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First published online August 18, 2003
doi: 10.1242/10.1242/dev.00680

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The Drosophila proneural gene amos promotes olfactory sensillum formation and suppresses bristle formation

Petra I. zur Lage, David R. A. Prentice, Eimear E. Holohan and Andrew P. Jarman*

The Wellcome Trust Centre for Cell Biology, Institute of Cell and Molecular Biology, University of Edinburgh, King's Buildings, Edinburgh EH9 3JR, UK



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  		Fig. 1. Antennal defects in amos mutants. Third antennal segments are shown. (A) Wild-type antenna, with trichodea indicated (t). (B) Higher power view with examples marked of sensilla coeloconica (c), basiconica (b) and trichodea (t). (C) amos1/Df(2L)M36-S6. Third segment is reduced because of missing basiconica and trichodea. Ectopic mechanosensory bristles are indicated (br). (D) Higher power view showing an abnormal domed sensillum (arrow) and a normal sensillum coeloconicum (arrowhead). Sensilla are very sparse. (E) amos2 with numerous abnormal sensilla (arrows). (F) Double mutant for amos and ato (clone of amos1 tissue in ato1 fly: eyFLP; FRT-amos1/FRT-nlsGFP; ato1). The clone patch contains no sense organs except bristles. (G) Double mutant for amos and sc10-1 (which removes both ac and sc function). The extra bristles of amos1 are largely dependent on ac/sc function. (H) Double mutant for amos and lz. The extra bristles of amos1 are absent and therefore depend on lz function.

 


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  		Fig. 2. Olfactory receptor neurons in amos mutants. Confocal projection images of late pupal antennae stained to detect sensory neurons. (A) Wild type. Clusters of cell bodies and their dendrites can be seen. (B) amos3 mutant showing far fewer clusters. (C,D) Higher magnification views. Although most ORNs are clustered, as seen by the multiple dendrites (*) (representing sensilla coeloconica), some sensilla appear to be mono-innervated (arrows) and may represent the bristles. (E) Wild-type confocal section showing the three olfactory nerve bundles. (F) Confocal section of amos mutant, with the three bundles labelled (a small section has been pasted in from another confocal plane to show clearly the second bundle).

 


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  		Fig. 3. Amos expression during olfactory precursor formation. (A-L) Time course of Amos protein expression relative to olfactory precursor formation. In all cases we concentrate on precursors in the third segment, although a large number of chordotonal precursors are also visible in the surrounding second segment (carets; see also the summary in Fig. 4). (A,E,I) At 0 hours APF, the first wave of precursors appear (arrow). (B,F,J) At 4 hours APF, the second wave of precursors appears in a highly characteristic pattern (bracket). (C,G,K) At 8 hours APF, the third wave of precursors accumulate between the rows of the second wave, eventually obscuring any clear pattern by 16 hours APF (D,H,L). (A-D) Amos expression is detected throughout this time, but the expression is ectodermal from 0-4 hours APF, and then it co-labels with some of the precursors between 4-16 hours APF. Amos continues to be expressed in some cells at 16 hours APF, and these cells seem to be a mixture of ectodermal cells and precursors.

 


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  		Fig. 4. Olfactory precursors and amos/ato expression. (A,B) ato is expressed in the wave 1 precursors. The arrows mark the ends of this semicircular line of olfactory precursors. ato-dependent arista precursors (*) and chordotonal precursors (caret) are also marked. (C,D) ato is expressed in the wave 2 olfactory precursors (bracket). It is not expressed in the third wave, the first cells of which can be seen between the Ato rows (arrows in D). (E,F) amos is expressed in a complementary way to ato in the third wave of precursors. (E) Confocal projection of a stack of images showing Amos detection in a similar disc to D. (F) Deep confocal section from E showing nuclei of amos-expressing precursors (arrows) underlying the main amos proneural expression domain. These correspond to the non-Ato-expressing precursors (arrows in D). (G) Schematic summary of amos- and ato-dependent precursor pattern at ~8 hours APF.

