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The lengthening phase of ventral furrow formation in Drosophila gastrulation is driven by cytoplasmic flows triggered by apical constriction of mesoderm cells independent of the mechanical inputs from the basolateral membranes. A role for viscosity on cellular morphogenesis Using Drosophila gastrulation as a model, Eric Wieschaus and colleagues have studied the respective contributions of cortical cellular forces and cytoplasmic viscous forces to the generation of complex three-dimensional morphogenetic processes. They found that the lengthening phase in furrow formation is driven by cytoplasmic flows triggered by apical constriction of mesoderm cells, and that cell individualization is dispensable. These findings favour a viscous component in morphogenesis, rather than one based solely on mechanical inputs from cell membranes. Epithelial folding mediated by apical constriction converts flat epithelial sheets into multilayered, complex tissue structures and is used throughout development in most animals.sup.1. Little is known, however, about how forces produced near the apical surface of the tissue are transmitted within individual cells to generate the global changes in cell shape that characterize tissue deformation. Here we apply particle tracking velocimetry in gastrulating Drosophila embryos to measure the movement of cytoplasm and plasma membrane during ventral furrow formation.sup.2,3. We find that cytoplasmic redistribution during the lengthening phase of ventral furrow formation can be precisely described by viscous flows that quantitatively match the predictions of hydrodynamics. Cell membranes move with the ambient cytoplasm, with little resistance to, or driving force on, the flow. Strikingly, apical constriction produces similar flow patterns in mutant embryos that fail to form cells before gastrulation ('acellular' embryos), such that the global redistribution of cytoplasm mirrors the summed redistribution occurring in individual cells of wild-type embryos. Our results indicate that during the lengthening phase of ventral furrow formation, hydrodynamic behaviour of the cytoplasm provides the predominant mechanism transmitting apically generated forces deep into the tissue and that cell individualization is dispensable., Author(s): Bing He [sup.1] , Konstantin Doubrovinski [sup.1] , Oleg Polyakov [sup.2] , Eric Wieschaus [sup.1] [sup.3] Author Affiliations: (1) Department of Molecular Biology, Princeton University, Princeton, USA (2) Department [...]

The generation of acoustic communication signals is widespread across the animal kingdom (1,2), and males of many species, including Drosophilidae, produce patterned courtship songs to increase their chance of success [...]

Two-photon calcium imaging experiments reveal that ring neurons in the Drosophila central complex represent visual features and show direction-selective orientation tuning, resembling simple cells in mammalian primary visual cortex; future fly studies may enhance our understanding of circuit computations underlying visually guided action selection. Visual recognition in the insect brain Insects can perform complex visual pattern recognition and memory tasks, capabilities that equip them for challenging motor behaviours such as flight. But whether and how the neurons of the fly central complex represent visual features are unknown. Now Johannes Seelig and Vivek Jayaraman have imaged neurons of the Drosophila melanogaster central complex in head-fixed walking and flying flies, and show that ring neurons there represent visual features such as direction-selective orientation tuning, thus resembling so-called simple cells found in the primary visual cortex of mammals. This work lays the foundations for the genetic investigation of the neuronal circuits used for visually guided motor commands. Many animals, including insects, are known to use visual landmarks to orient in their environment. In Drosophila melanogaster, behavioural genetics studies have identified a higher brain structure called the central complex as being required for the fly's innate responses to vertical visual features.sup.1 and its short- and long-term memory for visual patterns.sup.2,3,4. But whether and how neurons of the fly central complex represent visual features are unknown. Here we use two-photon calcium imaging in head-fixed walking and flying flies to probe visuomotor responses of ring neurons--a class of central complex neurons that have been implicated in landmark-driven spatial memory in walking flies.sup.2,3 and memory for visual patterns in tethered flying flies.sup.5. We show that dendrites of ring neurons are visually responsive and arranged retinotopically. Ring neuron receptive fields comprise both excitatory and inhibitory subfields, resembling those of simple cells in the mammalian primary visual cortex. Ring neurons show strong and, in some cases, direction-selective orientation tuning, with a notable preference for vertically oriented features similar to those that evoke innate responses in flies.sup.1,2. Visual responses were diminished during flight, but, in contrast with the hypothesized role of the central complex in the control of locomotion.sup.6, not modulated during walking. Taken together, these results indicate that ring neurons represent behaviourally relevant visual features in the fly's environment, enabling downstream central complex circuits to produce appropriate motor commands.sup.6. More broadly, this study opens the door to mechanistic investigations of circuit computations underlying visually guided action selection in the Drosophila central complex., Author(s): Johannes D. Seelig [sup.1] , Vivek Jayaraman [sup.1] Author Affiliations: (1) Janelia Farm Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, Virginia 20147, USA, Main Flies display [...]

