Odors are key sensory signals for social communication and food search in animals including insects. Drosophila melanogaster, is a powerful neurogenetic model commonly used to reveal molecular and cellular mechanisms involved in odorant detection. Males use olfaction together with other sensory modalities to find their mates. Here, we review known olfactory signals, their related olfactory receptors, and the corresponding neuronal architecture impacting courtship. OR67d receptor detects 11-cis-Vaccenyl Acetate (cVA), a male specific pheromone transferred to the female during copulation. Transferred cVA is able to reduce female attractiveness for other males after mating, and is also suspected to decrease male-male courtship. cVA can also serve as an aggregation signal, maybe through another OR. OR47b was shown to be activated by fly odors, and to enhance courtship depending on taste pheromones. IR84a detects phenylacetic acid (PAA) and phenylacetaldehyde (PA). These two odors are not pheromones produced by flies, but are present in various fly food sources. PAA enhances male courtship, acting as a food aphrodisiac. Drosophila males have thus developed complementary olfactory strategies to help them to select their mates.
courtship; Drosophila; olfaction; receptor; nervous system
The molecular and cellular events mediating complex behaviors in animals are largely unknown. Elucidating the circuits underlying behaviors in simple model systems may shed light on how these circuits function. In Drosophila, courtship behavior provides a tractable model for studying the underlying basis of innate behavior. The male-specific pheromone 11-cis-vaccenyl acetate (cVA) modulates courtship behavior and is detected by T1 neurons, located on the antenna of male and female flies. The T1 neurons express the odorant receptor Or67d, and are exquisitely tuned to cVA pheromone. However, cVA-induced changes in mating behavior have also been reported upon manipulation of olfactory neurons expressing odorant receptor Or65a. These findings raise the issue of whether multiple olfactory-driven circuits underlie cVA-induced behavioral responses, and what role these circuits play in behavior. Here, we engineered flies in which the Or67d circuit is specifically activated in the absence of cVA in order to determine the role of this circuit in behavior. We created transgenic flies that express a dominant-active, pheromone-independent variant of the extracellular pheromone receptor, LUSH. We found that, similar to the behaviors elicited by cVA, engineered male flies have dramatically reduced courtship, while engineered females showed enhanced courtship. Furthermore, cVA exposure did not enhance the dominant LUSH-triggered effects on behavior in the engineered flies. Finally, we show the effects of both cVA and dominant LUSH on courtship are reversed by genetically removing Or67d. These findings demonstrate that the T1/Or67d circuit is necessary and sufficient to mediate sexually dimorphic courtship behaviors.
lush; Or67d; cVA; courtship; pheromone; olfaction
Remarkably little is known about the molecular and cellular basis of mate recognition in Drosophila . We systematically examine one of the three major types of sensilla that house olfactory receptor neurons (ORNs) on the Drosophila antenna, the trichoid sensilla, by electrophysiological analysis. We find that none respond strongly to food odors, but all respond to fly odors. Two subtypes of trichoid sensilla contain ORNs that respond to cis-vaccenyl acetate (cVA), an anti-aphrodisiac pheromone present in males and transferred to females during mating [2–4]. All trichoid sensilla yield responses to a male extract; a subset yield responses to a virgin female extract as well. Thus males can be distinguished from virgin females by the activity they elicit among the trichoid ORN population. We then systematically test all members of the Odor receptor (Or) gene family [5–7] that are expressed in trichoid sensilla , using an in vivo expression system . Four receptors respond to fly odors in this system: two respond to extracts of both males and virgin females, and two respond to cVA. We propose a model for how these receptors might be used by a male to distinguish suitable from unsuitable mating partners through a simple logic.
The olfactory response of the vinegar fly Drosophila melanogaster to food odor is modulated by starvation. Here we show that this modulation is not restricted to food odors and their detecting sensory neurons but rather increases the behavioral response to odors as different as food odors, repellents and pheromones. The increased behavioral responsiveness is paralleled by an increased physiological sensitivity of sensory neurons regardless whether they express olfactory or ionotropic receptors and regardless whether they are housed in basiconic, coeloconic, or trichoid sensilla. Silencing several genes that become up-regulated under starvation confirmed the involvement of the short neuropeptide f receptor in the starvation effect. In addition it revealed that the CCHamide-1 receptor is another important factor governing starvation-induced olfactory modifications.