 


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  		Fig. 5. Olfactory precursors in amos and ato mutants. These discs should be compared with the corresponding wild-type discs in Figs 3 and 4. (A,D) The early precursors are specifically lost in ato mutants. The remaining olfactory precursors correspond to the third wave (B,C) and align very closely with the amos expression domains (E,F). In the second segment, the chordotonal precursors are also missing and only a few bristle precursors remain (*, A-C). (G-L) The late precursors are specifically lost in amos mutants. (G,J) Early precursor pattern resembles wild type, with mutant Amos1 protein detectable between the rows of precursors (brackets). Caret in G indicates chordotonal precursors. (H,K) At 8 hours APF, the pattern remains unchanged as the third wave SOPs are not formed (c.f. Fig. 4C). These early precursors mostly express Ato, although a number of non-Ato expressing SOPs appear between the early rows, which could correspond to the bristle SOPs (arrows in K). (I,L) The early pattern is still apparent at 16 hours APF as it has not been obliterated by the third wave of SOPs (the early SOPs have now been replaced by PSCs, some of which are ringed). (M,N) Cut expression appears prematurely activated in amos mutants. (M) Wild type at 8 hours APF. No Cut expression is detectable in the third segment SOPs; however, Cut stains very strongly in the surrounding tissue (caret). (N) amos1 mutant at 8 hours APF. Some Cut labelling (arrows) appears in SOPs derived from the Amos-expressing domains (in this case expressing non-functional Amos1 protein). These cells seem to correspond to the ato-independent cells in K.

 


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  		Fig. 6. Fate of amos-expressing cells during olfactory development. (A-D,G) Activity of an amos SOP enhancer driving GFP expression. (A,B) At 8 hours APF, GFP can be detected in the third wave of olfactory precursors, some co-labelled SOPs are indicated by arrows (co-labelled with Sens and Amos in the separation in B). (C,D) The amos-GFP expressing cells contribute to late differentiating sensilla, as shown by lack of co-labelling with a neuronal marker (22C10) at 24 hours APF. C is a projection of many sections whereas D is a confocal section with some of the differentiating neurons marked by asterisks, these do not express GFP. (E,F) Later expression of amos does not correspond to PSC cells. (E) At 16 hours APF, Amos expression is fading, but there is no overlap with A101 ß-galactosidase expression in the PSCs, some of which are ringed. (F) There is no overlap of Amos expression with that of Pros, a marker of one of the PSCs. (G) amos-GFP construct at 30 hours APF: a large number of sensilla retain GFP. Protein appears to be in sensillar groups (as indicated by rings), and includes the outer support cells, so that sensilla trichodea (t) and basiconica (b) can clearly be discerned. (H,I) Analysis of amos-GFP in confocal sections of antennae at 24 hour APF relative to the component cells of the sensilla (see insets). n, neuron; sh, sheath; os, outer support cells. amos-GFP labels rows of cells corresponding to each sensillum basiconicum or trichodeum (some are ringed), whereas presumptive coeloconica (c) do not express GFP. (H) GFP is expressed in neurons (marked by Elav) and sheath cells (marked by Pros). (I) GFP is expressed in outer support cells (marked by stronger expression of Cut).

 


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  		Fig. 7. Expression of sc and sc target genes in the antenna. (A-D) sc mRNA detected by in situ hybridisation. (A) Wild type, with sc expressed not only in the second antennal segment (caret) but also in the third segment (arrows). (B) Wild type, with sc RNA detected by immunofluorescence (green). (C) amos1 mutant. sc mRNA is increased and is present in SOPs (arrows). (D) The second segment sc expression is reduced in lz34 mutants. (E,F) Ac expression is present in some SOPs in amos mutants. (E) Wild type at 8 hours APF, showing very little Ac expression in the third segment (first precursor wave marked by arrow) (some is visible in the second segment; caret). (F) amos3 mutant at 8 hours APF, showing some Ac expression in second segment (arrow). (G,H) GFP expression from sc-E1-GFP reporter transgene. (G) Wild type, showing no expression in third segment. (H) amos3 mutant showing expression (*).

 


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  		Fig. 8. amos misexpression represses bristle formation. (A) Wild type dorsal thorax. (B) Dorsal thorax from 109-68Gal4/UAS-amos fly. amos misexpression driven in sc PNCs by this driver line results in loss of many bristles (mainly the large macrochaetae). (C) Summary of proneural functions in antenna. Diversity of sense organs laid down by function of three proneural gene systems. Blue, atonal; red, amos; green, achaete/scute.

 

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© The Company of Biologists Ltd 2003