Development of the Drosophila olfactory sense organs utilizes cell-cell interactions as well as lineage

Legacies of salient environmental experiences-insights from chemosensation

Evidence for parental environments profoundly influencing the physiology, biology, and neurobiology of future generations has been accumulating in the literature. Recent efforts to understand this phenomenon and its underlying mechanisms have sought to use species like rodents and insects to model multi-generational legacies of parental experiences like stress and nutritional exposures. From these studies, we have come to appreciate that parental exposure to salient environmental experiences impacts the cadence of brain development, hormonal responses to stress, and the expression of genes that govern cellular responses to stress in offspring. Recent studies using chemosensory exposure have emerged as a powerful tool to shed new light on how future generations come to be influenced by environments to which parents are exposed. With a specific focus on studies that have leveraged such use of salient chemosensory experiences, this review synthesizes our current understanding of the concept, causes, and consequences of the inheritance of chemosensory legacies by future generations and how this field of inquiry informs the larger picture of how parental experiences can influence offspring biology.

Evolution of coeloconic sensilla in the peripheral olfactory system of Drosophila mojavensis

Populations inhabiting habitats with different environmental conditions, such as climate, resource availability, predation, competition, can undergo selection for traits that are adaptive in one habitat and not the other, leading to divergence between populations. Changes in the olfactory systems of insects that rely on different host plants, for example, can occur in response to differences in sensory stimuli between habitats. In this study, we investigate the evolution of host preference by characterizing the coeloconic sensilla in Drosophila mojavensis, a species that breeds on different necrotic cacti across its geographic range. These cactus species differ in the volatile chemicals they emit, a primary sensory cue for host plant discrimination. Analysis of odor-evoked responses identified four coeloconic sensilla that were qualitatively similar to those of Drosophila melanogaster, but varied in the breadth and strength of their olfactory sensory neuron responses to some acids and amines. Variation in responses to certain odorants among D. mojavensis populations was also observed. Compared to D. melanogaster, there was a lack of sensitivity of antennal coeloconic type 3 (ac3) sensilla to primary ligands of OR35a across all populations. Consistent with this result was a lack of detectable Or35a gene expression. Using a comparative approach, we then examined odor specificity of ac3 sensilla for seven additional Drosophila species, and found that OR35a-like sensitivity may be limited to the melanogaster subgroup. The variation in specificity that was observed among species is not clearly attributable to the degree of ecological specialization, nor to the ecological niche.

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.

Evolutionary variation in neural gene expression in the developing sense organs of the crustacean Daphnia magna

Arthropods have numerous sense organs, which are adapted to their habitat. While some sense organs are similar in structure and function in all arthropod groups, structural differences in functionally related sense organs have been described, as well as the absence of particular sense organ subtypes in individual arthropod groups. Here we address the question of how the diverse structures of arthropod sense organs have evolved by analysing the underlying molecular developmental processes in a crustacean, an arthropod group that has been neglected so far. We have investigated the development of four types of chemo- and mechanosensory sense organs in the branchiopod Daphnia magna (Cladocera) that either cannot be found in arthropods other than crustaceans or represent adaptations to an aquatic environment. The formation of the sensory organ precursors shows greater similarity to the arthropod taxa Chelicerata and Myriapoda than to the more closely related insects. All analysed sense organ types co-express the proneural genes ASH and atonal regardless of their structure and function. In contrast, in Drosophila melanogaster, ASH and atonal expression does not overlap and the genes confer different sense organ subtype identities. We performed experimental co-expression studies in D. melanogaster and found that the combinatorial expression of ato and ASH can change the external structure of sense organs. Our results indicate a central role for ASH and Atonal family members in the emergence of structural variations in arthropod sense organs.

Establishing and maintaining gene expression patterns: insights from sensory receptor patterning

In visual and olfactory sensory systems with high discriminatory power, each sensory neuron typically expresses one, or very few, sensory receptor genes, excluding all others. Recent studies have provided insights into the mechanisms that generate and maintain sensory receptor expression patterns. Here, we review how this is achieved in the fly retina and compare it with the mechanisms controlling sensory receptor expression patterns in the mouse retina and in the mouse and fly olfactory systems.