Most animals are endowed with an olfactory system that is essential for finding foods, avoiding predators, and locating mating partners. The olfactory system must encode the identity and intensity of behaviorally relevant stimuli in a dynamic environmental landscape. How is olfactory information represented? How is a large dynamic range of odor concentrations represented in the olfactory system? How is this representation modulated to meet the demands of different internal physiological states? Recent studies have found that sensory terminals are important targets for neuromodulation. The emerging evidence suggests that presynaptic inhibition scales with sensory input and thus, provides a mechanism to increase dynamic range of odor representation. In addition, presynaptic facilitation could be a mechanism to alter behavioral responses in hungry animals. This review will focus on the GABAB receptor-mediated presynaptic inhibition, and neuropeptide-mediated presynaptic modulation in Drosophila.
Early sensory processing can play a critical role in sensing environmental cues. We have investigated the physiological and behavioral function of gain control at the first synapse of olfactory processing in Drosophila. We report that olfactory receptor neurons (ORNs) express the GABAB receptor (GABABR) and its expression expands the dynamic range of ORN synaptic transmission that is preserved in projection neuron responses. Strikingly, we find that different ORN channels have unique baseline levels of GABABR expression. ORNs that sense the aversive odorant CO2 do not express GABABRs nor exhibit any presynaptic inhibition. In contrast, pheromone-sensing ORNs express a high level of GABABRs and exhibit strong presynaptic inhibition. Furthermore, a behavioral significance of presynaptic inhibition was revealed by a courtship behavior in which pheromone-dependent mate localization is impaired in flies that lack GABABRs in specific ORNs. Together, these findings indicate that different olfactory receptor channels may employ heterogeneous presynaptic gain control as a mechanism to allow an animal’s innate behavioral responses to match its ecological needs.
Drosophila; olfaction; GABAB; presynaptic inhibition; gain control; dynamic range; two-photon imaging
Aggression is regulated by pheromones in many animal species1,2,3. However in no system have aggression pheromones, their cognate receptors and corresponding sensory neurons been identified. Here we show that 11-cis-vaccenyl acetate (cVA), a male-specific volatile pheromone, robustly promotes male-male aggression in the vinegar fly Drosophila melanogaster. The aggression-promoting effect of synthetic cVA requires olfactory sensory neurons (OSNs) expressing the receptor Or67d4,5,6, as well as the receptor itself. Activation of Or67d-expressing OSNs, either by genetic manipulation of their excitability or by exposure to male pheromones in the absence of other classes of OSNs, is sufficient to promote aggression. High densities of male flies can promote aggression through release of volatile cVA. In turn, cVA-promoted aggression can promote male fly dispersal from a food resource, in a manner dependent upon Or67d-expressing OSNs. These data suggest that cVA may mediate negative feedback control of male population density, through its effect on aggression. Identification of a pheromone-OSN pair controlling aggression in a genetic organism opens the way to unraveling the neurobiology of this evolutionarily conserved behavior.
Recognition of conspecifics and mates is based on a variety of sensory cues that are specific to the species, sex and social status of each individual. The courtship and mating activity of Drosophila melanogaster flies is thought to depend on the olfactory perception of a male-specific volatile pheromone, cis-vaccenyl acetate (cVA), and the gustatory perception of cuticular hydrocarbons (CHs), some of which are sexually dimorphic. Using two complementary sampling methods (headspace Solid Phase Micro-Extraction [SPME] and solvent extraction) coupled with GC-MS analysis, we measured the dispersion of pheromonal CHs in the air and on the substrate around the fly. We also followed the variations in CHs that were induced by social and sexual interactions. We found that all CHs present on the fly body were deposited as a thin layer on the substrate, whereas only a few of these molecules were also detected in the air. Moreover, social experience during early adult development and in mature flies strongly affected male volatile CHs but not cVA, whereas sexual interaction only had a moderate influence on dispersed CHs. Our study suggests that, in addition to their role as contact cues, CHs can influence fly behavior at a distance and that volatile, deposited and body pheromonal CHs participate in a three-step recognition of the chemical identity and social status of insects.