Animal behaviour arises from computations in neuronal circuits, but our understanding of these computations has been frustrated by the lack of detailed synaptic connection maps, or connectomes. For example, despite intensive investigations over half a century, the neuronal implementation of local motion detection in the insect visual system remains elusive. Here we develop a semi-automated pipeline using electron microscopy to reconstruct a connectome, containing 379 neurons and 8,637 chemical synaptic contacts, within the Drosophila optic medulla. By matching reconstructed neurons to examples from light microscopy, we assigned neurons to cell types and assembled a connectome of the repeating module of the medulla. Within this module, we identified cell types constituting a motion detection circuit, and showed that the connections onto individual motion-sensitive neurons in this circuit were consistent with their direction selectivity. Our results identify cellular targets for future functional investigations, and demonstrate that connectomes can provide key insights into neuronal computations., Vision in insects has been subject to intense behavioural (1),physiological (2) and anatomical (3) investigations, yet our understanding of its underlying neural computations is still far from complete. One such [...]

The extraction of directional motion information from changing retinal images is one of the earliest and most important processing steps in any visual system. In the fly optic lobe, two parallel processing streams have been anatomically described, leading from two first-order interneurons, L1 and L2, via T4 and T5 cells onto large, wide-field motion-sensitive interneurons of the lobula plate (1). Therefore, T4 and T5 cells are thought to have a pivotal role in motion processing; however, owing to their small size, it is difficult to obtain electrical recordings of T4 and T5 cells, leaving their visual response properties largely unknown. We circumvent this problem by means of optical recording from these cells in Drosophila, using the genetically encoded calcium indicator GCaMP5 (ref. 2). Here we find that specific subpopulations of T4 and T5 cells are directionally tuned to one of the four cardinal directions; that is, front-to-back, back-to-front, upwards and downwards. Depending on their preferred direction, T4 and T5 cells terminate in specific sublayers of the lobula plate. T4 and T5 functionally segregate with respect to contrast polarity: whereas T4 cells selectively respond to moving brightness increments (ON edges), T5 cells only respond to moving brightness decrements (OFF edges). When the output from T4 or T5 cells is blocked, the responses of postsynaptic lobula plate neurons to moving ON (T4 block) or OFF edges (T5 block) are selectively compromised. The same effects are seen in turning responses of tethered walking flies. Thus, starting with L1 and L2, the visual input is split into separate ON and OFF pathways, and motion along all four cardinal directions is computed separately within each pathway. The output of these eight different motion detectors is then sorted such that ON (T4) and OFF (T5) motion detectors with the same directional tuning converge in the same layer of the lobula plate, jointly providing the input to downstream circuits and motion-driven behaviours., Most of the neurons in the fly brain are dedicated to image processing. The respective part of the head ganglion, called the optic lobe, consists of several layers of neuropile [...]

When an odour activates a fly's antennae asymmetrically, more neurotransmitter is released from olfactory receptor neuron axon branches ipsilateral to the antenna than from contralateral branches. This causes ipsilateral central olfactory neurons to begin spiking earlier and at a higher rate than contralateral neurons, thereby enabling a walking fly to turn towards the odour. Fruitflies' sense of direction A fruitfly knows right from left when confronted with an asymmetric odour plume. This study demonstrates a neuronal basis for that ability. Rachel Wilson and colleagues show that Drosophila are able to discriminate between odours presented to the right antenna versus the left antenna because each spike arising from the right antenna releases only about two-thirds as much neurotransmitter in the left brain, as compared to the right brain. As a result, central neurons on the right spike a few milliseconds before neurons on the left, and neurons on the right also spike at a slightly higher rate. The brain of a walking fly can detect this subtle asymmetry and initiate a turn to the right before the fly even completes one stride. In Drosophila, most individual olfactory receptor neurons (ORNs) project bilaterally to both sides of the brain.sup.1,2. Having bilateral rather than unilateral projections may represent a useful redundancy. However, bilateral ORN projections to the brain should also compromise the ability to lateralize odours. Nevertheless, walking or flying Drosophila reportedly turn towards the antenna that is more strongly stimulated by odour.sup.3,4,5. Here we show that each ORN spike releases approximately 40% more neurotransmitter from the axon branch ipsilateral to the soma than from the contralateral branch. As a result, when an odour activates the antennae asymmetrically, ipsilateral central neurons begin to spike a few milliseconds before contralateral neurons, and at a 30 to 50% higher rate than contralateral neurons. We show that a walking fly can detect a 5% asymmetry in total ORN input to its left and right antennal lobes, and can turn towards the odour in less time than it requires the fly to complete a stride. These results demonstrate that neurotransmitter release properties can be tuned independently at output synapses formed by a single axon onto two target cells with identical functions and morphologies. Our data also show that small differences in spike timing and spike rate can produce reliable differences in olfactory behaviour., Author(s): Quentin Gaudry [sup.1] , Elizabeth J. Hong [sup.1] , Jamey Kain [sup.2] , Benjamin L. de Bivort [sup.2] [sup.3] , Rachel I. Wilson [sup.1] Author Affiliations: (1) Department of [...]

