Antennal lobe projection destinations of Helicoverpa zea male olfactory receptor neurons responsive to heliothine sex pheromone components

Functional map of the macroglomerular complex of male Helicoverpa armigera

The mechanism of sex pheromone reception in the male cotton bollworm Helicoverpa armigera has been extensively studied because it has become an important model system for understanding insect olfaction. However, the pathways of pheromone processing from the antenna to the primary olfactory center in H. armigera have not yet been clarified. Here, the physiology and morphology of male H. armigera olfactory sensory neurons (OSNs) were studied using single sensillum recording along with anterograde filling and intracellular recording with retrograde filling. OSNs localized in type A sensilla responded to the major pheromone component cis-11-hexadecenal, and the axonal terminals projected to the cumulus (Cu) of the macroglomerular complex (MGC). The OSNs in type B sensilla responded to the behavioral antagonist cis-9-tetradecenal, and the axonal terminals projected to the dorsomedial anterior (DMA) unit of the MGC. In type C sensilla, there were 2 OSNs: one that responded to cis-9-tetradecenal and cis-11-hexadecenol with the axonal terminals projecting to the DMA, and another that responded to the secondary pheromone components cis-9-hexadecenal and cis-9-tetradecenal with the axonal terminals projecting to the dorsomedial posterior (DMP) unit of the MGC. Type A and type B sensilla also housed the secondary OSNs, which were silent neurons with axonal terminals projected to the glomerulus G49 and DMP. Overall, the neural pathways that carry information on attractiveness and aversiveness in response to female pheromone components in H. armigera exhibit distinct projections to the MGC units.

Anatomical Comparison of Antennal Lobes in Two Sibling Ectropis Moths: Emphasis on the Macroglomerular Complex

Ectropis obliqua and Ectropis grisescens are two sibling moth species of tea plantations in China. The male antennae of both species can detect shared and specific sex pheromone components. Thus, the primary olfactory center, i.e., the antennal lobe (AL), plays a vital role in distinguishing the sex pheromones. To provide evidence for the possible mechanism allowing this distinction, in this study, we compared the macroglomerular complex (MGC) of the AL between the males of the two species by immunostaining using presynaptic antibody and propidium iodide (PI) with antennal backfills, and confocal imaging and digital 3D-reconstruction. The results showed that MGC of both E. obliqua and E. grisescens contained five glomeruli at invariant positions between the species. However, the volumes of the anterior-lateral glomerulus (ALG) and posterior-ventral (PV) glomerulus differed between the species, possibly related to differences in sensing sex pheromone compounds and their ratios between E. obliqua and E. grisescens. Our results provide an important basis for the mechanism of mating isolation between these sibling moth species.

A reversal in sensory processing accompanies ongoing ecological divergence and speciation in Rhagoletis pomonella

Changes in behaviour often drive rapid adaptive evolution and speciation. However, the mechanistic basis for behavioural shifts is largely unknown. The tephritid fruit fly Rhagoletis pomonella is an example of ecological specialization and speciation in action via a recent host plant shift from hawthorn to apple. These flies primarily use specific odours to locate fruit, and because they mate only on or near host fruit, changes in odour preference for apples versus hawthorns translate directly to prezygotic reproductive isolation, initiating speciation. Using a variety of techniques, we found a reversal between apple and hawthorn flies in the sensory processing of key odours associated with host fruit preference at the first olfactory synapse, linking changes in the antennal lobe of the brain with ongoing ecological divergence. Indeed, changes to specific neural pathways of any sensory modality may be a broad mechanism for changes in animal behaviour, catalysing the genesis of new biodiversity.

Detection of heliothine sex pheromone components in the Australian budworm moth, Helicoverpa punctigera: electrophysiology, neuroanatomy, and behavior

This study examined electrophysiological responses of the Australian budworm moth Helicoverpa punctigera, to heliothine sex pheromone components, via single sensillum recordings (SSR), and examined male neuroanatomy using confocal microscopy and 3D imaging tools. We found that male H. punctigera have three distinct regions of the macroglomerular complex (MGC) in the antennal lobe. Male antennae have only two functional types of sensilla trichoidea (A and C) and type A sensilla contain an olfactory sensory neuron (OSN) that responds to the major sex pheromone component (Z)-11-hexadecenal (Z11-16:Ald) with axons projecting to the cumulus of the macroglomerular complex (MGC) in the antennal lobe. Type C sensilla contained large-spiking receptor neurons which responded primarily to (Z)-9-tetradecenal (Z9-14:Ald) and to a lesser degree to (Z)-11-hexadecenol (Z11-16:OH). These were co-compartmentalized with small-spiking receptor neurons in type C sensilla which responded strongly to Z9-14:Ald and (Z)-9-hexadecenal (Z9-16:Ald), and to a lesser degree to (Z)-11-hexadecenyl acetate (Z11-16:OAc) and Z11-16:OH. Axons from the two co-localized neurons in Type C sensilla projected to the two small MGC units, the dorsomedial anterior and dorsomedial posterior, respectively. In wind tunnel assays, the addition of Z9-16:Ald to an otherwise attractive blend completely shut down male H. punctigera upwind flight.

