Functional dissection of the neural substrates for gravitaxic maze behavior in Drosophila melanogaster

A conditional glutamatergic synaptic vesicle marker for Drosophila

Glutamate is a principal neurotransmitter used extensively by the nervous systems of all vertebrate and invertebrate animals. It is primarily an excitatory neurotransmitter that has been implicated in nervous system development, as well as a myriad of brain functions from the simple transmission of information between neurons to more complex aspects of nervous system function including synaptic plasticity, learning, and memory. Identification of glutamatergic neurons and their sites of glutamate release are thus essential for understanding the mechanisms of neural circuit function and how information is processed to generate behavior. Here, we describe and characterize smFLAG-vGlut, a conditional marker of glutamatergic synaptic vesicles for the Drosophila model system. smFLAG-vGlut is validated for functionality, conditional expression, and specificity for glutamatergic neurons and synaptic vesicles. The utility of smFLAG-vGlut is demonstrated by glutamatergic neurotransmitter phenotyping of 26 different central complex neuron types of which nine were established to be glutamatergic. This illumination of glutamate neurotransmitter usage will enhance the modeling of central complex neural circuitry and thereby our understanding of information processing by this region of the fly brain. The use of smFLAG for glutamatergic neurotransmitter phenotyping and identification of glutamate release sites can be extended to any Drosophila neuron(s) represented by a binary transcription system driver.

Fertility and Iron Bioaccumulation in Drosophila melanogaster Fed with Magnetite Nanoparticles Using a Validated Method

Research on nanomaterial exposure-related health risks is still quite limited; this includes standardizing methods for measuring metals in living organisms. Thus, this study validated an atomic absorption spectrophotometry method to determine fertility and bioaccumulated iron content in Drosophila melanogaster flies after feeding them magnetite nanoparticles (Fe₃O₄NPs) dosed in a culture medium (100, 250, 500, and 1000 mg kg⁻¹). Some NPs were also coated with chitosan to compare iron assimilation. Considering both accuracy and precision, results showed the method was optimal for concentrations greater than 20 mg L⁻¹. Recovery values were considered optimum within the 95–105% range. Regarding fertility, offspring for each coated and non-coated NPs concentration decreased in relation to the control group. Flies exposed to 100 mg L⁻¹ of coated NPs presented the lowest fertility level and highest bioaccumulation factor. Despite an association between iron bioaccumulation and NPs concentration, the 500 mg L⁻¹ dose of coated and non-coated NPs showed similar iron concentrations to those of the control group. Thus, Drosophila flies’ fertility decreased after NPs exposure, while iron bioaccumulation was related to NPs concentration and coating. We determined this method can overcome sample limitations and biological matrix-associated heterogeneity, thus allowing for bioaccumulated iron detection regardless of exposure to coated or non-coated magnetite NPs, meaning this protocol could be applicable with any type of iron NPs.

A Neural Network for Wind-Guided Compass Navigation

Spatial maps in the brain are most accurate when they are linked to external sensory cues. Here, we show that the compass in the Drosophila brain is linked to the direction of the wind. Shifting the wind rightward rotates the compass as if the fly were turning leftward, and vice versa. We describe the mechanisms of several computations that integrate wind information into the compass. First, an intensity-invariant representation of wind direction is computed by comparing left-right mechanosensory signals. Then, signals are reformatted to reduce the coding biases inherent in peripheral mechanics, and wind cues are brought into the same circular coordinate system that represents visual cues and self-motion signals. Because the compass incorporates both mechanosensory and visual cues, it should enable navigation under conditions where no single cue is consistently reliable. These results show how local sensory signals can be transformed into a global, multimodal, abstract representation of space.

Honey bees flexibly use two navigational memories when updating dance distance information

Honey bees can communicate navigational information which makes them unique amongst all prominent insect navigators. Returning foragers recruit nest mates to a food source by communicating flight distance and direction using a small scale walking pattern: the waggle dance. It is still unclear how bees transpose flight information to generate corresponding dance information. In single feeder shift experiments, we monitored for the first time how individual bees update dance duration after a shift of feeder distance. Interestingly, the majority of bees (86%) needed two or more foraging trips to update dance duration. This finding demonstrates that transposing flight navigation information to dance information is not a reflexive behavior. Furthermore, many bees showed intermediate dance durations during the update process, indicating that honey bees highly likely use two memories: (i) a recently acquired navigation experience and (ii) a previously stored flight experience. Double-shift experiments, in which the feeder was moved forward and backward, created an experimental condition in which honey bee foragers did not update dance duration; suggesting the involvement of more complex memory processes. Our behavioral paradigm allows the dissociation of foraging and dance activity and opens the possibility of studying the molecular and neural processes underlying the waggle dance behavior.

