Genesis of an organ: molecular analysis of the pha-1 gene.

The organisation of organ formation is still an unsolved problem. Mutations in the zygotic lethal gene pha-1 affect a late step during organ development in the nematode C. elegans. In mutant embryos all tissues in the pharynx fail to undergo terminal differentiation and morphogenesis. The expression of an early differentiation marker in pharyngeal muscle precursors is not impaired in mutant embryos, which suggests that pharynx cells still acquire their identity. Therefore the gene defines an organ-specific terminal differentiation function. We cloned and sequenced the pha-1 gene and found that the deduced protein sequence contains features characteristic of the bZIP family of transcription factors. During embryogenesis a transgenic pha-1 reporter construct is expressed transiently in all pharynx precursor cells at the time when these cells become restricted to form the pharynx organ. A mosaic analysis of the requirement of pha-1 activity during pharynx formation is consistent with the notion that pha-1 acts cell-autonomously in all cells of the pharynx primordium. The data suggest that pha-1 initiates and coordinates programs required for cytodifferentiation and morphogenesis in all cell types of the entire organ on the transcriptional level. We propose that organs are independent developmental units whose identity is reflected on the gene regulatory level.