Determinants of the Drosophila odorant receptor pattern

In most olfactory systems studied to date, neurons that express the same odorant receptor (Or) gene are scattered across sensory epithelia, intermingled with neurons that express different Or genes. In Drosophila, olfactory sensilla that express the same Or gene are dispersed on the antenna and the maxillary palp. Here we show that Or identity is specified in a spatially stereotyped pattern by the cell-autonomous activity of the transcriptional regulators Engrailed and Dachshund. Olfactory sensilla then become highly motile and disperse beneath the epidermis. Thus, positional information and cell motility underlie the dispersed patterns of Drosophila Or gene expression.

Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

The olfactory organ of the Caribbean spiny lobster Panulirus argus maintains lifelong proliferation and turnover of olfactory receptor neurons (ORNs). Towards examining the molecular basis of this adult neurogenesis, we search for expression of homologs of proneural, neurogenic, and pre-pattern genes in this olfactory organ. We report here a homolog of the proneural Achaete-Scute family, called splash (spiny lobster achaete-scute homolog), and a homolog of the pre-pattern and neurogenic hairy-enhancer of split family, called splhairy (spiny lobster hairy). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicates a molt stage dependence of the levels of expression of splash and splhairy mRNA in the olfactory organ, with higher expression in premolt than in postmolt or intermolt animals, which is positively correlated with rates of neurogenesis. splash and splhairy mRNA are expressed not only in the olfactory organ but also in other tissues, albeit at lower levels, irrespective of molt stage. We conclude that the expression of achaete-scute and hairy-enhancer of split in the proliferation zone of the olfactory organ of spiny lobsters and their enhanced expression in premolt animals suggest that they play a role in the proliferation of ORNs and that their expression in regions of the olfactory organ populated by mature ORNs and in other tissues suggests that they have additional functions.

The olfactory sensory map in Drosophila

The fruit fly (Drosophila melanogaster) exhibits robust odor-evoked behaviors in response to cues from diverse host plants and pheromonal cues from other flies. Understanding how the adult olfactory system supports the perception of these odorous chemicals and translates them into appropriate attraction or avoidance behaviors is an important goal in contemporary sensory neuroscience. Recent advances in genomics and molecular neurobiology have provided an unprecedented level of detail into how the adult Drosophila olfactory system is organized. Volatile odorants are sensed by two bilaterally symmetric olfactory sensory appendages, the third segment of the antenna and the maxillary palps, which respectively contain approximately 1200 and 120 olfactory sensory neurons (OSNs) each. These OSNs express a divergent family of seven transmembrane domain odorant receptors (ORs) with no homology to vertebrate ORs, which determine the odor specificity of a given OSN. Drosophila was the first animal for which all OR genes were cloned, their patterns of gene expression determined and axonal projections of most OSNs elucidated. In vivo electrophysiology has been used to decode the ligand response profiles of most of the ORs, providing insight into the initial logic of olfactory coding in the fly. This chapter will review the molecular biology, neuroanatomy and function of the peripheral olfactory system of Drosophila.

Development of the Drosophila olfactory system

The olfactory system throughout the animal kingdom is characterized by a large number of highly specialized neuronal cell types. Olfactory receptor neurons (ORNs) in the peripheral sensory epithelium display two main differentiation features: the selective expression of a single odorant receptor out of a large genomic repertoire of receptor genes and the synaptic connection to a single type of relay neuron in the primary olfactory CNS target area. In the mouse olfactory system, odorant receptors themselves play a central role in the coordination of both types of ORN differentiation. The olfactory system of Drosophila, although similar in structural and functional organization compared to mammals, does not seem to involve odorant receptors in the selection of OR gene expression and target cell recognition, suggesting distinct developmental control mechanisms. In this chapter we summarize recent findings in Drosophila of how gene networks regulate ORN specification and differentiation in the peripheral sensory organs as well as how different cellular interactions and patterning signals organize the class-specific axonal and dendritic connectivity in the CNS target area.

