Regulation of the spalt/spalt-related gene complex and its function during sensory organ development in the Drosophila thorax.
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The nuclear proteins Spalt and Spalt-related belong to a conserved family of transcriptional regulators characterised by the presence of double zinc-finger domains. In the wing, they are regulated by the secreted protein Decapentaplegic and participate in the positioning of the wing veins. Here, we identify regulatory regions in the spalt/spalt-related gene complex that direct expression in the wing disc. The regulatory sequences are organised in independent modules, each of them responsible for expression in particular domains of the wing imaginal disc. In the thorax, spalt and spalt-related are expressed in a restricted domain that includes most proneural clusters of the developing sensory organs in the notum, and are regulated by the signalling molecules Wingless, Decapentaplegic and Hedgehog. We find that spalt/spalt-related participate in the development of sensory organs in the thorax, mainly in the positioning of specific proneural clusters. Later, the expression of at least spalt is eliminated from the sensory organ precursor cells and this is a requisite for the differentiation of these cells. We postulate that spalt and spalt-related belong to a category of transcriptional regulators that subdivide the thorax into expression domains (prepattern) required for the localised activation of proneural genes. (+info)
Odor coding in a model olfactory organ: the Drosophila maxillary palp.
(10/535)
Odor coding relies on the activity of different classes of receptor neurons, each with distinct response characteristics. We have examined odor coding in a model olfactory organ, the maxillary palp of Drosophila. This organ contains only 120 olfactory receptor neurons, compartmentalized in sensory hairs called sensilla, and provides an opportunity to characterize all neurons in an entire olfactory organ. Extensive extracellular recordings from single sensilla reveal that the neurons fall into six functional classes. Each of the 60 sensilla houses two neurons, which observe a pairing rule: each sensillum combines neurons of two particular classes, thereby yielding three sensillum types. The sensillum types are intermingled on the surface of the palp, but their distribution is not random. The neurons exhibit diverse response characteristics, providing the basis for an olfactory code. A particular odor can excite one neuron and inhibit another, and a particular neuron can be excited by one odor and inhibited by another. Some excitatory responses continue beyond the end of odor delivery, but responses to most odors terminate abruptly after the end of odor delivery, with some followed by a period of poststimulus quiescence. The specificity of odor response is examined in detail for the neurons of one sensillum, which were found to differ in their relative responses to a homologous series of esters. Adaptation and cross-adaptation are documented, and cross-adaptation experiments demonstrate that the two neurons within one type of sensillum can function independently. The analysis of all neuronal types in this model olfactory organ is discussed in terms of its functional organization and the mechanisms by which it encodes olfactory information. (+info)
Antagonism of EGFR and notch signalling in the reiterative recruitment of Drosophila adult chordotonal sense organ precursors.
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The selection of Drosophila melanogaster sense organ precursors (SOPs) for sensory bristles is a progressive process: each neural equivalence group is transiently defined by the expression of proneural genes (proneural cluster), and neural fate is refined to single cells by Notch-Delta lateral inhibitory signalling between the cells. Unlike sensory bristles, SOPs of chordotonal (stretch receptor) sense organs are tightly clustered. Here we show that for one large adult chordotonal SOP array, clustering results from the progressive accumulation of a large number of SOPs from a persistent proneural cluster. This is achieved by a novel interplay of inductive epidermal growth factor-receptor (EGFR) and competitive Notch signals. EGFR acts in opposition to Notch signalling in two ways: it promotes continuous SOP recruitment despite lateral inhibition, and it attenuates the effect of lateral inhibition on the proneural cluster equivalence group, thus maintaining the persistent proneural cluster. SOP recruitment is reiterative because the inductive signal comes from previously recruited SOPs. (+info)
Responses of Xenopus laevis water nose to water-soluble and volatile odorants.
(12/535)
Using the whole-cell mode of the patch-clamp technique, we recorded action potentials, voltage-activated cationic currents, and inward currents in response to water-soluble and volatile odorants from receptor neurons in the lateral diverticulum (water nose) of the olfactory sensory epithelium of Xenopus laevis. The resting membrane potential was -46.5 +/- 1.2 mV (mean +/- SEM, n = 68), and a current injection of 1-3 pA induced overshooting action potentials. Under voltage-clamp conditions, a voltage-dependent Na+ inward current, a sustained outward K+ current, and a Ca2+-activated K+ current were identified. Application of an amino acid cocktail induced inward currents in 32 of 238 olfactory neurons in the lateral diverticulum under voltage-clamp conditions. Application of volatile odorant cocktails also induced current responses in 23 of 238 olfactory neurons. These results suggest that the olfactory neurons respond to both water-soluble and volatile odorants. The application of alanine or arginine induced inward currents in a dose-dependent manner. More than 50% of the single olfactory neurons responded to multiple types of amino acids, including acidic, neutral, and basic amino acids applied at 100 microM or 1 mM. These results suggest that olfactory neurons in the lateral diverticulum have receptors for amino acids and volatile odorants. (+info)
Revisiting the Drosophila microchaete lineage: a novel intrinsically asymmetric cell division generates a glial cell.