Olfactory coding of intra- and interspecific pheromonal messages by the male Mythimna separata in North China

Moths often use multi-component pheromones with fixed ratios to keep intraspecific communication and interspecific isolation. Unusually, the Oriental armyworm Mythimna separata in North China use only Z11-16:Ald as the essential component of its sex pheromone to find mates. To understand how this species keeps behavioral isolation from other species sharing Z11-16:Ald as a major pheromone component, we study the olfactory coding of intra- and interspecific pheromonal messages in the males of M. separata. Firstly, we functionally characterized the long trichoid sensilla in male antennae by single sensillum recording. Two types of sensilla were classified: the A type sensilla responded to Z11-16:Ald and Z9-14:Ald, and the B type sensilla mainly to Z9-14:Ald, and also to Z11-16:Ac, Z11-16:OH, and Z9-16:Ald. Next, we examined the glomerulus responses in the antennal lobes to these compounds by using in vivo optical imaging. The results showed that among the three subunits of the macroglomerular complex (MGC), Z11-16:Ald activated the cumulus, Z9-14:Ald activated the dorso-anterior and the cumulus, Z11-16:OH and Z11-16:Ac activated the dorso-anterior and dorso-posterior, respectively. However, Z9-16:Ald activated an ordinary glomerulus. Thirdly, we tested the behavioral responses of the males to these compounds in the wind tunnel. Addition of Z9-14:Ald at the ratio of 1:10 greatly reduced the attractiveness of Z11-16:Ald, addition of Z9-16:Ald or Z11-16:OH at the ratio of 1:1 also had behavioral antagonistic effects, while addition of Z11-16:Ac had no effect on the attractiveness of Z11-16:Ald. Finally, we used antennal transcriptome data and the Xenopus expression system to identify the receptor of Z9-14:Ald in M. separata. The Xenopus oocytes co-expressing MsepOR2 and MsepORco showed a strong response to Z9-14:Ald. Two-color fluorescence in situ hybridization validated that the cells expressing MsepOR2 and MsepOR3, tuned to Z9-14:Ald and Z11-16:Ald respectively, were localized in the different sensilla of male antennae. Comparing the sex pheromone communication channel of the related species, our results suggest that the conserved olfactory pathways for behavioral antagonists play a crucial role in behavioral isolation of noctuid species.

Dissecting sex pheromone communication of Mythimna separata (Walker) in North China from receptor molecules and antennal lobes to behavior

The Oriental armyworm, Mythimna separata, has been described to emit geographic population-specific sex pheromones, with either Z11-16:Ald or Z11-16:Ac as the major component. Using a comprehensive set of electrophysiological, behavioral, and genetic analyses, we study the sex pheromone communication of M. separata in North China from pheromone receptors and antennal lobe to behavior. GC-EAD results show that Z11-16:Ald is the only compound eliciting electrophysiological responses in pheromone gland extracts. Further in vivo optical imaging studies reveal that Z11-16:Ald activates the cumulus of the MGC and show dose-dependent responses. The wind tunnel tests demonstrate that Z11-16:Ald alone is sufficient to induce the entire sequence of male sexual behaviors. Transcriptome and q-PCR results show that MsepOR3 is specifically and abundantly expressed in male antennae. By using the Xenopus oocytes and two-electrode voltage-clamp recording, we finally validate that the oocytes expressing MsepOR3/ORco gave dose dependent responses to Z11-16:Ald. We suggest single Z11-16:Ald could be used for monitoring the population of M. separata in North China.