The role of Notch signalling and numb function in mechanosensory organ formation in the spider Cupiennius salei

In the spider Cupiennius salei the mechanosensory organs of the legs are generated from epithelial sensory precursor groups which are specified by elevated levels of the achaete-scute homologues CsASH1 and CsASH2. Neural precursors delaminate from the groups and occupy positions basal and proximal to the accessory cells which remain in the epithelium. Here we analyse the role of Notch signalling and numb function in the development of the mechanosensory organs of the spider. We show that Notch signalling is required for several processes: the selection of the sensory precursor groups, the maintenance of undifferentiated sensory precursors, the binary cell fate decision between accessory and neural fate and the differentiation of sensory neurons. Our data suggest that Numb antagonises Notch signalling in the neural precursors, which results in activation of the neural cell fate determinant Prospero and delamination of the neural precursors from the epithelium. Prospero is expressed de novo in sensory neural precursors and we assume that the expression of the gene is regulated by the Notch to Numb ratio within the sensory precursors. Interestingly, the spider numb RNAi phenotype resembles the numb/numblike loss of function phenotypes in the mammalian nervous system, indicating that the interaction between Notch signalling and Numb might play a similar role in both systems.

Engrailed expression in subsets of adult Drosophila sensory neurons: an enhancer-trap study

Engrailed (En) has an important role in neuronal development in vertebrates and invertebrates. In adult Drosophila, although En expression persists throughout adulthood, a detailed description of its expression in sensory neurons has not been made. In this study, en-GAL4 was used to drive UAS-CD8::GFP expression and the projections of sensory neurons were examined with confocal microscopy. En protein expression was confirmed using immunocytochemistry. In the antenna, En is present in subsets of Johnston's organ neurons and of olfactory neurons. En-driven GFP is expressed in axons projecting to 18 identified olfactory glomeruli, originating from basiconic, trichoid and coeloconic sensilla. In most cases both neurons of a sensillum express En. En expression overlaps with that of Acj6, another transcription factor. En-driven GFP is also expressed in a subset of maxillary palp olfactory neurons and in all mechanosensory and gustatory sensilla in the posterior compartment of the labial palps. In the legs and halteres, en-driven GFP is expressed in only a subset of the sensory neurons of different modalities that arise in the posterior compartment. Finally, en-driven GFP is expressed in a single multidendritic sensory neuron in each abdominal segment.

Sensory cell proliferation within the olfactory epithelium of developing adult Manduca sexta (Lepidoptera)

BACKGROUND

Insects detect a multitude of odors using a broad array of phenotypically distinct olfactory organs referred to as olfactory sensilla. Each sensillum contains one to several sensory neurons and at least three support cells; these cells arise from mitotic activities from one or a small group of defined precursor cells. Sensilla phenotypes are defined by distinct morphologies, and specificities to specific odors; these are the consequence of developmental programs expressed by associated neurons and support cells, and by selection and expression of subpopulations of olfactory genes encoding such proteins as odor receptors, odorant binding proteins, and odor degrading enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We are investigating development of the olfactory epithelium of adult M. sexta, identifying events which might establish sensilla phenotypes. In the present study, antennal tissue was examined during the first three days of an 18 day development, a period when sensory mitotic activity was previously reported to occur. Each antenna develops as a cylinder with an outward facing sensory epithelium divided into approximately 80 repeat units or annuli. Mitotic proliferation of sensory cells initiated about 20-24 hrs after pupation (a.p.), in pre-existing zones of high density cells lining the proximal and distal borders of each annulus. These high density zones were observed as early as two hr. a.p., and expanded with mitotic activity to fill the mid-annular regions by about 72 hrs a.p. Mitotic activity initiated at a low rate, increasing dramatically after 40-48 hrs a.p.; this activity was enhanced by ecdysteroids, but did not occur in animals entering pupal diapause (which is also ecdysteroid sensitive).

CONCLUSIONS/SIGNIFICANCE

Sensory proliferation initiates in narrow zones along the proximal and distal borders of each annulus; these zones rapidly expand to fill the mid-annular regions. These zones exist prior to any mitotic activity as regions of high density cells which form either at or prior to pupation. Mitotic sensitivity to ecdysteroids may be a regulatory mechanism coordinating olfactory development with the developmental choice of diapause entry.

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.

