In both insects and mammals, olfactory receptor neurons (ORNs) expressing specific olfactory receptors converge their axons onto specific glomeruli,creating a spatial map in the brain. We have previously shown that second order projection neurons (PNs) in Drosophila are prespecified by lineage and birth order to send their dendrites to one of ∼50 glomeruli in the antennal lobe. How can a given class of ORN axons match up with a given class of PN dendrites? Here, we examine the cellular and developmental events that lead to this wiring specificity. We find that, before ORN axon arrival,PN dendrites have already created a prototypic map that resembles the adult glomerular map, by virtue of their selective dendritic localization. Positional cues that create this prototypic dendritic map do not appear to be either from the residual larval olfactory system or from glial processes within the antennal lobe. We propose instead that this prototypic map might originate from both patterning information external to the developing antennal lobe and interactions among PN dendrites.

In recent years, Drosophila melanogaster has emerged as a powerful model for neuronal circuit development, pathology, and function. A major impediment to these studies has been the lack of a genetically encoded, specific, universal, and phenotypically neutral marker of the somatodendritic compartment. We have developed such a marker and show that it is effective and specific in all neuronal populations tested in the peripheral and central nervous system. The marker, which we name DenMark (Dendritic Marker), is a hybrid protein of the mouse protein ICAM5/Telencephalin and the red fluorescent protein mCherry. We show that DenMark is a powerful tool for revealing novel aspects of the neuroanatomy of developing dendrites, identifying previously unknown dendritic arbors, and elucidating neuronal connectivity.T o discover neuronal circuit architecture, genetic tools that specifically mark the pre-and postsynaptic cells and compartments are necessary. Drosophila is a leading genetic model organism in this regard; however, most neuronal circuits remain unmapped. Of particular note is the lack of a universal, phenotypically neutral, and specific marker of the somatodendritic and postsynaptic compartments. Several molecular differences between dendrites and axons, including the presence of different membrane and cytoskeletal proteins in neuronal subregions, have been identified (1, 2). Drosophila neurons exhibit the major kinds of compartmentalization present in mammalian neurons and the fly has emerged as a powerful system to study the establishment and maintenance of neuronal connections (3, 4). Almost all studies of neuronal circuits in the fly have relied on genetic markers such as CD8::GFP that outline the morphology of entire cells rather than particular subcellular compartments (5), as well as presynaptic markers such as Synaptotagmin, Synaptobrevin, and Bruchpilot GFP fusion proteins (6-11). However, more accurate identification and mapping of novel neuronal circuits has been hampered by the lack of a genetically encoded and phenotypically neutral dendritic marker. Over the years, many such markers have been proposed and several were recently examined (12), namely MAP2 (13, 14), Nod::YFP (4, 15-18), Homer::GFP (19), and DSCAM17.1::GFP (20, 21). The analysis of these markers reveals that none of them labels the entire somatodendritic field. Furthermore, it remains unclear whether the markers tested are neutral with respect to dendritic morphology.Intercellular adhesion molecules (ICAMs) mediate neuronal migration, axon elongation, and fasciculation, synaptogenesis, and synaptic plasticity (22). ICAM5, or Telencephalin, is a 130-kDa type I transmembrane glycoprotein comprising a characteristic extracellular domain, a single transmembrane region, and a short cytoplasmic region (23). The expression of ICAM5 is restricted to the mammalian brain telencephalon (24) but there is no homolog in invertebrates and lower vertebrates. The developmental appearance of ICAM5 parallels the time of dendritic elongation, branching, a...

The basic design of the larval olfactory system is similar to the adult one. However, ORNs and projection neurons lack cellular redundancy and do not exhibit any convergent or divergent connectivity; 21 ORNs confront essentially similar numbers of antennal-lobe glomeruli, projection neurons, and calycal glomeruli. Hence, we propose the Drosophila larva as an "elementary" olfactory model system.

