Drosophila Antenna Research Articles

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.

Distal-less and homothorax regulate multiple targets to pattern the Drosophila antenna.

The Drosophila antenna is a highly derived appendage required for a variety of sensory functions including olfaction and audition. To investigate how this complex structure is patterned, we examine the specific functions of genes required for antenna development. The nuclear factors, Homothorax, Distal-less and Spineless, are each required for particular aspects of antennal fate. Coexpression of Homothorax, necessary for nuclear localization of its ubiquitously expressed partner Extradenticle, with Distal-less is required to establish antenna fate. Here we test which antenna patterning genes are targets of Homothorax, Distal-less and/or Spineless. We report that the antennal expression of dachshund, atonal, spalt, and cut requires Homothorax and/or Distal-less, but not Spineless. We conclude that Distal-less and Homothorax specify antenna fates via regulation of multiple genes. We also report for the first time phenotypic consequences of losing either dachshund or spalt and spalt-related from the antenna. We find that dachshund and spalt/spalt-related are essential for proper joint formation between particular antennal segments. Furthermore, the spalt/spalt-related null antennae are defective in hearing. Hearing defects are also associated with the human diseases Split Hand/Split Foot Malformation and Townes-Brocks Syndrome, which are linked to human homologs of Distal-less and spalt, respectively. We therefore propose that there are significant genetic similarities between the auditory organs of humans and flies.

A new role for cryptochrome in a Drosophila circadian oscillator.

Cryptochromes are flavin/pterin-containing proteins that are involved in circadian clock function in Drosophila and mice. In mice, the cryptochromes Cry1 and Cry2 are integral components of the circadian oscillator within the brain and contribute to circadian photoreception in the retina. In Drosophila, cryptochrome (CRY) acts as a photoreceptor that mediates light input to circadian oscillators in both brain and peripheral tissue. A Drosophila cry mutant, cryb, leaves circadian oscillator function intact in central circadian pacemaker neurons but renders peripheral circadian oscillators largely arrhythmic. Although this arrhythmicity could be caused by a loss of light entrainment, it is also consistent with a role for CRY in the oscillator. A peripheral oscillator drives circadian olfactory responses in Drosophila antennae. Here we show that CRY contributes to oscillator function and physiological output rhythms in the antenna during and after entrainment to light-dark cycles and after photic input is eliminated by entraining flies to temperature cycles. These results demonstrate a photoreceptor-independent role for CRY in the periphery and imply fundamental differences between central and peripheral oscillator mechanisms in Drosophila.

Odor coding in the Drosophila antenna.

Odor coding in the Drosophila antenna is examined by a functional analysis of individual olfactory receptor neurons (ORNs) in vivo. Sixteen distinct classes of ORNs, each with a unique response spectrum to a panel of 47 diverse odors, are identified by extracellular recordings. ORNs exhibit multiple modes of response dynamics: an individual neuron can show either excitatory or inhibitory responses, and can exhibit different modes of termination kinetics, when stimulated with different odors. The 16 ORN classes are combined in stereotyped configurations within seven functional types of basiconic sensilla. One sensillum type contains four ORNs and the others contain two neurons, combined according to a strict pairing rule. We provide a functional map of ORNs, showing that each ORN class is restricted to a particular spatial domain on the antennal surface.

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.

The inositol 1,4,5-trisphosphate receptor is required for maintenance of olfactory adaptation in Drosophila antennae.

A role for inositol 1,4,5-trisphosphate (IP(3)) as a second messenger during olfactory transduction has been postulated in both vertebrates and invertebrates. However, given the absence of either suitable pharmacological reagents or mutant alleles specific for the IP(3) signaling pathway, an unequivocal demonstration of IP(3) function in olfaction has not been possible. Here we have investigated the role of a well-established cellular target of IP(3)-the IP(3) receptor (IP(3)R)-in olfactory transduction in Drosophila. For this purpose we tested existing viable combinations of IP(3)R mutant alleles, as well as a newly generated set of viable itpr alleles, for olfactory function. In all of the viable allelic combinations primary olfactory responses were found to be normal. However, a subset of itpr alleles (including a null allele) exhibit faster recovery after a strong pulse of odor, indicating that the IP(3)R is required for maintenance of olfactory adaptation. Interestingly, this defect in adaptation is dominant for two of the alleles tested, suggesting that the mechanism of adaptation is sensitive to levels of the IP(3)R.

Irregular chiasm-C-roughest, a member of the immunoglobulin superfamily, affects sense organ spacing on the Drosophila antenna by influencing the positioning of founder cells on the disc ectoderm.

We describe a role for Irregular chiasmC-roughest (IrreC-rst), an immunoglobulin (Ig) superfamily member, in patterning sense organs on the Drosophila antenna. IrreC-rst protein is initially expressed homogeneously on apical profiles of ectodermal cells in regions of the antennal disc. During specification of founder cells (FCs), the intracellular protein distribution changes and becomes concentrated in regions where specific intercellular contacts presumably occur. Loss of function mutations as well as misexpression of irreC-rst results in an altered arrangement of FCs within the disc compared to wildtype. Sense organ development occurs normally, although spacing is affected. Unlike its role in interommatidial spacing, irreC-rst does not affect apoptosis during antennal development. We propose that IrreC-rst affects the spatial relationship between sensory and ectodermal cells during FC delamination.

Olfactory coding in a compound nose. Coexpression of odorant-binding proteins in Drosophila.

Odorant-binding proteins (OBPs) are small, soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. We have examined the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We found that the OS-E and OS-F transcription units are located < 1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is spatially restricted to the ventrolateral region of the Drosophila antenna. Within this region, both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. This finding has potential implications for the roles of odorant-binding proteins in olfactory coding.