In Drosophila, most individual olfactory receptor neurons (ORNs) project bilaterally to both sides of the brain (1,2). Having bilateral rather than unilateral projections may represent a useful redundancy. However, bilateral [...]

Dopamine is synonymous with reward and motivation in mammals [12]. However, only recently has dopamine been linked to motivated behaviour and rewarding reinforcement in fruitflies [3,4]. Instead, octopamine has historically been considered to be the signal for reward in insects [5-7]. Here we show, using temporal control of neural function in Drosophila, that only short-term appetitive memory is reinforced by octopamine. Moreover, octopamine-dependent memory formation requires signalling through dopamine neurons. Part of the octopamine signal requires the a-adrenergic-like OAMB receptor in an identified subset of mushroom-body-targeted dopamine neurons. Octopamine triggers an increase in intracellular calcium in these dopamine neurons, and their direct activation can substitute for sugar to form appetitive memory, even in flies lacking octopamine. Analysis of the β-adrenergic-like OCT02R receptor reveals that octopamine-dependent reinforcement also requires an interaction with dopamine neurons that control appetitive motivation. These data indicate that sweet taste engages a distributed octopamine signal that reinforces memory through discrete subsets of mushroom-body-targeted dopamine neurons. In addition, they reconcile previous findings with octopamine and dopamine and suggest that reinforcement systems in flies are more similar to mammals than previously thought., Fruitfly octopamine is synthesized from tyrosine via two steps catalysed by tyrosine decarboxylase (TDC) and tyramine b-hydroxylase (Tbh) (8,9). The Tdc2 gene encodes the neuronal TDC and a Tdc2-GAL4 construct [...]

Adult stem cells support tissue homeostasis and repair throughout the life of an individual. During ageing, numerous intrinsic and extrinsic changes occur that result in altered stem-cell behaviour and reduced tissue maintenance and regeneration. In the Drosophila testis, ageing results in a marked decrease in the self-renewal factor Unpaired (Upd), leading to a concomitant loss of germline stem cells. Here we demonstrate that IGF-II messenger RNA binding protein (Imp) counteracts endogenous small interfering RNAs to stabilize upd (also known as os) RNA. However, similar to upd, Imp expression decreases in the hub cells of older males, which is due to the targeting of Imp by the heterochronic microRNA let-7. In the absence of Imp, upd mRNA therefore becomes unprotected and susceptible to degradation. Understanding the mechanistic basis for ageing-related changes in stem-cell behaviour will lead to the development of strategies to treat age-onset diseases and facilitate stem-cell-based therapies in older individuals. In the Drosophila testis, IGF-II messenger RNA binding protein (Imp) is shown to promote stem-cell niche maintenance by stabilizing unpaired (upd) RNA; Imp levels decrease in the hub cells of older males, owing to regulation by the microRNA let-7. Stem-cell loss with age In the Drosophila testis, a cluster of cells called the apical hub secretes the self-renewal factor Unpaired (Upd), which promotes stem-cell maintenance. Leanne Jones and colleagues now show that the evolutionarily conserved RNA binding protein Imp stabilizes upd messenger RNA, thereby contributing to maintenance of the stem-cell niche. During ageing, Imp is targeted by the microRNA let-7, causing a decrease in Imp expression and degradation of Upd by endogenous small interfering RNA, and decreased stem-cell maintenance with age., Author(s): Hila Toledano [sup.1] [sup.5] , Cecilia D'Alterio [sup.1] , Benjamin Czech [sup.2] , Erel Levine [sup.3] , D. Leanne Jones [sup.1] Author Affiliations: (1) Laboratory of Genetics, The Salk [...]

Adult stem cells support tissue homeostasis and repair throughout the life of an individual. During ageing, numerous intrinsic and extrinsic changes occur that result in altered stem-cell behaviour and reduced tissue maintenance and regeneration. In the Drosophila testis, ageing results in a marked decrease in the self-renewal factor Unpaired (Upd), leading to a concomitant loss of germline stem cells. Here we demonstrate that IGF-II messenger RNA binding protein (Imp) counteracts endogenous small interfering RNAs to stabilize upd (also known as os) RNA. However, similar to upd, Imp expression decreases in the hub cells of older males, which is due to the targeting of Imp by the heterochronic microRNA let-7. In the absence of Imp, upd mRNA therefore becomes unprotected and susceptible to degradation. Understanding the mechanistic basis for ageing-related changes in stem-cell behaviour will lead to the development of strategies to treat age-onset diseases and facilitate stem-cell-based therapies in older individuals., Many stem cells lose the capacity for self-renewal when removed from their local microenvironment (or niche), indicating that the niche has a major role in controlling stem-cell fate (1). Changes [...]