Constancy and variability of glomerular organization in the antennal lobe of the silkmoth

Glomerular organization of the antennal lobe in the fall webworm Hyphantria cunea (Drury, 1770)

The fall webworm (Hyphantria cunea), a typical destructive invasive pest, has caused substantial damage to both the ecological environment and economy in China. H. cunea uses primarily its antennae to locate food and perceive pheromones through stimulation of olfactory receptor neurons. These receptor neurons project their axons into glomeruli within the antennal lobes, the primary olfactory center in the brain. The projection patterns of sensory antennal neurons into the antennal lobe and its precise structure have not been described so far. To decipher the primary organization behind olfactory recognition in H. cunea, this study employed synaptic antibody immunostaining, as well as mass staining of olfactory sensory neurons (OSNs), and computer-based reconstruction to establish a three-dimensional olfactory glomerular map of the moth's antennal lobes. A total of 74 male and 81 female antennal lobe glomeruli were identified, including 3 male-specific glomeruli (Macroglomerular complex, MGC) and 8 female-specific glomeruli (DL1-DL8). While the Cumulus (Cu) volume was largest in MGC, the differences in volume among dorsomedial anterior and dorsomedial posterior were minimal. These findings lay the groundwork for a better understanding of the olfactory anatomical organization in H. cunea.

Odorant Receptors Expressing and Antennal Lobes Architecture Are Linked to Caste Dimorphism in Asian Honeybee, Apis cerana (Hymenoptera: Apidae)

Insects heavily rely on the olfactory system for food, mating, and predator evasion. However, the caste-related olfactory differences in Apis cerana, a eusocial insect, remain unclear. To explore the peripheral and primary center of the olfactory system link to the caste dimorphism in A. cerana, transcriptome and immunohistochemistry studies on the odorant receptors (ORs) and architecture of antennal lobes (ALs) were performed on different castes. Through transcriptomesis, we found more olfactory receptor genes in queens and workers than in drones, which were further validated by RT-qPCR, indicating caste dimorphism. Meanwhile, ALs structure, including volume, surface area, and the number of glomeruli, demonstrated a close association with caste dimorphism. Particularly, drones had more macroglomeruli possibly for pheromone recognition. Interestingly, we found that the number of ORs and glomeruli ratio was nearly 1:1. Also, the ORs expression distribution pattern was very similar to the distribution of glomeruli volume. Our results suggest the existence of concurrent plasticity in both the peripheral olfactory system and ALs among different castes of A. cerana, highlighting the role of the olfactory system in labor division in insects.

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.

Organization of the parallel antennal-lobe tracts in the moth

The olfactory pathways of the insect brain have been studied comprehensively for more than 40 years, yet the last decade has included a particularly large accumulation of new information relating to this system's structure. In moths, sharp intracellular recording and staining has been used to elucidate the anatomy and physiology of output neurons from the primary olfactory center, the antennal lobe. This review concentrates on the connection patterns characterizing these projection neurons, which follow six separate antennal-lobe tracts. In addition to highlighting the connections between functionally distinct glomerular clusters and higher-order olfactory neuropils, we discuss how parallel tracts in the male convey distinct features of the social signals released by conspecific and heterospecific females. Finally, we consider the current state of knowledge regarding olfactory processing in the moth's protocerebrum and make suggestions as to how the information concerning antennal-lobe output may be used to design future studies.

Shedding Light on Inter-Individual Variability of Olfactory Circuits in Drosophila

Inter-individual differences in behavioral responses, anatomy or functional properties of neuronal populations of animals having the same genotype were for a long time disregarded. The majority of behavioral studies were conducted at a group level, and usually the mean behavior of all individuals was considered. Similarly, in neurophysiological studies, data were pooled and normalized from several individuals. This approach is mostly suited to map and characterize stereotyped neuronal properties between individuals, but lacks the ability to depict inter-individual variability regarding neuronal wiring or physiological characteristics. Recent studies have shown that behavioral biases and preferences to olfactory stimuli can vary significantly among individuals of the same genotype. The origin and the benefit of these diverse "personalities" is still unclear and needs to be further investigated. A perspective taken into account the inter-individual differences is needed to explore the cellular mechanisms underlying this phenomenon. This review focuses on olfaction in the vinegar fly Drosophila melanogaster and summarizes previous and recent studies on odor-guided behavior and the underlying olfactory circuits in the light of inter-individual variability. We address the morphological and physiological variabilities present at each layer of the olfactory circuitry and attempt to link them to individual olfactory behavior. Additionally, we discuss the factors that might influence individuality with regard to olfactory perception.

