The survival advantage of olfaction in a competitive environment

Asymmetric dynamics of GABAergic system and paradoxical responses of GABAergic neurons in piriform seizures

OBJECTIVE

The piriform cortex (PC) plays a critical role in ictogenesis, where an excitation/inhibition imbalance contributes to epilepsy etiology. However, the epileptic dynamics of the gamma-aminobutyric acid (GABA) system and the precise role of GABAergic neurons within the PC in epilepsy remain unclear.

METHODS

We combined Ca2+ and GABA sensors to investigate the dynamics of Gad2-expressing neurons and GABA levels, and selectively manipulated GABAergic neurons in the PC through chemogenetic inhibition and caspase3-mediated apoptosis targeting Gad2 interneurons.

RESULTS

GABAergic system dynamics in the PC were bidirectional and asymmetric, accompanied by PC optokindling-induced seizures, notably characterized by a robust response of Gad2 neurons but a rapid descent of GABA content during seizures. Chemogenetic inhibition of PC Gad2 neurons induced seizure-like behavior, with a discrepancy between the GABAergic neuron activities and GABA levels, signifying a transition from interictal to ictal states. Surprisingly, selective inhibition of Gad2 neurons in the PC produced paradoxical activation in a subset of Gad2 neurons. Moreover, the chronic deficiency of PC Gad2 neurons triggered spontaneous recurrent seizures.

SIGNIFICANCE

Our findings uncover the dynamic interplay within PC inhibitory components and elaborate counteractive mechanisms in seizure regulation. These insights could inform future therapeutic strategies targeting GABAergic neurons to control epileptic activity.

Noninvasive Diagnostic Method to Objectively Measure Olfaction and Diagnose Smell Disorders by a Molecularly Targeted Fluorescence Imaging Agent

Despite the recent advances in understanding the mechanisms of olfaction, no tools are currently available to noninvasively identify loss of smell. Because of the substantial increase in patients presenting with coronavirus disease 2019-related loss of smell, the pandemic has highlighted the urgent need to develop quantitative methods. Methods: Our group investigated the use of a novel fluorescent probe named Tsp1a-IR800P as a tool to diagnose loss of smell. Tsp1a-IR800P targets sodium channel 1.7, which plays a critical role in olfaction by aiding the signal propagation to the olfactory bulb. Results: Intuitively, we have identified that conditions leading to loss of smell, including chronic inflammation and coronavirus disease 2019, correlate with the downregulation of sodium channel 1.7 expression in the olfactory epithelium, both at the transcript and at the protein levels. We demonstrated that lower Tsp1a-IR800P fluorescence emissions significantly correlate with loss of smell in live animals-thus representing a potential tool for its semiquantitative assessment. Currently available methods rely on delayed subjective behavioral studies. Conclusion: This method could aid in significantly improving preclinical and clinical studies by providing a way to objectively diagnose loss of smell and therefore aid the development of therapeutic interventions.

Tracking the Navigation Behavior of Drosophila Larvae in Real and Virtual Odor Gradients by Using the Raspberry Pi Virtual Reality (PiVR) System

In a closed-loop experimental paradigm, an animal experiences a modulation of its sensory input as a function of its own behavior. Tools enabling closed-loop experiments are crucial for delineating causal relationships between the activity of genetically labeled neurons and specific behavioral responses. We have recently developed an experimental platform known as "Raspberry Pi Virtual Reality" (PiVR) that is used to perform closed-loop optogenetic stimulation of neurons in unrestrained animals. PiVR is a system that operates at high temporal resolution (>30-Hz) and with low latencies. Larvae of the fruit fly Drosophila melanogaster are ideal to study the role of individual neurons in modulating behavior to aid the understanding of the neural pathways underlying various guided behaviors. Here, we introduce larval chemotaxis as an example of a navigational behavior in which an animal seeks to locate a target-in this case, the attractive source of an odor-by tracking a concentration gradient. The methodologies that we describe here combine the use of PiVR with the study of larval chemotaxis in real and virtual odor gradients, but these can also be readily adapted to other sensory modalities.

Neuroregulation of foraging behavior mediated by the olfactory co-receptor Orco in termites

Olfaction is critical for survival because it allows animals to look for food and detect pheromonal cues. Neuropeptides modulate olfaction and behaviors in insects. While how the neuroregulation of olfactory recognition affects foraging behavior in termites is still unclear. Here, we analyzed the change after silencing the olfactory co-receptor gene (Orco) and the neuropeptide Y gene (NPY), and then investigated the impact of olfactory recognition on foraging behavior in Odontotermes formosanus under different predation pressures. The knockdown of Orco resulted in the reduced Orco protein expression in antennae and the decreased EAG response to trail pheromones. In addition, NPY silencing led to the damaged ability of olfactory response through downregulating Orco expression. Both dsOrco- and dsNPY-injected worker termites showed significantly reduced walking activity and foraging success. Additionally, we found that 0.1 pg/cm trail pheromone and nestmate soldiers could provide social buffering to relieve the adverse effect of predator ants on foraging behavior in worker termites with the normal ability of olfactory recognition. Our orthogonal experiments further verified that Orco/NPY genes are essential in manipulating termite olfactory recognition during foraging under different predation pressures, suggesting that the neuroregulation of olfactory recognition plays a crucial role in regulating termite foraging behavior.

Identification and functional analysis of odorant-binding proteins of the parasitoid wasp Scleroderma guani reveal a chemosensory synergistic evolution with the host Monochamus alternatus

Scleroderma guani is a generalist ectoparasitoid of wood-boring insects. The chemosensory genes expressed in its antennae play crucial roles in host-seeking. In the present study, we identified 14 OBP genes for the first time from the antennae transcriptomes and genomic data of S. guani. The expression profiles of 14 OBPs were tested by RT-qPCR, and the RT-qPCR results showed that SguaOBP2/5/6/11/12/13 were specifically highly expressed in the female antennae. Then we performed ligand binding assays to test the interactions between six selected SguaOBPs with host specific chemical compounds from M. alternatus and pines. The binding results indicated that SguaOBP12 had a higher binding affinity with longifolene, β-caryophyllene, α-pinene, β-pinene, myrcene, butylated hydroxytoluene, and 3-carene. SguaOBP11 had a high or medium binding affinity with them. Furthermore, both SguaOBP11 and SguaOBP12 had a medium binding affinity with the aggregation pheromone of Monochamus species, 2-undecyloxy-1-ethanol. Finally, by using molecular docking and RNAi, we further explored the molecular interactions and behavioral functions of SguaOBP11 and SguaOBP12 with these vital odor molecules. Our study contributes to the further understanding of chemical communications between S. guani and its host, and further exploration for its role as a more effective biological control agent.

