The circadian system comprises multiple clocks, including central and peripheral clocks. The central clock generally governs peripheral clocks to synchronize circadian rhythms throughout the animal body. However, whether the peripheral clock influences the central clock is unclear. This issue can be addressed through a system comprising a peripheral clock (compound eye clock [CE clock]) and central clock (the optic lobe [OL] clock) in the cricket Gryllus bimaculatus. We previously found that the compound eye regulates the free-running period (τ) and the stability of locomotor rhythms driven by the OL clock, as measured by the daily deviation of τ at 30°C. However, the role of the CE clock in this regulation remains unexplored. In this study, we investigated the importance of the CE clock in this regulation using RNA interference (RNAi) of the period (per) gene localized to the compound eye (perCE-RNAi). The perCE-RNAi abolished the compound eye rhythms of the electroretinogram (ERG) amplitude and clock gene expression but the locomotor rhythm driven by the OL clock was maintained. The locomotor rhythm of the tested crickets showed a significantly longer τ and greater daily variation of τ than those of control crickets treated with dsDsRed2. The variation of τ was comparable with that of crickets with the optic nerve severed. The τ was considerably longer but was comparable with that of crickets with the optic nerve severed. These results suggest that the CE clock regulates the OL clock to maintain and stabilize τ.

The arthropod mushroom body is well-studied as an expansion layer representing olfactory stimuli and linking them to contingent events. However, 8% of mushroom body Kenyon cells in Drosophila melanogaster receive predominantly visual input, and their function remains unclear. Here, we identify inputs to visual Kenyon cells using the FlyWire adult whole-brain connectome. Input repertoires are similar across hemispheres and connectomes with certain inputs highly overrepresented. Many visual neurons presynaptic to Kenyon cells have large receptive fields, while interneuron inputs receive spatially restricted signals that may be tuned to specific visual features. Individual visual Kenyon cells randomly sample sparse inputs from combinations of visual channels, including multiple optic lobe neuropils. These connectivity patterns suggest that visual coding in the mushroom body, like olfactory coding, is sparse, distributed, and combinatorial. However, the specific input repertoire to the smaller population of visual Kenyon cells suggests a constrained encoding of visual stimuli.

The brain is consisted of diverse neurons arising from a limited number of neural stem cells. Drosophila neural stem cells called neuroblasts (NBs) produces specific neural lineages of various lineage sizes depending on their location in the brain. In the Drosophila visual processing centre - the optic lobes (OLs), medulla NBs derived from the neuroepithelium (NE) give rise to neurons and glia cells of the medulla cortex. The timing and the mechanisms responsible for the cessation of medulla NBs are so far not known. In this study, we show that the termination of medulla NBs during early pupal development is determined by the exhaustion of the NE stem cell pool. Hence, altering NE-NB transition during larval neurogenesis disrupts the timely termination of medulla NBs. Medulla NBs terminate neurogenesis via a combination of apoptosis, terminal symmetric division via Prospero, and a switch to gliogenesis via Glial Cell Missing (Gcm); however, these processes occur independently of each other. We also show that temporal progression of the medulla NBs is mostly not required for their termination. As the Drosophila OL shares a similar mode of division with mammalian neurogenesis, understanding when and how these progenitors cease proliferation during development can have important implications for mammalian brain size determination and regulation of its overall function.
Every cell in the body can be traced back to a stem cell. For instance, most cells in the adult brains of fruit flies come from a type of stem cell known as a neuroblast. This includes neurons and glial cells (which support and protect neurons) in the optic lobe, the part of the brain that processes visual information. The numbers of neurons and glia in the optic lobe are tightly regulated such that when the right numbers are reached, the neuroblasts stop making more and are terminated. But how and when this occurs is poorly understood. To investigate, Nguyen and Cheng studied when neuroblasts disappear in the optic lobe over the course of development. This revealed that the number of neuroblasts dropped drastically 12 to 18 hours after the fruit fly larvae developed in to pupae, and were completely gone by 30 hours in to pupae life. Further experiments revealed that the timing of this decrease is influenced by neuroepithelium cells, the pool of stem cells that generate neuroblasts during the early stages of development. Nguyen and Cheng found that speeding up this transition so that neuroblasts arise from the neuroepithelium earlier, led neuroblasts to disappear faster from the optic lobe; whereas delaying the transition caused neuroblasts to persist for much longer. Thus, the time at which neuroblasts are born determines when they are terminated. Furthermore, Nguyen and Cheng showed that the neuroblasts were lost through a combination of means. This includes dying via a process called apoptosis, dividing to form two mature neurons, or switching to a glial cell fate. These findings provide a deeper understanding of the mechanisms regulating stem cell pools and their conversion to different cell types, a process that is crucial to the proper development of the brain. How cells divide to form the optic lobe of fruit flies is similar to how new neurons arise in the mammalian brain. Understanding how and when stem cells in the fruit fly brain stop proliferating could therefore provide new insights in to the development of the human brain.

