Modular subdivision of mushroom bodies by Kenyon cells in the silkmoth

Single-cell transcriptomes provide insights into expansion of glial cells in Bombyx mori

The diversity of cell types in the brain and how these change during different developmental stages, remains largely unknown. The life cycle of insects is short and goes through 4 distinct stages including embryonic, larval, pupal, and adult stages. During postembryonic life, the larval brain transforms into a mature adult version after metamorphosis. The silkworm, Bombyx mori, is a lepidopteran model insect. Here, we characterized the brain cell repertoire of larval and adult B. mori by obtaining 50 708 single-cell transcriptomes. Seventeen and 12 cell clusters from larval and adult brains were assigned based on marker genes, respectively. Identified cell types include Kenyon cells, optic lobe cells, monoaminergic neurons, surface glia, and astrocyte glia. We further assessed the cell type compositions of larval and adult brains. We found that the transition from larva to adult resulted in great expansion of glial cells. The glial cell accounted for 49.8% of adult midbrain cells. Compared to flies and ants, the mushroom body kenyon cell is insufficient in B. mori, which accounts for 5.4% and 3.6% in larval and adult brains, respectively. Analysis of neuropeptide expression showed that the abundance and specificity of expression varied among individual neuropeptides. Intriguingly, we found that ion transport peptide was specifically expressed in glial cells of larval and adult brains. The cell atlas dataset provides an important resource to explore cell diversity, neural circuits and genetic profiles.

3D reconstruction of larval and adult brain neuropils of two giant silk moth species: Hyalophora cecropia and Antheraea pernyi

We present a detailed analysis of the brain anatomy of two saturniid species, the cecropia silk moth, Hyalophora cecropia, and the Chinese oak silk moth, Antheraea pernyi, including 3D reconstructions of the major brain neuropils in the larva and in male and female adults. The 3D reconstructions, prepared from high-resolution optical sections, showed that the corresponding neuropils of these saturniid species are virtually identical. Similarities between the two species include a pronounced sexual dimorphism in the adults in the form of a male-specific assembly of markedly enlarged glomeruli forming the so-called macroglomerular complex. From the reports published to date, it can be concluded that the neuropil architecture of saturniids resembles that of other nocturnal moths, including the sibling family Sphingidae. In addition, compared with previous anatomical data on diurnal lepidopteran species, significant differences were observed in the two saturniid species, which include the thickness of the Y-tract of the mushroom body, the size of the main neuropils of the optic lobes, and the sexual dimorphisms of the antennal lobes.

Brain composition in Godyris zavaleta, a diurnal butterfly, Reflects an increased reliance on olfactory information

Interspecific comparisons of brain structure can inform our functional understanding of brain regions, identify adaptations to species-specific ecologies, and explore what constrains adaptive changes in brain structure, and coevolution between functionally related structures. The value of such comparisons is enhanced when the species considered have known ecological differences. The Lepidoptera have long been a favored model in evolutionary biology, but to date descriptions of brain anatomy have largely focused on a few commonly used neurobiological model species. We describe the brain of Godyris zavaleta (Ithomiinae), a member of a subfamily of Neotropical butterflies with enhanced reliance on olfactory information. We demonstrate for the first time the presence of sexually dimorphic glomeruli within a distinct macroglomerular complex (MGC) in the antennal lobe of a diurnal butterfly. This presents a striking convergence with the well-known moth MGC, prompting a discussion of the potential mechanisms behind the independent evolution of specialized glomeruli. Interspecific analyses across four Lepidoptera further show that the relative size of sensory neuropils closely mirror interspecific variation in sensory ecology, with G. zavaleta displaying levels of sensory investment intermediate between the diurnal monarch butterfly (Danaus plexippus), which invests heavily in visual neuropil, and night-flying moths, which invest more in olfactory neuropil. We identify several traits that distinguish butterflies from moths, and several that distinguish D. plexippus and G. zavaleta. Our results illustrate that ecological selection pressures mold the structure of invertebrate brains, and exemplify how comparative analyses across ecologically divergent species can illuminate the functional significance of variation in brain structure.

