Chemotaxis behavior mediated by single larval olfactory neurons in Drosophila

Nucleotide and copy-number polymorphism at the odorant receptor genes Or22a and Or22b in Drosophila melanogaster

In Drosophila, odorant receptors are encoded by an old and moderately sized multigene family. Or22a and Or22b are two tandemly arranged genes of this family that have proved to be the result of a rather young duplication. Nucleotide variation in the region spanning both duplicates was surveyed in four natural populations (two African and two non-African) of Drosophila melanogaster and also analyzed in species of the melanogaster subgroup. The intraspecific survey revealed a particular copy-number polymorphism in some of the studied populations, with the two genes (Or22a and Or22b) present in the long variant and a single chimeric gene (Or22ab) present in the short variant. Estimated nucleotide diversity was higher in the short than in the long variant, despite the ancestral character of the latter variant in D. melanogaster. The general skew toward low-frequency variants detected in the non-African long variant and its reduced level of silent polymorphism relative to divergence is consistent with the recent fixation of an advantageous mutation at, or nearby, the Or22 long variant region. The nonnegligible frequency of the short variant and the presence of a highly divergent haplotype in the East African sample would point to direct or indirect selection for its maintenance in the species. There was evidence for a generally more rapid evolution of the Or22b copy at both synonymous and nonsynonymous sites. However, an excess of nonsynonymous substitutions was only detected in the early history of this copy.

First Contact to Odors: Our Current Knowledge about Odorant Receptor

Chemical senses – especially smell – are known to be important for the fundamental life events such as sensing predators, selecting mates, as well as finding food. The chemical senses are decoded in the olfactory system which is able to detect and differentiate thousands of odorous substances comprised of chemically divergent structures (i.e. odorants). The high selectivity of the olfactory system is heavily dependent on the receptors for each odorants (i.e. odorant receptors). Thus, studying odorant receptors may not only facilitate our understanding the initial events of olfaction but provide crucial knowledge for developing a novel, odorant receptor-based biosensor for chemical screening. Here we provide a review of recent advances in our understanding of odorant receptors.

The survival advantage of olfaction in a competitive environment

Olfaction is generally assumed to be critical for survival because this sense allows animals to detect food and pheromonal cues. Although the ability to sense sex pheromones [1, 2, 3] is likely to be important for insects, the contribution of general odor detection to survival is unknown. We investigated the extent to which the olfactory system confers a survival advantage on Drosophila larvae foraging for food under conditions of limited resources and competition from other larvae.

Comparing peripheral olfactory coding with host preference in the rhagoletis species complex

Recent studies have shown that flies from sympatric populations of Rhagoletis pomonella infesting hawthorn, apple, and flowering dogwood fruit can distinguish among unique volatile blends identified from each host. Analysis of peripheral chemoreception in Rhagoletis flies suggests that changes in receptor specificity and/or receptor neuron sensitivity could impact olfactory preference among the host populations and their hybrids. In an attempt to validate these claims, we have combined flight tunnel analyses and single sensillum electrophysiology in F(2) and backcross hybrids displaying a variety of behavioral phenotypes. Results show that differences in peripheral chemoreception among second-generation adults do not provide a direct correlation between peripheral coding and olfactory behavior. We conclude that either the plasticity of the central nervous system in Rhagoletis can compensate for significant alterations in peripheral coding or that peripheral changes present subtle effects on behavior not easily detectable with current techniques. The results of this study imply that the basis for olfactory behavior in Rhagoletis has a complicated genetic and neuronal basis, even for populations with a recent divergence in preference.

Translation of sensory input into behavioral output via an olfactory system

We investigate the logic by which sensory input is translated into behavioral output. First we provide a functional analysis of the entire odor receptor repertoire of an olfactory system. We construct tuning curves for the 21 functional odor receptors of the Drosophila larva and show that they sharpen at lower odor doses. We construct a 21-dimensional odor space from the responses of the receptors and find that the distance between two odors correlates with the extent to which one odor masks the other. Mutational analysis shows that different receptors mediate the responses to different concentrations of an odorant. The summed response of the entire receptor repertoire correlates with the strength of the behavioral response. The activity of a small number of receptors is a surprisingly powerful predictor of behavior. Odors that inhibit more receptors are more likely to be repellents. Odor space is largely conserved between two dissimilar olfactory systems.