Inbreeding-Driven Innate Behavioral Changes in Drosophila melanogaster

Drosophila melanogaster has long been used to demonstrate the effect of inbreeding, particularly in relation to reproductive fitness and stress tolerance. In comparison, less attention has been given to exploring the influence of inbreeding on the innate behavior of D. melanogaster. In this study, multiple replicates of six different types of crosses were set in pair conformation of the laboratory-maintained wild-type D. melanogaster. This resulted in progeny with six different levels of inbreeding coefficients. Larvae and adult flies of varied inbreeding coefficients were subjected to different behavioral assays. In addition to the expected inbreeding depression in the-egg to-adult viability, noticeable aberrations were observed in the crawling and phototaxis behaviors of larvae. Negative geotactic behavior as well as positive phototactic behavior of the flies were also found to be adversely affected with increasing levels of inbreeding. Interestingly, positively phototactic inbred flies demonstrated improved learning compared to outbred flies, potentially the consequence of purging. Flies with higher levels of inbreeding exhibited a delay in the manifestation of aggression and courtship. In summary, our findings demonstrate that inbreeding influences the innate behaviors in D. melanogaster, which in turn may affect the overall biological fitness of the flies.

Identification and expression analyses of the olfactory-related genes in different tissues' transcriptome of a predacious soldier beetle, Podabrus annulatus (Coleoptera, Cantharidae)

We sequenced and analyzed the transcriptomes from different tissues of the soldier beetle, Podabrus annulatus (Coleoptera: Cantharidae), and obtained 75.74 Gb clean reads which were assembled into 95,274 unigenes. Among these transcripts, 25,484 unigenes of highly quality were annotated. Based on annotation and tBLASTn results, we identified a total of 101 candidate olfactory-related genes for the first time, including 11 putative odorant-binding proteins (OBPs), 6 chemosensory proteins (CSP), 50 olfactory receptors (ORs), 25 gustatory receptors (GRs), 6 ionotropic receptors (IRs), and 3 sensory neuron membrane proteins (SNMPs). BLASTX best-hit results indicated that these chemosensory genes were most identical to their respective orthologs from Photinus pyralis. Phylogenetic analyses also revealed that the ORs, GRs, and IRs of Podabrus annulatus are closely related to those of Photinus pyralis. The fragment per kilobase per million mapped fragments (FPKM) values showed that the PannOBP2, PannOBP3, and PannOBP10 were predominantly expressed in the antennae, PannOBP1 in the abdomen-thorax, while others were not identified to be tissue-specific. These olfactory-related differentially expressed genes (DEGs) demonstrated different roles in the olfactory system of Podabrus annulatus. This study establishes the groundwork for future research into the molecular mechanism of olfactory recognition in Podabrus annulatus.

Enhancement of Pathogen Toxicity by Feeding Reticulitermes chinensis Snyder Sonicated Bacteria Expressing Double-Stranded RNA That Interferes with Olfaction

Reticulitermes chinensis Snyder is a serious pest in China, and the odorant receptor co-receptor gene RcOrco plays a crucial role in olfaction. However, the function of RcOrco in the resistance of termites to entomopathogens has not been reported. We constructed dsRcOrco-HT115 engineered bacteria based on the RcOrco sequence from the full-length transcriptome data of R. chinensis. The engineered bacteria expressed dsRNA of RcOrco. Sonication was used to inactivate the dsRNA-HT115 strain and obtain a large amount of dsRcOrco. The dsRcOrco produced using this method overcame the problem that genetically engineered bacteria could not be applied directly and improved its effectiveness against termites. Bioassays using the dsRcOrco generated using this method showed that dsRcOrco significantly increased the toxicity of the bacterial and fungal pathogens to R. chinensis. The present study showed, for the first time, the function of Orco in termite resistance to pathogens, and the results provide a theoretical basis for the development and application of termite RNA biopesticides.

A mobile APP-based, customizable automated device for self-administered olfactory testing and an implementation of smell identification test

Olfactory tests are used for the evaluation of ability to detect and identify common odors in humans psychophysically. Olfactory tests are currently administered by professionals with a set of given odorants. Manual administration of such tests can be labor and cost intensive and data collected as such are confounded with experimental variables, which adds personnel costs and introduces potential errors and data variability. For large-scale and longitudinal studies, manually recorded data must be collected and compiled from multiple sites. It is difficult to standardize the way data are collected and recorded. There is a need for a computerized smell test system for psychophysical and clinical applications. A mobile digital olfactory testing system (DOTS) was developed, consisting of an odor delivery system (DOTS-ODD) and a mobile application program (DOTS-APP) connected wirelessly. The University of Pennsylvania Smell Identification Test was implemented in DOTS and compared to its commercial product on a cohort of 80 normosmic subjects and a clinical cohort of 12 Parkinson's disease patients. A test-retest was conducted on 29 subjects of the normal cohort. The smell identification scores obtained from the DOTS and standard UPSIT commercial test are highly correlated (r = 0.714, P < 0.001), and test-retest reliability coefficient was 0.807 (r = 0.807, P < 0.001). The DOTS is customizable and mobile compatible, which allows for the implementation of standardized olfactory tests and the customization of investigators' experimental paradigms. The DOTS-APP on mobile devices offers capabilities for a broad range of on-site, online, or remote clinical and scientific chemosensory applications.

The role of food odor in invertebrate foraging

Foraging for food is an integral part of animal survival. In small insects and invertebrates, multisensory information and optimized locomotion strategies are used to effectively forage in patchy and complex environments. Here, the importance of olfactory cues for effective invertebrate foraging is discussed in detail. We review how odors are used by foragers to move toward a likely food source and the recent models that describe this sensory-driven behavior. We argue that smell serves a second function by priming an organism for the efficient exploitation of food. By appraising food odors, invertebrates can establish preferences and better adapt to their ecological niches, thereby promoting survival. The smell of food pre-prepares the gastrointestinal system and primes feeding motor programs for more effective ingestion as well. Optimizing resource utilization affects longevity and reproduction as a result, leading to drastic changes in survival. We propose that models of foraging behavior should include odor priming, and illustrate this with a simple toy model based on the marginal value theorem. Lastly, we discuss the novel techniques and assays in invertebrate research that could investigate the interactions between odor sensing and food intake. Overall, the sense of smell is indispensable for efficient foraging and influences not only locomotion, but also organismal physiology, which should be reflected in behavioral modeling.

Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae)

An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.

Silencing Orco Impaired the Ability to Perceive Trail Pheromones and Affected Locomotion Behavior in Two Termite Species

Sophisticated social behaviors in termite colonies are mainly regulated via chemical communication of a wide range of pheromones. Trail pheromones play important roles in foraging behavior and building tunnels and nests in termites. However, it is almost unclear how termites perceive trail pheromones. Here, we cloned and sequenced of olfactory co-receptor (Orco) genes from the two termites Reticulitermes chinensis Snyder (Isoptera: Rhinotermitidae) and Odontotermes formosanus (Shiraki) (Isoptera: Termitidae), and then examined their responses to trail pheromones after silencing Orco through RNA interference (RNAi). We found that Orco knockdown impaired their ability to perceive trail pheromones and resulted in the disability of following pheromone trails in the two termite species. Our locomotion behavior assays further showed that Orco knockdown significantly decreased the distance and velocity in the two termite species, but significantly increased the angular velocity and turn angle in the termite R. chinensis. These findings strongly demonstrated that Orco is essential for termites to perceive their trail pheromones, which provides a potential way to control termite pests by damaging olfactory system.

Braitenberg Vehicles as Computational Tools for Research in Neuroscience

Valentino Braitenberg reported his seminal thought experiment in 1984 using reactive automatons or vehicles with relatively simple sensorimotor connections as models for seemingly complex cognitive processes in biological brains. Braitenberg's work, meant as a metaphor for biological life encompassed a deep knowledge of and served as an analogy for the multitude of neural processes and pathways that underlie animal behavior, suggesting that seemingly complex behavior may arise from relatively simple designs. Braitenberg vehicles have been adopted in robotics and artificial life research for sensor-driven navigation behaviors in robots, such as localizing sound and chemical sources, orienting toward or away from current flow under water etc. The neuroscience community has benefitted from applying Braitenberg's bottom-up approach toward understanding analogous neural mechanisms underpinning his models of animal behavior. We present a summary of the latest studies of Braitenberg vehicles for bio-inspired navigation and relate the results to experimental findings on the neural basis of navigation behavior in animals. Based on these studies, we motivate the important role of Braitenberg vehicles as computational tools to inform research in behavioral neuroscience.

Mini-brain computations converting dynamic olfactory inputs into orientation behavior

The neural logic underlying the conversion of non-stationary (dynamic) olfactory inputs into odor-search behaviors has been difficult to crack due to the distributed nature of the olfactory code - food odors typically co-activate multiple olfactory sensory neurons. In the Drosophila larva, the activity of a single olfactory sensory neuron is sufficient to direct accurate reorientation maneuvers in odor gradients (chemotaxis). In this reduced sensory system, a descending pathway essential for larval chemotaxis has been delineated from the peripheral olfactory system down to the premotor system. Here, I review how anatomical and functional inspections of this pathway have advanced our understanding of the neural mechanisms that convert behaviorally relevant sensory signals into orientation responses.

Identification of Candidate Olfactory Genes in the Antennal Transcriptome of the Stink Bug Halyomorpha halys

The brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae), is a serious agricultural and urban pest that has become an invasive species in many parts of the world. Olfaction plays an indispensable role in regulating insect behaviors, such as host plant location, partners searching, and avoidance of predators. In this study, we sequenced and analyzed the antennal transcriptomes of both male and female adults of H. halys to better understand the olfactory mechanisms in this species. A total of 241 candidate chemosensory genes were identified, including 138 odorant receptors (ORs), 24 ionotropic receptors (IRs), 15 gustatory receptors (GRs), 44 odorant-binding proteins (OBPs), 17 chemosensory proteins (CSPs), and three sensory neuron membrane proteins (SNMPs). The results of semi-quantitative reverse transcription PCR (RT-PCR) assays showed that some HhalOBP and HhalCSP genes have tissue-specific and sex-biased expression patterns. Our results provide an insight into the molecular mechanisms of the olfactory system in H. halys and identify potential novel targets for pest control strategies.

Development of a Korean Culture-Friendly Olfactory Function Test and Optimization of a Diagnostic Cutoff Value

Objectives

Cultural familiarity and safety must be considered when assessing olfactory ability. The YSK olfactory function (YOF) test is a new olfactory function test using culturally familiar odorants to Koreans.

Methods

The YOF test comprises three subtests for threshold (T), discrimination (D), and identification (I). The identification test included eight universal and four Korean culture-friendly odorants, which were selected considering eight major functional groups. Data were obtained from 1,127 subjects over 19 years old. Subjects were classified as having normosmia (n=542), hyposmia (n=472), and anosmia (n=113) by self-reported olfactory function. The YOF test and the Korean version of the Sniffin’ stick test (KVSS-II) were performed on the same day in random order. Diagnostic cutoffs for anosmia and hyposmia were calculated using the Youden index (J).

Results

The mean values for each T/D/I subtest and the total TDI score were as follows: normosmia (T, 4.6±2.3; D, 8.6±2.1; I, 11.1±1.7; TDI score, 24.2±4.5); hyposmia (T, 3.3±2.2; D, 7.1±2.5; I, 9.2±3.1; TDI score, 19.5±6.4); and anosmia (T, 1.7±1.2; D, 5.1±2.5; I, 5.0±3.2; TDI score, 11.8±5.6). The correlation coefficients between the YOF test and KVSS-II were 0.57, 0.65, 0.80, and 0.86 for T, D, I, and the TDI score, respectively (P<0.001). The diagnostic cutoffs were a TDI score ≤14.5 (J=0.67) for anosmia and 14.5(TDI score ≤21.0 (J=0.38) for hyposmia. The diagnostic efficacy of the YOF test (area under the curve [AUC], 0.88) was equivalent to that of the KVSS-II (AUC, 0.88; P=0.843; DeLong method).

Conclusion

The YOF test is a new olfactory test using safe and Korean culture-friendly odorants. It showed equivalent validity with the conventional olfactory function test. Furthermore, the YOF test provides information on the major functional groups of odorants, potentially enabling a more comprehensive interpretation for patients with olfactory disorders.