A new antigenic marker specifically labels a subpopulation of the class II Kenyon cells in the brain of the European honeybee Apis mellifera

The mushroom bodies are the higher-order integration center in the insect brain and are involved in higher brain functions such as learning and memory. In the social hymenopteran insects such as honeybees, the mushroom bodies are the prominent brain structures. The mushroom bodies are composed of lobed neuropils formed by thousands of parallel-projecting axons of intrinsic neurons, and the lobes are divided into parallel subdivisions. In the present paper, we report a new antigenic marker to label a single layer in the vertical lobes of the European honeybee Apis mellifera. In the brain of A. mellifera, a monoclonal antibody (mAb) 15C3, which was originally developed against an insect ecdysone receptor (EcR) protein, immunolabels a single layer of the vertical lobes that correspond to the most dorsal layer of the γ-lobe. The 15C3 mAb recognizes a single ~200 kDa protein expressed in the adult honeybee brain. In addition, the 15C3 mAb immunoreactivity was also observed in the lobes of the developing pupal mushroom bodies. Since γ-lobe is well known to their extensive reorganization that occurs during metamorphosis in Drosophila, the novel antigenic marker for the honeybee γ-lobe allows us to investigate morphological changes of the mushroom bodies during metamorphosis.

Molecular and neural mechanisms of sex pheromone reception and processing in the silkmoth Bombyx mori

Male moths locate their mates using species-specific sex pheromones emitted by conspecific females. One striking feature of sex pheromone recognition in males is the high degree of specificity and sensitivity at all levels, from the primary sensory processes to behavior. The silkmoth Bombyx mori is an excellent model insect in which to decipher the underlying mechanisms of sex pheromone recognition due to its simple sex pheromone communication system, where a single pheromone component, bombykol, elicits the full sexual behavior of male moths. Various technical advancements that cover all levels of analysis from molecular to behavioral also allow the systematic analysis of pheromone recognition mechanisms. Sex pheromone signals are detected by pheromone receptors expressed in olfactory receptor neurons in the pheromone-sensitive sensilla trichodea on male antennae. The signals are transmitted to the first olfactory processing center, the antennal lobe (AL), and then are processed further in the higher centers (mushroom body and lateral protocerebrum) to elicit orientation behavior toward females. In recent years, significant progress has been made elucidating the molecular mechanisms underlying the detection of sex pheromones. In addition, extensive studies of the AL and higher centers have provided insights into the neural basis of pheromone processing in the silkmoth brain. This review describes these latest advances, and discusses what these advances have revealed about the mechanisms underlying the specific and sensitive recognition of sex pheromones in the silkmoth.

Concentric zones for pheromone components in the mushroom body calyx of the moth brain

The spatial distribution of input and output neurons in the mushroom body (MB) calyx was investigated in the silkmoth Bombyx mori. In Lepidoptera, the brain has a specialized system for processing sex pheromones. How individual pheromone components are represented in the MB has not yet been elucidated. Toward this end, we first compared the distribution of the presynaptic boutons of antennal lobe projection neurons (PNs), which transfer odor information from the antennal lobe to the MB calyx. The axons of PNs that innervate pheromonal glomeruli were confined to a relatively small area within the calyx. In contrast, the axons of PNs that innervate nonpheromonal glomeruli were more widely distributed. PN axons for the minor pheromone component covered a larger area than those for the major pheromone component and partially overlapped with those innervating nonpheromonal glomeruli, suggesting the integration of the minor pheromone component with plant odors. Overall, we found that PN axons innervating pheromonal and nonpheromonal glomeruli were organized into concentric zones. We then analyzed the dendritic fields of Kenyon cells (KCs), which receive inputs from PNs. Despite the strong regional localization of axons of different PN classes, the dendrites of KCs were less well classified. Finally, we estimated the connectivity between PNs and KCs and suggest that the dendritic field may be organized to receive different amounts of pheromonal and nonpheromonal inputs. PNs for multiple pheromone components and plant odors enter the calyx in a concentric fashion, and they are read out by the elaborate dendritic field of KCs.