Hedonic taste in Drosophila revealed by olfactory receptors expressed in taste neurons

Taste and olfaction are each tuned to a unique set of chemicals in the outside world, and their corresponding sensory spaces are mapped in different areas in the brain. This dichotomy matches categories of receptors detecting molecules either in the gaseous or in the liquid phase in terrestrial animals. However, in Drosophila olfactory and gustatory neurons express receptors which belong to the same family of 7-transmembrane domain proteins. Striking overlaps exist in their sequence structure and in their expression pattern, suggesting that there might be some functional commonalities between them. In this work, we tested the assumption that Drosophila olfactory receptor proteins are compatible with taste neurons by ectopically expressing an olfactory receptor (OR22a and OR83b) for which ligands are known. Using electrophysiological recordings, we show that the transformed taste neurons are excited by odor ligands as by their cognate tastants. The wiring of these neurons to the brain seems unchanged and no additional connections to the antennal lobe were detected. The odor ligands detected by the olfactory receptor acquire a new hedonic value, inducing appetitive or aversive behaviors depending on the categories of taste neurons in which they are expressed i.e. sugar- or bitter-sensing cells expressing either Gr5a or Gr66a receptors. Taste neurons expressing ectopic olfactory receptors can sense odors at close range either in the aerial phase or by contact, in a lipophilic phase. The responses of the transformed taste neurons to the odorant are similar to those obtained with tastants. The hedonic value attributed to tastants is directly linked to the taste neurons in which their receptors are expressed.

Gene discovery in an invasive tephritid model pest species, the Mediterranean fruit fly, Ceratitis capitata

Background

The medfly, Ceratitis capitata, is a highly invasive agricultural pest that has become a model insect for the development of biological control programs. Despite research into the behavior and classical and population genetics of this organism, the quantity of sequence data available is limited. We have utilized an expressed sequence tag (EST) approach to obtain detailed information on transcriptome signatures that relate to a variety of physiological systems in the medfly; this information emphasizes on reproduction, sex determination, and chemosensory perception, since the study was based on normalized cDNA libraries from embryos and adult heads.

Results

A total of 21,253 high-quality ESTs were obtained from the embryo and head libraries. Clustering analyses performed separately for each library resulted in 5201 embryo and 6684 head transcripts. Considering an estimated 19% overlap in the transcriptomes of the two libraries, they represent about 9614 unique transcripts involved in a wide range of biological processes and molecular functions. Of particular interest are the sequences that share homology with Drosophila genes involved in sex determination, olfaction, and reproductive behavior. The medfly transformer2 (tra2) homolog was identified among the embryonic sequences, and its genomic organization and expression were characterized.

Conclusion

The sequences obtained in this study represent the first major dataset of expressed genes in a tephritid species of agricultural importance. This resource provides essential information to support the investigation of numerous questions regarding the biology of the medfly and other related species and also constitutes an invaluable tool for the annotation of complete genome sequences. Our study has revealed intriguing findings regarding the transcript regulation of tra2 and other sex determination genes, as well as insights into the comparative genomics of genes implicated in chemosensory reception and reproduction.

The molecular and cellular basis of olfactory-driven behavior in Anopheles gambiae larvae

The mosquito Anopheles gambiae is the principal Afrotropical vector for human malaria. A central component of its vectorial capacity is the ability to maintain sufficient populations of adults. During both adult and preadult (larval) stages, the mosquitoes depend on the ability to recognize and respond to chemical cues that mediate feeding and survival. In this study, we used a behavioral assay to identify a range of odorant-specific responses of An. gambiae larvae that are dependent on the integrity of the larval antennae. Parallel molecular analyses have identified a subset of the An. gambiae odorant receptors (AgOrs) that are localized to discrete neurons within the larval antennae and facilitate odor-evoked responses in Xenopus oocytes that are consistent with the larval behavioral spectrum. These studies shed light on chemosensory-driven behaviors and represent molecular and cellular characterization of olfactory processes in mosquito larvae. These advances may ultimately enhance the development of vector control strategies, targeting olfactory pathways in larval-stage mosquitoes to reduce the catastrophic effects of malaria and other diseases.