Re-emergence and diversification of a specialized antennal lobe morphology in ithomiine butterflies

How an organism's sensory system functions is central to how it navigates its environment. The insect olfactory system is a prominent model for investigating how ecological factors impact sensory reception and processing. Notably, work in Lepidoptera led to the discovery of vastly expanded structures, termed macroglomerular complexes (MGCs), within the primary olfactory processing centre. MGCs typically process pheromonal cues, are usually larger in males, and provide classic examples of how variation in the size of neural structures reflects the importance of sensory cues. Though prevalent across moths, MGCs were lost during the origin of butterflies, consistent with evidence that courtship initiation in butterflies is primarily reliant on visual cues, rather than long distance chemical signals. However, an MGC was recently described in a species of ithomiine butterfly, suggesting that this once lost neural adaptation has re-emerged in this tribe. Here, we show that MGC-like morphologies are widely distributed across ithomiines, but vary in both their structure and prevalence of sexual dimorphism. Based on this interspecific variation we suggest that the ithomiine MGC is involved in processing both plant and pheromonal cues, which have similarities in their chemical constitution, and co-evolved with an increased importance of plant derived chemical compounds.

Inverse resource allocation between vision and olfaction across the genus Drosophila

Divergent populations across different environments are exposed to critical sensory information related to locating a host or mate, as well as avoiding predators and pathogens. These sensory signals generate evolutionary changes in neuroanatomy and behavior; however, few studies have investigated patterns of neural architecture that occur between sensory systems, or that occur within large groups of closely-related organisms. Here we examine 62 species within the genus Drosophila and describe an inverse resource allocation between vision and olfaction, which we consistently observe at the periphery, within the brain, as well as during larval development. This sensory variation was noted across the entire genus and appears to represent repeated, independent evolutionary events, where one sensory modality is consistently selected for at the expense of the other. Moreover, we provide evidence of a developmental genetic constraint through the sharing of a single larval structure, the eye-antennal imaginal disc. In addition, we examine the ecological implications of visual or olfactory bias, including the potential impact on host-navigation and courtship.

Marked interspecific differences in the neuroanatomy of the male olfactory system of honey bees (genus Apis)

All honey bee species (genus Apis) display a striking mating behavior with the formation of male (drone) congregations, in which virgin queens mate with many drones. Bees' mating behavior relies on olfactory communication involving queen-but also drone pheromones. To explore the evolution of olfactory communication in Apis, we analyzed the neuroanatomical organization of the antennal lobe (primary olfactory center) in the drones of five species from the three main lineages (open-air nesting species: dwarf honey bees Apis florea and giant honey bees Apis dorsata; cavity-nesting species: Apis mellifera, Apis kochevnikovi, and Apis cerana) and from three populations of A. cerana (Borneo, Thailand, and Japan). In addition to differences in the overall number of morphological units, the glomeruli, our data reveal marked differences in the number and position of macroglomeruli, enlarged units putatively dedicated to sex pheromone processing. Dwarf and giant honey bee species possess two macroglomeruli while cavity-nesting bees present three or four macroglomeruli, suggesting an increase in the complexity of sex communication during evolution in the genus Apis. The three A. cerana populations showed differing absolute numbers of glomeruli but the same three macroglomeruli. Overall, we identified six different macroglomeruli in the genus Apis. One of these (called MGb), which is dedicated to the detection of the major queen compound 9-ODA in A. mellifera, was conserved in all species. We discuss the implications of these results for our understanding of sex communication in honey bees and propose a putative scenario of antennal lobe evolution in the Apis genus.

