A Functionally Conserved Gene Regulatory Network Module Governing Olfactory Neuron Diversity

BarH1 regulates the expression of conserved odorant-binding protein 22 from Dastarcus helophoroides

Chemical signals are pivotal in establishing tritrophic interactions among host plants, herbivorous insects, and natural enemies. Previous studies have shown that evolutionarily conserved MaltOBPs in Monochamus alternatus and DhelOBPs in Dastarcus helophoroides contribute to the establishment of pine -pest - natural enemy tritrophic interactions by recognizing the same volatile emitted by the host during crucial developmental stages. We hypothesized that the transcriptional regulatory mechanisms of evolutionarily conserved OBPs respectively from pests and enemies are similar. In this study, we identified the promoter region of DhelOBP22 through chromosome walking and discovered that transcription factor BarH1, which have been proved to regulate the expression of MaltOBP19 in M. alternatus, regulates the expression of DhelOBP22 by binding to its promoter region, as evidenced by dual-luciferase assays and electrophoretic mobility shift assay (EMSA). When the BarH1 gene was silenced using RNAi, the expression of DhelOBP22 was inhibited, leading to the disappearance of the attracted behavior of mated female D. helophoroides adults towards camphene. This study underscores that conserved transcriptional regulation mechanisms play a role in regulating the expression of evolutionarily conserved OBPs respectively from herbivores and natural enemies.

Using Chromatin Immunoprecipitation (ChIP) to Study the Chromatin State in Drosophila

The chromatin state plays an important role in regulating gene expression, which affects organismal development and plasticity. Proteins, including transcription factors, chromatin modulatory proteins, and histone proteins, usually with modifications, interact with gene loci involved in cellular differentiation, function, and modulation. One molecular method used to characterize protein-DNA interactions is chromatin immunoprecipitation (ChIP). ChIP uses antibodies to immunoprecipitate specific proteins cross-linked to DNA fragments. This approach, in combination with quantitative PCR (qPCR) or high-throughput DNA sequencing, can determine the enrichment of a certain protein or histone modification around specific gene loci or across the whole genome. ChIP has been used in Drosophila to characterize the binding pattern of transcription factors and to elucidate the roles of regulatory proteins in gene expression during development and in response to environment stimuli. This review outlines ChIP procedures using tissues from the Drosophila nervous system as an example and discusses all steps and the necessary optimization.

Olfactory receptor coexpression and co-option in the dengue mosquito

The olfactory sensory neurons of vinegar flies and mice tend to express a single ligand-specific receptor. While this 'one neuron-one receptor' motif has long been expected to apply broadly across insects, recent evidence suggests it may not extend to mosquitoes. We sequenced and analyzed the transcriptomes of 46,000 neurons from antennae of the dengue mosquito Aedes aegypti to resolve all olfactory, thermosensory, and hygrosensory neuron subtypes and identify the receptors expressed therein. We find that half of all olfactory subtypes coexpress multiple receptors. However, coexpression occurs almost exclusively among genes from the same family-among odorant receptors (ORs) or among ionotropic receptors (IRs). Coexpression of ORs with IRs is exceedingly rare. Many coexpressed receptors are recent duplicates. In other cases, the recruitment or co-option of single receptors by multiple neuron subtypes has placed these genes together in the same cells with distant paralogs. Close examination of data from Drosophila reveal rare cases of both phenomena, indicating that the olfactory systems of these two species are not fundamentally different, but instead fall at different locations along a continuum likely to encompass diverse insects.

Social experience and pheromone receptor activity reprogram gene expression in sensory neurons