The red flour beetle's large nose: an expanded odorant receptor gene family in Tribolium castaneum

The Tribolium castaneum genome sequence reveals a large number of odorant receptor (Or) genes compared to those found in other insects whose olfactory genomes have been studied-341 Or genes and pseudogenes, encoding 259 intact odorant receptor proteins. An RT-PCR study of larvae and adults revealed that only 145 (64%) of 233 genes with successful genomic DNA amplifications were expressed. No expression of the other 87 genes was detected at any age, suggesting either that these genes are not expressed in this particular strain, or that they are induced only in certain environmental or developmental conditions. TcOR1, the ortholog of the Drosophila Or83b (DmOr83b) gene, which is required for the function of olfactory receptor proteins in Drosophila, was expressed in extracts from adult and larval heads and in extracts from adult bodies. Expression of 41 TcOr genes was detected in extracts from larval head tissue and 111 in extracts from adult head tissue (both figures exclude TcOr1). Twenty-eight TcOrs were detected only in adult bodies. Beetle pupae were injected with TcOr1 dsRNA; unlike sham-injected and control beetles, these knock-down beetles showed no significant response to the Tribolium aggregation pheromone, supporting the hypothesis that TcOr1 plays a similar decisive role in olfaction to DmOr83b. The substantial number of Ors poses the question of why Tribolium has such a large olfactory receptor repertoire, and underlines the need for more studies of the natural history of this species.

Crossmodal visual input for odor tracking during fly flight

Flies generate robust and high-performance olfactory and visual behaviors. Adult fruit flies can distinguish small differences in odor concentration across antennae separated by less than 1 mm [1], and a single olfactory sensory neuron is sufficient for near-normal gradient tracking in larvae [2]. During flight a male housefly chasing a female executes a corrective turn within 40 ms after a course deviation by its target [3]. The challenges imposed by flying apparently benefit from the tight integration of unimodal sensory cues. Crossmodal interactions reduce the discrimination threshold for unimodal memory retrieval by enhancing stimulus salience [4], and dynamic crossmodal processing is required for odor search during free flight because animals fail to locate an odor source in the absence of rich visual feedback [5]. The visual requirements for odor localization are unknown. We tethered a hungry fly in a magnetic field, allowing it to yaw freely, presented odor plumes, and examined how visual cues influence odor tracking. We show that flies are unable to use a small-field object or landmark to assist plume tracking, whereas odor activates wide-field optomotor course control to enable accurate orientation toward an attractive food odor.

High-resolution measurement of odor-driven behavior in Drosophila larvae

Olfactory responses in Drosophila larvae have been traditionally studied in Petri dishes comprising a single peripheral odor source. In this behavioral paradigm, the experimenter usually assumes that the rapid diffusion of odorant molecules from the source leads to the creation of a stable gradient in the dish. To establish a quantitative correlation between sensory inputs and behavioral responses, it is necessary to achieve a more thorough characterization of the odorant stimulus conditions. In this video article, we describe a new method allowing the construction of odorant gradients with stable and controllable geometries. We briefly illustrate how these gradients can be used to screen for olfactory defects (full and partial anosmia) and to study more subtle features of chemotaxis behavior.

Insect olfaction: receptors, signal transduction, and behavior

The insect olfactory system is a suitable model for exploring molecular function of odorant receptors, axonal projection of olfactory receptor neurons onto secondary neurons, and the neural circuit for odor perception. Recent progress in the study of insect olfaction revealed that the heteromeric insect olfactory receptor complex forms a cation nonselective ion channel directly gated by odor or pheromone ligands independent of known G-protein signaling pathways. Despite fundamental differences in transduction machineries between insects and vertebrates, the anatomical and functional features of insect odor-coding strategy are similar and thus justify any consideration of mammalian olfaction in the study of insects. The understanding of the molecular mechanism of insect olfaction will help in the development of insect repellents for controlling insect pest and vector populations for a wide range of pathogens.