Plant-associated odor perception and processing in two parasitoid species with different degrees of host specificity: Implications for host location strategies

Microplitis croceipes and Cotesia marginiventris (Hymenoptera: Braconidae) are parasitoids of lepidopteran larvae with different degrees of host specificity. Both parasitoid species rely on host-related plant volatiles as odor cues to locate their herbivore hosts. To better understand mechanisms of odor processing in parasitoids, we tested responses of olfactory sensory neurons (OSNs) in the antennal sensilla placodea of female parasitoids to select plant volatiles and mixtures. The compounds tested include two green leaf volatiles (i.e., cis-3-hexenol and hexanal) and three herbivore-induced plant volatiles (i.e., cis-3-hexenyl butyrate, cis-3-hexenyl acetate and linalool). Single-sensillum recording showed that the test compounds elicited activity in large and small amplitude neurons housed in the short sensilla placodea of both parasitoid species. In general, C. marginiventris showed greater OSN responses to a low dose while M. croceipes showed greater responses to a high dose of test compounds. Binary mixtures of cis-3-hexenol and linalool inhibited OSN activity in M. croceipes, but not in C. marginiventris. These differences may have implications for odor discrimination in the two parasitoid species. In addition, anterograde neurobiotin stainings were performed to map glomerular projections of OSNs in the antennal lobe of the parasitoids. In M. croceipes, a mixture of cis-3-hexenol and linalool inhibited activity of the glomerulus activated by cis-3-hexenol alone. In C. marginiventris, a mixture of cis-3-hexenol and cis-3-hexenyl acetate showed intense labeling in their respective glomeruli, possibly suggesting a synergistic interaction. These differences in detection and coding of single compounds and mixtures may impact host location strategies in the two parasitoid species.

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.

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.

Antennal-lobe tracts in the noctuid moth, Heliothis virescens: new anatomical findings

As in other insects, three main tracts in the moth brain form parallel connections between the antennal lobe and the protocerebrum. These tracts, which consist of the antennal-lobe projection-neuron axons, target two main areas in the protocerebrum, the calyces of the mushroom bodies and the lateral horn. In spite of the solid neuroanatomical knowledge already established, there are still unresolved issues regarding the antennal-lobe tracts of the moth. One is the proportion of lateral-tract neurons targeting the calyces. In the study presented here, we have performed both retrograde and anterograde labeling of the antennal-lobe projection neurons in the brain of the moth, Heliothis virescens. The results from the retrograde staining, obtained by applying dye in the calyces, demonstrated that the direct connection between the antennal lobe and this neuropil is maintained primarily by the medial antennal-lobe tract; only a few axons confined to the lateral tract were found to innervate the calyces. In addition, these staining experiments, which allowed us to explore the arborization pattern of labeled neurons within the antennal lobe, resulted in new findings regarding anatomical arrangement of roots and cell body clusters linked to the medial tract. The results from the anterograde staining, obtained by applying dye into the antennal lobe, visualized the total assembly of axons passing along the antennal-lobe tracts. In addition to the three classical tracts, we found a transverse antennal-lobe tract not previously described in the moth. Also, these staining experiments revealed an organized neuropil in the lateral horn formed by terminals of the four antennal-lobe tracts.

Olfactory perception and behavioral effects of sex pheromone gland components in Helicoverpa armigera and Helicoverpa assulta

Two sympatric species Helicoverpa armigera and Helicoverpa assulta use (Z)-11-hexadecenal and (Z)-9-hexadecenal as sex pheromone components in reverse ratio. They also share several other pheromone gland components (PGCs). We present a comparative study on the olfactory coding mechanism and behavioral effects of these additional PGCs in pheromone communication of the two species using single sensillum recording, in situ hybridization, calcium imaging, and wind tunnel. We classify antennal sensilla types A, B and C into A, B1, B2, C1, C2 and C3 based on the response profiles, and identify the glomeruli responsible for antagonist detection in both species. The abundance of these sensilla types when compared with the number of OSNs expressing each of six pheromone receptors suggests that HarmOR13 and HassOR13 are expressed in OSNs housed within A type sensilla, HarmOR14b within B and C type sensilla, while HassOR6 and HassOR16 within some of C type sensilla. We find that for H. armigera, (Z)-11-hexadecenol and (Z)-11-hexadecenyl acetate act as behavioral antagonists. For H. assulta, instead, (Z)-11-hexadecenyl acetate acts as an agonist, while (Z)-9-hexadecenol, (Z)-11-hexadecenol and (Z)-9-hexadecenyl acetate are antagonists. The results provide an overall picture of intra- and interspecific olfactory and behavioral responses to all PGCs in two sister species.