Antennal transcriptome analyses and olfactory protein identification in an important wood-boring moth pest, Streltzoviella insularis (Lepidoptera: Cossidae)

Olfaction plays key roles in insect survival and reproduction, such as feeding, courtship, mating, and oviposition. The olfactory-based control strategies have been developed an important means for pest management. Streltzoviella insularis is a destructive insect pest of many street tree species, and characterization of its olfactory proteins could provide targets for the disruption of their odour recognition processes and for urban forestry protection. In this study, we assembled the antennal transcriptome of S. insularis by next-generation sequencing and annotated the main olfactory multi-gene families, including 28 odorant-binding proteins (OBPs), 12 chemosensory proteins (CSPs), 56 odorant receptors (ORs), 11 ionotropic receptors (IRs), two sensory neuron membrane proteins (SNMPs), and 101 odorant-degrading enzymes (ODEs). Sequence and phylogenetic analyses confirmed the characteristics of these proteins. We further detected tissue- and sex-specific expression patterns of OBPs, CSPs and SNMPs by quantitative real time-PCR. Most OBPs were highly and differentially expressed in the antennae of both sexes. SinsCSP10 was expressed more highly in male antennae than in other tissues. Two SNMPs were highly expressed in the antennae, with no significant difference in expression between the sexes. Our results lay a solid foundation for understanding the precise molecular mechanisms underlying S. insularis odour recognition.

Identification and expression analysis of odorant binding proteins and chemosensory proteins from dissected antennae and mouthparts of the rice bug Leptocorisa acuta

The rice bug, Leptocorisa acuta (Tunberg) (Hemiptera: Alydidae), is a notorious pest in Asia, and it is significantly attracted by the volatiles derived from host plants. However, it remains unknown how L. acuta recognizes host volatile compounds at the molecular level. Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are thought to be responsible for the initial biochemical recognition during olfactory perception. Here, we followed the RNA Sequencing (RNA-Seq) approach to identify candidate genes encoding OBPs and CSPs from dissected antennae and mouthparts of L. acuta. In total, 26 unigenes were identified coding for OBPs (22 Classic OBPs and four Plus-C OBPs), and 17 unigenes coding for CSPs. Real-time quantitative PCR (RT-qPCR) revealed that 11 OBPs (LacuOBP1, 5, 6, 7, 9, 11, 13, 14, 17, 20 and 23) and nine CSPs (LacuCSP2, 3, 4, 5, 6, 8, 9, 10 and 12) were predominantly expressed in antennae, indicating that they might be essential for detection of general odorants and pheromones. Among these antennae-predominantly expressed genes, LacuOBP11 and LacuOBP13 showed male-biased expression and therefore may play crucial roles in the detection of sex pheromones. Seven LacuOBPs (LacuOBP4, 8, 10, 12, 21, 25 and 26) and two CSPs (LacuCSP7 and LacuOBP11) were predominantly expressed in mouthparts, suggesting that these genes might be involved in taste perception. Our work provides a starting point to facilitate functional study of these OBPs and CSPs in L. acuta at the molecular level in the future.

The genomic architecture and molecular evolution of ant odorant receptors

The massive expansions of odorant receptor (OR) genes in ant genomes are notable examples of rapid genome evolution and adaptive gene duplication. However, the molecular mechanisms leading to gene family expansion remain poorly understood, partly because available ant genomes are fragmentary. Here, we present a highly contiguous, chromosome-level assembly of the clonal raider ant genome, revealing the largest known OR repertoire in an insect. While most ant ORs originate via local tandem duplication, we also observe several cases of dispersed duplication followed by tandem duplication in the most rapidly evolving OR clades. We found that areas of unusually high transposable element density (TE islands) were depauperate in ORs in the clonal raider ant, and found no evidence for retrotransposition of ORs. However, OR loci were enriched for transposons relative to the genome as a whole, potentially facilitating tandem duplication by unequal crossing over. We also found that ant OR genes are highly AT-rich compared to other genes. In contrast, in flies, OR genes are dispersed and largely isolated within the genome, and we find that fly ORs are not AT-rich. The genomic architecture and composition of ant ORs thus show convergence with the unrelated vertebrate ORs rather than the related fly ORs. This might be related to the greater gene numbers and/or potential similarities in gene regulation between ants and vertebrates as compared to flies.

A Gustatory Receptor Used for Rapid Detection of Tyrophagus putrescentiae in Fungi Hosts

The storage mite, Tyrophagus putrescentiae, found worldwide in many habitats, is an important pest of edible fungi in China. Storage mites are tiny and difficult to observe, especially when they occur in fungi composts. In this study, one gustatory receptor protein (TputGR1) was identified from the transcriptome of T. putrescentiae. Phylogenetic analysis of GRs families from 10 arthropod species revealed that TputGR1 had high homology with the SccaGR1 of Sarcoptes scabiei and TurtGR1-2 of Tetranychus urticae, but low homology with other insect species, Drosophila melanogaster, Anopheles gambiae, Bombyx mori, Aedes aegypti, Culex quinquefasciatus, and Pediculus humanus. We developed a detection system for the mite on fungi hosts using the GR protein and the loop-mediated isothermal amplification (LAMP). This procedure was rapid (60 min from sampling to result) and had high sensitivity (0.5 ng/mL). LAMP provided rapid and reliable detection of T. putrescentiae. It has good specificity for single samples and for large-scale surveys.

Species-specific modulation of food-search behavior by respiration and chemosensation in Drosophila larvae

Animals explore their environment to encounter suitable food resources. Despite its vital importance, this behavior puts individuals at risk by consuming limited internal energy during locomotion. We have developed a novel assay to investigate how food-search behavior is organized in Drosophila melanogaster larvae dwelling in hydrogels mimicking their natural habitat. We define three main behavioral modes: resting at the gel's surface, digging while feeding near the surface, and apneic dives. In unstimulated conditions, larvae spend most of their time digging. By contrast, deep and long exploratory dives are promoted by olfactory stimulations. Hypoxia and chemical repellents impair diving. We report remarkable differences in the dig-and-dive behavior of D. melanogaster and the fruit-pest D. suzukii. The present paradigm offers an opportunity to study how sensory and physiological cues are integrated to balance the limitations of dwelling in imperfect environmental conditions and the risks associated with searching for potentially more favorable conditions.

orco Mutagenesis Causes Loss of Antennal Lobe Glomeruli and Impaired Social Behavior in Ants

Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here, we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the ∼500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain. VIDEO ABSTRACT.