Neuroarchitecture of the Drosophila central complex: A catalog of nodulus and asymmetrical body neurons and a revision of the protocerebral bridge catalog

The central complex, a set of neuropils in the center of the insect brain, plays a crucial role in spatial aspects of sensory integration and motor control. Stereotyped neurons interconnect these neuropils with one another and with accessory structures. We screened over 5,000 Drosophila melanogaster GAL4 lines for expression in two neuropils, the noduli (NO) of the central complex and the asymmetrical body (AB), and used multicolor stochastic labeling to analyze the morphology, polarity, and organization of individual cells in a subset of the GAL4 lines that showed expression in these neuropils. We identified nine NO and three AB cell types and describe them here. The morphology of the NO neurons suggests that they receive input primarily in the lateral accessory lobe and send output to each of the six paired noduli. We demonstrate that the AB is a bilateral structure which exhibits asymmetry in size between the left and right bodies. We show that the AB neurons directly connect the AB to the central complex and accessory neuropils, that they target both the left and right ABs, and that one cell type preferentially innervates the right AB. We propose that the AB be considered a central complex neuropil in Drosophila. Finally, we present highly restricted GAL4 lines for most identified protocerebral bridge, NO, and AB cell types. These lines, generated using the split-GAL4 method, will facilitate anatomical studies, behavioral assays, and physiological experiments.

Elucidating the "Gravome": Quantitative Proteomic Profiling of the Response to Chronic Hypergravity in Drosophila

Altered gravity conditions, such as experienced by organisms during spaceflight, are known to cause transcriptomic and proteomic changes. We describe the proteomic changes in whole adult Drosophila melanogaster (fruit fly) but focus specifically on the localized changes in the adult head in response to chronic hypergravity (3 g) treatment. Canton S adult female flies (2 to 3 days old) were exposed to chronic hypergravity for 9 days and compared with 1 g controls. After hypergravity treatment, either whole flies (body + head) or fly-head-only samples were isolated and evaluated for quantitative comparison of the two gravity conditions using an isobaric tagging liquid chromatography-tandem mass spectrometry approach. A total of 1948 proteins from whole flies and 1480 proteins from fly heads were differentially present in hypergravity-treated flies. Gene Ontology analysis of head-specific proteomics revealed host immune response, and humoral stress proteins were significantly upregulated. Proteins related to calcium regulation, ion transport, and ATPase were decreased. Increased expression of cuticular proteins may suggest an alteration in chitin metabolism and in chitin-based cuticle development. We therefore present a comprehensive quantitative survey of proteomic changes in response to chronic hypergravity in Drosophila, which will help elucidate the underlying molecular mechanism(s) associated with altered gravity environments.

Neuroarchitecture and neuroanatomy of the Drosophila central complex: A GAL4-based dissection of protocerebral bridge neurons and circuits

Insects exhibit an elaborate repertoire of behaviors in response to environmental stimuli. The central complex plays a key role in combining various modalities of sensory information with an insect's internal state and past experience to select appropriate responses. Progress has been made in understanding the broad spectrum of outputs from the central complex neuropils and circuits involved in numerous behaviors. Many resident neurons have also been identified. However, the specific roles of these intricate structures and the functional connections between them remain largely obscure. Significant gains rely on obtaining a comprehensive catalog of the neurons and associated GAL4 lines that arborize within these brain regions, and on mapping neuronal pathways connecting these structures. To this end, small populations of neurons in the Drosophila melanogaster central complex were stochastically labeled using the multicolor flip-out technique and a catalog was created of the neurons, their morphologies, trajectories, relative arrangements, and corresponding GAL4 lines. This report focuses on one structure of the central complex, the protocerebral bridge, and identifies just 17 morphologically distinct cell types that arborize in this structure. This work also provides new insights into the anatomical structure of the four components of the central complex and its accessory neuropils. Most strikingly, we found that the protocerebral bridge contains 18 glomeruli, not 16, as previously believed. Revised wiring diagrams that take into account this updated architectural design are presented. This updated map of the Drosophila central complex will facilitate a deeper behavioral and physiological dissection of this sophisticated set of structures. J. Comp. Neurol. 523:997–1037, 2015. © 2014 Wiley Periodicals, Inc.