Social experience and pheromone signaling in olfactory neurons affect neuronal responses and male courtship behaviors in Drosophila. We previously showed that social experience and pheromone signaling modulate chromatin around behavioral switch gene fruitless, which encodes a transcription factor necessary and sufficient for male sexual behaviors. Fruitless drives social experience-dependent modulation of courtship behaviors and physiological sensory neuron responses to pheromone; however, the molecular mechanisms underlying this modulation of neural responses remain less clear. To identify the molecular mechanisms driving social experience-dependent changes in neuronal responses, we performed RNA-seq from antennal samples of mutants in pheromone receptors and fruitless, as well as grouped or isolated wild-type males. Genes affecting neuronal physiology and function, such as neurotransmitter receptors, ion channels, ion and membrane transporters, and odorant binding proteins are differentially regulated by social context and pheromone signaling. While we found that loss of pheromone detection only has small effects on differential promoter and exon usage within fruitless gene, many of the differentially regulated genes have Fruitless-binding sites or are bound by Fruitless in the nervous system. Recent studies showed that social experience and juvenile hormone signaling co-regulate fruitless chromatin to modify pheromone responses in olfactory neurons. Interestingly, genes involved in juvenile hormone metabolism are also misregulated in different social contexts and mutant backgrounds. Our results suggest that modulation of neuronal activity and behaviors in response to social experience and pheromone signaling likely arise due to large-scale changes in transcriptional programs for neuronal function downstream of behavioral switch gene function.

The effect of Drosophila attP40 background on the glomerular organization of Or47b olfactory receptor neurons

Bacteriophage integrase-directed insertion of transgenic constructs into specific genomic loci has been widely used by Drosophila community. The attP40 landing site located on the second chromosome gained popularity because of its high inducible transgene expression levels. Here, unexpectedly, we found that homozygous attP40 chromosome disrupts normal glomerular organization of Or47b olfactory receptor neuron (ORN) class in Drosophila. This effect is not likely to be caused by the loss of function of Msp300, where the attP40 docking site is inserted. Moreover, the attP40 background seems to genetically interact with the second chromosome Or47b-GAL4 driver, which results in a similar glomerular defect. Whether the ORN phenotype is caused by the neighbouring genes around Msp300 locus in the presence of attP40-based insertions or a second unknown mutation in the attP40 background remains elusive. Our findings tell a cautionary tale about using this popular transgenic landing site, highlighting the importance of rigorous controls to rule out the attP40 landing site-associated background effects.

Drosophila olfaction: past, present and future

Among the many wonders of nature, the sense of smell of the fly Drosophila melanogaster might seem, at first glance, of esoteric interest. Nevertheless, for over a century, the 'nose' of this insect has been an extraordinary system to explore questions in animal behaviour, ecology and evolution, neuroscience, physiology and molecular genetics. The insights gained are relevant for our understanding of the sensory biology of vertebrates, including humans, and other insect species, encompassing those detrimental to human health. Here, I present an overview of our current knowledge of D. melanogaster olfaction, from molecules to behaviours, with an emphasis on the historical motivations of studies and illustration of how technical innovations have enabled advances. I also highlight some of the pressing and long-term questions.

Insights from analyses of low complexity regions with canonical methods for protein sequence comparison

Low complexity regions are fragments of protein sequences composed of only a few types of amino acids. These regions frequently occur in proteins and can play an important role in their functions. However, scientists are mainly focused on regions characterized by high diversity of amino acid composition. Similarity between regions of protein sequences frequently reflect functional similarity between them. In this article, we discuss strengths and weaknesses of the similarity analysis of low complexity regions using BLAST, HHblits and CD-HIT. These methods are considered to be the gold standard in protein similarity analysis and were designed for comparison of high complexity regions. However, we lack specialized methods that could be used to compare the similarity of low complexity regions. Therefore, we investigated the existing methods in order to understand how they can be applied to compare such regions. Our results are supported by exploratory study, discussion of amino acid composition and biological roles of selected examples. We show that existing methods need improvements to efficiently search for similar low complexity regions. We suggest features that have to be re-designed specifically for comparing low complexity regions: scoring matrix, multiple sequence alignment, e-value, local alignment and clustering based on a set of representative sequences. Results of this analysis can either be used to improve existing methods or to create new methods for the similarity analysis of low complexity regions.