Wiring specificity in the olfactory system

The fruitfly brain learns about the olfactory world by reading the activity of about 50 distinct channels of incoming information. The receptor neurons that compose each channel have their own distinctive odour response profile governed by a specific receptor molecule. These receptor neurons form highly specific connections in the first olfactory relay of the fly brain, each synapsing with specific second order partner neurons. We use this system to discuss the logic of wiring specificity in the brain and to review the cellular and molecular mechanisms that allow such precise wiring to develop.

Programmed cell death and context dependent activation of the EGF pathway regulate gliogenesis in the Drosophila olfactory system

In the Drosophila antenna, sensory lineages selected by the basic helix-loop-helix transcription factor Atonal are gliogenic while those specified by the related protein Amos are not. What are the mechanisms that cause the two lineages to act differentially? We found that ectopic expression of the Baculovirus inhibitor of apoptosis protein (p35) rescues glial cells from the Amos-derived lineages, suggesting that precursors are removed by programmed cell death. In the wildtype, glial precursors express the extracellular-signal regulated kinase transiently, and antagonism of Epidermal Growth Factor (EGF) pathway signaling compromises their development. We suggest that all sensory lineages on the antenna are competent to produce glia but only those specified by Atonal respond to EGF signaling and survive. These results underscore the importance of developmental context of cell lineages in their responses to non-autonomous signaling in the choice between survival and death.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

The worm's sense of smell

Animals sense their chemical environment using multiple chemosensory neuron types, each of which exhibits characteristic response properties. The chemosensory neurons of the nematode Caenorhabditis elegans provide an excellent system in which to explore the developmental mechanisms giving rise to this functional diversity. In this review, we discuss the principles underlying the patterning, generation, differentiation, and diversification of chemosensory neuron subtypes in C. elegans. Current knowledge of the molecular mechanisms underlying each of these individual steps is derived from work in different model organisms. It is essential to describe the complete developmental pathways in each organism to determine whether functional diversification in chemosensory systems is achieved via conserved or novel mechanisms. Such a complete description may be possible in C. elegans.

Combinatorial expression of Prospero, Seven-up, and Elav identifies progenitor cell types during sense-organ differentiation in the Drosophila antenna

The Drosophila antenna has a diversity of chemosensory organs within a single epidermal field. We have some idea from recent studies of how the three broad categories of sense-organs are specified at the level of progenitor choice. However, little is known about how cell fates within single sense-organs are specified. Selection of individual primary olfactory progenitors is followed by organization of groups of secondary progenitors, which divide in a specific order to form a differentiated sensillum. The combinatorial expression of Prospero Elav, and Seven-up allows us to distinguish three secondary progenitor fates. The lineages of these cells have been established by clonal analysis and marker distribution following mitosis. High Notch signaling and the exclusion of these markers identifies PIIa; this cell gives rise to the shaft and socket. The sheath/neuron lineage progenitor PIIb, expresses all three markers; upon division, Prospero asymmetrically segregates to the sheath cell. In the coeloconica, PIIb undergoes an additional division to produce glia. PIIc is present in multiinnervated sense-organs and divides to form neurons. An understanding of the lineage and development of olfactory sense-organs provides a handle for the analysis of how olfactory neurons acquire distinct terminal fates.

Precocious expression of the Glide/Gcm glial-promoting factor in Drosophila induces neurogenesis

Neurons and glial cells depend on similar developmental pathways and often originate from common precursors; however, the differentiation of one or the other cell type depends on the activation of cell-specific pathways. In Drosophila, the differentiation of glial cells depends on a transcription factor, Glide/Gcm. This glial-promoting factor is both necessary and sufficient to induce the central and peripheral glial fates at the expense of the neuronal fate. In a screen for mutations affecting the adult peripheral nervous system, we have found a dominant mutation inducing supernumerary sensory organs. Surprisingly, this mutation is allelic to glide/gcm and induces precocious glide/gcm expression, which, in turn, activates the proneural genes. As a consequence, sensory organs are induced. Thus, temporal misregulation of the Glide/Gcm glial-promoting factor reveals a novel potential for this cell fate determinant. At the molecular level, this implies unpredicted features of the glide/gcm pathway. These findings also emphasize the requirement for both spatial and temporal glide/gcm regulation to achieve proper cell specification within the nervous system.