It's All in the Mix: Blend-Specific Behavioral Response to a Sexual Pheromone in a Butterfly

Among insects, sexual pheromones are typically mixtures of two to several components, all of which are generally required to elicit a behavioral response. Here we show for the first time that a complete blend of sexual pheromone components is needed to elicit a response also in a butterfly. Males of the Green-veined White, Pieris napi, emit an aphrodisiac pheromone, citral, from wing glands. This pheromone is requisite for females to accept mating with a courting male. Citral is a mixture of the two geometric isomers geranial (E-isomer) and neral (Z-isomer) in an approximate 1:1 ratio. We found that both these compounds are required to elicit acceptance behavior, which indicates synergistic interaction between processing of the isomers. Using functional Ca²⁺ imaging we found that geranial and neral evoke significantly different but overlapping glomerular activity patterns in the antennal lobe, which suggests receptors with different affinity for the two isomers. However, these glomeruli were intermingled with glomeruli responding to, for example, plant-related compounds, i.e., no distinct subpopulation of pheromone-responding glomeruli as in moths and other insects. In addition, these glomeruli showed lower specificity than pheromone-activated glomeruli in moths. We could, however, not detect any mixture interactions among four identified glomeruli, indicating that the synergistic effect may be generated at a higher processing level. Furthermore, correlations between glomerular activity patterns evoked by the single isomers and the blend did not change over time.

Transplant Antennae and Host Brain Interact to Shape Odor Perceptual Space in Male Moths

Behavioral responses to odors rely first upon their accurate detection by peripheral sensory organs followed by subsequent processing within the brain’s olfactory system and higher centers. These processes allow the animal to form a unified impression of the odor environment and recognize combinations of odorants as single entities. To investigate how interactions between peripheral and central olfactory pathways shape odor perception, we transplanted antennal imaginal discs between larval males of two species of moth Heliothis virescens and Heliothis subflexa that utilize distinct pheromone blends. During metamorphic development olfactory receptor neurons originating from transplanted discs formed connections with host brain neurons within olfactory glomeruli of the adult antennal lobe. The normal antennal receptor repertoire exhibited by males of each species reflects the differences in the pheromone blends that these species employ. Behavioral assays of adult transplant males revealed high response levels to two odor blends that were dissimilar from those that attract normal males of either species. Neurophysiological analyses of peripheral receptor neurons and central olfactory neurons revealed that these behavioral responses were a result of: 1. the specificity of H. virescens donor olfactory receptor neurons for odorants unique to the donor pheromone blend and, 2. central odor recognition by the H. subflexa host brain, which typically requires peripheral receptor input across 3 distinct odor channels in order to elicit behavioral responses.

Specific olfactory neurons and glomeruli are associated to differences in behavioral responses to pheromone components between two Helicoverpa species

Sex pheromone communication of moths helps to understand the mechanisms underlying reproductive isolation and speciation. Helicoverpa armigera and Helicoverpa assulta use (Z)-11-hexadecenal (Z11-16:Ald) and (Z)-9-hexadecenal (Z9-16:Ald) as pheromone components in reversed ratios, 97:3 and 5:95, respectively. H. armigera also produces trace amount of (Z)-9-tetradecenal (Z9-14:Ald) in the sex pheromone gland, but H. assulta does not. Wind tunnel studies revealed that the addition of small amounts (0.3%) of Z9-14:Ald to the main pheromone blend of H. armigera increased the males' attraction, but at higher doses (1%, 10%) the same compound acted as an inhibitor. In H. assulta, Z9-14:Ald reduced male attraction when presented as 1% to the pheromone blend, but was ineffective at lower concentrations (0.3%). Three types (A–C) of sensilla trichodea in antennae were identified by single sensillum recording, responding to Z11-16:Ald, Z9-14:Ald, and both Z9-16:Ald and Z9-14:Ald, respectively. Calcium imaging in the antennal lobes (ALs) revealed that the input information of the three chemicals was transmitted to three units of the macroglomerular complex (MGC) in ALs in both species: a large glomerulus for the major pheromone components, a small one for the minor pheromone components, and a third one for the behavioral antagonists. The type A and C neurons tuned to Z11-16:Ald and Z9-16:Ald had a reversed target in the MGC between the two species. In H. armigera, low doses (1, 10 μg) of Z9-14:Ald dominantly activated the glomerulus which processes the minor pheromone component, while a higher dose (100 μg) also evoked an equal activity in the antagonistic glomerulus. In H. assulta, instead, Z9-14:Ald always strongly activated the antagonistic glomerulus. These results suggest that Z9-14:Ald plays different roles in the sexual communication of two Helicoverpa species through activation of functionally different olfactory pathways.

Processing of Pheromone Information in Related Species of Heliothine Moths

In heliothine moths, the male-specific olfactory system is activated by a few odor molecules, each of which is associated with an easily identifiable glomerulus in the primary olfactory center of the brain. This arrangement is linked to two well-defined behavioral responses, one ensuring attraction and mating behavior by carrying information about pheromones released by conspecific females and the other inhibition of attraction via signal information emitted from heterospecifics. The chance of comparing the characteristic properties of pheromone receptor proteins, male-specific sensory neurons and macroglomerular complex (MGC)-units in closely-related species is especially intriguing. Here, we review studies on the male-specific olfactory system of heliothine moths with particular emphasis on five closely related species, i.e., Heliothis virescens, Heliothis subflexa, Helicoverpa zea, Helicoverpa assulta and Helicoverpa armigera.