Glomerular Organization in the Antennal Lobe of the Oriental Fruit Fly Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis is one of the most destructive pests of horticultural crops in tropical and subtropical Asia. The insect relies heavily on its olfactory system to select suitable hosts for development and reproduction. To understand the neural basis of its odor-driven behaviors, it is fundamental to characterize the anatomy of its olfactory system. In this study, we investigated the anatomical organization of the antennal lobe (AL), the primary olfactory center, in B. dorsalis, and constructed a 3D glomerular atlas of the AL based on synaptic antibody staining combined with computerized 3D reconstruction. To facilitate identification of individual glomeruli, we also applied mass staining of olfactory sensory neurons (OSNs) and projection neurons (PNs). In total, 64 or 65 glomeruli are identifiable in both sexes based on their shape, size, and relative spatial relationship. The overall glomerular volume of two sexes is not statistically different. However, eight glomeruli are sexually dimorphic: four (named AM2, C1, L2, and L3) are larger in males, and four are larger in females (A3, AD1, DM3, and M1). The results from anterograde staining, obtained by applying dye in the antennal lobe, show that three typical medial, media lateral, and lateral antennal-lobe tracts form parallel connections between the antennal lobe and protocerebrum. In addition to these three tracts, we also found a transverse antennal-lobe tract. Based on the retrograde staining of the calyx in the mushroom body, we also characterize the arrangement of roots and cell body clusters linked to the medial antennal-lobe tracts. These data provide a foundation for future studies on the olfactory processing of host odors in B. dorsalis.

Morphology and physiology of antennal lobe projection neurons in the hawkmoth Agrius convolvuli

The neuronal pathways involved in the processing of sex pheromone information were investigated in the hawkmoth Agrius convolvuli (Lepidoptera: Sphingidae), which uses (E,E)-11,13-hexadecadienal (E11,E13-16:Ald) as the single sex pheromone component. We first clarified the anatomical organization of the antennal lobe of A. convolvuli. Subsequently, central neurons in the antennal lobe that responded to E11,E13-16:Ald were identified. The dendritic processes of these neurons were confined within a specific glomerulus (cumulus) in the antennal lobe. The axons of these neurons projected to the inferior lateral protocerebrum and mushroom body calyx. Although the anatomical organization and morphology of individual neurons in A. convolvuli were similar to other species in the superfamily Bombycoidea, the use of cumulus as the single pathway for sex pheromone information processing was characteristic to this species.

Organization of the antennal lobes in the praying mantis (Tenodera aridifolia)

Olfaction in insects plays pivotal roles in searching for food and/or for sexual partners. Although many studies have focused on the olfactory processes of nonpredatory insect species, little is known about those in predatory insects. Here, we investigated the anatomical features of the primary olfactory center (antennal lobes) in an insect predator whose visual system is well developed, the praying mantis Tenodera aridifolia. Both sexes of T. aridifolia were found to possess 54 glomeruli, and each glomerulus was identified based on its location and size. Moreover, we found a sexual dimorphism in three glomeruli (macroglomeruli) located at the entrance of the antennal nerves, which are 15 times bigger in males than their homologs in females. We additionally deduced the target glomeruli of olfactory sensory neurons housed in cognate types of sensilla by degenerating the sensory afferents. The macroglomeruli received sensory inputs from grooved peg sensilla, which are present in a large number at the proximal part of the males' antennae. Furthermore, our findings suggest that glomeruli at the posteriodorsal part of the antennal lobes receive sensory information from putative hygro- and thermosensitive sensilla. The origins of projections connected to the protocerebrum are also discussed. J. Comp. Neurol. 525:1685-1706, 2017. © 2016 Wiley Periodicals, Inc.

Dendritic Arborization Patterns of Small Juxtaglomerular Cell Subtypes within the Rodent Olfactory Bulb