The rich variety of behaviours observed in animals arises through the interplay between sensory processing and motor control. To understand these sensorimotor transformations, it is useful to build models that predict not only neural responses to sensory input1-5 but also how each neuron causally contributes to behaviour6,7. Here we demonstrate a novel modelling approach to identify a one-to-one mapping between internal units in a deep neural network and real neurons by predicting the behavioural changes that arise from systematic perturbations of more than a dozen neuronal cell types. A key ingredient that we introduce is \'knockout training\', which involves perturbing the network during training to match the perturbations of the real neurons during behavioural experiments. We apply this approach to model the sensorimotor transformations of Drosophila melanogaster males during a complex, visually guided social behaviour8-11. The visual projection neurons at the interface between the optic lobe and central brain form a set of discrete channels12, and prior work indicates that each channel encodes a specific visual feature to drive a particular behaviour13,14. Our model reaches a different conclusion: combinations of visual projection neurons, including those involved in non-social behaviours, drive male interactions with the female, forming a rich population code for behaviour. Overall, our framework consolidates behavioural effects elicited from various neural perturbations into a single, unified model, providing a map from stimulus to neuronal cell type to behaviour, and enabling future incorporation of wiring diagrams of the brain15 into the model.

In the perception of color, wavelengths of light reflected off objects are transformed into the derived quantities of brightness, saturation and hue. Neurons responding selectively to hue have been reported in primate cortex, but it is unknown how their narrow tuning in color space is produced by upstream circuit mechanisms. We report the discovery of neurons in the Drosophila optic lobe with hue-selective properties, which enables circuit-level analysis of color processing. From our analysis of an electron microscopy volume of a whole Drosophila brain, we construct a connectomics-constrained circuit model that accounts for this hue selectivity. Our model predicts that recurrent connections in the circuit are critical for generating hue selectivity. Experiments using genetic manipulations to perturb recurrence in adult flies confirm this prediction. Our findings reveal a circuit basis for hue selectivity in color vision.

Visual circuit development is characterized by subdivision of neuropils into layers that house distinct sets of synaptic connections. We find that, in the Drosophila medulla, this layered organization depends on the axon guidance regulator Plexin A. In Plexin A null mutants, synaptic layers of the medulla neuropil and arborizations of individual neurons are wider and less distinct than in controls. Analysis of semaphorin function indicates that Semaphorin 1a, acting in a subset of medulla neurons, is the primary partner for Plexin A in medulla lamination. Removal of the cytoplasmic domain of endogenous Plexin A has little effect on the formation of medulla layers; however, both null and cytoplasmic domain deletion mutations of Plexin A result in an altered overall shape of the medulla neuropil. These data suggest that Plexin A acts as a receptor to mediate morphogenesis of the medulla neuropil, and as a ligand for Semaphorin 1a to subdivide it into layers. Its two independent functions illustrate how a few guidance molecules can organize complex brain structures by each playing multiple roles.