Peripheral coding of sex pheromone blends with reverse ratios in two helicoverpa species

The relative proportions of components in a pheromone blend play a major role in sexual recognition in moths. Two sympatric species, Helicoverpa armigera and Helicoverpa assulta, use (Z)-11-hexadecenal (Z11-16: Ald) and (Z)-9-hexadecenal (Z9-16: Ald) as essential sex pheromone components but in very different ratios, 97∶3 and 7∶93 respectively. Using wind tunnel tests, single sensillum recording and in vivo calcium imaging, we comparatively studied behavioral responses and physiological activities at the level of antennal sensilla and antennal lobe (AL) in males of the two species to blends of the two pheromone components in different ratios (100∶0, 97∶3, 50∶50, 7∶93, 0∶100). Z11-16: Ald and Z9-16: Ald were recognized by two populations of olfactory sensory neurons (OSNs) in different trichoid sensilla on antennae of both species. The ratios of OSNs responding to Z11-16:Ald and Z9-16:Ald OSNs were 100∶28.9 and 21.9∶100 in H. armigera and H. assulta, respectively. The Z11-16:Ald OSNs in H. armigera exhibited higher sensitivity and efficacy than those in H. assulta, while the Z9-16:Ald OSNs in H. armigera had the same sensitivity but lower efficacy than those in H. assulta. At the dosage of 10 µg, Z11-16: Ald and Z9-16: Ald evoked calcium activity in 8.5% and 3.0% of the AL surface in H. armigera, while 5.4% and 8.6% of AL in H. assulta, respectively. The calcium activities in the AL reflected the peripheral input signals of the binary pheromone mixtures and correlated with the behavioral output. These results demonstrate that the binary pheromone blends were precisely coded by the firing frequency of individual OSNs tuned to Z11-16: Ald or Z9-16: Ald, as well as their population sizes. Such information was then accurately reported to ALs of H. armigera and H. assulta, eventually producing different behaviors.

Antennal lobe organization in the slender pigeon louse, Columbicola columbae (Phthiraptera: Ischnocera)

This study reports on the structure of the antennal lobe of the pigeon louse, Columbicola columbae. Anterograde staining of antennal receptor neurons revealed an antennal lobe with a few diffuse compartments, an organization distinct from the typical spheroidal glomerular structure found in the olfactory bulb of vertebrates and the antennal lobe of many other insects. This anatomical arrangement of neuronal input is somewhat reminiscent of the aglomerular antennal lobe previously reported in psyllids and aphids. As in psyllids, reports on the odor-mediated behavior of C. columbae suggest that the olfactory sense is important in these animals and indicates that a glomerular organization of the antennal lobe may not be necessary to subtend odor-mediated behaviors in all insects. The diffuse or aglomerular antennal lobe organization found in these two Paraneopteran insect orders might represent an independently evolved reduction due to similar ecological constraints.

Anatomical organization of antennal-lobe glomeruli in males and females of the scarab beetle Holotrichia diomphalia (Coleoptera: Melolonthidae)

The glomerular organization of the primary olfactory brain center, the antennal lobe, was studied in males and females of Holotrichia diomphalia adults using serial histological sections labeled by the reduced silver-stain technique. The results revealed an apparent sexual dimorphism. Whereas an enlarged cap-shaped glomerulus was found at the antennal nerve entrance into the antennal lobe in males, no such unit was present in females. Also the size of the antennal lobe differed between the sexes, the antennal lobe of males being larger than that of females. We estimated the total number of glomeruli at approximately 60 units in the female antennal lobe. In males, we could discriminate only those glomeruli that were located in the anterior area of the antennal lobe.