Pheromones regulate male social behaviors in Drosophila, but the identities and behavioral role(s) of these chemosensory signals, and how they interact, are incompletely understood. Here we show that (Z)-7-tricosene (7-T), a male-enriched cuticular hydrocarbon (CH) previously shown to inhibit male-male courtship, is also essential for normal levels of aggression. The opposite influences of 7-T on aggression and courtship are independent, but both require the gustatory receptor Gr32a. Surprisingly, sensitivity to 7-T is required for the aggression-promoting effect of 11-cis-vaccenyl acetate (cVA), an olfactory pheromone, but 7-T sensitivity is independent of cVA. 7-T and cVA therefore regulate aggression in a hierarchical manner. Furthermore, the increased courtship caused by depletion of male CHs is suppressed by a mutation in the olfactory receptor Or47b. Thus, male social behaviors are controlled by gustatory pheromones that promote and suppress aggression and courtship, respectively, and whose influences are dominant to olfactory pheromones that enhance these behaviors.
In many insect species, cuticular hydrocarbons serve as pheromones that can mediate complex social behaviors. In Drosophila melanogaster, several hydrocarbons including the male sex pheromone 11-cis-vaccenyl acetate (cVA) and female-specific 7,11-dienes influence courtship behavior and can function as cues for short-term memory associated with the mating experience. Behavioral and physiological studies suggest that other unidentified chemical communication cues are likely to exist. To more fully characterize the hydrocarbon profile of the D. melanogaster cuticle, we applied direct ultraviolet laser desorption/ionization orthogonal time-of-flight mass spectrometry (UV-LDI-o-TOF MS) and analyzed the surface of intact fruit flies at a spatial resolution of approximately 200 μm.
We report the chemical and spatial characterization of 28 species of cuticular hydrocarbons, including a new major class of oxygen-containing compounds. Using UV-LDI MS, pheromones previously shown to be expressed exclusively by one sex, e.g. cVA, 7,11-heptacosadiene, and 7,11-nonacosadiene, appear to be found on both male and female flies. In males, cVA co-localizes at the tip of the ejaculatory bulb with a second acetylated hydrocarbon named CH503. We describe the chemical structure of CH503 as 3-O-acetyl-1,3-dihydroxy-octacosa-11,19-diene and show one behavioral role for this compound as a long-lived inhibitor of male courtship. Like cVA, CH503 is transferred from males to females during mating. Unlike cVA, CH503 remains on the surface of females for at least 10 days.
Oxygenated hydrocarbons comprise one major previously undescribed class of compounds on the Drosophila cuticular surface. In addition to cVA, a newly-discovered long chain acetate, CH503, serves as a mediator of courtship-related chemical communication.
What makes males and females behave differently? While genetic master-regulators commonly underlie physical differences, sexually dimorphic behavior is additionally influenced by sensory input such as olfactory cues. Olfaction requires both ligands for signaling and sensory neural circuits for detection. Specialized subsets of each interact to generate gender-dimorphic behavior. It has long been accepted that males and females emit sex-specific odor compounds that function as pheromones to promote stereotypic behavior. Significant advances have now been made in purifying and isolating several of these sex-specific olfactory ligands. In contrast, the neural mechanisms that enable a gender dimorphic response to these odors remain largely unknown. However, first progress has been made in identifying components of sexually dimorphic olfactory circuits in both Drosophila and the mouse.
Odorant-Degrading Enzymes (ODEs) are supposed to be involved in the signal inactivation step within the olfactory sensilla of insects by quickly removing odorant molecules from the vicinity of the olfactory receptors. Only three ODEs have been both identified at the molecular level and functionally characterized: two were specialized in the degradation of pheromone compounds and the last one was shown to degrade a plant odorant.
Previous work has shown that the antennae of the cotton leafworm Spodoptera littoralis, a worldwide pest of agricultural crops, express numerous candidate ODEs. We focused on an esterase overexpressed in males antennae, namely SlCXE7. We studied its expression patterns and tested its catalytic properties towards three odorants, i.e. the two female sex pheromone components and a green leaf volatile emitted by host plants.
