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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.
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