Mixture interactions in moth olfactory physiology: examining the effects of odorant mixture, concentration, distal stimulation, and antennal nerve transection on sensillar responses

The insect olfactory system is challenged to decipher valid signals from among an assortment of chemical cues present in the airborne environment. In the moth, Heliothis virescens, males rely upon detection and discrimination of a unique blend of components in the female sex pheromone to locate mates. The effect of variable odor mixtures was used to examine physiological responses from neurons within sensilla on the moth antenna sensitive to female sex pheromone components. Increasing concentrations of heliothine sex pheromone components applied in concert with the cognate stimulus for each neuronal type resulted in mixture suppression of activity, except for one odorant combination where mixture enhancement was apparent. Olfactory receptor neuron (ORN) responses were compared between moths with intact and transected antennal nerves to determine whether specific instances of suppression might be influenced by central mechanisms. Type A sensilla showed little variation in response between transected and intact preparations; however, recordings from type B sensilla with transected antennal nerves exhibited reduced mixture suppression. Testing by parallel stimulation of distal antennal segments while recording and stimulating proximal segments dismissed the possibility of interneuronal or ephaptic effects upon sensillar responses. The results indicate that increasing concentrations of "noncognate" odorants in an odor mixture or antennal nerve transection can produce variation in the intensity and temporal dynamics of physiological recordings from H. virescens ORNs.

Innate recognition of pheromone and food odors in moths: a common mechanism in the antennal lobe?

The survival of an animal often depends on an innate response to a particular sensory stimulus. For an adult male moth, two categories of odors are innately attractive: pheromone released by conspecific females, and the floral scents of certain, often co-evolved, plants. These odors consist of multiple volatiles in characteristic mixtures. Here, we review evidence that both categories of odors are processed as sensory objects, and we suggest a mechanism in the primary olfactory center, the antennal lobe (AL), that encodes the configuration of these mixtures and may underlie recognition of innately attractive odors. In the pheromone system, mixtures of two or three volatiles elicit upwind flight. Peripheral changes are associated with behavioral changes in speciation, and suggest the existence of a pattern recognition mechanism for pheromone mixtures in the AL. Moths are similarly innately attracted to certain floral scents. Though floral scents consist of multiple volatiles that activate a broad array of receptor neurons, only a smaller subset, numerically comparable to pheromone mixtures, is necessary and sufficient to elicit behavior. Both pheromone and floral scent mixtures that produce attraction to the odor source elicit synchronous action potentials in particular populations of output (projection) neurons (PNs) in the AL. We propose a model in which the synchronous output of a population of PNs encodes the configuration of an innately attractive mixture, and thus comprises an innate mechanism for releasing odor-tracking behavior. The particular example of olfaction in moths may inform the general question of how sensory objects trigger innate responses.

Transformation of the sex pheromone signal in the noctuid moth Agrotis ipsilon: from peripheral input to antennal lobe output

How information is transformed along synaptic processing stages is critically important to understand the neural basis of behavior in any sensory system. In moths, males rely on sex pheromone to find their mating partner. It is essential for a male to recognize the components present in a pheromone blend, their ratio, and the temporal pattern of the signal. To examine pheromone processing mechanisms at different levels of the olfactory pathway, we performed single-cell recordings of olfactory receptor neurons (ORNs) in the antenna and intracellular recordings of central neurons in the macroglomerular complex (MGC) of the antennal lobe of sexually mature Agrotis ipsilon male moths, using the same pheromone stimuli, stimulation protocol, and response analyses. Detailed characteristics of the ORN and MGC-neuron responses were compared to describe the transformation of the neuronal responses that takes place in the MGC. Although the excitatory period of the response is similar in both neuron populations, the addition of an inhibitory phase following the MGC neuron excitatory phase indicates participation of local interneurons (LN), which remodel the ORN input. Moreover, MGC neurons showed a wider tuning and a higher sensitivity to single pheromone components than ORNs.

Arrangement of output information from the 3 macroglomerular units in the heliothine moth Helicoverpa assulta: morphological and physiological features of male-specific projection neurons

Helicoverpa assulta is exceptional among heliothine species studied so far as concerns composition of the pheromone blend. Previous reports have accordingly pointed out distinct characteristics in the male-specific olfactory pathway of this species, peripherally by an unusual distribution of 2 sensillum categories and centrally by a particular anatomical arrangement of the male-specific glomeruli constituting the macroglomerular complex (MGC). In order to determine the physiological tuning of the 3 MGC units in this species, we have characterized male-specific antennal-lobe projection neurons morphologically and physiologically by use of the intracellular recording and staining technique combined with confocal microscopy. The results show 2 projection neuron types of equal numbers, one that responds to the primary pheromone component, cis-9-hexadecenal, and arborizes in the cumulus and one that responds to the interspecific signal, cis-9-tetradecenal, and arborizes in the dorsomedial unit. A third type responded to the secondary pheromone component, cis-11-hexadecenal, and innervated the smaller ventral unit. The results complement previous findings from tracing of physiologically identified receptor neurons and determine for the first time the functional specificity of each glomerulus in the MGC of H. assulta. The results are particularly interesting because heliothine moths are attractive objects for comparative studies addressing questions concerning divergence of male-specific olfactory characteristics related to speciation.