SlCXE7 expression was concomitant during development with male responsiveness to odorants and during adult scotophase with the period of male most active sexual behaviour. Furthermore, SlCXE7 transcription could be induced by male exposure to the main pheromone component, suggesting a role of Pheromone-Degrading Enzyme. Interestingly, recombinant SlCXE7 was able to efficiently hydrolyze the pheromone compounds but also the plant volatile, with a higher affinity for the pheromone than for the plant compound. In male antennae, SlCXE7 expression was associated with both long and short sensilla, tuned to sex pheromones or plant odours, respectively. Our results thus suggested that a same ODE could have a dual function depending of it sensillar localisation. Within the pheromone-sensitive sensilla, SlCXE7 may play a role in pheromone signal termination and in reduction of odorant background noise, whereas it could be involved in plant odorant inactivation within the short sensilla.
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.
floral scent; moths; neuroethology; olfaction; pheromone; sensory coding; sensory object; synchrony
A large set of xenobiotic-metabolizing enzymes (XMEs), such as the cytochrome P450 monooxygenases (CYPs), esterases and transferases, are highly expressed in mammalian olfactory mucosa (OM). These enzymes are known to catalyze the biotransformation of exogenous compounds to facilitate elimination. However, the functions of these enzymes in the olfactory epithelium are not clearly understood. In addition to protecting against inhaled toxic compounds, these enzymes could also metabolize odorant molecules, and thus modify their stimulating properties or inactivate them. In the present study, we investigated the in vitro biotransformation of odorant molecules in the rat OM and assessed the impact of this metabolism on peripheral olfactory responses. Rat OM was found to efficiently metabolize quinoline, coumarin and isoamyl acetate. Quinoline and coumarin are metabolized by CYPs whereas isoamyl acetate is hydrolyzed by carboxylesterases. Electro-olfactogram (EOG) recordings revealed that the hydroxylated metabolites derived from these odorants elicited lower olfactory response amplitudes than the parent molecules. We also observed that glucurono-conjugated derivatives induced no olfactory signal. Furthermore, we demonstrated that the local application of a CYP inhibitor on rat olfactory epithelium increased EOG responses elicited by quinoline and coumarin. Similarly, the application of a carboxylesterase inhibitor increased the EOG response elicited by isoamyl acetate. This increase in EOG amplitude provoked by XME inhibitors is likely due to enhanced olfactory sensory neuron activation in response to odorant accumulation. Taken together, these findings strongly suggest that biotransformation of odorant molecules by enzymes localized to the olfactory mucosa may change the odorant’s stimulating properties and may facilitate the clearance of odorants to avoid receptor saturation.
In vertebrates and invertebrates, sensory neurons adapt to variable ambient conditions, such as the duration or repetition of a stimulus, a physiological mechanism considered as a simple form of non-associative learning and neuronal plasticity. Although various signaling pathways, as cAMP, cGMP, and the inositol 1,4,5-triphosphate receptor (InsP3R) play a role in adaptation, their precise mechanisms of action at the cellular level remain incompletely understood. Recently, in Drosophila, we reported that odor-induced Ca2+-response in axon terminals of olfactory receptor neurons (ORNs) is related to odor duration. In particular, a relatively long odor stimulus (such as 5 s) triggers the induction of a second component involving intracellular Ca2+-stores.
We used a recently developed in-vivo bioluminescence imaging approach to quantify the odor-induced Ca2+-activity in the axon terminals of ORNs. Using either a genetic approach to target specific RNAs, or a pharmacological approach, we show that the second component, relying on the intracellular Ca2+-stores, is responsible for the adaptation to repetitive stimuli. In the antennal lobes (a region analogous to the vertebrate olfactory bulb) ORNs make synaptic contacts with second-order neurons, the projection neurons (PNs). These synapses are modulated by GABA, through either GABAergic local interneurons (LNs) and/or some GABAergic PNs. Application of GABAergic receptor antagonists, both GABAA or GABAB, abolishes the adaptation, while RNAi targeting the GABABR (a metabotropic receptor) within the ORNs, blocks the Ca2+-store dependent component, and consequently disrupts the adaptation. These results indicate that GABA exerts a feedback control. Finally, at the behavioral level, using an olfactory test, genetically impairing the GABABR or its signaling pathway specifically in the ORNs disrupts olfactory adapted behavior.
Taken together, our results indicate that a relatively long lasting form of adaptation occurs within the axon terminals of the ORNs in the antennal lobes, which depends on intracellular Ca2+-stores, attributable to a positive feedback through the GABAergic synapses.
