A simple mushroom body in an African scarabid beetle

Novel antennal lobe substructures revealed in the small hive beetle Aethina tumida

The small hive beetle, Aethina tumida, is an emerging pest of social bee colonies. A. tumida shows a specialized life style for which olfaction seems to play a crucial role. To better understand the olfactory system of the beetle, we used immunohistochemistry and 3-D reconstruction to analyze brain structures, especially the paired antennal lobes (AL), which represent the first integration centers for odor information in the insect brain. The basic neuroarchitecture of the A. tumida brain compares well to the typical beetle and insect brain. In comparison to other insects, the AL are relatively large in relationship to other brain areas, suggesting that olfaction is of major importance for the beetle. The AL of both sexes contain about 70 olfactory glomeruli with no obvious size differences of the glomeruli between sexes. Similar to all other insects including beetles, immunostaining with an antiserum against serotonin revealed a large cell that projects from one AL to the contralateral AL to densely innervate all glomeruli. Immunostaining with an antiserum against tachykinin-related peptides (TKRP) revealed hitherto unknown structures in the AL. Small TKRP-immunoreactive spherical substructures are in both sexes evenly distributed within all glomeruli. The source for these immunoreactive islets is very likely a group of about 80 local AL interneurons. We offer two hypotheses on the function of such structures.

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.

Metamorphosis and adult development of the mushroom bodies of the red flour beetle, Tribolium castaneum

The insect mushroom bodies play important roles in a number of higher processing functions such as sensory integration, higher level olfactory processing, and spatial and associative learning and memory. These functions have been established through studies in a handful of tractable model systems, of which only the fruit fly Drosophila melanogaster has been readily amenable to genetic manipulations. The red flour beetle Tribolium castaneum has a sequenced genome and has been subject to the development of molecular tools for the ready manipulation of gene expression; however, little is known about the development and organization of the mushroom bodies of this insect. The present account bridges this gap by demonstrating that the organization of the Tribolium mushroom bodies is strikingly like that of the fruit fly, with the significant exception that the timeline of neurogenesis is shifted so that the last population of Kenyon cells is born entirely after adult eclosion. Tribolium Kenyon cells are generated by two large neuroblasts per hemisphere and segregate into an early-born delta lobe subpopulation followed by clear homologs of the Drosophila gamma, alpha'/beta' and alpha/beta lobe subpopulations, with the larval-born cohorts undergoing dendritic reorganization during metamorphosis. BrdU labeling and immunohistochemical staining also reveal that a proportion of individual Tribolium have variable numbers of mushroom body neuroblasts. If heritable, this variation represents a unique opportunity for further studies of the genetic control of brain region size through the control of neuroblast number and cell cycle dynamics.

Ground plan of the insect mushroom body: functional and evolutionary implications

In most insects with olfactory glomeruli, each side of the brain possesses a mushroom body equipped with calyces supplied by olfactory projection neurons. Kenyon cells providing dendrites to the calyces supply a pedunculus and lobes divided into subdivisions supplying outputs to other brain areas. It is with reference to these components that most functional studies are interpreted. However, mushroom body structures are diverse, adapted to different ecologies, and likely to serve various functions. In insects whose derived life styles preclude the detection of airborne odorants, there is a loss of the antennal lobes and attenuation or loss of the calyces. Such taxa retain mushroom body lobes that are as elaborate as those of mushroom bodies equipped with calyces. Antennal lobe loss and calycal regression also typify taxa with short nonfeeding adults, in which olfaction is redundant. Examples are cicadas and mayflies, the latter representing the most basal lineage of winged insects. Mushroom bodies of another basal taxon, the Odonata, possess a remnant calyx that may reflect the visual ecology of this group. That mushroom bodies persist in brains of secondarily anosmic insects suggests that they play roles in higher functions other than olfaction. Mushroom bodies are not ubiquitous: the most basal living insects, the wingless Archaeognatha, possess glomerular antennal lobes but lack mushroom bodies, suggesting that the ability to process airborne odorants preceded the acquisition of mushroom bodies. Archaeognathan brains are like those of higher malacostracans, which lack mushroom bodies but have elaborate olfactory centers laterally in the brain.

Tritocerebral tract input to the insect mushroom bodies

Insect mushroom bodies, best known for their role in olfactory processing, also receive sensory input from other modalities. In crickets and grasshoppers, a tritocerebral tract containing afferents from palp mechanosensory and gustatory centers innervates the accessory calyx. The accessory calyx is uniquely composed of Class III Kenyon cells, and was shown by immunohistochemistry to be present sporadically across several insect orders. Neuronal tracers applied to the source of tritocerebral tract axons in several species of insects demonstrated that tritocerebral tract innervation of the mushroom bodies targeted the accessory calyx when present, the primary calyces when an accessory calyx was not present, or both. These results suggest that tritocerebral tract input to the mushroom bodies is likely ubiquitous, reflecting the importance of gustation for insect behavior. The scattered phylogenetic distribution of Class III Kenyon cells is also proposed to represent an example of generative homology, in which the developmental program for forming a structure is retained in all members of a lineage, but the program is not "run" in all branches.