Representations of odor plume flux are accentuated deep within the moth brain

Odor space, the representation of odor quality in the insect brain, is known to be optimally resolved when lateral inhibitory pathways are functioning normally. A new study published in the Journal of Biology now shows that odor time resolution also depends on the normal functioning of such pathways.

Balanced olfactory antagonism as a concept for understanding evolutionary shifts in moth sex pheromone blends

In the sex pheromone communication systems of moths, both heterospecific sex pheromone components and individual conspecific pheromone components may act as behavioral antagonists when they are emitted at excessive rates and ratios. In such cases, the resulting blend composition does not comprise the sex pheromone of a given species. That is, unless these compounds are emitted at optimal rates and ratios with other compounds, they act as behavioral antagonists. Thus, the array of blend compositions that are attractive to males is centered around the characterized female-produced sex pheromone blend of a species. I suggest here that the resulting optimal attraction of males to a sex pheromone is the result of olfactory antagonistic balance, compared to the would-be olfactory antagonistic imbalance imparted by behaviorally active compounds when they are emitted individually or in other off-ratio blends. Such balanced olfactory antagonism might be produced in any number of ways in olfactory pathways, one of which would be mutual, gamma-aminobutyric-acid-related disinhibition by local interneurons in neighboring glomeruli that receive excitatory inputs from pheromone-stimulated olfactory receptor neurons. Such mutual disinhibition would facilitate greater excitatory transmission to higher centers by projection interneurons arborizing in those glomeruli. I propose that in studies of moth sex pheromone olfaction, we should no longer artificially compartmentalize the olfactory effects of heterospecific behavioral antagonists into a special category distinct from olfaction involving conspecific sex pheromone components. Indeed, continuing to impose such a delineation among these compounds may retard advances in understanding how moth olfactory systems can evolve to allow males to exhibit correct behavioral responses (that is, attraction) to novel sex-pheromone-related compositions emitted by females.

Incomplete electrical isolation of sex-pheromone responsive olfactory receptor neurons from neighboring sensilla

In the long trichoid sensilla on male Helicoverpa zea antennae, approximately 40% of the sensilla having a large-spiking olfactory receptor neuron responding to the major pheromone component, (Z)-11-hexadecenal, also exhibit small-spiking action potentials that also seem to be responsive to this same compound. In this study, we investigated whether these small-spiking signals are a result of intrusive electrical signals generated from neighboring sensilla. Two methods were used for this study. First, the sensillum was completely covered by the saline-filled recording electrode to physically prevent the sensillum from being contacted by exposure to (Z)-11-hexadecenal. In this case, activation of the large-spiking neuron in response to the pheromone component was prevented, whereas the small-spiking activity continued to be influenced by the airborne delivery of the pheromone. In the second method the (Z)-11-hexadecenal was applied directly in solution through the cut tip of the sensillum through the recording electrode. In this case only large-spiking activity occurred in response to (Z)-11-hexadecenal, with no increase whatsoever in the firing frequency of the small spikes. We conclude that these long trichoid olfactory sensilla are not completely isolated electrically from neighboring sensilla and that small spikes in some recordings originate from large-spiking olfactory receptor neurons (ORNs) in neighboring sensilla.

Physiology and antennal lobe projections of olfactory receptor neurons from sexually isomorphic sensilla on male Heliothis virescens

The neurophysiology and antennal lobe projections of olfactory receptor neurons (ORNs) within sexually isomorphic short trichoid sensilla of male Heliothis virescens (Noctuidae: Lepidoptera) were investigated using cut-sensillum recording and cobalt-lysine staining. A total of 202 sensilla were sorted into 14 possible sensillar categories based on odor responses and physiology of ORNs within. Seventy-two percent of the sensilla identified contained ORNs stimulated by conspecific odors. In addition, a large number of ORNs were specifically sensitive to ss-caryophyllene, a plant-derived volatile (N = 41). Axons originating from ORNs associated with individual sensilla were stained with cobalt lysine (N = 67) and traced to individual glomeruli in the antennal lobe. ORNs with responses to female sex pheromone components exhibited similar axonal projections as those previously described from ORNs in long sensilla trichodea in male H. virescens. Antennal lobe axonal arborizations of ORNs sensitive to hairpencil components were also located in glomeruli near the base of the antennal nerve, whilst those sensitive to plant odorants projected to more medial glomeruli. Comparisons with ORNs described from female H. virescens supports the notion that glomeruli at the base of the antennal nerve are associated with conspecific and interspecific odorants, whereas those located medially are associated with plant volatiles.