Reproductive behavior in Drosophila has both stereotyped and plastic components that are driven by age- and sex-specific chemical cues. Males who unsuccessfully court virgin females subsequently avoid females that are of the same age as the trainer. In contrast, males trained with mature mated females associate volatile appetitive and aversive pheromonal cues and learn to suppress courtship of all females. Here we show that the volatile aversive pheromone that leads to generalized learning with mated females is (Z)-11-octadecenyl acetate (cis-vaccenyl acetate, cVA). cVA is a major component of the male cuticular hydrocarbon profile, but it is not found on virgin females. During copulation, cVA is transferred to the female in ejaculate along with sperm and peptides that decrease her sexual receptivity. When males sense cVA (either synthetic or from mated female or male extracts) in the context of female pheromone, they develop a generalized suppression of courtship. The effects of cVA on initial courtship of virgin females can be blocked by expression of tetanus toxin in Or65a, but not Or67d neurons, demonstrating that the aversive effects of this pheromone are mediated by a specific class of olfactory neuron. These findings suggest that transfer of cVA to females during mating may be part of the male’s strategy to suppress reproduction by competing males.
Learning and memory; olfaction; Drosophila; pheromones; cis-vaccenyl acetate
Anatomical and physiological experiments in the lamprey reveal the neural circuit involved in transforming olfactory inputs into motor outputs, which was previously unknown in a vertebrate.
It is widely recognized that animals respond to odors by generating or modulating specific motor behaviors. These reactions are important for daily activities, reproduction, and survival. In the sea lamprey, mating occurs after ovulated females are attracted to spawning sites by male sex pheromones. The ubiquity and reliability of olfactory-motor behavioral responses in vertebrates suggest tight coupling between the olfactory system and brain areas controlling movements. However, the circuitry and the underlying cellular neural mechanisms remain largely unknown. Using lamprey brain preparations, and electrophysiology, calcium imaging, and tract tracing experiments, we describe the neural substrate responsible for transforming an olfactory input into a locomotor output. We found that olfactory stimulation with naturally occurring odors and pheromones induced large excitatory responses in reticulospinal cells, the command neurons for locomotion. We have also identified the anatomy and physiology of this circuit. The olfactory input was relayed in the medial part of the olfactory bulb, in the posterior tuberculum, in the mesencephalic locomotor region, to finally reach reticulospinal cells in the hindbrain. Activation of this olfactory-motor pathway generated rhythmic ventral root discharges and swimming movements. Our study bridges the gap between behavior and cellular neural mechanisms in vertebrates, identifying a specific subsystem within the CNS, dedicated to producing motor responses to olfactory inputs.
Animal behaviors, including locomotion, can be driven by olfactory cues, such as pheromones or food sources. The neural substrate (neuroanatomical connections and physiological signals) that permits the transformation of olfactory inputs into locomotor responses is still unknown in vertebrates. In the present study, we identify such a neural substrate in the lamprey. Here, olfactory signals from the outside world are transmitted to the reticulospinal neurons in the lower brainstem, which provide the descending locomotor command to the spinal cord. We found that this circuit originates in the medial portion of the olfactory bulb and that connections are made in the posterior tuberculum, a ventral diencephalic structure. These inputs are then conveyed to the mesencephalic locomotor region, known to project extensively to brainstem reticulospinal neurons and thereby activate locomotion. Our results illuminate a specific dedicated neural substrate in the brain of lampreys that underlies olfactory-motor responses, which is activated by both food-related or pheromonal olfactory cues. It will be of interest to determine whether such a pathway is preserved in all vertebrates.
Turbulent fluid landscapes impose temporal patterning upon chemical signals, and the dynamical neuronal responses to patterned input vary across the olfactory receptor repertoire in flies, moths, and locusts. Sensory transformations exhibit low pass filtering that ultimately results in perceptual fusion of temporally transient sensory signals. For example, humans perceive a sufficiently fast flickering light as continuous, but the frequency threshold at which this fusion occurs varies with wavelength. Although the summed frequency sensitivity of the fly antenna has been examined to a considerable extent, it is unknown how intermittent odor signals are integrated to influence plume tracking behavior independent of wind cues, and whether temporal fusion for behavioral tracking might vary according to the odor encountered.