Maturation of peptide-positive synaptic arbors in the medicinal leech requires rhythmic target activity

The formation and refinement of synaptic connections are dependent on the activity that emerges from nascent synaptic connections. Such activity has the effect of regulating the production and release of specific neurotransmitters. To determine the role of activity in regulating the production of peptide-positive synapses, we used antibodies against Phe-Met-Arg-Phe-NH₂ and acetylated α-tubulin as well as intracellular injections of Neurobiotin to examine varicosities belonging to heart excitor (HE) neurons on the heart tubes of medicinal leeches, Hirudo spp. We found that the production of peptide-positive varicosities increased considerably during the last week of embryogenesis, which coincided with the emergence of rhythmic activity of the heart tube. When we compromised central input to HE neurons with bicuculline or by surgical ablation of the central pattern generator during early embryogenesis, we found that activity in the heart tubes and its rhythmicity were greatly diminished. Furthermore, the activity of HE neurons had also lost its rhythmicity and appeared tonic, and production of peptide-positive varicosities was substantially reduced as well. Partial surgical ablations that preserved rhythmic activity in the heart tube while disrupting heart tube innervation by some HE neurons still resulted in peptide-positive varicosity production. Taken together, our results suggest that postsynaptic rhythmic activity of the heart tube is necessary and sufficient for the development and maturation of peptide-positive synapses.

Insect-machine hybrid system for understanding and evaluating sensory-motor control by sex pheromone in Bombyx mori

To elucidate the dynamic information processing in a brain underlying adaptive behavior, it is necessary to understand the behavior and corresponding neural activities. This requires animals which have clear relationships between behavior and corresponding neural activities. Insects are precisely such animals and one of the adaptive behaviors of insects is high-accuracy odor source orientation. The most direct way to know the relationships between neural activity and behavior is by recording neural activities in a brain from freely behaving insects. There is also a method to give stimuli mimicking the natural environment to tethered insects allowing insects to walk or fly at the same position. In addition to these methods an 'insect-machine hybrid system' is proposed, which is another experimental system meeting the conditions necessary for approaching the dynamic processing in the brain of insects for generating adaptive behavior. This insect-machine hybrid system is an experimental system which has a mobile robot as its body. The robot is controlled by the insect through its behavior or the neural activities recorded from the brain. As we can arbitrarily control the motor output of the robot, we can intervene at the relationship between the insect and the environmental conditions.

Neuronal organization of the hemiellipsoid body of the land hermit crab, Coenobita clypeatus: correspondence with the mushroom body ground pattern

Malacostracan crustaceans and dicondylic insects possess large second-order olfactory neuropils called, respectively, hemiellipsoid bodies and mushroom bodies. Because these centers look very different in the two groups of arthropods, it has been debated whether these second-order sensory neuropils are homologous or whether they have evolved independently. Here we describe the results of neuroanatomical observations and experiments that resolve the neuronal organization of the hemiellipsoid body in the terrestrial Caribbean hermit crab, Coenobita clypeatus, and compare this organization with the mushroom body of an insect, the cockroach Periplaneta americana. Comparisons of the morphology, ultrastructure, and immunoreactivity of the hemiellipsoid body of C. clypeatus and the mushroom body of the cockroach P. americana reveal in both a layered motif provided by rectilinear arrangements of extrinsic and intrinsic neurons as well as a microglomerular organization. Furthermore, antibodies raised against DC0, the major catalytic subunit of protein kinase A, specifically label both the crustacean hemiellipsoid bodies and insect mushroom bodies. In crustaceans lacking eyestalks, where the entire brain is contained within the head, this antibody selectively labels hemiellipsoid bodies, the superior part of which approximates a mushroom body's calyx in having large numbers of microglomeruli. We propose that these multiple correspondences indicate homology of the crustacean hemiellipsoid body and insect mushroom body and discuss the implications of this with respect to the phylogenetic history of arthropods. We conclude that crustaceans, insects, and other groups of arthropods share an ancestral neuronal ground pattern that is specific to their second-order olfactory centers.

Activity in Neurons of a Putative Protocerebral Circuit Representing Information about a 10 Component Plant Odor Blend in Heliothis virescens

The olfactory pathway in the insect brain is anatomically well described from the antennal lobe (AL) to the mushroom bodies and the lateral protocerebrum (LP) in several species. Less is known about the further connections of the olfactory network in protocerebrum and how information about relevant plant odorants and mixtures are represented in this network, resulting in output information mediated by descending neurons. In the present study we have recorded intracellularly followed by dye injections from neurons in the LP and superior protocerebrum (SP) of the moth, Heliothis virescens. As relevant stimuli, we have used selected primary plant odorants and mixtures of them. The results provide the morphology and physiological responses of neurons involved in a putative circuit connecting the mushroom body lobes, the SP, and the LP, as well as input to SP and LP by one multiglomerular AL neuron and output from the LP by one descending neuron. All neurons responded to a particular mixture of ten primary plant odorants, some of them also to single odorants of the mixture. Altogether, the physiological data indicate integration in protocerebral neurons of information from several of the receptor neuron types functionally described in this species.