Non-canonical odor coding in the mosquito

Aedes aegypti mosquitoes are a persistent human foe, transmitting arboviruses including dengue when they feed on human blood. Mosquitoes are intensely attracted to body odor and carbon dioxide, which they detect using ionotropic chemosensory receptors encoded by three large multi-gene families. Genetic mutations that disrupt the olfactory system have modest effects on human attraction, suggesting redundancy in odor coding. The canonical view is that olfactory sensory neurons each express a single chemosensory receptor that defines its ligand selectivity. We discovered that Ae. aegypti uses a different organizational principle, with many neurons co-expressing multiple chemosensory receptor genes. In vivo electrophysiology demonstrates that the broad ligand-sensitivity of mosquito olfactory neurons depends on this non-canonical co-expression. The redundancy afforded by an olfactory system in which neurons co-express multiple chemosensory receptors may increase the robustness of the mosquito olfactory system and explain our long-standing inability to disrupt the detection of humans by mosquitoes.

Chemoreceptor co-expression in Drosophila melanogaster olfactory neurons

Drosophila melanogaster olfactory neurons have long been thought to express only one chemosensory receptor gene family. There are two main olfactory receptor gene families in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). The dozens of odorant-binding receptors in each family require at least one co-receptor gene in order to function: Orco for ORs, and Ir25a, Ir8a, and Ir76b for IRs. Using a new genetic knock-in strategy, we targeted the four co-receptors representing the main chemosensory families in D. melanogaster (Orco, Ir8a, Ir76b, Ir25a). Co-receptor knock-in expression patterns were verified as accurate representations of endogenous expression. We find extensive overlap in expression among the different co-receptors. As defined by innervation into antennal lobe glomeruli, Ir25a is broadly expressed in 88% of all olfactory sensory neuron classes and is co-expressed in 82% of Orco+ neuron classes, including all neuron classes in the maxillary palp. Orco, Ir8a, and Ir76b expression patterns are also more expansive than previously assumed. Single sensillum recordings from Orco-expressing Ir25a mutant antennal and palpal neurons identify changes in olfactory responses. We also find co-expression of Orco and Ir25a in Drosophila sechellia and Anopheles coluzzii olfactory neurons. These results suggest that co-expression of chemosensory receptors is common in insect olfactory neurons. Together, our data present the first comprehensive map of chemosensory co-receptor expression and reveal their unexpected widespread co-expression in the fly olfactory system.

BarH1 regulates odorant-binding proteins expression and olfactory perception of Monochamus alternatus Hope

Insect odorant-binding proteins (OBPs) are a class of small soluble proteins that can be found in various tissues wherein binding and transport of small molecules are required. Thus, OBPs are not only involved in typical olfactory function by specific activities with odorants but also participate in other physiological processes in non-chemosensory tissues. To better understand the complex biological functions of OBPs, it is necessary to study the transcriptional regulation of their expression patterns. In this paper, an apparent gradient expression pattern of Obp19, that was highly and specifically expressed in antennae and played an essential role in the detection of camphene, was defined in the antennae of the Japanese pine sawyer. Further, the transcription factor BarH1, that also presented gradient expression pattern in antennae, was found to regulate expression of Obp19 directly through binding to its upstream DNA sequence. The condition of BarH1 gene silence, the gene expression levels of Obp19 significantly decreased. At the same time, additional olfactory genes also were regulated and thus influence camphene reception. These findings provide us an opportunity to incorporate Obps in the gene regulatory networks of insects, which contribute to a better understanding of the multiplicity and diversity of OBPs and the olfactory mediated behaviors.

An ammonium transporter is a non-canonical olfactory receptor for ammonia

Numerous hematophagous insects are attracted to ammonia, a volatile released in human sweat and breath.1-3 Low levels of ammonia also attract non-biting insects such as the genetic model organism Drosophila melanogaster and several species of agricultural pests.4,5 Two families of ligand-gated ion channels function as olfactory receptors in insects,6-10 and studies have linked ammonia sensitivity to a particular olfactory receptor in Drosophila.5,11,12 Given the widespread importance of ammonia to insect behavior, it is surprising that the genomes of most insects lack an ortholog of this gene.6 Here, we show that canonical olfactory receptors are not necessary for responses to ammonia in Drosophila. Instead, we demonstrate that a member of the ancient electrogenic ammonium transporter family, Amt, is likely a new type of olfactory receptor. We report two hitherto unidentified olfactory neuron populations that mediate neuronal and behavioral responses to ammonia in Drosophila. Their endogenous ammonia responses are lost in Amt mutant flies, and ectopic expression of either Drosophila or Anopheles Amt confers ammonia sensitivity. These results suggest that Amt is the first transporter known to function as an olfactory receptor in animals and that its function may be conserved across insect species.