Neuronal assemblies of the Drosophila mushroom body

The mushroom body (MB) of the insect brain has important roles in odor learning and memory and in diverse other brain functions. To elucidate the anatomical basis underlying its function, we studied how the MB of Drosophila is organized by its intrinsic and extrinsic neurons. We screened for the GAL4 enhancer-trap strains that label specific subsets of these neurons and identified seven subtypes of Kenyon cells and three other intrinsic neuron types. Laminar organization of the Kenyon cell axons divides the pedunculus into at least five concentric strata. The alpha', beta', alpha, and beta lobes are each divided into three strata, whereas the gamma lobe appears more homogeneous. The outermost stratum of the alpha/beta lobes is specifically connected with a small, protruded subregion of the calyx, the accessory calyx, which does not receive direct olfactory input. As for the MB extrinsic neurons (MBENs), we found three types of antennal lobe projection neurons, among which two are novel. In addition, we resolved 17 other types of MBENs that arborize in the calyx, lobes, and pedunculus. Lobe-associated MBENs arborize in only specific areas of the lobes, being restricted along their longitudinal axes, forming two to five segmented zones in each lobe. The laminar arrangement of the Kenyon cell axons and segmented organization of the MBENs together divide the lobes into smaller synaptic units, possibly facilitating characteristic interaction between intrinsic and extrinsic neurons in each unit for different functional activities along the longitudinal lobe axes and between lobes. Structural differences between lobes are also discussed.

Global and local modulatory supply to the mushroom bodies of the moth Spodoptera littoralis

The moth Spodoptera littoralis, is a major pest of agriculture whose olfactory system is tuned to odorants emitted by host plants and conspecifics. As in other insects, the paired mushroom bodies are thought to play pivotal roles in behaviors that are elicited by contextual and multisensory signals, amongst which those of specific odors dominate. Compared with species that have elaborate behavioral repertoires, such as the honey bee Apis mellifera or the cockroach Periplaneta americana, the mushroom bodies of S. littoralis were originally viewed as having a simple cellular organization. This has been since challenged by observations of putative transmitters and neuromodulators. As revealed by immunocytology, the spodopteran mushroom bodies, like those of other taxa, are subdivided longitudinally into discrete neuropil domains. Such divisions are further supported by the present study, which also demonstrates discrete affinities to different mushroom body neuropils by antibodies raised against two putative transmitters, glutamate and gamma-aminobutyric acid, and against three putative neuromodulatory substances: serotonin, A-type allatostatin, and tachykinin-related peptides. The results suggest that in addition to longitudinal divisions of the lobes, circuits in the calyces and lobes are likely to be independently modulated.

Structure of the mushroom bodies of the insect brain

The past decade has produced an explosion of new information on the development, neuroanatomy, and possible functions of the mushroom bodies. This review provides a concise, contemporary overview of the structure of the mushroom bodies. Two topics are highlighted: the volume plasticity of mushroom body neuropils evident in the brains of some adult insects and a possible essential role for the gamma lobe in olfactory memory.

Organization of Kenyon cells in subdivisions of the mushroom bodies of a lepidopteran insect

The mushroom bodies are paired structures in the insect brain involved in complex functions such as memory formation, sensory integration, and context recognition. In many insects these centers are elaborate, sometimes comprising several hundred thousand neurons. The present account describes the mushroom bodies of Spodoptera littoralis, a moth extensively used for studies of olfactory processing and conditioning. The mushroom bodies of Spodoptera consist of only about 4,000 large-diameter Kenyon cells. However, these neurons are recognizably similar to morphological classes of Kenyon cells identified in honey bees, Drosophila, and cockroaches. The spodopteran mushroom body is equipped with three major divisions of its vertical and medial lobe, one of which, the gamma lobe, is supplied by clawed class II Kenyon cells as in other described taxa. Of special interest is the presence of a discrete tract (the Y tract) of axons leading from the calyx, separate from the pedunculus, that innervates lobelets above and beneath the medial lobe, close to the latter's origin from the pedunculus. This tract is comparable to tracts and resultant lobelets identified in cockroaches and termites. The article discusses possible functional roles of the spodopteran mushroom body against the background of olfactory behaviors described from this taxon and discusses the possible functional relevance of mushroom body structure, emphasizing similarities and dissimilarities with mushroom bodies of other species, in particular the fruit fly, Drosophila melanogaster.