Developmental expression of cell recognition molecules in the mushroom body and antennal lobe of the locust Locusta migratoria

We examined the development of olfactory neuropils in the hemimetabolous insect Locusta migratoria with an emphasis on the mushroom bodies, protocerebral integration centers implicated in memory formation. Using a marker of the cyclic adenosine monophosphate (cAMP) signaling cascade and lipophilic dye labeling, we obtained new insights into mushroom body organization by resolving previously unrecognized accessory lobelets arising from Class III Kenyon cells. We utilized antibodies against axonal guidance cues, such as the cell surface glycoproteins Semaphorin 1a (Sema 1a) and Fasciclin I (Fas I), as embryonic markers to compile a comprehensive atlas of mushroom body development. During embryogenesis, all neuropils of the olfactory pathway transiently expressed Sema 1a. The immunoreactivity was particularly strong in developing mushroom bodies. During late embryonic stages, Sema 1a expression in the mushroom bodies became restricted to a subset of Kenyon cells in the core region of the peduncle. Sema 1a was differentially sorted to the Kenyon cell axons and absent in the dendrites. In contrast to Drosophila, locust mushroom bodies and antennal lobes expressed Fas I, but not Fas II. While Fas I immunoreactivity was widely distributed in the midbrain during embryogenesis, labeling persisted into adulthood only in the mushroom bodies and antennal lobes. Kenyon cells proliferated throughout the larval stages. Their neurites retained the embryonic expression pattern of Sema 1a and Fas I, suggesting a role for these molecules in developmental mushroom body plasticity. Our study serves as an initial step toward functional analyses of Sema 1a and Fas I expression during locust mushroom body formation.

Neuropeptides in insect mushroom bodies

Owing to their experimental amenability, insect nervous systems continue to be in the foreground of investigations into information processing in - ostensibly - simple neuronal networks. Among the cerebral neuropil regions that hold a particular fascination for neurobiologists are the paired mushroom bodies, which, despite their function in other behavioral contexts, are most renowned for their role in learning and memory. The quest to understand the processes that underlie these capacities has been furthered by research focusing on unraveling neuroanatomical connections of the mushroom bodies and identifying key players that characterize the molecular machinery of mushroom body neurons. However, on a cellular level, communication between intrinsic and extrinsic mushroom body neurons still remains elusive. The present account aims to provide an overview on the repertoire of neuropeptides expressed in and utilized by mushroom body neurons. Existing data for a number of insect representatives is compiled and some open gaps in the record are filled by presenting additional original data.

The neurobiology of insect olfaction: sensory processing in a comparative context

The simplicity and accessibility of the olfactory systems of insects underlie a body of research essential to understanding not only olfactory function but also general principles of sensory processing. As insect olfactory neurobiology takes advantage of a variety of species separated by millions of years of evolution, the field naturally has yielded some conflicting results. Far from impeding progress, the varieties of insect olfactory systems reflect the various natural histories, adaptations to specific environments, and the roles olfaction plays in the life of the species studied. We review current findings in insect olfactory neurobiology, with special attention to differences among species. We begin by describing the olfactory environments and olfactory-based behaviors of insects, as these form the context in which neurobiological findings are interpreted. Next, we review recent work describing changes in olfactory systems as adaptations to new environments or behaviors promoting speciation. We proceed to discuss variations on the basic anatomy of the antennal (olfactory) lobe of the brain and higher-order olfactory centers. Finally, we describe features of olfactory information processing including gain control, transformation between input and output by operations such as broadening and sharpening of tuning curves, the role of spiking synchrony in the antennal lobe, and the encoding of temporal features of encounters with an odor plume. In each section, we draw connections between particular features of the olfactory neurobiology of a species and the animal's life history. We propose that this perspective is beneficial for insect olfactory neurobiology in particular and sensory neurobiology in general.