Information flow, cell types and stereotypy in a full olfactory connectome

The hemibrain connectome provides large-scale connectivity and morphology information for the majority of the central brain of Drosophila melanogaster. Using this data set, we provide a complete description of the Drosophila olfactory system, covering all first, second and lateral horn-associated third-order neurons. We develop a generally applicable strategy to extract information flow and layered organisation from connectome graphs, mapping olfactory input to descending interneurons. This identifies a range of motifs including highly lateralised circuits in the antennal lobe and patterns of convergence downstream of the mushroom body and lateral horn. Leveraging a second data set we provide a first quantitative assessment of inter- versus intra-individual stereotypy. Comparing neurons across two brains (three hemispheres) reveals striking similarity in neuronal morphology across brains. Connectivity correlates with morphology and neurons of the same morphological type show similar connection variability within the same brain as across two brains.

bric à brac controls sex pheromone choice by male European corn borer moths

The sex pheromone system of ~160,000 moth species acts as a powerful form of assortative mating whereby females attract conspecific males with a species-specific blend of volatile compounds. Understanding how female pheromone production and male preference coevolve to produce this diversity requires knowledge of the genes underlying change in both traits. In the European corn borer moth, pheromone blend variation is controlled by two alleles of an autosomal fatty-acyl reductase gene expressed in the female pheromone gland (pgFAR). Here we show that asymmetric male preference is controlled by cis-acting variation in a sex-linked transcription factor expressed in the developing male antenna, bric à brac (bab). A genome-wide association study of preference using pheromone-trapped males implicates variation in the 293 kb bab intron 1, rather than the coding sequence. Linkage disequilibrium between bab intron 1 and pgFAR further validates bab as the preference locus, and demonstrates that the two genes interact to contribute to assortative mating. Thus, lack of physical linkage is not a constraint for coevolutionary divergence of female pheromone production and male behavioral response genes, in contrast to what is often predicted by evolutionary theory.

Single-cell transcriptomes of developing and adult olfactory receptor neurons in Drosophila

Recognition of environmental cues is essential for the survival of all organisms. Transcriptional changes occur to enable the generation and function of the neural circuits underlying sensory perception. To gain insight into these changes, we generated single-cell transcriptomes of Drosophila olfactory- (ORNs), thermo-, and hygro-sensory neurons at an early developmental and adult stage using single-cell and single-nucleus RNA sequencing. We discovered that ORNs maintain expression of the same olfactory receptors across development. Using receptor expression and computational approaches, we matched transcriptomic clusters corresponding to anatomically and physiologically defined neuron types across multiple developmental stages. We found that cell-type-specific transcriptomes partly reflected axon trajectory choices in development and sensory modality in adults. We uncovered stage-specific genes that could regulate the wiring and sensory responses of distinct ORN types. Collectively, our data reveal transcriptomic features of sensory neuron biology and provide a resource for future studies of their development and physiology.

Single cell RNA sequencing identifies early diversity of sensory neurons forming via bi-potential intermediates

Somatic sensation is defined by the existence of a diversity of primary sensory neurons with unique biological features and response profiles to external and internal stimuli. However, there is no coherent picture about how this diversity of cell states is transcriptionally generated. Here, we use deep single cell analysis to resolve fate splits and molecular biasing processes during sensory neurogenesis in mice. Our results identify a complex series of successive and specific transcriptional changes in post-mitotic neurons that delineate hierarchical regulatory states leading to the generation of the main sensory neuron classes. In addition, our analysis identifies previously undetected early gene modules expressed long before fate determination although being clearly associated with defined sensory subtypes. Overall, the early diversity of sensory neurons is generated through successive bi-potential intermediates in which synchronization of relevant gene modules and concurrent repression of competing fate programs precede cell fate stabilization and final commitment.

