the rules governing such behavior have been difficult to elucidate [2]. Drosophila larvae are known to socially aggregate [3, 4] and use vision, mechanosensation, and gustation to recognize each other [5-8]. We describe here a model experimental system of cooperative behavior involving Drosophila larvae. While foraging in liquid food, larvae are observed to align themselves and coordinate their movements in order to drag a common air cavity and dig deeper. Large-scale cooperation is required to maintain contiguous air contact across the posterior breathing spiracles. On the basis of a directed genetic screen we find that vision plays a key role in cluster dynamics. Our experiments show that blind larvae form fewer clusters and dig less efficiently than wild-type and that socially isolated larvae behave as if they were blind. Furthermore, we observed that blind and socially isolated larvae do not integrate effectively into wild-type clusters. Behavioral data indicate that vision and social experience are required to coordinate precise movements between pairs of larvae, therefore increasing the degree of cooperativity within a cluster. Hence, we hypothesize that vision and social experience allow Drosophila larvae to assemble cooperative digging groups leading to more effective feeding and potential evasion of predators. Most importantly, these results indicate that control over membership of such a cooperative group can be regulated.

Drosophila melanogaster, the fruit fly, is a commonly used model organism because of its homology to mammals and facile genetic manipulations. However, the size of the nervous system is very small. We report a method to evoke and detect rapid changes in extracellular dopamine in a single nerve cord isolated from a Drosophila larva. Flies were genetically modified to express Channelrhodopsin-2, a blue-light activated cation channel, in only dopaminergic neurons. Extracellular dopamine changes were measured with fast-scan cyclic voltammetry at an implanted carbon-fiber microelectrode. Seven-second stimulations with blue light result in an average peak dopamine concentration of 810 ± 60 nM, similar to electrically-stimulated release in mammals. Stimulations repeated at 15-minute intervals are stable for 65 minutes, allowing pharmacological experiments in the same sample. Peak duration is extended after cocaine or nisoxetine, inhibitors of the dopamine transporter (DAT). Release was reduced upon exposure to reserpine, which inhibits vesicular packaging. Chronic administration of NSD-1015, a dopamine synthesis inhibitor, decreased dopamine release and inhibited pupation, showing a link between neurotransmission and physiology. This is the first method to measure endogenous dopamine in an intact larval Drosophila nervous system and will allow studies of genetic and pharmacological manipulations of dopamine release and uptake.

Serotonin is a classical small-molecule neurotransmitter with known effects on developmental processes. Previous studies have shown a developmental role for serotonin in the fly peripheral nervous system. In this study, we show that serotonin can modulate the development of serotonergic varicosities within the fly central nervous system. We have developed a system to examine the development of serotonergic varicosities in the larval CNS. We use this method to describe the normal serotonergic development in the A7 abdominal ganglion. From first to third instar larvae, the volume of the neuropil and number of serotonergic varicosities increase substantially while the varicosity density remains relatively constant. We hypothesize that serotonin is an autoregulator for serotonergic varicosity density. We tested the sensitivity of serotonergic varicosities to serotonin by adding neurotransmitter at various stages to isolated larval ventral nerve cords. Addition of excess exogenous serotonin decreases native varicosity density in older larvae, and these acute effects are reversible. The effects of serotonin appear to be selective for serotonergic varicosities, as dopaminergic and corazonergic varicosities remain qualitatively intact following serotonin application.

Gene 10 of bacteriophage T7, which encodes the most abundant capsid protein, has two products: a major product, 10A (36 kDa), and a minor product, 10B (41 kDa). 10B is produced by frameshifting into the -1 frame near the end of the 10A coding frame and is incorporated into the capsid. The frameshift occurs at a frequency of about 10% and is conserved in bacteriophage T3. This study shows that sequences important to frameshifting include the originally proposed frameshift site, consisting of overlapping phenylalanine codons and the 3' noncoding region that includes the transcriptional terminator over 200 bases downstream of the frameshift site. The frameshift occurs at the overlapping phenylalanine codons as determined from peptide sequencing data. Complementation studies show that there is only a very weak phenotype associated with phage infections in which there is no 10A frameshifting. Capsids from such infections are devoid of 10B and are as stable as wild-type capsids.The accuracy by which genetic information is copied into a protein sequence is probably a trade-off between the final requirement of a certain amount of active product and the energy cost of avoiding errors during synthesis. Careful studies of the translational apparatus have revealed a background error frequency that is, as expected, very low (for an excellent review, see reference 12). However, there are now a number of examples of what appear to be mistakes that occur at frequencies far higher than expected. Perhaps the most characterized of these are the frameshifts (1). In these cases a significant proportion of ribosomes switch reading frames at a specific site in a coding region, thus entering a new frame. The result is that a single mRNA can produce two products, a protein from the normal coding frame and a transframe fusion protein. In many cases it is known that the frameshift product is essential (7). Thus it appears that, where it is advantageous to synthesize two such products, specific properties of the frame-maintaining process are exploited. Knowledge of the signals that subvert normal frame maintenance could be very informative about how the translational apparatus works and how it can be regulated.Probably the most studied of the frameshifts are the retroviral gag-pol and coronaviral examples and the Escherichia coli release factor 2 (RF2) gene (2, 4, 7). In RF2, there is a special frameshift site bounded on the 3' side by a stop codon and on the 5' side by a Shine-Dalgamo-like sequence. It has been shown that the Shine-Dalgarno sequence must interact with the 3' end of the 16S rRNA for frameshifting (21). All three signals are required for the final high-level frameshift reaction. The essential features of the retroviral and coronaviral cases are the requirement for a homopolymeric run of bases at the frameshift site and a downstream mRNA secondary structure. The function of the run of bases seems simple in that it allows the maintenance of tRNA codon-anticodon pairing before and after the frameshift. An emerging view of fram...

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