Here we have adopted a virtual reality flight simulator to study the dynamics of plume tracking under different experimental conditions. Flies tethered in a magnetic field actively track continuous (non-intermittent) plumes of vinegar, banana, or ethyl butyrate with equal precision. However, pulsing these plumes at varying frequency reveals that the threshold rate, above which flies track the plume as if it were continuous, is unique for each odorant tested. Thus, the capability of a fly to navigate an intermittent plume depends on the particular odorant being tracked during flight. Finally, we measured antennal field potential responses to an intermittent plume, found that receptor dynamics track the temporal pattern of the odor stimulus and therefore do not limit the observed behavioral temporal fusion limits.
This study explores the flies' ability to track odor plumes that are temporally intermittent. We were surprised to find that the perceptual critical fusion limit, determined behaviorally, is strongly dependent on odor identity. Antennal field potential recordings indicate that peripheral processing of temporal cues faithfully follow rapid odor transients above the rates that can be resolved behaviorally. These results indicate that (1) higher order circuits create a perceptually continuous signal from an intermittent sensory one, and that (2) this transformation varies with odorant rather than being constrained by sensory-motor integration, thus (3) offering an entry point for examining the mechanisms of rapid olfactory decision making in an ecological context.
A unifying feature of mammalian and insect olfactory systems is that olfactory sensory neurons (OSNs) expressing the same unique odorant receptor gene converge onto the same glomeruli in the brain (1–7). Most odorants activate a combination of receptors and thus distinct patterns of glomeruli, forming a proposed combinatorial spatial code that could support discrimination between a large number of odorants (8–11). OSNs also exhibit odor-evoked responses with complex temporal dynamics (11), but the contribution of this activity to behavioral odor discrimination has received little attention (12). Here we investigated the importance of spatial encoding in the relatively simple Drosophila antennal lobe. We show that Drosophila can learn to discriminate between two odorants with one functional class of Or83b-expressing OSNs. Furthermore, these flies encode one odorant from a mixture, and cross-adapt to odorants that activate the relevant OSN class, demonstrating that they discriminate odorants using the same OSNs. Lastly, flies with a single class of Or83b-expressing OSNs recognize a specific odorant across a range of concentration indicating that they encode odorant identity. Therefore flies can distinguish odorants without discrete spatial codes in the antennal lobe, implying an important role for odorant-evoked temporal dynamics in behavioral odorant discrimination.
Antennal olfaction is extremely important for insect survival, mediating key behaviors such as host preference, mate choice, and oviposition site selection. Multiple antennal proteins are involved in olfactory signal transduction pathways. Of these, odorant receptors (ORs) and ionotropic receptors (IRs) confer specificity on olfactory sensory neuron responses. In this study, we identified the olfactory gene repertoire of the economically important agricultural pest moth, Helicoverpa armigera, by assembling the adult male and female antennal transcriptomes. Within the male and female antennal transcriptomes we identified a total of 47 OR candidate genes containing 6 pheromone receptor candidates. Additionally, 12 IR genes as well as 26 odorant-binding proteins and 12 chemosensory proteins were annotated. Our results allow a systematic functional analysis across much of conventional ORs repertoire and newly reported IRs mediating the key olfaction-mediated behaviors of H. armigera.
Soon after birth all mammals must initiate milk suckling to survive. In rodents, this innate behavior is critically dependent on uncharacterized maternally-derived chemosensory ligands. Recently the first pheromone sufficient to initiate suckling was isolated from the rabbit. Identification of the olfactory cues that trigger first suckling in the mouse would provide the means to determine the neural mechanisms that generate innate behavior.
Here we use behavioral analysis, metabolomics, and calcium imaging of primary sensory neurons and find no evidence of ligands with intrinsic bioactivity, such as pheromones, acting to promote first suckling in the mouse. Instead, we find that the initiation of suckling is dependent on variable blends of maternal ‘signature odors’ that are learned and recognized prior to first suckling.
As observed with pheromone-mediated behavior, the response to signature odors releases innate behavior. However, this mechanism tolerates variability in both the signaling ligands and sensory neurons which may maximize the probability that this first essential behavior is successfully initiated. These results suggest that mammalian species have evolved multiple strategies to ensure the onset of this critical behavior.
