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Using the fruit fly Drosophila melanogaster as model host, we have identified mutants of the bacterium Pseudomonas aeruginosa with reduced virulence. Strikingly, all strains strongly impaired in fly killing also lacked twitching motility; most such strains had a mutation in pilGHIJKL chpABCDE, a gene cluster known to be required for twitching motility and potentially encoding a signal transduction system. The pil chp genes appear to control the expression of additional virulence factors, however, since the wild-type fly-killing phenotype of a subset of mutants isolated on the basis of their compact colony morphology indicated that twitching motility itself was not required for full virulence in the fly.

The homeless (hls) gene of Drosophila is required for anteroposterior and dorsoventral axis formation during oogenesis. At a low frequency, females homozygous for mutations in hls generate early egg chambers in which the oocyte is positioned incorrectly within the cyst. At a high frequency, late-stage egg chambers exhibit a ventralized chorion. Sequence analysis of the hls cDNA predicts a protein with amino-terminal homology to members of the DE-H family of RNA-dependent ATPases and putative helicases. Similarity of 51% in the amino-terminal third of the protein was found to two yeast splicing factors, PRP2 and PRP16, and to Drosophila Maleless, which is required for dosage compensation. To analyze Hls function, RNA localization patterns were determined for seven different transcripts in hls mutant ovaries. Previtellogenic transport to the oocyte was unaffected for all transcripts examined. Transport and localization of bicoid and oskm messages during vitellogenic stages were strongly disrupted, and the distribution and/or quantity of gurken, orb, and fs(3)KIO mRNAs were also affected, but to a lesser degree. In contrast, hu-li tai shao and Biicaudul-D transcripts were transported and localized normally in hls mutants. In addition, Kinesin heavy chain:p-Galactosidase fusion protein failed to localize correctly to the posterior of the oocyte in vitellogenic egg chambers. Examination of the microtubule structure with anti-a-Tubulin antibodies revealed aberrant microtubule organizing center movement and an abnormally dense cytoplasmic microtubule meshwork. We discuss potential roles for Hls in organizing a cytoskeletal framework essential for localizing specific RNAs.

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The Drosophila egg chamber provides an excellent model for studying the link between patterning and morphogenesis. Late in oogenesis, a portion of the flat follicular epithelium remodels to form two tubes; secretion of eggshell proteins into the tube lumens creates the dorsal appendages. Two distinct cell types contribute to dorsal appendage formation: cells expressing the rhomboid-lacZ (rho-lacZ) marker form the ventral floor of the tube and cells expressing high levels of the transcription factor Broad form a roof over the rho-lacZ cells. In mutants that produce defective dorsal appendages (K10, Ras and ectopic decapentaplegic) both cell types are specified and reorganize to occupy their stereotypical locations within the otherwise defective tubes. Although the rho-lacZ and Broad cells rearrange to form a tube in wild type and mutant egg chambers, they never intermingle, suggesting that a boundary exists that prevents mixing between these two cell types. Consistent with this hypothesis, the Broad and rho-lacZ cells express different levels of the homophilic adhesion molecule Fasciclin 3. Furthermore, in the anterior of the egg, ectopic rhomboid is sufficient to induce both cell types, which reorganize appropriately to form an ectopic tube. We propose that signaling across a boundary separating the rho-lacZ and Broad cells choreographs the cell shape-changes and rearrangements necessary to transform an initially flat epithelium into a tube.

Boundaries establish and maintain separate populations of cells critical for organ formation. We show that Notch signaling establishes the boundary between two types of post-mitotic epithelial cells, the Rhomboid- and the Broad-positive cells. These cells will undergo morphogenetic movements to generate the two sides of a simple organ, the dorsal appendage tube of the Drosophila egg chamber. The boundary forms due to a difference in Notch levels in adjacent cells. The Notch expression pattern mimics the boundary; Notch levels are high in Rhomboid cells and low in Broad cells. Notch(-) mutant clones generate an ectopic boundary: ectopic Rhomboid cells arise in Notch(+) cells adjacent to the Notch(-) mutant cells but not further away from the clonal border. Pangolin, a component of the Wingless pathway, is required for Broad expression and for rhomboid repression. We further show that Broad represses rhomboid cell autonomously. Our data provide a foundation for understanding how a single row of Rhomboid cells arises adjacent to the Broad cells in the dorsal appendage primordia. Generating a boundary by the Notch pathway might constitute an evolutionarily conserved first step during organ formation in many tissues.

The Drosophila gene tramtrack (ttk) encodes two transcriptional repressors, Ttk69 and Ttk88, which are required for normal embryogenesis and imaginal disc development. Here, we characterize a novel female sterile allele of tramtrack called twin peaks (ttk(twk)) that, unlike othertramtrack alleles, has no effect on viability and produces no obvious morphological defects, except during oogenesis. Females homozygous for twin peaks produce small eggs with thin eggshells and short dorsal respiratory appendages. Complementation analyses, immunolocalization, and rescue data demonstrate that these defects are due to loss of Ttk69, which is expressed in the follicle cells and is required for normal chorion production and dorsal follicle-cell migration. Analyses of phenotypes produced by mutations in other loci that regulate eggshell synthesis suggest that the chorion production and follicle-cell migration defects are independent. We present evidence that twin peaks disrupts a promoter or promoters required for late-stage follicle-cell expression of Ttk69. We hypothesize that loss of Ttk69 in all follicle cells disrupts chorion gene expression and lack of function in dorsal anterior follicle cells inhibits morphogenetic changes required for elongating the dorsal appendages.