Support for (Z)-11-hexadecanal as a pheromone antagonist in Ostrinia nubilalis: flight tunnel and single sensillum studies with a New York population

The flight-tunnel response of male Z-strain European corn borer moths (ECB), Ostrinia nubilalis, from a population in New York State (USA), was significantly antagonized by addition of 1% (Z)-11-hexadecanal (Z11-16:Ald) to their sex pheromone (a 97:3 mix of (Z)- and (E)-11-tetradecenyl acetate [Z/E11-14:OAc]). The level of antagonism was equivalent to that observed for the previously identified ECB antagonist, (Z)-9-tetradecenyl acetate (Z9-14:OAc), and supports a recent report showing that Z11-16:Ald, a minor pheromone component of the Noctuid moth, Sesamia nonagrioides, caused antagonism of ECB pheromone communication in sympatric populations in the Iberian Peninsula. Single-sensillum recordings from ECB antennae, which included cross-adaptation experiments, showed that the same olfactory receptor neuron processing Z9-14:OAc inputs was responsible for detecting Z11-16:Ald, and that this neuron was not responsive to two other aldehydes, (Z)-9-tetradecanal (Z9-14:Ald) and (Z)-9-hexadecanal (Z9-16:Ald), found in other moth sex pheromones. Our results show that the antagonism is not confined to one geographic region, is specific for Z11-16:Ald, and that antagonist pathways might have the potential for processing a number of structurally similar compounds.

A specific male olfactory sensillum detects behaviorally antagonistic hairpencil odorants

Within insect species, olfactory signals play a vital role in communication, particularly in the context of mating. During courtship, males of many moth species release pheromones that function as aphrodisiacs for conspecific females, or repellants to competing conspecific males. The physiology and antennal lobe projections are described of olfactory receptor neurons within an antennal sensillum present on male Heliothis virescens F. (Lepidoptera: Noctuidae) moths sensitive to conspecific male H. virescens-produced pheromone components. Olfactory receptor neurons responded to hexadecanyl acetate and octadecanyl acetate hairpencil components, and Z11-hexadecenyl acetate, an odorant used by closely related heliothine species in their female produced pheromone, which is antagonistic to male H. virescens responses. This acetate-sensitive sensillum appears homologous to a sensillum type previously described in females of this species, sharing similar physiology and glomerular projection targets within the antennal lobe. Wind tunnel observations indicate that H. virescens hairpencil odors (hexadecanyl acetate, octadecanyl acetate) function to antagonize responses of conspecific males following a female sex pheromone plume. Thus, male-male flight antagonism in H. virescens appears to be mediated by this particular sensillum type.

Inheritance of olfactory preferences II. Olfactory receptor neuron responses from Heliothis subflexa x Heliothis virescens hybrid male moths

Single-cell electrophysiological recordings were obtained from olfactory receptor neurons (ORNs) in sensilla trichodea on male antennae of hybrids formed mainly by crossing female Heliothis subflexa with male Heliothis virescens ('SV hybrids'). We recorded from the A-, B-, and C-type sensilla trichodea, with the latter two types housing ORNs exhibiting response profiles to different pheromone components that we had previously found to be characteristic for each species. For both the B- and the C-type SV hybrid sensilla, most of the ORNs exhibited a spike amplitude and ORN co-compartmentalization within sensilla that more strongly resembled the ORNs of parental H. subflexa rather than those of H. virescens. The overall mean dose-response profiles of the ORNs in hybrid C- and B-type sensilla were intermediate between those of the H. virescens and H. subflexa parental type ORNs. However, not all hybrid ORNs were intermediate in their tuning spectra, but rather ranged from those that closely resembled H. subflexa or H. virescens parental types to those that were intermediate, even on the same antenna. The most noteworthy shift in ORN responsiveness in hybrid males was an overall increase in sensitivity to Z9-14:Ald exhibited by Z9-16:Ald-responsive ORNs. Heightened cross-responsiveness to Z9-14:Ald by hybrid ORNs correlates well with observed behavioral cross-responsiveness of hybrids in which Z9-14:Ald could substitute for Z9-16:Ald in the pheromone blend, a behavior not observed in parental types. The hybrid ORN shifts involving greater sensitivity to Z9- 14:Ald also correlate well with studies of hybrid male antennal lobe interneurons that exhibited a shift toward greater cross-responsiveness to Z9-14:Ald and Z9- 16:Ald. We propose that the differences between parental H. virescens, H. subflexa, and SV hybrid male pheromone ORN responsiveness to Z9-16:Ald and Z9-14:Ald are most logically explained by an increased or decreased co-expression of two different odorant receptors for each of these compounds on the same ORN.