Control of antennal versus leg development in Drosophila.

During the evolution of insects from a millipede-like ancestor, the Hox genes are thought to have promoted the diversification of originally identical body structures. In Drosophila melanogaster, antennae and legs are homologous structures that differ from each other as a result of the Hox gene Antennapedia (Antp), which promotes leg identities by repressing unknown antennal-determining genes. Here we present four lines of evidence that identify extradenticle (exd) and homothorax (hth) as antennal-determining genes. First, removing the function of exd or hth, which is required for the nuclear localization of Exd protein, transforms the antenna into leg; such transformations occur without activation of Antp. Second, hth is expressed and Exd is nuclear in most antennal cells, whereas both are restricted to proximal cells of the leg. Third, Antp is a repressor of hth. Fourth, ectopic expression of Meis1, a murine hth homologue, can trigger antennal development elsewhere in the fly. Taken together, these data indicate that hth is an antennal selector gene, and that Antp promotes leg development by repressing hth and consequently nuclear Exd.

Odorant response of individual sensilla on the Drosophila antenna.

We have documented odor responses of all morphological classes of sensilla on the surface of the Drosophila antenna: sensilla basiconica, sensilla trichodea, and sensilla coeloconica. Both subtypes of s. basiconica, large and small, respond to odors. S. trichodea fall into different functional types. Type 1 appears narrowly tuned, as it responded only to cis-vaccenyl acetate, believed to be a pheromone. Type 2 responded to trans-2-hexenal and 4-methyl cyclohexanol. These two types of s. trichodea are differentially distributed on the antennal surface, and have dramatically different frequencies of spontaneous action potentials. Likewise, there are multiple types of s. coeloconica. One type if broadly tuned, responding most strongly to a test stimulus of butyric acid, but also to a variety of other odors; it is restricted to the dorso-medial portion of the third antennal segment. A second type gave detectable responses only to trans-2-hexenal. These results demonstrate that all classes of sensilla are olfactory, and they reveal the organizational complexity of the Drosophila olfactory system.

The Drosophila antenna: ultrastructural and physiological studies in wild-type and lozenge mutants.

The physiology and ultrastructure of the antenna in Drosophila melanogaster have been examined in wild-type and lozenge mutants. Scanning electron microscopy (SEM) of sensilla on the antennal surface has revealed that in the wild-type the basiconic sensilla contain linear arrays of pores connected by longitudinal furrows and transected by shorter furrows. Sensilla trichodea also are shown to have pores, as revealed by examining transverse sections by transmission electron microscopy (TEM); these data directly address a longstanding controversy. Coeloconic sensilla, previously described as "pit sensilla" and as "grooved" sensilla, are shown to rise directly from the antennal surface, as opposed to lying below the antennal surface in pits; the previously observed grooves correspond to the junctions between bundled, finger-like projections. This description of coeloconic sensilla is supported by analysis of lz mutants, in which the projections of coeloconic sensilla splay apart. Coeloconic sensilla are also shown to undergo duplication on the lz3 antenna. Physiological recordings from the antenna show that responses to all odorants tested are severely decreased in lz mutants. Measurements made from different parts of the antenna show similar defects. Evidence is provided that both the physiological and ultrastructural defects map to the lz locus.

Time- and Concentration-Dependent Response of the Drosophila Antenna Imaginal Disc to Antennapedia

The response of the antenna imaginal disc to ectopic Antennapedia gene expression was explored in a heat shock Antennapedia (hsAntp) transgenic strain and in strains doubly transgenic for hsAntp and downstream enhancer trap targets, The distal to proximal changes in morphological transformation in response to Antennapedia product at different developmental stages were correlated with changing expression patterns of transgene targets from antenna to leg-like patterns. Dose-response studies indicated changing thresholds of response to Antennapedia. At particular stages and doses of Antennapedia product, cell differentiation of leg bristles was uncoupled from transformation of the third antennal segment to tarsus. The results suggest that determination for bristle type does not depend on a prior determination decision for organ type. The results also provide an avenue for exploring the nature of "competence" at cellular and molecular levels.

Olfactory physiology in the Drosophila antenna and maxillary palp: acj6 distinguishes two classes of odorant pathways.

This article provides characterization of the electrical response to odorants in the Drosophila antenna and provides physiological evidence that a second organ, the maxillary palp, also has olfactory function in Drosophila. The acj6 mutation, previously isolated by virtue of defective olfactory behavior, affects olfactory physiology in the maxillary palp as well as in the antenna. Interestingly, abnormal chemosensory jump 6 (acj6) reduces response in the maxillary palp to all odorants tested except benzaldehyde (odor of almond), as if response to benzaldehyde is mediated through a different type of odorant pathway from the other odorants. In other experiments, different parts of the antenna are shown to differ with respect to odorant sensitivity. Evidence is also provided that antennal response to odorants varies with age, and that odorants differ in their age dependence.

Comparative electrophoretic studies of third antennal segments of Drosophila melanogaster

1. Intact third segments of Drosophila antennae, where the sensilla involved in chemoreception are located, have been isolated. 2. Homogenates from flies differing in age and sex have been subjected to denaturing and non-denaturing electrophoresis. 3. Comparison of the polypeptide patterns shows differences related to developmental changes as well as specific female and male features pointing out sexual dimorphism, especially in adult flies. 4. These results also suggest that the technique of isolation as well as the comparative studies of third antennal segments should be useful for further investigations on Drosophila chemoreception.