Fig. 7. Model for the function of ceh-16/engrailed in C. elegans. (A) ceh-16/engrailed keeps the seam cells in a linear organization (green), due to its repression of seam cell fusion and cell migrations. (B) During elongation, hyp7 (dorsal) start to fuse; ceh-16/engrailed acts as a fusion repressor and as a regulator of seam-cell differentiation in order to maintain a straight boundary. At this stage genes important for `seam-cell-fate' are turned on/regulated by ceh-16/engrailed. (C) Transcriptional cascade regulating seam cell fate. (D) Summary of genetic regulatory cascades in the three rows of epidermal cells preceding and during the 1.5-fold stage: Dorsal: eff-1 is expressed and required for the fusion of hyp7 into a syncytium. It is not known which genes regulate eff-1 expression in the dorsal epidermis. Medial or lateral: in the seam cells ceh-16/engrailed represses the expression of eff-1 (directly or indirectly). This is necessary to allow ceh-16/engrailed to activate genes required for correct positioning of the seam cells (cell adhesion molecules) and for the differentiation of the seam cells (elt-5 etc.). Genes that regulate ceh-16/engrailed in C. elegans are unknown. Ventral: analogous to ceh-16/engrailed in the seam cells, lin-39/HoxD4/Dfd and ceh-20/Exd are required for the regulation of elt-5 and for the repression of eff-1 to allow vulva formation (Koh et al., 2002; Shemer et al., 2004). During postembryonic development, the seam cells act as a belt with two straight boundaries that are lost in the absence of ceh-16/engrailed.