Whole-Brain Monosynaptic Afferent Inputs to Basal Forebrain Cholinergic System

The basal forebrain cholinergic system (BFCS) robustly modulates many important behaviors, such as arousal, attention, learning and memory, through heavy projections to cortex and hippocampus. However, the presynaptic partners governing BFCS activity still remain poorly understood. Here, we utilized a recently developed rabies virus-based cell-type-specific retrograde tracing system to map the whole-brain afferent inputs of the BFCS. We found that the BFCS receives inputs from multiple cortical areas, such as orbital frontal cortex, motor cortex, and insular cortex, and that the BFCS also receives dense inputs from several subcortical nuclei related to motivation and stress, including lateral septum, central amygdala, paraventricular nucleus of hypothalamus, dorsal raphe, and parabrachial nucleus. Interestingly, we found that the BFCS receives inputs from the olfactory areas and the entorhinal–hippocampal system. These results greatly expand our knowledge about the connectivity of the mouse BFCS and provided important preliminary indications for future exploration of circuit function.

Identification and expression profile of odorant-binding proteins in Halyomorpha halys (Hemiptera: Pentatomidae)

The brown marmorated stink bug, Halyomorpha halys, is a devastating invasive species in the USA. Similar to other insects, olfaction plays an important role in its survival and reproduction. As odorant-binding proteins (OBPs) are involved in the initial semiochemical recognition steps, we used RNA-Sequencing (RNA-Seq) to identify OBPs in its antennae, and studied their expression pattern in different body parts under semiochemical stimulation by either aggregation or alarm pheromone or food odorants. Thirty full-length putative HhalOBPs were identified, corresponding to 22 'classic' OBPs and eight 'Plus-C' OBPs. The similarity amongst them ranged from 4.95-70.92%, and with another 325 hemipteran OBPs similarity ranged from 1.94-91.51%, the highest levels being with other stink bug OBPs. Phylogenetic analysis confirmed the monophyly of seven groups of stink bug and other hemipteran OBPs. All 30 HhalOBPs were expressed and about 2/3 were expressed primarily in antennae. The expression of 21 HhalOBPs was higher in the antennae under alarm pheromone stimulus, indicating that multiple OBPs may be responding to this pheromone. Two were highest in antennae under aggregation pheromone stimulus. These findings should provide a basis for understanding the physiological functions of HhalOBPs and the chemosensory perception of this pest, which may help to uncover new control targets for behavioural interference.

Chemosensory proteins involved in host recognition in the stored-food mite Tyrophagus putrescentiae

BACKGROUND

Chemosensory proteins (CSPs) have been proposed to transport a range of aliphatic compounds, esters and other long-chain compounds. A large number of CSPs from different gene subfamilies have been identified and annotated in arthropods; however, the CSP genes in mites remain unknown. Tyrophagus putrescentiae Schrank is an important stored-product and house-dust pest.

RESULTS

By analysing the transcriptome, two putative CSPs were identified, namely TputCSP1 and TputCSP2 (14.9 kDa and 12.1 kDa respectively). The phylogenetic tree showed that the two TputCSPs shared most homology with CSPs in Ixodes scapularis and partially with Diptera, including Anopheles gambiae, Drosophila melanogaster, D. pseudoobscura, D. simulans, Delia antiqua and Culex quinquefasciatus. Additionally, they had similar secondary structure. The 3D models revealed that there are six α-helices enclosing the hydrophobic ligand binding pocket. Based on a docking study, we found that three ligands, (-)-alloaromadendrene, 2-methylnaphthalene and cyclopentadecane, had high binding affinities for TputCSP1. Moreover, the TputCSP2 protein had a higher inhibition constant with different affinities to all test ligands from host volatile substances.

CONCLUSION

The two CSPs have distinct physiological functions. TputCSP1 may mediate host recognition. © 2015 Society of Chemical Industry.

Follow your nose: leaf odour as an important foraging cue for mammalian herbivores

Studies of odour-driven foraging by mammals focus on attractant cues emitted by flowers, fruits, and fungi. Yet, the leaves of many plant species worldwide produce odour, which could act as a cue for foraging mammalian herbivores. Leaf odour may thus improve foraging efficiency for such herbivores in many ecosystems by reducing search time, particularly but not only, for plants that are visually obscured. We tested the use of leaf odour by a free-ranging mammalian browser, the swamp wallaby (Wallabia bicolor) to find and browse palatable tree seedlings (Eucalyptus pilularis). Wallabies visited patches non-randomly with respect to the presence of seedlings. In the absence of visual plant cues, they used leaf odour (cut seedlings in vials) to find patches earlier, and visited and investigated them more often than control patches (empty vials), supporting the hypothesis that wallabies used seedling odour to enhance search efficiency. In contrast, the grey kangaroo (Macropus giganteus), a grazer, showed no response to seedling odour. When the availability of seedling visual and olfactory cues was manipulated, wallabies browsed seedlings equally quickly in all treatments: upright (normal cues), pinned to the ground (reduced visual cues), and upright plus pinned seedlings (double olfactory cues). Odour cues play a critical role in food-finding by swamp wallabies, and these animals are finely tuned to detecting these cues with their threshold for detection reached by odours from only a single plant. The global significance of leaf odour in foraging by mammalian herbivores consuming conifers, eucalypts, and other odour-rich species requires greater attention.

Insights Into the Evolution of Chemoreceptor Genes Superfamily in Tyrophagus putrescentiae (Acari: Acaridae)

All living organisms, including animals, plants, fungi, and bacteria, use the olfactory system to recognize chemicals or pheromone from their environment. Insects detect a volatile substance using odorant receptors (ORs) or gustatory receptors (GRs) and ionotropic receptors (IRs). The gene families of the olfactory system in Acari are still not clear. In this study, we identified seven ORs, one GR, and five IRs from the transcriptome of the storage mite, Tyrophagus putrescentiae Schrank. No olfactory coreceptor was found in this transcriptome. Phylogenetic analysis of these gene families with other Arthropoda species revealed the conservation of carbon dioxide receptors in all tested flying insects and T. putrescentiae Most of these ORs and GRs were unique to three mosquitoes (Anopheles gambiae Giles, Culex quinquefasciatus Say, and Aedes aegypti L.), Ixodes scapularis Say and Pediculus humanus L., indicating their involvement in specific aspects of both gustatory and olfactory perception. Some clades contained receptors obtained from all tested insect vector species, indicating a degree of conservation among some vector-dependent OR lineages. IRs family was a highly dynamic and independent original of the chemoreceptor genes subfamily. Our findings would make it possible for future research on the chemosensory recognition mechanism in Acari.