Within the glomerular layer of the rodent olfactory bulb, numerous subtypes of local interneurons contribute to early processing of incoming sensory information. Here we have investigated dopaminergic and other small local juxtaglomerular cells in rats and mice and characterized their dendritic arborization pattern with respect to individual glomeruli by fluorescent labeling via patching and reconstruction of dendrites and glomerular contours from two-photon imaging data. Dopaminergic neurons were identified in a transgenic mouse line where the expression of dopamine transporter (DAT) was labeled with GFP. Among the DAT+ cells we found a small short-axon cell (SAC) subtype featuring hitherto undescribed dendritic specializations. These densely ramifying structures clasped mostly around somata of other juxtaglomerular neurons, which were also small, non-dopaminergic and to a large extent non-GABAergic. Clasping SACs were observed also in wild-type mice and juvenile rats. In DAT+ SAC dendrites, single backpropagating action potentials evoked robust calcium entry throughout both clasping and non-clasping compartments. Besides clasping SACs, most other small neurons either corresponded to the classical periglomerular cell type (PGCs), which was never DAT+, or were undersized cells with a small dendritic tree and low excitability. Aside from the presence of clasps in SAC dendrites, many descriptors of dendritic morphology such as the number of dendrites and the extent of branching were not significantly different between clasping SACs and PGCs. However, a detailed morphometric analysis in relation to glomerular contours revealed that the dendrites of clasping SACs arborized mostly in the juxtaglomerular space and never entered more than one glomerulus (if at all), whereas most PGC dendrites were restricted to their parent glomerulus, similar to the apical tufts of mitral cells. These complementary arborization patterns might underlie a highly complementary functional connectivity. The morphometric approach may serve to differentiate also other subtypes of juxtaglomerular neurons, help to identify putative synaptic partners and thus to establish a more refined picture of glomerular network interactions during odor sensing.

A global-wide search for sexual dimorphism of glomeruli in the antennal lobe of female and male Helicoverpa armigera

By using immunostaining and three-dimensional reconstruction, the anatomical organization of the antennal lobe glomeruli of the female cotton bollworm Helicoverpa armigera was investigated. Eighty-one glomeruli were identified, 15 of which were not previously discovered. The general anatomical organization of the AL of female is similar to that of male and all glomeruli were classified into four sub-groups, including the female-specific glomerular complex, posterior complex, labial-palp pit organ glomerulus, and ordinary glomeruli. A global-wide comparison on the complete glomerular map of female and male was performed and for the first time the quantitative difference in volume for each individual homologous glomerulus was analyzed. We found that the sexual dimorphism includes not only the sex-specific glomeruli but also some of the other glomeruli. The findings in the present study may provide a reference to examine the antennal-lobe organization more in detail and to identify new glomeruli in other moth species. In addition, the complete identification and global-wide comparison of the sexes provide an important basis for mapping the function of distinct glomeruli and for understanding neural mechanisms underlying sexually dimorphic olfactory behaviors.

Morphological characterization of the antennal lobes in the Mediterranean fruit fly Ceratitis capitata

The medfly Ceratitis capitata is one of the most important pests for horticulture worldwide. The knowledge about anatomy and function of the medfly olfactory system is still limited. The first brain structure to process olfactory information in insects is the antennal lobe (AL), which is composed of its functional and morphological units, the olfactory glomeruli. Here, we present a morphological three-dimensional reconstruction of AL glomeruli in adult brains. We used unilateral antennal backfills of olfactory receptor neurons (ORNs) with neural tracers, revealing the AL structure. We recorded confocal stacks acquired from whole-mount specimens, and analyzed them with the software AMIRA. The ALs in C. capitata are organized in glomeruli which are more tightly packed in the anterior part than the posterior one. Axons of ORNs bilaterally connect the ALs through a commissure between the two ALs. This commissure is formed by several distinct fascicles. Contralateral dye transfer suggests the presence of gap junctions connecting ORNs from both antennae. There was no statistical difference between the average volumes of female ALs (204,166 ± 12,554 μm(3)) and of male ALs (190,287 ± 11,823 μm(3)). In most specimens, we counted 53 glomeruli in each AL, seven of which were sexually dimorphic in size.

Targeted disruption of a single sex pheromone receptor gene completely abolishes in vivo pheromone response in the silkmoth