Organization and functional roles of the central complex in the insect brain

The central complex is a group of modular neuropils across the midline of the insect brain. Hallmarks of its anatomical organization are discrete layers, an organization into arrays of 16 slices along the right-left axis, and precise inter-hemispheric connections via chiasmata. The central complex is connected most prominently with the adjacent lateral complex and the superior protocerebrum. Its developmental appearance corresponds with the appearance of compound eyes and walking legs. Distinct dopaminergic neurons control various forms of arousal. Electrophysiological studies provide evidence for roles in polarized light vision, sky compass orientation, and integration of spatial information for locomotor control. Behavioral studies on mutant and transgenic flies indicate roles in spatial representation of visual cues, spatial visual memory, directional control of walking and flight, and place learning. The data suggest that spatial azimuthal directions (i.e., where) are represented in the slices, and cue information (i.e., what) are represented in different layers of the central complex.

Phototactic personality in fruit flies and its suppression by serotonin and white

Drosophila typically move toward light (phototax positively) when startled. The various species of Drosophila exhibit some variation in their respective mean phototactic behaviors; however, it is not clear to what extent genetically identical individuals within each species behave idiosyncratically. Such behavioral individuality has indeed been observed in laboratory arthropods; however, the neurobiological factors underlying individual-to-individual behavioral differences are unknown. We developed "FlyVac," a high-throughput device for automatically assessing phototaxis in single animals in parallel. We observed surprising variability within every species and strain tested, including identically reared, isogenic strains. In an extreme example, a domesticated strain of Drosophila simulans harbored both strongly photopositive and strongly photonegative individuals. The particular behavior of an individual fly is not heritable and, because it persists for its lifetime, constitutes a model system for elucidating the molecular mechanisms of personality. Although all strains assayed had greater than expected variation (assuming binomial sampling), some had more than others, implying a genetic basis. Using genetics and pharmacology, we identified the metabolite transporter White and white-dependent serotonin as suppressors of phototactic personality. Because we observed behavioral idiosyncrasy in all experimental groups, we suspect it is present in most behaviors of most animals.

Drosophila auditory organ genes and genetic hearing defects

The Drosophila auditory organ shares equivalent transduction mechanisms with vertebrate hair cells, and both are specified by atonal family genes. Using a whole-organ knockout strategy based on atonal, we have identified 274 Drosophila auditory organ genes. Only four of these genes had previously been associated with fly hearing, yet one in five of the genes that we identified has a human cognate that is implicated in hearing disorders. Mutant analysis of 42 genes shows that more than half of them contribute to auditory organ function, with phenotypes including hearing loss, auditory hypersusceptibility, and ringing ears. We not only discover ion channels and motors important for hearing, but also show that auditory stimulus processing involves chemoreceptor proteins as well as phototransducer components. Our findings demonstrate mechanosensory roles for ionotropic receptors and visual rhodopsins and indicate that different sensory modalities utilize common signaling cascades.

The central complex of the flesh fly, Neobellieria bullata: recordings and morphologies of protocerebral inputs and small-field neurons

The central complex in the brains of insects is a series of midline neuropils involved in motor control, sensory integration, and associative learning. To understand better the role of this center and its supply of sensory information, intracellular recordings and dye fills were made of central complex neurons in the fly, Neobellieria bullata. Recordings were obtained from 24 neurons associated with the ellipsoid body, fan-shaped body, and protocerebral bridge, all of which receive both visual and mechanosensory information from protocerebral centers. One neuron with dendrites in an area of the lateral protocerebrum associated with motion-sensitive outputs from the optic lobes invades the entire protocerebral bridge and was driven by visual motion. Inputs to the fan-shaped body and ellipsoid body responded both to visual stimuli and to air puffs directed at the head and abdomen. Intrinsic neurons in both of these structures respond to changes in illumination. A putative output neuron connecting the protocerebral bridge, the fan-shaped body, and one of the lateral accessory lobes showed opponent responses to moving visual stimuli. These recordings identify neurons with response properties previously known only from extracellular recordings in other species. Dye injections into neurons connecting the central complex with areas of the protocerebrum suggest that some classes of inputs into the central complex are electrically coupled.