Chromatin-based reprogramming of a courtship regulator by concurrent pheromone perception and hormone signaling

To increase fitness, animals use both internal and external states to coordinate reproductive behaviors. The molecular mechanisms underlying this coordination remain unknown. Here, we focused on pheromone-sensing Drosophila Or47b neurons, which exhibit age- and social experience-dependent increase in pheromone responses and courtship advantage in males. FruitlessM (FruM), a master regulator of male courtship behaviors, drives the effects of social experience and age on Or47b neuron responses and function. We show that simultaneous exposure to social experience and age-specific juvenile hormone (JH) induces chromatin-based reprogramming of fruM expression in Or47b neurons. Group housing and JH signaling increase fruM expression in Or47b neurons and active chromatin marks at fruM promoter. Conversely, social isolation or loss of JH signaling decreases fruM expression and increases repressive marks around fruM promoter. Our results suggest that fruM promoter integrates coincident hormone and pheromone signals driving chromatin-based changes in expression and ultimately neuronal and behavioral plasticity.

Evolution, developmental expression and function of odorant receptors in insects

Animals rely on their chemosensory system to discriminate among a very large number of attractive or repulsive chemical cues in the environment, which is essential to respond with proper action. The olfactory sensory systems in insects share significant similarities with those of vertebrates, although they also exhibit dramatic differences, such as the molecular nature of the odorant receptors (ORs): insect ORs function as heteromeric ion channels with a common Orco subunit, unlike the G-protein-coupled olfactory receptors found in vertebrates. Remarkable progress has recently been made in understanding the evolution, development and function of insect odorant receptor neurons (ORNs). These studies have uncovered the diversity of olfactory sensory systems among insect species, including in eusocial insects that rely extensively on olfactory sensing of pheromones for social communication. However, further studies, notably functional analyses, are needed to improve our understanding of the origins of the Orco-OR system, the mechanisms of ORN fate determination, and the extraordinary diversity of behavioral responses to chemical cues.

Sensory neuron lineage mapping and manipulation in the Drosophila olfactory system

Nervous systems exhibit myriad cell types, but understanding how this diversity arises is hampered by the difficulty to visualize and genetically-probe specific lineages, especially at early developmental stages prior to expression of unique molecular markers. Here, we use a genetic immortalization method to analyze the development of sensory neuron lineages in the Drosophila olfactory system, from their origin to terminal differentiation. We apply this approach to define a fate map of nearly all olfactory lineages and refine the model of temporal patterns of lineage divisions. Taking advantage of a selective marker for the lineage that gives rise to Or67d pheromone-sensing neurons and a genome-wide transcription factor RNAi screen, we identify the spatial and temporal requirements for Pointed, an ETS family member, in this developmental pathway. Transcriptomic analysis of wild-type and Pointed-depleted olfactory tissue reveals a universal requirement for this factor as a switch-like determinant of fates in these sensory lineages.

Few tools exist to study molecular diversity during neurodevelopment. Here the authors apply a genetic immortalization method in Drosophila to generate a fate map of olfactory sensory lineages, examine the relationships of this map and the neuroanatomical, molecular and evolutionary properties of the mature circuits, and identify a novel factor controlling lineage development.

Strength in diversity: functional diversity among olfactory neurons of the same type

Most animals depend upon olfaction to find food, mates, and to avoid predators. An animal's olfactory circuit helps it sense its olfactory environment and generate critical behavioral responses. The general architecture of the olfactory circuit, which is conserved across species, is made up of a few different neuronal types including first-order receptor neurons, second- and third-order neurons, and local interneurons. Each neuronal type differs in their morphology, physiology, and neurochemistry. However, several recent studies have suggested that there is intrinsic diversity even among neurons of the same type and that this diversity is important for neural function. In this review, we first examine instances of intrinsic diversity observed among individual types of olfactory neurons. Next, we review potential genetic and experience-based plasticity mechanisms that underlie this diversity. Finally, we consider the implications of intrinsic neuronal diversity for circuit function. Overall, we hope to highlight the importance of intrinsic diversity as a previously underestimated property of circuit function.

The Two Main Olfactory Receptor Families in Drosophila, ORs and IRs: A Comparative Approach

Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.

