title: The four-jointed gene is required in the Drosophila eye for ommatidial polarity specification.
abstract: BACKGROUND
  The Drosophila eye is composed of about 800 ommatidia, each of which becomes dorsoventrally polarised in a process requiring signalling through the Notch, JAK/STAT and Wingless pathways. These three pathways are thought to act by setting up a gradient of a signalling molecule (or molecules) often referred to as the 'second signal'. Thus far, no candidate for a second signal has been identified.
  
  
  RESULTS
  The four-jointed locus encodes a type II transmembrane protein that is expressed in a dorsoventral gradient in the developing eye disc. We have analysed the function and regulation of four-jointed during eye patterning. Loss-of-function clones or ectopic expression of four-jointed resulted in strong non-autonomous defects in ommatidial polarity on the dorsoventral axis. Ectopic expression experiments indicated that localised four-jointed expression was required at the time during development when ommatidial polarity was being determined. In contrast, complete removal of four-jointed function resulted in only a mild ommatidial polarity defect. Finally, we found that four-jointed expression was regulated by the Notch, JAK/STAT and Wingless pathways, consistent with it mediating their effects on ommatidial polarity.
  
  
  CONCLUSIONS
  The clonal phenotypes, time of requirement and regulation of four-jointed are consistent with it acting in ommatidial polarity determination as a second signal downstream of Notch, JAK/STAT and Wingless. Interestingly, it appears to act redundantly with unknown factors in this process, providing an explanation for the previous failure to identify a second signal.
authors.affiliation: Department of Genetics, Harvard Medical School, Boston, 02115, USA.
authors.forename: M P
authors.initials: MP
authors.lastname: Zeidler
authors.forename: N
authors.initials: N
authors.lastname: Perrimon
authors.forename: D I
authors.initials: DI
authors.lastname: Strutt
mesh_terms: D000818:Animals; D001483:Base Sequence; D019521:Body Patterning / Q000235:genetics*; D003001:Cloning, Molecular; D004268:DNA-Binding Proteins / Q000378:metabolism; D004330:Drosophila / Q000196:embryology*; D029721:Drosophila Proteins; D005123:Eye / Q000196:embryology* / Q000378:metabolism; D051157:Frizzled Receptors; D019476:Insect Proteins / Q000502:physiology*; D053616:Janus Kinase 3; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D009154:Mutation; D016133:Polymerase Chain Reaction; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled; D051880:Receptors, Notch; D050794:STAT1 Transcription Factor; D015398:Signal Transduction; D015534:Trans-Activators / Q000378:metabolism; D051155:Wnt1 Protein
chemical_list: D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D051157:Frizzled Receptors; D019476:Insect Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; C497759:N protein, Drosophila; D011518:Proto-Oncogene Proteins; D043562:Receptors, G-Protein-Coupled; D051880:Receptors, Notch; D050794:STAT1 Transcription Factor; D015534:Trans-Activators; D051155:Wnt1 Protein; C095900:fj protein, Drosophila; C058784:fz protein, Drosophila; C074622:wg protein, Drosophila; D011505:Protein-Tyrosine Kinases; D053616:Janus Kinase 3