Delta signaling from the germ line controls the proliferation and differentiation of the somatic follicle cells during Drosophila oogenesis

The body axes of Drosophila are established during oogenesis through reciprocal interactions between the germ line cells and the somatic follicle cells that surround them. The Notch pathway is required at two stages in this process: first, for the migration of the follicle cells around the germ line cyst and, later, for the polarization of the anterior-posterior (A-P) axis of the oocyte. Its function in these events, however, has remained controversial. Using clonal analysis, we show that Notch signaling controls cell proliferation and differentiation in the whole follicular epithelium. Notch mutant follicle cells remain in a precursor state and fail to switch from the mitotic cell cycle to the endocycle. Furthermore, removal of Delta from the germ line produces an identical phenotype, showing that Delta signals from the germ cells to control the timing of follicle cell differentiation. This explains the axis formation defects in Notch mutants, which arise because undifferentiated posterior follicle cells cannot signal to polarize the oocyte. Delta also signals from the germ line to Notch in the soma earlier in oogenesis to control the differentiation of the polar and stalk follicle cells. The germ line therefore regulates the development of the follicle cells through two complementary signaling pathways: Gurken signals twice to control spatial patterning, whereas Delta signals twice to exert temporal control.

Postembryonic proliferation in the spiny lobster antennular epithelium: rate of genesis of olfactory receptor neurons is dependent on molt stage

Olfactory systems undergo continuous growth and turnover in many animals. Many decapod crustaceans, such as lobsters and crayfish, have indeterminate growth, and in these animals, turnover of both peripheral and central components of the olfactory system occurs continuously throughout life. In this study, we examine the dynamics of olfactory receptor neuron (ORN) proliferation in the antennule of the Caribbean spiny lobster, Panulirus argus, using in vivo incorporation of the cell proliferation marker BrdU. We show that addition of ORNs occurs in a "proximal proliferation zone" (PPZ), which exists on the proximo-lateral margin of the existing ORN population. The PPZ is spatially and temporally dynamic in that it travels as a wave in the proximal and lateral directions in the antennule. This wave results in continuous addition of ORNs throughout the molt cycle. The rate of proliferation, as measured by the size and shape of the PPZ, changes depending on the animal's molt stage. The rate is highest during premolt and lowest during intermolt. ORNs are the most prominent cell-type produced in the PPZ, but other cell types, including glia, are also produced. Patches of proliferating epithelial cells occur immediately proximal to the PPZ, suggesting that neuronal and glial precursors reside in this region. Possible mechanisms for peripheral and central modulation of ORN development are discussed.

Sense organ identity in the Drosophila antenna is specified by the expression of the proneural gene atonal

We have shown that the basic helix-loop-helix transcription factor Atonal is sufficient for specification of one of the three subsets of olfactory sense organs on the Drosophila antenna. Misexpression of Atonal in all sensory precursors in the antennal disc results in their conversion to coeloconic sensilla. The mechanism by which specific sense organ fate is triggered remains unclear. We have shown that the homeodomain transcription factor Cut which acts in the chordotonal-external sense organ choice does not play a role in olfactory sense organ development. The expression of atonal in specific domains of the antennal disc is regulated by an interplay of the patterning genes, Hedgehog and Wingless, and Drosophila epidermal growth factor receptor pathway.

Mechanisms underlying olfactory neuronal connectivity in Drosophila-the atonal lineage organizes the periphery while sensory neurons and glia pattern the olfactory lobe

Patterning of the antennal lobe of adult Drosophila occurs through a complex interaction between sensory neurons, glia, and central neurons of larval and adult origin. Neurons from the olfactory sense organs are organized into distinct fascicles lined by glial cells. The glia originate from one of the three types of sensory lineages-specified by the proneural gene atonal. Gain-of-function as well as loss-of-function analysis validates a role for cells of the Atonal lineage in the ordered fasciculation of sensory neurons. Upon entry of the antennal nerve to central regions, sensory neurons at first remain closely associated with central glia which lie around the periphery of the lobe anlage. Coincident with the arrival of sensory neurons into the brain, glial precursors undergo mitosis and neural precursors expressing Dachshund appear around the lobe. Sensory neurons and glial cells project into the lobe at around the same time and are likely to coordinate the correct localization of different glomeruli. The influence of sensory neurons on the development of the olfactory lobe could serve to match and lock peripheral and central properties important for the generation of olfactory behavior.