Molecular Characterization and Expression Profiling of Odorant-Binding Proteins in Apolygus lucorum

Apolygus lucorum (Meyer-Dür) (Hemiptera: Miridae) is one of the most important agricultural pests, with broad host range and cryptic feeding habits in China. Chemosensory behavior plays an important role in many crucial stages in the life of A. lucorum, such as the detection of sex pheromone cues during mate pursuit and fragrant odorants during flowering host plant localization. Odorant-binding proteins (OBPs) are involved in the initial biochemical recognition steps in semiochemical perception. In the present study, a transcriptomics-based approach was used to identify potential OBPs in A. lucorum. In total, 38 putative OBP genes were identified, corresponding to 26 ‘classic’ OBPs and 12 ‘Plus-C’ OBPs. Phylogenetic analysis revealed that A. lucorum OBP proteins are more closely related to the OBP proteins of other mirid bugs as the same family OBP clustering together. Quantitative real-time PCR analysis for the first reported 23 AlucOBPs revealed that the expression level of 11 AlucOBP genes were significantly higher in antennae of both sexes than in other tissues. Three of them were male antennae-biased and six were female antennae-biased, suggesting their putative roles in the detection of female sex pheromones and host plant volatiles. In addition, three, four, two and one AlucOBPs had the highest degree of enrichment in the stylet, head, leg, and in abdomen tissues, respectively. Two other OBPs were ubiquitously expressed in the main tissues, including antennae, stylets, heads, legs and wings. Most orthologs had similar expression patterns, strongly indicating that these genes have the same function in olfaction and gustation.

Discovery of Chemosensory Genes in the Oriental Fruit Fly, Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis, is a devastating fruit fly pest in tropical and sub-tropical countries. Like other insects, this fly uses its chemosensory system to efficiently interact with its environment. However, our understanding of the molecular components comprising B. dorsalis chemosensory system is limited. Using next generation sequencing technologies, we sequenced the transcriptome of four B. dorsalis developmental stages: egg, larva, pupa and adult chemosensory tissues. A total of 31 candidate odorant binding proteins (OBPs), 4 candidate chemosensory proteins (CSPs), 23 candidate odorant receptors (ORs), 11 candidate ionotropic receptors (IRs), 6 candidate gustatory receptors (GRs) and 3 candidate sensory neuron membrane proteins (SNMPs) were identified. The tissue distributions of the OBP and CSP transcripts were determined by RT-PCR and a subset of nine genes were further characterized. The predicted proteins from these genes shared high sequence similarity to Drosophila melanogaster pheromone binding protein related proteins (PBPRPs). Interestingly, one OBP (BdorOBP19c) was exclusively expressed in the sex pheromone glands of mature females. RT-PCR was also used to compare the expression of the candidate genes in the antennae of male and female B. dorsalis adults. These antennae-enriched OBPs, CSPs, ORs, IRs and SNMPs could play a role in the detection of pheromones and general odorants and thus could be useful target genes for the integrated pest management of B. dorsalis and other agricultural pests.

Chemosensory gene families in adult antennae of Anomala corpulenta Motschulsky (Coleoptera: Scarabaeidae: Rutelinae)

BACKGROUND

The metallic green beetle, Anomala corpulenta (Coleoptera: Scarabaeidae: Rutelinae), is a destructive pest in agriculture and horticulture throughout Asia, including China. Olfaction plays a crucial role in the survival and reproduction of A. corpulenta. As a non-model species, A. corpulenta is poorly understood, and information regarding the molecular mechanisms underlying olfaction in A. corpulenta and other scarab species is scant.

METHODOLOGY/PRINCIPLE FINDINGS

We assembled separate antennal transcriptome for male and female A. corpulenta using Illumina sequencing technology. The relative abundance of transcripts with gene ontology annotations, including those related to olfaction in males and females was highly similar. Transcripts encoding 15 putative odorant binding proteins, five chemosensory proteins, one sensory neuron membrane protein, 43 odorant receptors, eight gustatory receptors, and five ionotropic receptors were identified. The sequences of all of these chemosensory-related transcripts were confirmed using reverse transcription polymerase chain reaction (RT-PCR), and direct DNA sequencing. The expression patterns of 54 putative chemosensory genes were analyzed using quantitative real time RT-PCR (qRT-PCR). Antenna-specific expression was detected for many of these genes, suggesting that they may have important functions in semiochemical detection.

CONCLUSIONS

The identification of a large number of chemosensory proteins provides a major resource for the study of the molecular mechanism of odorant detection in A. corpulenta and its chemical ecology. The genes identified, especially those that were expressed at high levels in the antennae may represent novel molecular targets for the development of population control strategies based on the manipulation of chemoreception-driven behaviors.

Olfactory control of blood progenitor maintenance

Drosophila hematopoietic progenitor maintenance involves both near neighbor and systemic interactions. This study shows that olfactory receptor neurons (ORNs) function upstream of a small set of neurosecretory cells that express GABA. Upon olfactory stimulation, GABA from these neurosecretory cells is secreted into the circulating hemolymph and binds to metabotropic GABAB receptors expressed on blood progenitors within the hematopoietic organ, the lymph gland. The resulting GABA signal causes high cytosolic Ca(2+), which is necessary and sufficient for progenitor maintenance. Thus, the activation of an odorant receptor is essential for blood progenitor maintenance, and consequently, larvae raised on minimal odor environments fail to sustain a pool of hematopoietic progenitors. This study links sensory perception and the effects of its deprivation on the integrity of the hematopoietic and innate immune systems in Drosophila. PAPERCLIP:

Graded encoding of food odor value in the Drosophila brain

Odors are highly evocative, yet how and where in the brain odors derive meaning remains unknown. Our analysis of the Drosophila brain extends the role of a small number of hunger-sensing neurons to include food-odor value representation. In vivo two-photon calcium imaging shows the amplitude of food odor-evoked activity in neurons expressing Drosophila neuropeptide F (dNPF), the neuropeptide Y homolog, strongly correlates with food-odor attractiveness. Hunger elevates neural and behavioral responses to food odors only, although food odors that elicit attraction in the fed state also evoke heightened dNPF activity in fed flies. Inactivation of a subset of dNPF-expressing neurons or silencing dNPF receptors abolishes food-odor attractiveness, whereas genetically enhanced dNPF activity not only increases food-odor attractiveness but promotes attraction to aversive odors. Varying the amount of presented odor produces matching graded neural and behavioral curves, which can function to predict preference between odors. We thus demonstrate a possible motivationally scaled neural "value signal" accessible from uniquely identifiable cells.