Male moths use species-specific sex pheromones to identify and orientate toward conspecific females. Odorant receptors (ORs) for sex pheromone substances have been identified as sex pheromone receptors in various moth species. However, direct in vivo evidence linking the functional role of these ORs with behavioural responses is lacking. In the silkmoth, Bombyx mori, female moths emit two sex pheromone components, bombykol and bombykal, but only bombykol elicits sexual behaviour in male moths. A sex pheromone receptor BmOR1 is specifically tuned to bombykol and is expressed in specialized olfactory receptor neurons (ORNs) in the pheromone sensitive long sensilla trichodea of male silkmoth antennae. Here, we show that disruption of the BmOR1 gene, mediated by transcription activator-like effector nucleases (TALENs), completely removes ORN sensitivity to bombykol and corresponding pheromone-source searching behaviour in male moths. Furthermore, transgenic rescue of BmOR1 restored normal behavioural responses to bombykol. Our results demonstrate that BmOR1 is required for the physiological and behavioural response to bombykol, demonstrating that it is the receptor that mediates sex pheromone responses in male silkmoths. This study provides the first direct evidence that a member of the sex pheromone receptor family in moth species mediates conspecific sex pheromone information for sexual behaviour.

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.

Antennal lobe organization and pheromone usage in bombycid moths

We investigated the neuroanatomy of the macroglomerular complex (MGC), which is involved in sex pheromone processing, in five species in the subfamily Bombycinae, including Ernolatia moorei, Trilocha varians, Rondotia menciana, Bombyx mandarina and Bombyx mori. The glomerulus located at the dorsal-most part of the olfactory centre shows the largest volume in moth species examined to date. Such normal glomerular organization has been observed in E. moorei and T. varians, which use a two-component mixture and includes the compound bombykal as a mating signal. By contrast, the other three species, which use another component as a single attractant, exhibited a modified arrangement of the MGC. This correlation between pheromone usage and neural organization may be useful for understanding the process of speciation.

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.

Anatomical and functional analysis of domestication effects on the olfactory system of the silkmoth Bombyx mori

The silkmoth Bombyx mori is the main producer of silk worldwide and has furthermore become a model organism in biological research, especially concerning chemical communication. However, the impact domestication might have had on the silkmoth's olfactory sense has not yet been investigated. Here, we show that the pheromone detection system in B. mori males when compared with their wild ancestors Bombyx mandarina seems to have been preserved, while the perception of environmental odorants in both sexes of domesticated silkmoths has been degraded. In females, this physiological impairment was mirrored by a clear reduction in olfactory sensillum numbers. Neurophysiological experiments with hybrids between wild and domesticated silkmoths suggest that the female W sex chromosome, so far known to have the sole function of determining femaleness, might be involved in the detection of environmental odorants. Moreover, the coding of odorants in the brain, which is usually similar among closely related moths, differs strikingly between B. mori and B. mandarina females. These results indicate that domestication has had a strong impact on odour detection and processing in the olfactory model species B. mori.

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.

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.

Population coding is essential for rapid information processing in the moth antennal lobe

We investigated how odorant information is transmitted by neurons in the moth antennal lobe (AL). The neurons were repeatedly stimulated by three different odorants and their activity was intracellularly recorded. First, the response properties of single neurons were analyzed. The neurons exhibited highly reliable responses to the odorants and 43% of AL neurons responded to two or three odorants. The population distribution of firing rates in response to odorant stimulation was relatively broad in moth AL neurons, which is consistent across insects. Second, we attempted to decode the odorant identity from the activity of the recorded neurons using the maximum likelihood method. The decoding performance rapidly improves with increasing the number of neurons. Notably, an increase in the size of neural population results in faster transfer of information and increased the duration to retain odorant information. In conclusion, the AL neurons encode odorant information reliably and the population coding can transmit odorant information to olfactory centers. Population coding allows AL to encode and transmit olfactory information faster than the discrimination latency demonstrated in behavioral experiments. This article is part of a Special Issue entitled Neural Coding 2012.

Organization of the olfactory system of nymphalidae butterflies

Olfaction is in many species the most important sense, essential for food search, mate finding, and predator avoidance. Butterflies have been considered a microsmatic group of insects that mainly rely on vision due to their diurnal lifestyle. However, an emerging number of studies indicate that butterflies indeed use the sense of smell for locating food and oviposition sites. To unravel the neural substrates for olfaction, we performed an anatomical study of 2 related butterfly species that differ in food and host plant preference. We found many of the anatomical structures and pathways, as well as distribution of neuroactive substances, to resemble that of their nocturnal relatives among the Lepidoptera. The 2 species differed in the number of one type of olfactory sensilla, thus indicating a difference in sensitivity to certain compounds. Otherwise no differences could be observed. Our findings suggest that the olfactory system in Lepidoptera is well conserved despite the long evolutionary time since butterflies and moths diverged from a common ancestor.