Olfaction in Drosophila

The fruit fly, Drosophila melanogaster, is equipped with a sophisticated olfactory sensory system that permits it to recognize and discriminate hundreds of discrete odorants. The perception of these odorants is essential for the animal to identify relevant food sources and suitable sites for egg-laying. Advances in the last year have begun to define the molecular basis of this insect's discriminatory power. The identification of a large multi-gene family of candidate Drosophila odorant receptors suggests that, as in other animals, a multitude of distinct odorants is recognized by a diversity of ligand-binding receptors. How olfactory signals are transduced and interpreted by the brain remains an important question for future analysis. The availability of genetic tools and a complete genome sequence makes Drosophila a particularly attractive organism for studying the molecular basis of olfaction.

An olfactory sensory map in the fly brain

We have isolated the "complete" repertoire of genes encoding the odorant receptors in Drosophila and employ these genes to provide a molecular description of the organization of the peripheral olfactory system. The repertoire of Drosophila odorant receptors is encoded by 57 genes. Individual sensory neurons are likely to express only a single receptor gene. Neurons expressing a given gene project axons to one or two spatially invariant glomeruli in the antennal lobe. The insect brain therefore retains a two-dimensional map of receptor activation such that the quality of an odor may be encoded by different spatial patterns of activity in the antennal lobe.

amos, a proneural gene for Drosophila olfactory sense organs that is regulated by lozenge

In a variety of organisms, early neurogenesis requires the function of basic-helix-loop-helix (bHLH) transcription factors. For the Drosophila PNS, such transcription factors are encoded by the proneural genes (atonal and the achaete-scute complex, AS-C). We have identified a proneural gene, amos, that has strong similarity with atonal in its bHLH domain. We present evidence that amos is required for olfactory sensilla and is regulated by the prepattern gene lozenge. Between them, amos, atonal, and the AS-C can potentially account for the origin of the entire PNS.

A spatial map of olfactory receptor expression in the Drosophila antenna

Insects provide an attractive system for the study of olfactory sensory perception. We have identified a novel family of seven transmembrane domain proteins, encoded by 100 to 200 genes, that is likely to represent the family of Drosophila odorant receptors. Members of this gene family are expressed in topographically defined subpopulations of olfactory sensory neurons in either the antenna or the maxillary palp. Sensory neurons express different complements of receptor genes, such that individual neurons are functionally distinct. The isolation of candidate odorant receptor genes along with a genetic analysis of olfactory-driven behavior in insects may ultimately afford a system to understand the mechanistic link between odor recognition and behavior.

The odor specificities of a subset of olfactory receptor neurons are governed by Acj6, a POU-domain transcription factor

Little is known about how the odor specificities of olfactory neurons are generated, a process essential to olfactory coding. We have found that neuronal identity relies on the abnormal chemosensory jump 6 (acj6) gene, originally identified by a defect in olfactory behavior. Physiological analysis of individual olfactory neurons shows that in acj6 mutants, a subset of neurons acquires a different odorant response profile. Certain other neurons do not respond to any tested odors in acj6. Molecular analysis of acj6 shows that it encodes a POU-domain transcription factor expressed in olfactory neurons. Our data suggest that the odor response spectrum of an olfactory neuron, and perhaps the choice of receptor genes, is determined through a process requiring the action of Acj6.

A novel family of divergent seven-transmembrane proteins: candidate odorant receptors in Drosophila

Although insects have proven to be valuable models for exploring the function, organization, and development of the olfactory system, the receptor molecules that bind odors have not been identified in any insect. We have developed a novel search algorithm, used it to search the Drosophila genomic sequence database, and identified a large multigene family encoding seven transmembrane domain proteins that are expressed in olfactory organs. We show that expression is restricted to subsets of olfactory receptor neurons (ORNs) for a number of these genes. Different members of the family initiate expression at different times during antennal development. Some of the genes are not expressed in a mutant of the Acj6 POU-domain transcription factor, a mutant in which a subset of ORNs show abnormal odorant specificities.