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The bristle mechanosensory organs of the adult fly are composed of four different cells that originate from a single precursor cell, pI, via two rounds of asymmetric cell division. Here, we have examined the pattern of cell divisions in this lineage by time-lapse confocal microscopy using GFP imaging and by immunostaining analysis. pI divided within the plane of the epithelium and along the anteroposterior axis to give rise to an anterior cell, pIIb, and a posterior cell, pIIa. pIIb divided prior to pIIa to generate a small subepithelial cell and a larger daughter cell, named pIIIb. This unequal division, oriented perpendicularly to the epithelium plane, has not been described previously. pIIa divided after pIIb, within the plane of the epithelium and along the AP axis, to produce a posterior socket cell and an anterior shaft cell. Then pIIIb divided perpendicularly to the epithelium plane to generate a basal neurone and an apical sheath cell. The small subepithelial pIIb daughter cell was identified as a sense organ glial cell: it expressed glial cell missing, a selector gene for the glial fate and migrated away from the sensory cluster along extending axons. We propose that mechanosensory organ glial cells, the origin of which was until now unknown, are generated by the asymmetric division of pIIb cells. Both Numb and Prospero segregated specifically into the basal glial and neuronal cells during the pIIb and pIIIb divisions, respectively. This revised description of the sense organ lineage provides the basis for future studies on how polarity and fate are regulated in asymmetrically dividing cells. (+info)
The Drosophila Ral GTPase regulates developmental cell shape changes through the Jun NH(2)-terminal kinase pathway.
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The Ral GTPase is activated by RalGDS, which is one of the effector proteins for Ras. Previous studies have suggested that Ral might function to regulate the cytoskeleton; however, its in vivo function is unknown. We have identified a Drosophila homologue of Ral that is widely expressed during embryogenesis and imaginal disc development. Two mutant Drosophila Ral (DRal) proteins, DRal(G20V) and DRal(S25N), were generated and analyzed for nucleotide binding and GTPase activity. The biochemical analyses demonstrated that DRal(G20V) and DRal(S25N) act as constitutively active and dominant negative mutants, respectively. Overexpression of the wild-type DRal did not cause any visible phenotype, whereas DRal(G20V) and DRal(S25N) mutants caused defects in the development of various tissues including the cuticular surface, which is covered by parallel arrays of polarized structures such as hairs and sensory bristles. The dominant negative DRal protein caused defects in the development of hairs and bristles. These phenotypes were genetically suppressed by loss of function mutations of hemipterous and basket, encoding Drosophila Jun NH(2)-terminal kinase kinase (JNKK) and Jun NH(2)-terminal kinase (JNK), respectively. Expression of the constitutively active DRal protein caused defects in the process of dorsal closure during embryogenesis and inhibited the phosphorylation of JNK in cultured S2 cells. These results indicate that DRal regulates developmental cell shape changes through the JNK pathway. (+info)
Negative regulation of dorsoventral signaling by the homeotic gene Ultrabithorax during haltere development in Drosophila.
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Growth and patterning during Drosophila wing development are mediated by signaling from its dorsoventral (D/V) organizer. In the metathorax, wing development is essentially suppressed by the homeotic selector gene Ultrabithorax (Ubx) to mediate development of a pair of tiny balancing organs, the halteres. Here we show that expression of Ubx in the haltere D/V boundary down-regulates its D/V organizer signaling compared to that of the wing D/V boundary. Somatic loss of Ubx from the haltere D/V boundary thus results in the formation of a wing-type D/V organizer in the haltere field. Long-distance signaling from this organizer was analyzed by assaying the ability of a Ubx(-) clone induced in the haltere D/V boundary to effect homeotic transformation of capitellum cells away from the boundary. The clonally restored wing D/V organizer in mosaic halteres not only enhanced the homeotic transformation of Ubx(-) cells in the capitellum but also caused homeotic transformation of even Ubx(+) cells in a genetic background known to induce excessive cell proliferation in the imaginal discs. In addition to demonstrating a non-cell-autonomous role for Ubx during haltere development, these results reveal distinct spatial roles of Ubx during maintenance of cell fate and patterning in the halteres. (+info)
The spineless-aristapedia and tango bHLH-PAS proteins interact to control antennal and tarsal development in Drosophila.
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The Drosophila spineless (ss) gene encodes a basic-helix-loop-helix-PAS transcription factor that is required for proper specification of distal antennal identity, establishment of the tarsal regions of the legs, and normal bristle growth. ss is the closest known homolog of the mammalian aryl hydrocarbon receptor (Ahr), also known as the dioxin receptor. Dioxin and other aryl hydrocarbons bind to the PAS domain of Ahr, causing Ahr to translocate to the nucleus, where it dimerizes with another bHLH-PAS protein, the aryl hydrocarbon receptor nuclear translocator (Arnt). Ahr:Arnt heterodimers then activate transcription of target genes that encode enzymes involved in metabolizing aryl hydrocarbons. In this report, we present evidence that Ss functions as a heterodimer with the Drosophila ortholog of Arnt, Tango (Tgo). We show that the ss and tgo genes have a close functional relationship: loss-of-function alleles of tgo were recovered as dominant enhancers of a ss mutation, and tgo-mutant somatic clones show antennal, leg, and bristle defects almost identical to those caused by ss(-) mutations. The results of yeast two-hybrid assays indicate that the Ss and Tgo proteins interact directly, presumably by forming heterodimers. Coexpression of Ss and Tgo in Drosophila SL2 cells causes transcriptional activation of reporters containing mammalian Ahr:Arnt response elements, indicating that Ss:Tgo heterodimers are very similar to Ahr:Arnt heterodimers in DNA-binding specificity and transcriptional activation ability. During embryogenesis, Tgo is localized to the nucleus at sites of ss expression. This localization is lost in a ss null mutant, suggesting that Tgo requires heterodimerization for translocation to the nucleus. Ectopic expression of ss causes coincident ectopic nuclear localization of Tgo, independent of cell type or developmental stage. This suggests that the interaction of Ss and Tgo does not require additional signals, unlike the ligand-dependent interaction of Ahr and Arnt. Despite the very different biological roles of Ahr and Arnt in insects and mammals, the molecular mechanisms by which these proteins function appear to be largely conserved. (+info)