Combinations of DIPs and Dprs control organization of olfactory receptor neuron terminals in Drosophila

In Drosophila, 50 classes of olfactory receptor neurons (ORNs) connect to 50 class-specific and uniquely positioned glomeruli in the antennal lobe. Despite the identification of cell surface receptors regulating axon guidance, how ORN axons sort to form 50 stereotypical glomeruli remains unclear. Here we show that the heterophilic cell adhesion proteins, DIPs and Dprs, are expressed in ORNs during glomerular formation. Many ORN classes express a unique combination of DIPs/dprs, with neurons of the same class expressing interacting partners, suggesting a role in class-specific self-adhesion between ORN axons. Analysis of DIP/Dpr expression revealed that ORNs that target neighboring glomeruli have different combinations, and ORNs with very similar DIP/Dpr combinations can project to distant glomeruli in the antennal lobe. DIP/Dpr profiles are dynamic during development and correlate with sensilla type lineage for some ORN classes. Perturbations of DIP/dpr gene function result in local projection defects of ORN axons and glomerular positioning, without altering correct matching of ORNs with their target neurons. Our results suggest that context-dependent differential adhesion through DIP/Dpr combinations regulate self-adhesion and sort ORN axons into uniquely positioned glomeruli.

In the human brain there are over 80 billion neurons that form approximately 100 trillion specific connections. How the brain organizes the axon terminals of these neurons into distinct synaptic units on such a large scale is largely unknown. In Drosophila, 50 classes of olfactory receptor neurons (ORNs) connect to 50 class-specific and uniquely positioned glomeruli in the antennal lobe, providing a complex yet workable model to understand the organization of glomerular structures and morphology. Here we show that the heterophilic cell adhesion proteins, DIPs and Dprs, are expressed in ORNs during glomerular formation. Many ORN classes express a unique combination of DIPs/dprs, with neurons of the same class expressing interacting partners, suggesting a role in class-specific self-adhesion between ORN axons. Analysis of DIP/Dpr expression revealed that ORNs that target neighboring glomeruli have different combinations, and ORNs with very similar DIP/Dpr combinations can project to distant glomeruli in the antennal lobe. Perturbations of DIP/dpr gene function result in local projection defects of ORN axons and glomerular positioning, without altering correct matching of ORNs with their target neurons. Our results suggest that context-dependent differential adhesion through DIP/Dpr combinations regulate self-adhesion and sort ORN axons into uniquely positioned glomeruli.

Preparing Developing Peripheral Olfactory Tissue for Molecular and Immunohistochemical Analysis in Drosophila

The olfactory system of Drosophila is a widely used system in developmental neurobiology, systems neuroscience, as well as neurophysiology, behavior, and behavioral evolution. Drosophila olfactory tissues house the olfactory receptor neurons (ORNs) that detect volatile chemical cues in addition to hydro- and thermo-sensory neurons. In this protocol, we describe the dissection of developing peripheral olfactory tissue of the adult Drosophila species. We first describe how to stage and age Drosophila larvae, followed by the dissection of the antennal disc from early pupal stages, followed by the dissection of the antennae from mid-pupal stages and adults. We also show methods where preparations can be utilized in molecular techniques, such as the RNA extraction for qRT-PCR, RNAseq, or immunohistochemistry. These methods can also be applied to other Drosophila species after species-specific pupal development times are determined, and respective stages are calculated for appropriate aging.

Patterns of transcriptional parallelism and variation in the developing olfactory system of Drosophila species

Organisms have evolved strikingly parallel phenotypes in response to similar selection pressures suggesting that there may be shared constraints limiting the possible evolutionary trajectories. For example, the behavioral adaptation of specialist Drosophila species to specific host plants can exhibit parallel changes in their adult olfactory neuroanatomy. We investigated the genetic basis of these parallel changes by comparing gene expression during the development of the olfactory system of two specialist Drosophila species to that of four other generalist species. Our results suggest that the parallelism observed in the adult olfactory neuroanatomy of ecological specialists extends more broadly to their developmental antennal expression profiles, and to the transcription factor combinations specifying olfactory receptor neuron (ORN) fates. Additionally, comparing general patterns of variation for the antennal transcriptional profiles in the adult and developing olfactory system of the six species suggest the possibility that specific, non-random components of the developmental programs underlying the Drosophila olfactory system harbor a disproportionate amount of interspecies variation. Further examination of these developmental components may be able to inform a deeper understanding of how traits evolve.