Bilateral olfactory sensory input enhances chemotaxis behavior

Neural comparisons of bilateral sensory inputs are essential for visual depth perception and accurate localization of sounds in space. All animals, from single-cell prokaryotes to humans, orient themselves in response to environmental chemical stimuli, but the contribution of spatial integration of neural activity in olfaction remains unclear. We investigated this problem in Drosophila melanogaster larvae. Using high-resolution behavioral analysis, we studied the chemotaxis behavior of larvae with a single functional olfactory neuron on either the left or right side of the head, allowing us to examine unilateral or bilateral olfactory input. We developed new spectroscopic methods to create stable odorant gradients in which odor concentrations were experimentally measured. In these controlled environments, we observed that a single functional neuron provided sufficient information to permit larval chemotaxis. We found additional evidence that the overall accuracy of navigation is enhanced by the increase in the signal-to-noise ratio conferred by bilateral sensory input.

A toolbox for light control of Drosophila behaviors through Channelrhodopsin 2-mediated photoactivation of targeted neurons

In order to study the function of specific neural circuits, we generated UAS-Channelrhodopsin2 (ChR2) transgenic Drosophila and established a ChR2-based system that enables specific activation of targeted neurons in larval and adult fruit flies with blue light illumination, under the control of a newly designed light source that provides fully programmable stimulation patterns. We showed that stimulating selectively the nociceptor of larvae expressing ChR2 elicited light-induced 'pain' response, confined freely behaving larvae in defined area and directed larva migration along a preset route. In freely behaving adult flies, rapid photoactivation of targeted gustatory sensory neurons, dopaminergic modulatory neurons and motor neurons triggered the proboscis extension response, escaping reflex and changes in the locomotion pattern, respectively, with precise temporal control. This non-invasive method for remote control of animal behaviors also provides a potential tool for conducting 'gain of function' studies toward understanding how animal behaviors are controlled by neural activity.

Molecular Architecture of Smell and Taste inDrosophila

The chemical senses-smell and taste-allow animals to evaluate and distinguish valuable food resources from dangerous substances in the environment. The central mechanisms by which the brain recognizes and discriminates attractive and repulsive odorants and tastants, and makes behavioral decisions accordingly, are not well understood in any organism. Recent molecular and neuroanatomical advances in Drosophila have produced a nearly complete picture of the peripheral neuroanatomy and function of smell and taste in this insect. Neurophysiological experiments have begun to provide insight into the mechanisms by which these animals process chemosensory cues. Given the considerable anatomical and functional homology in smell and taste pathways in all higher animals, experimental approaches in Drosophila will likely provide broad insights into the problem of sensory coding. Here we provide a critical review of the recent literature in this field and comment on likely future directions.

Architecture of the primary taste center ofDrosophila melanogasterlarvae

A simple nervous system combined with stereotypic behavioral responses to tastants, together with powerful genetic and molecular tools, have turned Drosophila larvae into a very promising model for studying gustatory coding. Using the Gal4/UAS system and confocal microscopy for visualizing gustatory afferents, we provide a description of the primary taste center in the larval central nervous system. Essentially, gustatory receptor neurons target different areas of the subesophageal ganglion (SOG), depending on their segmental and sensory organ origin. We define two major and two smaller subregions in the SOG. One of the major areas is a target of pharyngeal sensilla, the other one receives inputs from both internal and external sensilla. In addition to such spatial organization of the taste center, circumstantial evidence suggests a subtle functional organization: aversive and attractive stimuli might be processed in the anterior and posterior part of the SOG, respectively. Our results also suggest less coexpression of gustatory receptors than proposed in prior studies. Finally, projections of putative second-order taste neurons seem to cover large areas of the SOG. These neurons may thus receive multiple gustatory inputs. This suggests broad sensitivity of secondary taste neurons, reminiscent of the situation in mammals.

Olfactory processing and behavior downstream from highly selective receptor neurons

In both the vertebrate nose and the insect antenna, most olfactory receptor neurons (ORNs) respond to multiple odors. However, some ORNs respond to just a single odor, or at most to a few highly related odors. It has been hypothesized that narrowly tuned ORNs project to narrowly tuned neurons in the brain, and that these dedicated circuits mediate innate behavioral responses to a particular ligand. Here we have investigated neural activity and behavior downstream from two narrowly tuned ORN types in Drosophila melanogaster. We found that genetically ablating either of these ORN types impairs innate behavioral attraction to their cognate ligand. Neurons in the antennal lobe postsynaptic to one of these ORN types are, like their presynaptic ORNs, narrowly tuned to a pheromone. However, neurons postsynaptic to the second ORN type are broadly tuned. These results demonstrate that some narrowly tuned ORNs project to dedicated central circuits, ensuring a tight connection between stimulus and behavior, whereas others project to central neurons that participate in the ensemble representations of many odors.