A subpopulation of mushroom body intrinsic neurons is generated by protocerebral neuroblasts in the tobacco hornworm moth, Manduca sexta (Sphingidae, Lepidoptera)

Subpopulations of Kenyon cells, the intrinsic neurons of the insect mushroom bodies, are typically sequentially generated by dedicated neuroblasts that begin proliferating during embryogenesis. When present, Class III Kenyon cells are thought to be the first born population of neurons by virtue of the location of their cell somata, farthest from the position of the mushroom body neuroblasts. In the adult tobacco hornworm moth Manduca sexta, the axons of Class III Kenyon cells form a separate Y tract and dorsal and ventral lobelet; surprisingly, these distinctive structures are absent from the larval Manduca mushroom bodies. BrdU labeling and immunohistochemical staining reveal that Class III Kenyon cells are in fact born in the mid-larval through adult stages. The peripheral position of their cell bodies is due to their genesis from two previously undescribed protocerebral neuroblasts distinct from the mushroom body neuroblasts that generate the other Kenyon cell types. These findings challenge the notion that all Kenyon cells are produced solely by the mushroom body neuroblasts, and may explain why Class III Kenyon cells are found sporadically across the insects, suggesting that when present, they may arise through de novo recruitment of neuroblasts outside of the mushroom bodies. In addition, lifelong neurogenesis by both the Class III neuroblasts and the mushroom body neuroblasts was observed, raising the possibility that adult neurogenesis may play a role in mushroom body function in Manduca.

3D Standard Brain of the Red Flour Beetle Tribolium Castaneum: A Tool to Study Metamorphic Development and Adult Plasticity

The red flour beetle Tribolium castaneum is emerging as a further standard insect model beside Drosophila. Its genome is fully sequenced and it is susceptible for genetic manipulations including RNA-interference. We use this beetle to study adult brain development and plasticity primarily with respect to the olfactory system. In the current study, we provide 3D standard brain atlases of freshly eclosed adult female and male beetles (A0). The atlases include eight paired and three unpaired neuropils including antennal lobes (ALs), optic lobe neuropils, mushroom body calyces and pedunculi, and central complex. For each of the two standard brains, we averaged brain areas of 20 individual brains. Additionally, we characterized eight selected olfactory glomeruli from 10 A0 female and male beetles respectively, which we could unequivocally recognize from individual to individual owing to their size and typical position in the ALs. In summary, comparison of the averaged neuropil volumes revealed no sexual dimorphism in any of the reconstructed neuropils in A0 Tribolium brains. Both, the female and male 3D standard brain are also used for interspecies comparisons, and, importantly, will serve as future volumetric references after genetical manipulation especially regarding metamorphic development and adult plasticity.

Anisometric brain dimorphism revisited: Implementation of a volumetric 3D standard brain in Manduca sexta

Lepidopterans like the giant sphinx moth Manduca sexta are known for their conspicuous sexual dimorphism in the olfactory system, which is especially pronounced in the antennae and in the antennal lobe, the primary integration center of odor information. Even minute scents of female pheromone are detected by male moths, facilitated by a huge array of pheromone receptors on their antennae. The associated neuropilar areas in the antennal lobe, the glomeruli, are enlarged in males and organized in the form of the so-called macroglomerular complex (MGC). In this study we searched for anatomical sexual dimorphism more downstream in the olfactory pathway and in other neuropil areas in the central brain. Based on freshly eclosed animals, we created a volumetric female and male standard brain and compared 30 separate neuropilar regions. Additionally, we labeled 10 female glomeruli that were homologous to previously quantitatively described male glomeruli including the MGC. In summary, the neuropil volumes reveal an isometric sexual dimorphism in M. sexta brains. This proportional size difference between male and female brain neuropils masks an anisometric or disproportional dimorphism, which is restricted to the sex-related glomeruli of the antennal lobes and neither mirrored in other normal glomeruli nor in higher brain centers like the calyces of the mushroom bodies. Both the female and male 3D standard brain are also used for interspecies comparisons, and may serve as future volumetric reference in pharmacological and behavioral experiments especially regarding development and adult plasticity. J. Comp. Neurol. 517:210-225, 2009. (c) 2009 Wiley-Liss, Inc.

Reconstruction of virtual neural circuits in an insect brain

The reconstruction of large-scale nervous systems represents a major scientific and engineering challenge in current neuroscience research that needs to be resolved in order to understand the emergent properties of such systems. We focus on insect nervous systems because they represent a good compromise between architectural simplicity and the ability to generate a rich behavioral repertoire. In insects, several sensory maps have been reconstructed so far. We provide an overview over this work including our reconstruction of population activity in the primary olfactory network, the antennal lobe. Our reconstruction approach, that also provides functional connectivity data, will be refined and extended to allow the building of larger scale neural circuits up to entire insect brains, from sensory input to motor output.