The CCHamide 1 receptor modulates sensory perception and olfactory behavior in starved Drosophila

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.

Genetic architecture of olfactory behavior in Drosophila melanogaster: differences and similarities across development

In the holometabolous insect Drosophila melanogaster, genetic, physiological and anatomical aspects of olfaction are well known in the adult stage, while larval stages olfactory behavior has received some attention it has been less studied than its adult counterpart. Most of these studies focus on olfactory receptor (Or) genes that produce peripheral odor recognition. In this paper, through a loss-of-function screen using P-element inserted lines and also by means of expression analyses of larval olfaction candidate genes, we extended the uncovering of the genetic underpinnings of D. melanogaster larval olfactory behavior by demonstrating that larval olfactory behavior is, in addition to Or genes, orchestrated by numerous genes with diverse functions. Also, our results point out that the genetic architecture of olfactory behavior in D. melanogaster presents a dynamic and changing organization across environments and ontogeny.

Advantage of the Highly Restricted Odorant Receptor Expression Pattern in Chemosensory Neurons of Drosophila

A fundamental molecular feature of olfactory systems is that individual neurons express only one receptor from a large odorant receptor gene family. While numerous theories have been proposed, the functional significance and evolutionary advantage of generating a sophisticated one-receptor-per neuron expression pattern is not well understood. Using the genetically tractable Drosophila melanogaster as a model, we demonstrate that the breakdown of this highly restricted expression pattern of an odorant receptor in neurons leads to a deficit in the ability to exploit new food sources. We show that animals with ectopic co-expression of odorant receptors also have a competitive disadvantage in a complex environment with limiting food sources. At the level of the olfactory system, we find changes in both the behavioral and electrophysiological responses to odorants that are detected by endogenous receptors when an olfactory receptor is broadly misexpressed in chemosensory neurons. Taken together these results indicate that restrictive expression patterns and segregation of odorant receptors to individual neuron classes are important for sensitive odor-detection and appropriate olfactory behaviors.

Flies dynamically anti-track, rather than ballistically escape, aversive odor during flight

Tracking distant odor sources is crucial to foraging, courtship and reproductive success for many animals including fish, flies and birds. Upon encountering a chemical plume in flight, Drosophila melanogaster integrates the spatial intensity gradient and temporal fluctuations over the two antennae, while simultaneously reducing the amplitude and frequency of rapid steering maneuvers, stabilizing the flight vector. There are infinite escape vectors away from a noxious source, in contrast to a single best tracking vector towards an attractive source. Attractive and aversive odors are segregated into parallel neuronal pathways in flies; therefore, the behavioral algorithms for avoidance may be categorically different from tracking. Do flies plot random ballistic or otherwise variable escape vectors? Or do they instead make use of temporally dynamic mechanisms for continuously and directly avoiding noxious odors in a manner similar to tracking appetitive ones? We examine this question using a magnetic tether flight simulator that permits free yaw movements, such that flies can actively orient within spatially defined odor plumes. We show that in-flight aversive flight behavior shares all of the key features of attraction such that flies continuously 'anti-track' the noxious source.

Frequent recent origination of brain genes shaped the evolution of foraging behavior in Drosophila

The evolution of the brain and behavior are coupled puzzles. The genetic bases for brain evolution are widely debated, yet whether newly evolved genes impact the evolution of the brain and behavior is vaguely understood. Here, we show that during recent evolution in Drosophila, new genes have frequently acquired neuronal expression, particularly in the mushroom bodies. Evolutionary signatures combined with expression profiling showed that natural selection influenced the evolution of young genes expressed in the brain, notably in mushroom bodies. Case analyses showed that two young retrogenes are expressed in the olfactory circuit and facilitate foraging behavior. Comparative behavioral analysis revealed divergence in foraging behavior between species. Our data suggest that during adaptive evolution, new genes gain expression in specific brain structures and evolve new functions in neural circuits, which might contribute to the phenotypic evolution of animal behavior.

Comparative genomics of the odorant-binding and chemosensory protein gene families across the Arthropoda: origin and evolutionary history of the chemosensory system

Chemoreception is a biological process essential for the survival of animals, as it allows the recognition of important volatile cues for the detection of food, egg-laying substrates, mates, or predators, among other purposes. Furthermore, its role in pheromone detection may contribute to evolutionary processes, such as reproductive isolation and speciation. This key role in several vital biological processes makes chemoreception a particularly interesting system for studying the role of natural selection in molecular adaptation. Two major gene families are involved in the perireceptor events of the chemosensory system: the odorant-binding protein (OBP) and chemosensory protein (CSP) families. Here, we have conducted an exhaustive comparative genomic analysis of these gene families in 20 Arthropoda species. We show that the evolution of the OBP and CSP gene families is highly dynamic, with a high number of gains and losses of genes, pseudogenes, and independent origins of subfamilies. Taken together, our data clearly support the birth-and-death model for the evolution of these gene families with an overall high gene turnover rate. Moreover, we show that the genome organization of the two families is significantly more clustered than expected by chance and, more important, that this pattern appears to be actively maintained across the Drosophila phylogeny. Finally, we suggest the homologous nature of the OBP and CSP gene families, dating back their most recent common ancestor after the terrestrialization of Arthropoda (380--450 Ma) and we propose a scenario for the origin and diversification of these families.

