The worm digestive system can be subdivided into three regions: pharynx, gut and rectum. Formation of the pharynx and rectum depends on zygotic
pha-4 activity1. In
pha-4 mutants, rectal valves and rectal epithelial cells are often missing. In addition, markers for the earliest stage of pharynx development are not expressed. For example, a
pha-1::lacZ construct that is normally active in pharyngeal precursors and body wall muscles at the 200 cell stage2, is only expressed in body wall muscles in
pha-4 mutants. What happens to the pharyngeal and rectal progenitors in
pha-4 embryos?
pha-4 mutant heads contain extra hypodermal cells and glia, as shown by staining with anti- LIN-26 antibodies. To ask if cells that would normally develop as pharyngeal cells have now become glia and hypodermis, we counted LIN-26+ cells made by each 4-cell-stage blastomere after killing its neighbors with a laser. In wildtype embryos, ABa and EMS generate the pharynx, ABp produces the rectum, and P2 does not contribute descendants to the digestive tract. In
pha-4 mutants, EMS produces extra LIN-26+ cells, whereas P2 does not. (counts for AB descendants are in progress).These results suggest that cells that would normally become part of the pharynx develop as hypodermis or glia in
pha-4 mutants. We propose that
pha-4 acts at about the 200 cell stage to specify ectodermal cells as pharynx or rectum progenitors, and that in the absence of
pha-4, these cells develop as other types of ectoderm (hypodermis or glia) instead.
pha-4 maps on chromosome V, close to a fork head homolog identified in the McGhee
lab3. Antisense RNA injections of fork head phenocopy mutations in
pha-4: animals arrest as L1 larvae that lack a pharynx and contain excess LIN-26+ cells in their heads. To test whether
pha-4 encodes fork head, we are sequencing different
pha-4 alleles and testing a heatshock fork head construct for rescue. In Drosophila, fork head is required to form the foregut and hindgut; in the absence of fork head activity ectopic ectoderm is produced. The phenotypic and molecular similarities between C. elegans
pha-4 and Drosophila fork head suggest that formation of the digestive tract in diverse organisms may rely on conserved molecular mechanisms. 1. Mango et al., 1994. 2. Granato et al., 1994. 3. Azzaria et al., 1996. We thank John Kalb and Jim McGhee for generously providing the antisense cDNA construct used in this study. Many thanks also to Heinke and Ralf Schnabel for
pha-1::lacZ.