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased (ϳ10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

A simple nervous system combined with stereotypic behavioral responses to tastants, together with powerful genetic and molecular tools, have turned Drosophila larvae into a very promising model for studying gustatory coding. Using the Gal4/UAS system and confocal microscopy for visualizing gustatory afferents, we provide a description of the primary taste center in the larval central nervous system. Essentially, gustatory receptor neurons target different areas of the subesophageal ganglion (SOG), depending on their segmental and sensory organ origin. We define two major and two smaller subregions in the SOG. One of the major areas is a target of pharyngeal sensilla, the other one receives inputs from both internal and external sensilla. In addition to such spatial organization of the taste center, circumstantial evidence suggests a subtle functional organization: aversive and attractive stimuli might be processed in the anterior and posterior part of the SOG, respectively. Our results also suggest less coexpression of gustatory receptors than proposed in prior studies. Finally, projections of putative second-order taste neurons seem to cover large areas of the SOG. These neurons may thus receive multiple gustatory inputs. This suggests broad sensitivity of secondary taste neurons, reminiscent of the situation in mammals.

The sense organs of adult Drosophila, and holometabolous insects in general, derive essentially from imaginal discs and hence are adult specific. Experimental evidence presented here, however, suggests a different developmental design for the three largely gustatory sense organs located along the pharynx. In a comprehensive cellular analysis, we show that the posteriormost of the three organs derives directly from a similar larval organ and that the two other organs arise by splitting of a second larval organ. Interestingly, these two larval organs persist despite extensive reorganization of the pharynx. Thus, most of the neurons of the three adult organs are surviving larval neurons. However, the anterior organ includes some sensilla that are generated during pupal stages. Also, we observe apoptosis in a third larval pharyngeal organ. Hence, our experimental data show for the first time the integration of complex, fully differentiated larval sense organs into the nervous system of the adult fly and demonstrate the embryonic origin of their neurons. Moreover, they identify metamorphosis of this sensory system as a complex process involving neuronal persistence, generation of additional neurons and neuronal death. Our conclusions are based on combined analysis of reporter expression from P[GAL4] driver lines, horseradish peroxidase injections into blastoderm stage embryos, cell labeling via heat-shock-induced flip-out in the embryo, bromodeoxyuridine birth dating and staining for programmed cell death. They challenge the general view that sense organs are replaced during metamorphosis.

Size trade-offs of visual versus olfactory organs is a pervasive feature of animal evolution. This could result from genetic or functional constraints. We demonstrate that head sensory organ size tradeoffs in Drosophila are genetically encoded and arise through differential subdivision of the head primordium into visual versus non-visual fields. We discover that changes in the temporal regulation of the highly conserved eyeless/Pax6 gene expression during development is a conserved mechanism for sensory trade-offs within and between Drosophila species. We identify a natural single nucleotide polymorphism in the cis-regulatory region of eyeless in a binding site of its repressor Cut that is sufficient to alter its temporal regulation and eye size. Because eyeless/Pax6 is a conserved regulator of head sensory placode subdivision, we propose that its temporal regulation is key to define the relative size of head sensory organs.

Animals are characterized by a set of highly conserved developmental regulators. Changes in the cis-regulatory elements of these regulators are thought to constitute the major driver of morphological evolution. However, the role of coding sequence evolution remains unresolved. To address this question, we used the Atonal family of proneural transcription factors as a model. Drosophila atonal coding sequence was endogenously replaced with that of atonal homologues (ATHs) at key phylogenetic positions, non-ATH proneural genes, and the closest homologue to ancestral proneural genes. ATHs and the ancestral-like coding sequences rescued sensory organ fate in atonal mutants, in contrast to non-ATHs. Surprisingly, different ATH factors displayed different levels of proneural activity as reflected by the number and functionality of sense organs. This proneural potency gradient correlated directly with ATH protein stability, including in response to Notch signaling, independently of mRNA levels or codon usage. This establishes a distinct and ancient function for ATHs and demonstrates that coding sequence evolution can underlie quantitative variation in sensory development and function.DOI: http://dx.doi.org/10.7554/eLife.26402.001