Comparative analysis of behavioral and transcriptional variation underlying CO2 sensory neuron function and development in Drosophila

Carbon dioxide is an important environmental cue for many insects, regulating many behaviors including some that have direct human impacts. To further improve our understanding of how this system varies among closely related insect species, we examined both the behavioral response to CO₂ as well as the transcriptional profile of key developmental regulators of CO₂ sensory neurons in the olfactory system across the Drosophila genus. We found that CO₂ generally evokes repulsive behavior across most of the Drosophilids we examined, but this behavior has been lost or reduced in several lineages. Comparisons of transcriptional profiles from the developing and adult antennae for subset these species suggest that behavioral differences in some species may be due to differences in the expression of the CO₂ co-receptor Gr63a. Furthermore, these differences in Gr63a expression are correlated with changes in the expression of a few genes known to be involved in the development of the CO₂ circuit, namely dac, an important regulator of sensilla fate for sensilla that house CO₂ ORNs, and mip120, a member of the MMB/dREAM epigenetic regulatory complex that regulates CO₂ receptor expression. In contrast, most of the other known structural, molecular, and developmental components of the peripheral Drosophila CO₂ olfactory system seem to be well-conserved across all examined lineages. These findings suggest that certain components of CO₂ sensory ORN development may be more evolutionarily labile, and may contribute to differences in CO₂-evoked behavioral responses across species.

Mechanisms controlling diversification of olfactory sensory neuron classes

Animals survive in harsh and fluctuating environments using sensory neurons to detect and respond to changes in their surroundings. Olfactory sensory neurons are essential for detecting food, identifying danger, and sensing pheromones. The ability to sense a large repertoire of different types of odors is crucial to distinguish between different situations, and is achieved through neuronal diversity within the olfactory system. Here, we review the developmental mechanisms used to establish diversity of olfactory sensory neurons in various model organisms, including Caenorhabditis elegans, Drosophila, and vertebrate models. Understanding and comparing how different olfactory neurons develop within the nervous system of different animals can provide insight into how the olfactory system is shaped in humans.

Transcriptional profiling of olfactory system development identifies distal antenna as a regulator of subset of neuronal fates

Drosophila uses 50 different olfactory receptor neuron (ORN) classes that are clustered within distinct sensilla subtypes to decipher their chemical environment. Each sensilla subtype houses 1-4 ORN identities that arise through asymmetric divisions of a single sensory organ precursor (SOP). Despite a number of mutational studies investigating the regulation of ORN development, a majority of the transcriptional programs that lead to the different ORN classes in the developing olfactory system are unknown. Here we use transcriptional profiling across the time series of antennal development to identify novel transcriptional programs governing the differentiation of ORNs. We surveyed four critical developmental stages of the olfactory system: 3rd instar larval (prepatterning), 8 hours after puparium formation (APF, SOP selection), 40 hrs APF (neurogenesis), and adult antennae. We focused on the expression profiles of olfactory receptor genes and transcription factors-the two main classes of genes that regulate the sensory identity of ORNs. We identify distinct clusters of genes that have overlapping temporal expression profiles suggesting they have a key role during olfactory system development. We show that the expression of the transcription factor distal antenna (dan) is highly similar to other prepatterning factors and is required for the expression of a subset of ORs.

Age influences the olfactory profiles of the migratory oriental armyworm mythimna separate at the molecular level

Background

The oriental armyworm Mythimna separata (Walk) is a serious migratory pest; however, studies on its olfactory response and its underlying molecular mechanism are limited. To gain insights to the olfactory mechanism of migration, olfactory genes were identified using antennal transcriptome analysis. The olfactory response and the expression of olfactory genes for 1-day and 5-day-old moths were respectively investigated by EAG and RT-qPCR analyses.