Simple ways to measure behavioral responses of Drosophila to stimuli and use of these methods to characterize a novel mutant

The behavioral responses of adult Drosophila fruit flies to a variety of sensory stimuli – light, volatile and non-volatile chemicals, temperature, humidity, gravity, and sound - have been measured by others previously. Some of those assays are rather complex; a review of them is presented in the Discussion. Our objective here has been to find out how to measure the behavior of adult Drosophila fruit flies by methods that are inexpensive and easy to carry out. These new assays have now been used here to characterize a novel mutant that fails to be attracted or repelled by a variety of sensory stimuli even though it is motile.

The serotonin 5-HT7Dro receptor is expressed in the brain of Drosophila, and is essential for normal courtship and mating

The 5-HT(7) receptor remains one of the less well characterized serotonin receptors. Although it has been demonstrated to be involved in the regulation of mood, sleep, and circadian rhythms, as well as relaxation of vascular smooth muscles in mammals, the precise mechanisms underlying these functions remain largely unknown. The fruit fly, Drosophila melanogaster, is an attractive model organism to study neuropharmacological, molecular, and behavioral processes that are largely conserved with mammals. Drosophila express a homolog of the mammalian 5-HT(7) receptor, as well as homologs for the mammalian 5-HT(1A), and 5-HT(2), receptors. Each fly receptor couples to the same effector pathway as their mammalian counterpart and have been demonstrated to mediate similar behavioral responses. Here, we report on the expression and function of the 5-HT(7)Dro receptor in Drosophila. In the larval central nervous system, expression is detected postsynaptically in discreet cells and neuronal circuits. In the adult brain there is strong expression in all large-field R neurons that innervate the ellipsoid body, as well as in a small group of cells that cluster with the PDF-positive LNvs neurons that mediate circadian activity. Following both pharmacological and genetic approaches, we have found that 5-HT(7)Dro activity is essential for normal courtship and mating behaviors in the fly, where it appears to mediate levels of interest in both males and females. This is the first reported evidence of direct involvement of a particular serotonin receptor subtype in courtship and mating in the fly.

Quantitative trait locus mapping of gravitaxis behaviour in Drosophila melanogaster

Drosophila melanogaster, like other organisms, move and orient themselves in response to the earth's gravitational force. The ability to sense and respond to gravity is essential for an organism to navigate and thrive in its environment. The genes underlying this behaviour in Drosophila remain elusive. Using 88 recombinant inbred lines, we have identified four quantitative trait loci (QTLs) that contribute to adult gravitaxis (geotaxis) behaviour in Drosophila. Candidate genes of interest were selected from the QTLs of highest significance based on their function in chordotonal organ formation. Quantitative complementation tests with these candidate genes revealed a role for skittles in adult gravitaxis behaviour in D. melanogaster.

Object preference by walking fruit flies, Drosophila melanogaster, is mediated by vision and graviperception

Walking fruit flies, Drosophila melanogaster, use visual information to orient towards salient objects in their environment, presumably as a search strategy for finding food, shelter or other resources. Less is known, however, about the role of vision or other sensory modalities such as mechanoreception in the evaluation of objects once they have been reached. To study the role of vision and mechanoreception in exploration behavior, we developed a large arena in which we could track individual fruit flies as they walked through either simple or more topologically complex landscapes. When exploring a simple, flat environment lacking three-dimensional objects, flies used visual cues from the distant background to stabilize their walking trajectories. When exploring an arena containing an array of cones, differing in geometry, flies actively oriented towards, climbed onto, and explored the objects, spending most of their time on the tallest, steepest object. A fly's behavioral response to the geometry of an object depended upon the intrinsic properties of each object and not a relative assessment to other nearby objects. Furthermore, the preference was not due to a greater attraction towards tall, steep objects, but rather a change in locomotor behavior once a fly reached and explored the surface. Specifically, flies are much more likely to stop walking for long periods when they are perched on tall, steep objects. Both the vision system and the antennal chordotonal organs (Johnston's organs) provide sufficient information about the geometry of an object to elicit the observed change in locomotor behavior. Only when both these sensory systems were impaired did flies not show the behavioral preference for the tall, steep objects.