Influence of odorant receptor repertoire on odor perception in humans and fruit flies

The olfactory system is thought to recognize odors with multiple odorant receptors (ORs) that are activated by overlapping sets of odorous molecules, ultimately generating an odor percept in the brain. We investigated how the odor percept differs between humans and Drosophila melanogaster fruit flies, species with very different OR repertoires. We devised high-throughput single fly behavior paradigms to ask how a given OR contributes to the odor percept in Drosophila. Wild-type flies showed dose- and stimulus-dependent responses to 70 of 73 odors tested, whereas mutant flies missing one OR showed subtle behavioral deficits that could not be predicted from the physiological responses of the OR. We measured human and fly judgments of odor intensity and quality and found that intensity perception is conserved between species, whereas quality judgments are species-specific. This study bridges the gap between the activation of olfactory sensory neurons and the odor percept.

Mechanisms of odor receptor gene choice in Drosophila

A remarkable problem in neurobiology is how olfactory receptor neurons (ORNs) select, from among a large odor receptor repertoire, which receptors to express. We use computational algorithms and mutational analysis to define positive and negative regulatory elements that are required for selection of odor receptor (Or) genes in the proper olfactory organ of Drosophila, and we identify an element that is essential for selection in one ORN class. Two odor receptors are coexpressed by virtue of the alternative splicing of a single gene, and we identify dicistronic mRNAs that each encode two receptors. Systematic analysis reveals no evidence for negative feedback regulation, but provides evidence that the choices made by neighboring ORNs of a sensillum are coordinated via the asymmetric segregation of regulatory factors from a common progenitor. We show that receptor gene choice in Drosophila also depends on a combinatorial code of transcription factors to generate the receptor-to-neuron map.

Female-biased expression of odourant receptor genes in the adult antennae of the silkworm, Bombyx mori

Olfaction plays an important role in the life history of insects, including key behaviours such as host selection, oviposition and mate recognition. Odour perception by insects is primarily mediated by the large diverse family of odourant receptors (Ors) that are expressed on the dendrites of olfactory neurones housed within chemosensilla. However, few Or sequences have been identified from the Lepidoptera, an insect order that includes some of the most important pest species worldwide. We have identified 41 Or gene sequences from the silkworm (Bombyx mori) genome, more than double the number of published Or sequences from the Lepidoptera. Many silkworm Ors appear to be orthologs of the 17 published tobacco budworm (Heliothis virescens) Ors indicating that many Or lineages may be conserved within the Lepidoptera. The majority of the Or genes are expressed in adult female and male antennae (determined by quantitative real-time PCR analysis), supporting their probable roles in adult olfaction. Several Or genes are expressed at high levels in both male and female antennae, suggesting they mediate the perception of common host or conspecific volatiles important to both sexes. BmOrs 45-47 group together in the same phylogenetic branch and all three are expressed at moderate female-biased ratios, six to eight times higher in female compared to male moth antennae. Interestingly, BmOrs19 and 30 appear to be expressed predominantly in female antennae, opposite to that of the published silkworm pheromone receptors BmOrs 1 and 3 that are specific to male antennae. These results suggest that BmOr19 and 30 may detect odours critical to female behaviour, such as oviposition cues or male-produced courtship pheromones.