Odors elicit spatio-temporal patterns of activity in the brain. Spatial patterns arise from the specificity of the interaction between odorants and odorant receptors expressed in different olfactory receptor neurons (ORNs). But the origin of temporal patterns of activity and their role in odor coding remain unclear. We investigate how physiological aspects of ORN response and physical aspects of odor stimuli give rise to diverse responses in Drosophila ORNs. We show that odor stimuli have intrinsic dynamics that depend on odor type and strongly affect ORN response. Using linear-nonlinear modeling to remove the contribution of the stimulus dynamics from the ORN dynamics we study the physiological properties of the response to different odorants and concentrations. For several odorants and receptor types the ORN response dynamics normalized by the peak response are independent of stimulus intensity for a large portion of the neuron’s dynamic range. Adaptation to a background odor changes the gain and dynamic range of the response but does not affect normalized response dynamics. Stimulating ORNs with various odorants reveals significant odor-dependent delays in the ORN response functions. These differences however can be dominated by differences in stimulus dynamics. In one case the response of one ORN to two odorants is predicted solely from measurements of the odor signals. Within a large portion of their dynamic range ORNs can capture information about stimulus dynamics independently from intensity while introducing odor-dependent delays. How insects might use odor-specific stimulus dynamics and ORN dynamics in discrimination and navigation tasks remains an open question.
Many see fruit flies as an annoyance, invading our homes with a nagging persistence and efficiency. Yet from a scientific perspective, these tiny animals are a wonder of multisensory integration, capable of tracking fragmented odor plumes amidst turbulent winds and constantly varying visual conditions. The peripheral olfactory, mechanosensory, and visual systems of the fruit fly, Drosophila melanogaster, have been studied in great detail;1–4 however, the mechanisms by which fly brains integrate information from multiple sensory modalities to facilitate robust odor tracking remain elusive. Our studies on olfactory orientation by flying flies reveal that these animals do not simply follow their “nose“; rather, fruit flies require mechanosensory and visual input to track odors in flight.5,6 Collectively, these results shed light on the neural circuits involved in odor localization by fruit flies in the wild and illuminate the elegant complexity underlying a behavior to which the annoyed and amazed are familiar.
vision; olfaction; mechanosensory; antennae; visual processing; motor control; insect behavior; behavioral neuroscience; neuroethology; sensory ecology
Most living organisms use pheromones for inter-individual communication. In Drosophila melanogaster flies, several pheromones perceived either by contact/at a short distance (cuticular hydrocarbons, CHs), or at a longer distance (cis-vaccenyl acetate, cVA), affect courtship and mating behaviours. However, it has not previously been possible to precisely identify all potential pheromonal compounds and simultaneously monitor their variation on a time scale. To overcome this limitation, we combined Solid Phase Micro-Extraction with gas-chromatography coupled with mass-spectrometry. This allowed us (i) to identify 59 cuticular compounds, including 17 new CHs; (ii) to precisely quantify the amount of each compound that could be detected by another fly, and (iii) to measure the variation of these substances as a function of aging and mating. Sex-specific variation appeared with age, while mating affected cuticular compounds in both sexes with three possible patterns: variation was (i) reciprocal in the two sexes, suggesting a passive mechanical transfer during mating, (ii) parallel in both sexes, such as for cVA which strikingly appeared during mating, or (iii) unilateral, presumably as a result of sexual interaction. We provide a complete reassessment of all Drosophila CHs and suggest that the chemical conversation between male and female flies is far more complex than is generally accepted. We conclude that focusing on individual compounds will not provide a satisfactory understanding of the evolution and function of chemical communication in Drosophila.
Mechanisms for identifying appropriate mating partners are required for any species to survive. In many types of animals, males employ multiple sensory modalities to initially search for females and to subsequently determine if they are fit and/or receptive. In this paper we will detail the multiple types of sensory information that are used to initiate and drive courtship in Drosophila melanogaster and discuss the importance of context in the interpretation of chemosensory cues. We find that food-related olfactory cues increase the salience of the aversive pheromone cis-vaccenyl acetate.
Drosophila; olfaction; gustation; hearing; cis-vaccenyl acetate; pheromone