Distribution and functional organization of glomeruli in the olfactory bulbs of zebrafish (Danio rerio)

Odor molecules are transduced by thousands of olfactory sensory neurons (OSNs) located in the nasal cavity. Each OSN expresses a single functional odorant receptor protein and projects an axon from the sensory epithelia to an olfactory bulb glomerulus, which is selectively innervated by only one or a few OSN types. We used whole-mount immunocytochemistry to study the neurochemistry and anatomical organization of glomeruli in the zebrafish olfactory system. By employing combinations of antibodies against G-protein α subunits, calcium-binding proteins, and general neuronal markers, we selectively labeled various OSN types, their axonal projections to glomeruli, and the detailed anatomical distributions of individual glomeruli in different regions of the olfactory bulb. In this way we identified ≈140 glomeruli in each olfactory bulb of mature zebrafish. A small subset (27) of these glomeruli was unambiguously identifiable in nearly all animals examined. These units were large and, located mainly in the medial olfactory bulbs. Most glomeruli, however, were comparatively small, anatomically indistinguishable, and located in coarsely circumscribed regions; almost all of these latter glomeruli were innervated by OSNs that were labeled with anti-G(α s/olf) and/or anti-calretinin antibodies. Collectively, our results provide a uniquely detailed description of a vertebrate olfactory system and highlight anatomically distinct parallel neural pathways that mediate early aspects of olfactory processing in the zebrafish.

Heterogeneity in dendritic morphology of moth antennal lobe projection neurons

A population of projection neurons (PNs) in the antennal lobe (AL) integrates sensory information from the antenna and is essential for processing odor information in the insect brain. We examined the anatomy of this neuronal population in the brain of the silkmoth Bombyx mori. Using intracellular dye injection, we labeled a total of 246 PNs and systematically analyzed their morphological features, including the soma position, antennocerebral tract, and number of innervating glomeruli. For example, we analyzed PNs that had somata in the different cell clusters, innervated overlapping but different groups of glomeruli, and ran through different pathways. We also identified glomeruli innervated by PNs using a previously established procedure that first classifies glomeruli into regional groups and then identifies individual glomeruli. We analyzed uniglomerular PNs (75.6% of the total) and found heterogeneity in the dendritic morphology of the PNs that was dependent on the regions and/or the innervating glomeruli. For example, most PNs innervating the macroglomerular complex did not have extraglomerular processes, whereas most PNs innervating ordinary glomeruli did. Moreover, PNs innervating the toroid glomerulus showed heterogeneity in their dendritic morphology. These PNs had dendritic arborization in different areas within the glomerulus. We found that, in some cases, the innervation pattern of the PN dendrite correlated with individual variation in the glomerular organization. These results indicate that PNs are not homogeneous populations, and in some cases morphological heterogeneity in PNs correlated with change in glomerular organization in the silkmoth AL.

Olfactory coding in five moth species from two families

The aim of the present study was to determine what impact phylogeny and life history might have on the coding of odours in the brain. Using three species of hawk moths (Sphingidae) and two species of owlet moths (Noctuidae), we visualized neural activity patterns in the antennal lobe, the first olfactory neuropil in insects, evoked by a set of ecologically relevant plant volatiles. Our results suggest that even between the two phylogenetically distant moth families, basic olfactory coding features are similar. But we also found different coding strategies in the moths’ antennal lobe; namely, more specific patterns for chemically similar odorants in the two noctuid species than in the three sphingid species tested. This difference demonstrates the impact of the phylogenetic distance between species from different families despite some parallel life history traits found in both families. Furthermore, pronounced differences in larval and adult diet among the sphingids did not translate into differences in the olfactory code; instead, the three species had almost identical coding patterns.