Results

Putative 126 olfactory genes were identified in M. separata, which included 43 ORs, 13 GRs, 16 IRs, 37 OBPs, 14 CSPs, and 3 SNMPs. RPKM values of IR75d and 10 ORs were larger than co-receptors IR25a and ORco, and the RPKM value of PR2 was larger than that of other ORs. Expression of GR1 (sweet receptor) was higher than that of other GRs. Several sex pheromones activated evident EAG responses where the responses of 5-day-old male moths to the sex pheromones were significantly greater than those of female and 1-day old male moths. In accordance with the EAG response, 11 pheromone genes, including 6 PRs and 5 PBPs were identified in M. separate, and the expression levels of 7 pheromone genes in 5-day-old moths were significantly higher than those of females and 1-day-old moths. PR2 and PBP2 might be used in identifying Z11-16: Ald, which is the main sex pheromone component of M. separata. EAG responses to 16 plant volatiles and the expression levels of 43 olfactory genes in 1-day-old moths were significantly greater than that observed in the 5-day-old moths. Heptanal, Z6-nonenal, and benzaldehyde might be very important floral volatiles for host searching and recognized by several olfactory genes with high expression. Some plant volatiles might be important to male moths because the EAG response to 16 plant volatiles and the expression of 43 olfactory genes were significantly larger in males than in females.

Conclusions

The findings of the present study show the effect of adult age on olfactory responses and expression profile of olfactory genes in the migratory pest M. separate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3427-2) contains supplementary material, which is available to authorized users.

Genetic mapping of male pheromone response in the European corn borer identifies candidate genes regulating neurogenesis

The sexual pheromone communication system of moths is a model system for studies of the evolution of reproductive isolation. Females emit a blend of volatile components that males detect at a distance. Species differences in female pheromone composition and male response directly reinforce reproductive isolation in nature, because even slight variations in the species-specific pheromone blend are usually rejected by the male. The mechanisms by which a new pheromone signal-response system could evolve are enigmatic, because any deviation from the optimally attractive blend should be selected against. Here we investigate the genetic mechanisms enabling a switch in male response. We used a quantitative trait locus-mapping approach to identify the genetic basis of male response in the two pheromone races of the European corn borer, Ostrinia nubilalis Male response to a 99:1 vs. a 3:97 ratio of the E and Z isomers of the female pheromone is governed by a single, sex-linked locus. We found that the chromosomal region most tightly linked to this locus contains genes involved in neurogenesis but, in accordance with an earlier study, does not contain the odorant receptors expressed in the male antenna that detect the pheromone. This finding implies that differences in the development of neuronal pathways conveying information from the antenna, not differences in pheromone detection by the odorant receptors, are primarily responsible for the behavioral response differences among the males in this system. Comparison with other moth species reveals a previously unexplored mechanism by which male pheromone response can change in evolution.

Chromatin Modulatory Proteins and Olfactory Receptor Signaling in the Refinement and Maintenance of Fruitless Expression in Olfactory Receptor Neurons

During development, sensory neurons must choose identities that allow them to detect specific signals and connect with appropriate target neurons. Ultimately, these sensory neurons will successfully integrate into appropriate neural circuits to generate defined motor outputs, or behavior. This integration requires a developmental coordination between the identity of the neuron and the identity of the circuit. The mechanisms that underlie this coordination are currently unknown. Here, we describe two modes of regulation that coordinate the sensory identities of Drosophila melanogaster olfactory receptor neurons (ORNs) involved in sex-specific behaviors with the sex-specific behavioral circuit identity marker fruitless (fru). The first mode involves a developmental program that coordinately restricts to appropriate ORNs the expression of fru and two olfactory receptors (Or47b and Ir84a) involved in sex-specific behaviors. This regulation requires the chromatin modulatory protein Alhambra (Alh). The second mode relies on the signaling from the olfactory receptors through CamK and histone acetyl transferase p300/CBP to maintain ORN-specific fru expression. Our results highlight two feed-forward regulatory mechanisms with both developmentally hardwired and olfactory receptor activity-dependent components that establish and maintain fru expression in ORNs. Such a dual mechanism of fru regulation in ORNs might be a trait of neurons driving plastic aspects of sex-specific behaviors.