Image enhancement for tracking the translucent larvae of Drosophila melanogaster

Drosophila melanogaster larvae are model systems for studies of development, synaptic transmission, sensory physiology, locomotion, drug discovery, and learning and memory. A detailed behavioral understanding of larvae can advance all these fields of neuroscience. Automated tracking can expand fine-grained behavioral analysis, yet its full potential remains to be implemented for the larvae. All published methods are unable to track the larvae near high contrast objects, including the petri-dish edges encountered in many behavioral paradigms. To alleviate these issues, we enhanced the larval contrast to obtain complete tracks. Our method employed a dual approach of optical-contrast boosting and post-hoc image processing for contrast enhancement. We reared larvae on black food media to enhance their optical contrast through darkening of their digestive tracts. For image processing we performed Frame Averaging followed by Subtraction then Thresholding (FAST). This algorithm can remove all static objects from the movie, including petri-dish edges prior to processing by the image-tracking module. This dual approach for contrast enhancement also succeeded in overcoming fluctuations in illumination caused by the alternating current power source. Our tracking method yields complete tracks, including at the edges of the behavioral arena and is computationally fast, hence suitable for high-throughput fine-grained behavioral measurements.

Time to taste: circadian clock function in the Drosophila gustatory system

Circadian clocks keep time in the digestive, circulatory, reproductive, excretory and nervous systems even in absence of external cues. Central oscillators in the brain control locomotor activity of organisms ranging from fruit flies to man, but the functions of the clocks in peripheral nervous system are not well understood. The presence of autonomous peripheral oscillators in the major taste organ of Drosophila, the proboscis, prompted us to test whether gustatory responses are under control of the circadian clock. We find that synchronous rhythms in physiological and behavioral responses to attractive and aversive tastants are driven by oscillators in gustatory receptor neurons (GRNs); primary sensory neurons that carry taste information from the proboscis to the brain. During the middle of the night, high levels of G protein-coupled receptor kinase 2 (GPRK2) in the GRNs suppresses tastant-evoked responses. Flies with disrupted gustatory clocks are hyperphagic and hyperactive, recapitulating behaviors typically seen under the stress of starvation. Temporal plasticity in innate behaviors should offer adaptive advantages to flies. In this Extra View article we discuss how oscillators inside GRNs regulate responsiveness to tastants and influence feeding, metabolism and general activity.

Mechanisms of odor-tracking: multiple sensors for enhanced perception and behavior

Early in evolution, the ability to sense and respond to changing environments must have provided a critical survival advantage to living organisms. From bacteria and worms to flies and vertebrates, sophisticated mechanisms have evolved to enhance odor detection and localization. Here, we review several modes of chemotaxis. We further consider the relevance of a striking and recurrent motif in the organization of invertebrate and vertebrate sensory systems, namely the existence of two symmetrical olfactory sensors. By combining our current knowledge about the olfactory circuits of larval and adult Drosophila, we examine the molecular and neural mechanisms underlying robust olfactory perception and extend these analyses to recent behavioral studies addressing the relevance and function of bilateral olfactory input for gradient detection. Finally, using a comparative theoretical approach based on Braitenberg's vehicles, we speculate about the relationships between anatomy, circuit architecture and stereotypical orientation behaviors.

Towards plant-odor-related olfactory neuroethology in Drosophila

Drosophila melanogaster is today one of the three foremost models in olfactory research, paralleled only by the mouse and the nematode. In the last years, immense progress has been achieved by combining neurogenetic tools with neurophysiology, anatomy, chemistry, and behavioral assays. One of the most important tasks for a fruit fly is to find a substrate for eating and laying eggs. To perform this task the fly is dependent on olfactory cues emitted by suitable substrates as e.g. decaying fruit. In addition, in this area, considerable progress has been made during the last years, and more and more natural and behaviorally active ligands have been identified. The future challenge is to tie the progress in different fields together to give us a better understanding of how a fly really behaves. Not in a test tube, but in nature. Here, we review our present state of knowledge regarding Drosophila plant-odor-related olfactory neuroethology to provide a basis for new progress.

Production and biological function of volatile esters in Saccharomyces cerevisiae

The need to understand and control ester synthesis is driven by the fact that esters play a key role in the sensorial quality of fermented alcoholic beverages like beer, wine and sake. As esters are synthesized in yeast via several complex metabolic pathways, there is a need to gain a clear understanding of ester metabolism and its regulation. The individual genes involved, their functions and regulatory mechanisms have to be identified. In alcoholic beverages, there are two important groups of esters: the acetate esters and the medium‐chain fatty acid (MCFA) ethyl esters. For acetate ester synthesis, the genes involved have already been cloned and characterized. Also the biochemical pathways and the regulation of acetate ester synthesis are well defined. With respect to the molecular basis of MCFA ethyl ester synthesis, however, significant progress has only recently been made. Next to the characterization of the biochemical pathways and regulation of ester synthesis, a new and more important question arises: what is the advantage for yeast to produce these esters? Several hypotheses have been proposed in the past, but none was satisfactorily. This paper reviews the current hypotheses of ester synthesis in yeast in relation to the complex regulation of the alcohol acetyl transferases and the different factors that allow ester formation to be controlled during fermentation.

A circuit supporting concentration-invariant odor perception in Drosophila

BACKGROUND

Most odors are perceived to have the same quality over a large concentration range, but the neural mechanisms that permit concentration-invariant olfactory perception are unknown. In larvae of the vinegar fly Drosophila melanogaster, odors are sensed by an array of 25 odorant receptors expressed in 21 olfactory sensory neurons (OSNs). We investigated how subsets of larval OSNs with overlapping but distinct response properties cooperate to mediate perception of a given odorant across a range of concentrations.

RESULTS

Using calcium imaging, we found that ethyl butyrate, an ester perceived by humans as fruity, activated three OSNs with response thresholds that varied across three orders of magnitude. Whereas wild-type larvae were strongly attracted by this odor across a 500-fold range of concentration, individuals with only a single functional OSN showed attraction across a narrower concentration range corresponding to the sensitivity of each ethyl butyrate-tuned OSN. To clarify how the information carried by different OSNs is integrated by the olfactory system, we characterized the response properties of local inhibitory interneurons and projection neurons in the antennal lobe. Local interneurons only responded to high ethyl butyrate concentrations upon summed activation of at least two OSNs. Projection neurons showed a reduced response to odors when summed input from two OSNs impinged on the circuit compared to when there was only a single functional OSN.

CONCLUSIONS

Our results show that increasing odor concentrations induce progressive activation of concentration-tuned olfactory sensory neurons and concomitant recruitment of inhibitory local interneurons. We propose that the interplay of combinatorial OSN input and local interneuron activation allows animals to remain sensitive to odors across a large range of stimulus intensities.