The epidermis, or external epithelium, of C. elegans performs a pivotal role in generating the worm-like morphology of the animal during embryogenesis. After their birth, epidermal cells assemble into three rows on either side of the embryo, corresponding to dorsal, lateral and ventral positions. The dorsal and some ventral cells normally undergo limited, specific fusions during embryogenesis to generate several distinct syncytia. Neither P cells, the central cells in the ventral row, nor seam cells, the lateral cells, fuse during embryogenesis. The mechanisms that regulate epidermal cell fusion, ensuring that only the appropriate cells fuse, are not well understood.In a deficiency screen for zygotic loci required for establishment and patterning of the epidermis, we identified a few deficiencies that result in inappropriate fusion of epidermal cells (Terns et al., 1997). By screening for lethal mutations targeted to the region defined by one deficiency that leads to widespread cell fusion, we subsequently isolated a mutation in a gene we have provisionally called
fus-1. Immunostaining of terminally arrested
fus-1 embryos with MH27, which reacts with adherens junctions, showed that epidermal patterning is nearly normal in early embryos; however, most or all cells that normally contribute to the
hyp1-11 syncytia undergo hyperfusion later in embryogenesis. In the most severely affected embryos, all epidermal cells except the seam cells fuse into a single, large syncytium.A recent study has shown that a novel integral membrane protein, called EFF-1, is a key component required for cell fusion; all epidermal cell fusions are blocked in
eff-1 mutants (Mohler et al., 2002). To determine whether the apparent hyperfusion of
fus-1 embryos is mediated by
eff-1, we analyzed
eff-1;
fus-1 double mutant embryos. While an
eff-1 mutation does not rescue the embryonic lethality of the
fus-1 mutant, the hyperfusion phenotype of
fus-1 was suppressed, indicating that
fus-1 may be a negative regulator of
eff-1-mediated cell fusion. Progress on determining the molecular identity of
fus-1 will be reported.