ceh-16/engrailed patterns the embryonic epidermis of Caenorhabditis elegans

engrailed is a homeobox gene essential for developmental functions such as differentiation of cell populations and the onset of compartment boundaries in arthropods and vertebrates. We present the first functional study on engrailed in an unsegmented animal: the nematode Caenorhabditis elegans. In the developing worm embryo, ceh-16/engrailed is predominantly expressed in one bilateral row of epidermal cells (the seam cells). We show that ceh-16/engrailed primes a specification cascade through three mechanisms: (1) it suppresses fusion between seam cells and other epidermal cells by repressing eff-1/fusogen expression; (2) it triggers the differentiation of the seam cells through different factors, including the GATA factor elt-5; and (3) it segregates the seam cells into a distinct lateral cellular compartment, repressing cell migration toward dorsal and ventral compartments.