Neurons must be made in the correct proportions to communicate with the appropriate synaptic partners and form functional circuits. In the Drosophila visual system, multiple subtypes of distal medulla (Dm) inhibitory interneurons are made in distinct, reproducible numbers-from 5 to 800 per optic lobe. These neurons are born from a crescent-shaped neuroepithelium called the outer proliferation center (OPC), which can be subdivided into specific domains based on transcription factor and growth factor expression. We fate mapped Dm neurons and found that more abundant neural types are born from larger neuroepithelial subdomains, while less abundant subtypes are born from smaller ones. Additionally, morphogenetic Dpp/BMP signaling provides a second layer of patterning that subdivides the neuroepithelium into smaller domains to provide more granular control of cell proportions. Apoptosis appears to play a minor role in regulating Dm neuron abundance. This work describes an underappreciated mechanism for the regulation of neuronal stoichiometry.

Crickets serve as a well-established model organism in biological research spanning various fields, such as behavior, physiology, neurobiology, and ecology. Cricket circadian behavior was first reported over a century ago and prompted a wealth of studies delving into their chronobiology. Circadian rhythms have been described in relation to fundamental cricket behaviors, encompassing stridulation and locomotion, but also in hormonal secretion and gene expression. Here we review how changes in illumination patterns and light intensity differentially impact the different cricket behaviors as well as circadian gene expression. We further describe the cricket\'s circadian pacemaker. Ample anatomical manipulations support the location of a major circadian pacemaker in the cricket optic lobes and another in the central brain, possibly interconnected via signaling of the neuropeptide PDF. The cricket circadian machinery comprises a molecular cascade based on two major transcriptional/translational negative feedback loops, deviating somewhat from the canonical model of Drosophila and emphasizing the significance of exploring alternative models. Finally, the nocturnal nature of crickets has provided a unique avenue for investigating the repercussions of artificial light at night on cricket behavior and ecology, underscoring the critical role played by natural light cycles in synchronizing cricket behaviors and populations, further supporting the use of the cricket model in the study of the effects of light on insects. Some gaps in our knowledge and challenges for future studies are discussed.

Cell fate and growth require one-carbon units for the biosynthesis of nucleotides, methylation reactions and redox homeostasis, provided by one-carbon metabolism. Consistently, defects in one-carbon metabolism lead to severe developmental defects, such as neural tube defects. However, the role of this pathway during brain development and in neural stem cell regulation is poorly understood. To better understand the role of one carbon metabolism we focused on the enzyme Serine hydroxymethyl transferase (Shmt), a key factor in the one-carbon cycle, during Drosophila brain development. We show that, although loss of Shmt does not cause obvious defects in the central brain, it leads to severe phenotypes in the optic lobe. The shmt mutants have smaller optic lobe neuroepithelia, partly justified by increased apoptosis. In addition, shmt mutant neuroepithelia have morphological defects, failing to form a lamina furrow, which likely explains the observed absence of lamina neurons. These findings show that one-carbon metabolism is crucial for the normal development of neuroepithelia, and consequently for the generation of neural progenitor cells and neurons. These results propose a mechanistic role for one-carbon during brain development.

The large diversity of cell types in nervous systems presents a challenge in identifying the genetic mechanisms that encode it. Here, we report that nearly 200 distinct neurons in the Drosophila visual system can each be defined by unique combinations of on average 10 continuously expressed transcription factors. We show that targeted modifications of this terminal selector code induce predictable conversions of neuronal fates that appear morphologically and transcriptionally complete. Cis-regulatory analysis of open chromatin links one of these genes to an upstream patterning factor that specifies neuronal fates in stem cells. Experimentally validated network models describe the synergistic regulation of downstream effectors by terminal selectors and ecdysone signaling during brain wiring. Our results provide a generalizable framework of how specific fates are implemented in postmitotic neurons.