PHA-4, AN HNF-3 HOMOLOG, SPECIFIES PHARYNGEAL ORGAN IDENTITY IN CAENORHABDITIS-ELEGANS

To build complex organs, embryos have evolved mechanisms that integrat e the development of cells unrelated to one another by cell type or an cestry. Here we show that the pha-4 locus establishes organ identity f or the Caenorhabditis elegans pharynx. In pha-4 mutants, pharyngeal ce lls are transformed into ectoderm. Conversely, ectopic pha-4 expressio n produces excess pharyngeal cells, pha-4 encodes an HNF-3 homolog sel ectively expressed in the nascent digestive tract, including all phary nx precursors at the time they are restricted to a pharyngeal fate. We suggest that pha-4 is a key component of a transcription-based mechan ism to endow cells with pharyngeal organ identity.