Role of proneural genes in the formation of the larval olfactory organ of Drosophila

In this paper, we address the role of proneural genes in the formation of the dorsal organ in the Drosophila larva. This organ is an intricate compound comprising the multineuronal dome-the exclusive larval olfactory organ-and a number of mostly gustatory sensilla. We first determine the numbers of neurons and of the different types of accessory cells in the dorsal organ. From these data, we conclude that the dorsal organ derives from 14 sensory organ precursor cells. Seven of them appear to give rise to the dome, which therefore may be composed of seven fused sensilla, whereas the other precursors produce the remaining sensilla of the dorsal organ. By a loss-of-function approach, we then analyze the role of atonal, amos, and the achaete-scute complex (AS-C), which in the adult are the exclusive proneural genes required for chemosensory organ specification. We show that atonal and amos are necessary and sufficient in a complementary way for four and three of the sensory organ precursors of the dome, respectively. AS-C, on the other hand, is implicated in specifying the non-olfactory sensilla, partially in cooperation with atonal and/or amos. Similar links for these proneural genes with olfactory and gustatory function have been established in the adult fly. However, such conserved gene function is not trivial, given that adult and larval chemosensory organs are anatomically very different and that the development of adult olfactory sensilla involves cell recruitment, which is unlikely to play a role in dome formation.

Comparative chemosensation from receptors to ecology

Odour perception is initiated by specific interactions between odorants and a large repertoire of receptors in olfactory neurons. During the past few years, considerable progress has been made in tracing olfactory perception from the odorant receptor protein to the activity of olfactory neurons to higher processing centres and, ultimately, to behaviour. The most complete picture is emerging for the simplest olfactory system studied--that of the fruitfly Drosophila melanogaster. Comparison of rodent, insect and nematode olfaction reveals surprising differences and unexpected similarities among chemosensory systems.

Two chemosensory receptors together mediate carbon dioxide detection in Drosophila

Blood-feeding insects, including the malaria mosquito Anopheles gambiae, use highly specialized and sensitive olfactory systems to locate their hosts. This is accomplished by detecting and following plumes of volatile host emissions, which include carbon dioxide (CO2). CO2 is sensed by a population of olfactory sensory neurons in the maxillary palps of mosquitoes and in the antennae of the more genetically tractable fruitfly, Drosophila melanogaster. The molecular identity of the chemosensory CO2 receptor, however, remains unknown. Here we report that CO2-responsive neurons in Drosophila co-express a pair of chemosensory receptors, Gr21a and Gr63a, at both larval and adult life stages. We identify mosquito homologues of Gr21a and Gr63a, GPRGR22 and GPRGR24, and show that these are co-expressed in A. gambiae maxillary palps. We show that Gr21a and Gr63a together are sufficient for olfactory CO2-chemosensation in Drosophila. Ectopic expression of Gr21a and Gr63a together confers CO2 sensitivity on CO2-insensitive olfactory neurons, but neither gustatory receptor alone has this function. Mutant flies lacking Gr63a lose both electrophysiological and behavioural responses to CO2. Knowledge of the molecular identity of the insect olfactory CO2 receptors may spur the development of novel mosquito control strategies designed to take advantage of this unique and critical olfactory pathway. This in turn could bolster the worldwide fight against malaria and other insect-borne diseases.

Light-induced activation of distinct modulatory neurons triggers appetitive or aversive learning in Drosophila larvae

During classical conditioning, a positive or negative value is assigned to a previously neutral stimulus, thereby changing its significance for behavior. If an odor is associated with a negative stimulus, it can become repulsive. Conversely, an odor associated with a reward can become attractive. By using Drosophila larvae as a model system with minimal brain complexity, we address the question of which neurons attribute these values to odor stimuli. In insects, dopaminergic neurons are required for aversive learning, whereas octopaminergic neurons are necessary and sufficient for appetitive learning. However, it remains unclear whether two independent neuronal populations are sufficient to mediate such antagonistic values. We report the use of transgenically expressed channelrhodopsin-2, a light-activated cation channel, as a tool for optophysiological stimulation of genetically defined neuronal populations in Drosophila larvae. We demonstrate that distinct neuronal populations can be activated simply by illuminating the animals with blue light. Light-induced activation of dopaminergic neurons paired with an odor stimulus induces aversive memory formation, whereas activation of octopaminergic/tyraminergic neurons induces appetitive memory formation. These findings demonstrate that antagonistic modulatory subsystems are sufficient to substitute for aversive and appetitive reinforcement during classical conditioning.

