Anterior-posterior patterning within the Caenorhabditis elegans endoderm

The endoderm of higher organisms is extensively patterned along the anterio r/posterior axis. Although the endoderm (gut or E lineage) of the nematode Caenorhabditis elegans appears to be a simple uniform tube, cells in the an terior gut show several molecular and anatomical differences from cells in the posterior gut. In particular, the gut esterase ges-l gene, which is nor mally expressed in all cells of the endoderm, is expressed only in the ante rior-most gut cells when certain sequences in the ges-1 promoter are delete d. Using such a deleted ges-1 transgene as a biochemical marker of differen tiation, we have investigated the basis of anterior-posterior gut patternin g in C. elegans, Although homeotic genes are involved in endoderm patternin g in other organisms, we show that anterior gut markers are expressed norma lly in C. elegans embryos lacking genes of the homeotic cluster. Although s ignalling from the mesoderm is involved in endoderm patterning in other org anisms, we show that ablation of all non-gut blastomeres from the C. elegan s embryo does not affect anterior gut marker expression; furthermore, ectop ic guts produced by genetic transformation express anterior gut markers gen erally in the expected location and in the expected number of cells. We con clude that anterior gut fate requires no specific cell-cell contact but rat her is produced autonomously within the E lineage. Cytochalasin D blocking experiments fully support this conclusion. Finally, the HMG protein POP-1, a downstream component of the Wnt signalling pathway, has recently been sho wn to be important in many anterior/posterior fate decisions during C. eleg ans embryogenesis (Lin, R,, Hill, R, J, and Priess, J, R, (1998) Cell 92, 2 29-239), When RNA-mediated interference is used to eliminate pop-1 function from the embryo, gut is still produced but anterior gut marker expression is abolished. We suggest that the C. elegans endoderm is patterned by eleme nts of the Wnt/pop-1 signalling pathway acting autonomously within the E li neage.