Sex-linked transcription factor involved in a shift of sex-pheromone preference in the silkmoth Bombyx mori

In the sex-pheromone communication systems of moths, odorant receptor (Or) specificity as well as higher olfactory information processing in males should be finely tuned to the pheromone of conspecific females. Accordingly, male sex-pheromone preference should have diversified along with the diversification of female sex pheromones; however, the genetic mechanisms that facilitated the diversification of male preference are not well understood. Here, we explored the mechanisms involved in a drastic shift in sex-pheromone preference in the silkmoth Bombyx mori using spli mutants in which the genomic structure of the gene Bmacj6, which encodes a class IV POU domain transcription factor, is disrupted or its expression is repressed. B. mori females secrete an ∼11:1 mixture of bombykol and bombykal. Bombykol alone elicits full male courtship behavior, whereas bombykal alone shows no apparent activity. In the spli mutants, the behavioral responsiveness of males to bombykol was markedly reduced, whereas bombykal alone evoked full courtship behavior. The reduced response of spli males to bombykol was explained by the paucity of bombykol receptors on the male antennae. It was also found that, in the spli males, neurons projecting into the toroid, a compartment in the brain where bombykol receptor neurons normally project, responded strongly to bombykal. The present study highlights a POU domain transcription factor, Bmacj6, which may have caused a shift of sex-pheromone preference in B. mori through Or gene choice and/or axon targeting.

Odour maps in the brain of butterflies with divergent host-plant preferences

Butterflies are believed to use mainly visual cues when searching for food and oviposition sites despite that their olfactory system is morphologically similar to their nocturnal relatives, the moths. The olfactory ability in butterflies has, however, not been thoroughly investigated. Therefore, we performed the first study of odour representation in the primary olfactory centre, the antennal lobes, of butterflies. Host plant range is highly variable within the butterfly family Nymphalidae, with extreme specialists and wide generalists found even among closely related species. Here we measured odour evoked Ca²⁺ activity in the antennal lobes of two nymphalid species with diverging host plant preferences, the specialist Aglais urticae and the generalist Polygonia c-album. The butterflies responded with stimulus-specific combinations of activated glomeruli to single plant-related compounds and to extracts of host and non-host plants. In general, responses were similar between the species. However, the specialist A. urticae responded more specifically to its preferred host plant, stinging nettle, than P. c-album. In addition, we found a species-specific difference both in correlation between responses to two common green leaf volatiles and the sensitivity to these compounds. Our results indicate that these butterflies have the ability to detect and to discriminate between different plant-related odorants.

A single sex pheromone receptor determines chemical response specificity of sexual behavior in the silkmoth Bombyx mori

In insects and other animals, intraspecific communication between individuals of the opposite sex is mediated in part by chemical signals called sex pheromones. In most moth species, male moths rely heavily on species-specific sex pheromones emitted by female moths to identify and orient towards an appropriate mating partner among a large number of sympatric insect species. The silkmoth, Bombyx mori, utilizes the simplest possible pheromone system, in which a single pheromone component, (E, Z)-10,12-hexadecadienol (bombykol), is sufficient to elicit full sexual behavior. We have previously shown that the sex pheromone receptor BmOR1 mediates specific detection of bombykol in the antennae of male silkmoths. However, it is unclear whether the sex pheromone receptor is the minimally sufficient determination factor that triggers initiation of orientation behavior towards a potential mate. Using transgenic silkmoths expressing the sex pheromone receptor PxOR1 of the diamondback moth Plutella xylostella in BmOR1-expressing neurons, we show that the selectivity of the sex pheromone receptor determines the chemical response specificity of sexual behavior in the silkmoth. Bombykol receptor neurons expressing PxOR1 responded to its specific ligand, (Z)-11-hexadecenal (Z11-16:Ald), in a dose-dependent manner. Male moths expressing PxOR1 exhibited typical pheromone orientation behavior and copulation attempts in response to Z11-16:Ald and to females of P. xylostella. Transformation of the bombykol receptor neurons had no effect on their projections in the antennal lobe. These results indicate that activation of bombykol receptor neurons alone is sufficient to trigger full sexual behavior. Thus, a single gene defines behavioral selectivity in sex pheromone communication in the silkmoth. Our findings show that a single molecular determinant can not only function as a modulator of behavior but also as an all-or-nothing initiator of a complex species-specific behavioral sequence.