Genetic analysis of hook, a gene required for endocytic trafficking in drosophila

The Drosophila hook gene encodes a novel component of the endocytic compartment. Previously identified hook alleles, which still expressed truncated Hook proteins, affected the accumulation of internalized transmembrane ligands into multivesicular bodies (MVBs). To determine the hook null phenotype, we isolated nine new hook alleles on the basis of their characteristic hooked-bristle phenotype. At least one of these alleles, hk11, is a complete loss-of-function allele. Flies carrying the hk11 allele are viable and fertile but neither transmembrane ligands nor soluble ligands accumulate in MVBs. This effect on endocytosed ligands can be mimicked by the expression of Hook proteins truncated for the N- and C-terminal domains flanking the central coiled-coil region. The importance of all three domains for Hook function was confirmed by their conservation between two Drosophila and two human Hook proteins.

Math5 encodes a murine basic helix-loop-helix transcription factor expressed during early stages of retinal neurogenesis

We have identified Math5, a mouse basic helix-loop-helix (bHLH) gene that is closely related to Drosophila atonal and Xenopus Xath5 and is largely restricted to the developing eye. Math5 retinal expression precedes differentiation of the first neurons and persists within progenitor cells until after birth. To position Math5 in a hierarchy of retinal development, we compared Math5 and Hes1 expression in wild-type and Pax6-deficient (Sey) embryos. Math5 expression is downregulated in Sey/+ eyes and abolished in Sey/Sey eye rudiments, whereas the bHLH gene Hes1 is upregulated in a similar dose-dependent manner. These results link Pax6 to the process of retinal neurogenesis and provide the first molecular correlate for the dosage-sensitivity of the Pax6 phenotype. During retinogenesis, Math5 is expressed significantly before NeuroD, Ngn2 or Mash1. To test whether these bHLH genes influence the fates of distinct classes of retinal neurons, we ectopically expressed Math5 and Mash1 in Xenopus retinal progenitors. Unexpectedly, lipofection of either mouse gene into the frog retina caused an increase in differentiated bipolar cells. Directed expression of Math5, but not Xath5, in Xenopus blastomeres produced an expanded retinal phenotype. We propose that Math5 acts as a proneural gene, but has properties different from its most closely related vertebrate family member, Xath5.

Patterning an epidermal field: Drosophila lozenge, a member of the AML-1/Runt family of transcription factors, specifies olfactory sense organ type in a dose-dependent manner

Sense organ development in the Drosophila antenna is initiated by the selection of a founder cell from an epidermal field. This cell is believed to recruit neighbours to form a cluster of cells which then divides to form a mature sense organ. In most systems so far studied, sense organ type appears to be specified by the identity of proneural genes involved in the selection of precursors. The regulation of proneural gene expression is, in turn, controlled by the prepatterning genes. In the antenna, the only known proneural function is that of atonal, a gene that is involved in founder cell choice in the sensilla coeloconica, and no prepatterning gene function has yet been demonstrated. In this study, we show that Lozenge, a protein which possesses a DNA binding domain similar to that of the Acute myeloid leukemia-1/Runt transcription factors, functions in a dose-dependent manner to specify the fate of the other two types of sense organs in the antenna: the sensilla trichoidea and the sensilla basiconica. Our results suggest that Lozenge may act on the epidermal field, resulting in founder cells acquiring specific cell fates that lead to the development of an appropriate type of sense organ.

Regulation of armadillo by a Drosophila APC inhibits neuronal apoptosis during retinal development

We find that inactivation of a Drosophila homolog of the tumor suppressor APC (D-APC) causes retinal neuronal degeneration and pigment cell hypertrophy, a phenotype remarkably similar to that found in humans with germline APC mutations. Retinal degeneration in the D-APC mutant results from apoptotic cell death, which accompanies a defect in neuronal differentiation. Reduction in the Drosophila beta-catenin, Armadillo (Arm), rescues the differentiation defect and prevents apoptosis in the D-APC mutant, while Arm overexpression mimics D-APC inactivation. A mutation in dTCF, the DNA-binding protein required in Arm-mediated signal transduction, can eliminate the cell death without rescuing the differentiation defect in D-APC mutants. Uncoupling of these two Arm-induced processes suggests a novel role for the Arm/dTCF complex in the activation of apoptosis.

Xath5 participates in a network of bHLH genes in the developing Xenopus retina

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiation in the developing retina.