Multiple enhancers contribute to spatial but not temporal complexity in the expression of the proneural gene, amos

Background

The regulation of proneural gene expression is an important aspect of neurogenesis. In the study of the Drosophila proneural genes, scute and atonal, several themes have emerged that contribute to our understanding of the mechanism of neurogenesis. First, spatial complexity in proneural expression results from regulation by arrays of enhancer elements. Secondly, regulation of proneural gene expression occurs in distinct temporal phases, which tend to be under the control of separate enhancers. Thirdly, the later phase of proneural expression often relies on positive autoregulation. The control of these phases and the transition between them appear to be central to the mechanism of neurogenesis. We present the first investigation of the regulation of the proneural gene, amos.

Results

Amos protein expression has a complex pattern and shows temporally distinct phases, in common with previously characterised proneural genes. GFP reporter gene constructs were used to demonstrate that amos has an array of enhancer elements up- and downstream of the gene, which are required for different locations of amos expression. However, unlike other proneural genes, there is no evidence for separable enhancers for the different temporal phases of amos expression. Using mutant analysis and site-directed mutagenesis of potential Amos binding sites, we find no evidence for positive autoregulation as an important part of amos control during neurogenesis.

Conclusion

For amos, as for other proneural genes, a complex expression pattern results from the sum of a number of simpler sub-patterns driven by specific enhancers. There is, however, no apparent separation of enhancers for distinct temporal phases of expression, and this correlates with a lack of positive autoregulation. For scute and atonal, both these features are thought to be important in the mechanism of neurogenesis. Despite similarities in function and expression between the Drosophila proneural genes, amos is regulated in a fundamentally different way from scute and atonal.

The chemoreceptor superfamily in the honey bee, Apis mellifera: expansion of the odorant, but not gustatory, receptor family

The honey bee genome sequence reveals a remarkable expansion of the insect odorant receptor (Or) family relative to the repertoires of the flies Drosophila melanogaster and Anopheles gambiae, which have 62 and 79 Ors respectively. A total of 170 Or genes were annotated in the bee, of which seven are pseudogenes. These constitute five bee-specific subfamilies in an insect Or family tree, one of which has expanded to a total of 157 genes encoding proteins with 15%-99% amino acid identity. Most of the Or genes are in tandem arrays, including one with 60 genes. This bee-specific expansion of the Or repertoire presumably underlies their remarkable olfactory abilities, including perception of several pheromone blends, kin recognition signals, and diverse floral odors. The number of Apis mellifera Ors is approximately equal to the number of glomeruli in the bee antennal lobe (160-170), consistent with a general one-receptor/one-neuron/one-glomerulus relationship. The bee genome encodes just 10 gustatory receptors (Grs) compared with the D. melanogaster and A. gambiae repertoires of 68 and 76 Grs, respectively. A lack of Gr gene family expansion primarily accounts for this difference. A nurturing hive environment and a mutualistic relationship with plants may explain the lack of Gr family expansion. The Or family is the most dramatic example of gene family expansion in the bee genome, and characterizing their caste- and sex-specific gene expression may provide clues to their specific roles in detection of pheromone, kin, and floral odors.

The Drosophila larva as a model for studying chemosensation and chemosensory learning: a review

Understanding the relationship between brain and behavior is the fundamental challenge in neuroscience. We focus on chemosensation and chemosensory learning in larval Drosophila and review what is known about its molecular and cellular bases. Detailed analyses suggest that the larval olfactory system, albeit much reduced in cell number, shares the basic architecture, both in terms of receptor gene expression and neuronal circuitry, of its adult counterpart as well as of mammals. With respect to the gustatory system, less is known in particular with respect to processing of gustatory information in the central nervous system, leaving generalizations premature. On the behavioral level, a learning paradigm for the association of odors with food reinforcement has been introduced. Capitalizing on the knowledge of the chemosensory pathways, we review the first steps to reveal the genetic and cellular bases of olfactory learning in larval Drosophila. We argue that the simplicity of the larval chemosensory system, combined with the experimental accessibility of Drosophila on the genetic, electrophysiological, cellular, and behavioral level, makes this system suitable for an integrated understanding of chemosensation and chemosensory learning.

Olfactory coding: connecting odorant receptor expression and behavior in the Drosophila larva

The discovery of odorant receptors has significantly changed our understanding of how animals identify thousands of odorants. A recent study has shed new light on the central issue of how odor information is translated into meaningful behavior.