Like many animal species, moths use chemical signals called sex pheromones to communicate with conspecific individuals of the opposite sex in the context of reproduction. Typically, male moths depend on sex pheromones emitted by conspecific females to identify and locate their mates. Therefore, the behavioral preference of male moths to conspecific pheromones is a critical factor for successful reproduction. Sex pheromone receptor proteins expressed in specialized antennal olfactory receptor neurons reportedly play a central role in sex pheromone discrimination. However, the causal relationship between sex pheromone receptor specificity and behavioral preference remains to be proven. We have addressed this question in a genetically tractable moth species, the silkmoth (Bombyx mori), because this species possesses the simplest possible pheromone system in which a single pheromone substance, bombykol, elicits full sexual behavior. Using transgenic silkmoths expressing a sex pheromone receptor from another moth species, we revealed that solely the chemical specificity of the odorant receptors in bombykol receptor neurons determines the behavioral preference in male silkmoths. Our results show that the initiation of a complex programmed sexual behavior can depend on the properties of a single pheromone receptor gene expressed in a population of olfactory receptor neurons.

Complete mapping of glomeruli based on sensory nerve branching pattern in the primary olfactory center of the cockroach Periplaneta americana

Glomeruli are structural and functional units in the primary olfactory center in vertebrates and insects. In the cockroach Periplaneta americana, axons of different types of sensory neurons housed in sensilla on antennae form dorsal and ventral antennal nerves and then project to a number of glomeruli. In this study, we identified all antennal lobe (AL) glomeruli based on detailed innervation patterns of sensory tracts in addition to the shape, size, and locations in the cockroach. The number of glomeruli is approximately 205, and no sex-specific difference is observed. Anterograde dye injections into the antennal nerves revealed that axons supplying the AL are divided into 10 sensory tracts (T1-T10). Each of T1-T3 innervates small, oval glomeruli in the anteroventral region of the AL, with sensory afferents invading each glomerulus from multiple directions, whereas each of T4-T10 innervates large glomeruli with various shapes in the posterodorsal region, with a bundle of sensory afferents invading each glomerulus from one direction. The topographic branching patterns of all these tracts are conserved among individuals. Sensory afferents in a sub-tract of T10 had axon terminals in the dorsal margin of the AL and the protocerebrum, where they form numerous small glomerular structures. Sensory nerve branching pattern should reflect developmental processes to determine spatial arrangement of glomeruli, and thus the complete map of glomeruli based on sensory nerve branching pattern should provide a basis for studying the functional significance of spatial arrangement of glomeruli and its developmental basis.

Offset response of the olfactory projection neurons in the moth antennal lobe

We investigated a population activity of central olfactory neurons after the termination of odor input. Olfactory response of projection neurons in the moth primary olfactory center was characterized using in vivo intracellular recording and staining techniques. The population activity changed rapidly to the different states after the stimulus offset. The response after stimulus offset represents information regarding odor identity. We analyzed the spatial distribution of offset-activated glomeruli in a virtual neuronal population that was reconstructed using accumulated individual recordings obtained from different specimens. The offset-activated glomeruli tended to be widely distributed, whereas the onset-activated glomeruli were relatively clustered. These results suggest the importance of lateral interaction in shaping the offset olfactory response.

Integration of the antennal lobe glomeruli and three projection neurons in the standard brain atlas of the moth heliothis virescens

Digital three dimensional standard brain atlases (SBAs) are valuable tools for integrating neuroimaging data of different preparations. In insects, SBAs of five species are available, including the atlas of the female Heliothis virescens moth brain. Like for the other species, the antennal lobes (ALs) of the moth brain atlas were integrated as one material identity without internal structures. Different from the others, the H. virescens SBA exclusively included the glomerular layer of the AL. This was an advantage in the present study for performing a direct registration of the glomerular layer of individual preparations into the standard brain. We here present the H. virescens female SBA with a new model of the AL glomeruli integrated into the atlas, i.e. with each of the 66 glomeruli identified and labelled with a specific number. The new model differs from the previous H. virescens AL model both in respect to the number of glomeruli and the numbering system; the latter according to the system used for the AL atlases of two other heliothine species. For identifying female specific glomeruli comparison with the male AL was necessary. This required a new male AL atlas, included in this paper. As demonstrated by the integration of three AL projection neurons of different preparations, the new SBA with the integrated glomruli is a helpful tool for determining the glomeruli innervated as well as the relative position of the axonal projections in the protocerebrum.

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.