PDF receptor expression reveals direct interactions between circadian oscillators in Drosophila

Daily rhythms of behavior are controlled by a circuit of circadian pacemaking neurons. In Drosophila, 150 pacemakers participate in this network, and recent observations suggest that the network is divisible into M and E oscillators, which normally interact and synchronize. Sixteen oscillator neurons (the small and large lateral neurons [LNvs]) express a neuropeptide called pigment-dispersing factor (PDF) whose signaling is often equated with M oscillator output. Given the significance of PDF signaling to numerous aspects of behavioral and molecular rhythms, determining precisely where and how signaling via the PDF receptor (PDFR) occurs is now a central question in the field. Here we show that GAL4-mediated rescue of pdfr phenotypes using a UAS-PDFR transgene is insufficient to provide complete behavioral rescue. In contrast, we describe a approximately 70-kB PDF receptor (pdfr) transgene that does rescue the entire pdfr circadian behavioral phenotype. The transgene is widely but heterogeneously expressed among pacemakers, and also among a limited number of non-pacemakers. Our results support an important hypothesis: the small LNv cells directly target a subset of the other crucial pacemaker neurons cells. Furthermore, expression of the transgene confirms an autocrine feedback signaling by PDF back to PDF-expressing cells. Finally, the results present an unexpected PDF receptor site: the large LNv cells appear to target a population of non-neuronal cells that resides at the base of the eye.

The mushroom body of adult Drosophila characterized by GAL4 drivers

The mushroom body is required for a variety of behaviors of Drosophila melanogaster. Different types of intrinsic and extrinsic mushroom body neurons might underlie its functional diversity. There have been many GAL4 driver lines identified that prominently label the mushroom body intrinsic neurons, which are known as "Kenyon cells." Under one constant experimental condition, we analyzed and compared the the expression patterns of 25 GAL4 drivers labeling the mushroom body. As an internet resource, we established a digital catalog indexing representative confocal data of them. Further more, we counted the number of GAL4-positive Kenyon cells in each line. We found that approximately 2,000 Kenyon cells can be genetically labeled in total. Three major Kenyon cell subtypes, the gamma, alpha'/beta', and alpha/beta neurons, respectively, contribute to 33, 18, and 49% of 2,000 Kenyon cells. Taken together, this study lays groundwork for functional dissection of the mushroom body.

Wired for sex: the neurobiology of Drosophila mating decisions

Decisions about whom to mate with can sometimes be difficult, but making the right choice is critical for an animal's reproductive success. The ubiquitous fruit fly, Drosophila, is clearly very good at making these decisions. Upon encountering another fly, a male may or may not choose to court. He estimates his chances of success primarily on the basis of pheromone signals and previous courtship experience. The female decides whether to accept or reject the male, depending on her perception of his pheromone and acoustic signals, as well as her own readiness to mate. This simple and genetically tractable system provides an excellent model to explore the neurobiology of decision making.

Cracking neural circuits in a tiny brain: new approaches for understanding the neural circuitry of Drosophila

Genetic screens in Drosophila have identified many genes involved in neural development and function. However, until recently, it has been impossible to monitor neural signals in Drosophila central neurons, and it has been difficult to make specific perturbations to central neural circuits. This has changed in the past few years with the development of new tools for measuring and manipulating neural activity in the fly. Here we review how these new tools enable novel conceptual approaches to 'cracking circuits' in this important model organism. We discuss recent studies aimed at defining the cognitive demands on the fly brain, identifying the cellular components of specific neural circuits, mapping functional connectivity in those circuits and defining causal relationships between neural activity and behavior.

Development of Johnston's organ in Drosophila

Hearing is a specialized mechanosensory modality that is refined during evolution to meet the particular requirements of different organisms. In the fruitfly, Drosophila, hearing is mediated by Johnston's organ, a large chordotonal organ in the antenna that is exquisitely sensitive to the near-field acoustic signal of courtship songs generated by male wing vibration. We summarize recent progress in understanding the molecular genetic determinants of Johnston's organ development and discuss surprising differences from other chordotonal organs that likely facilitate hearing. We outline novel discoveries of active processes that generate motion of the antenna for acute sensitivity to the stimulus. Finally, we discuss further research directions that would probe remaining questions in understanding Johnston's organ development, function and evolution.