Expression of
eff-1p::gfp was silent through the first third of embryogenesis, first appearing about 230 min after first cleavage in a subset of epidermal precursor cells. Over the next 3 hr, these and additional fluorescent cells were observed to migrate over the ventral and dorsal surfaces of the embryo, and the majority of GFP-positive cells fused to form the
hyp6 and
hyp7 syncytia. As elongation progressed, GFP was also expressed in a pair of cells that fused to form the binucleate "tail spike" . After hatching,
eff-1p::gfp expression persisted in large epidermal syncytia through adulthood. Mononucleated epidermal cells-including the seam cells and the VPCs-remained nonfluorescent until shortly before undergoing larval fusion events. More specifically, GFP was seen in (1) nonstem daughters of the seam cells shortly before they fused into
hyp7; (2) vulval cells invaginating to form toroids during morphogenesis; and (3) the rays and fan of the adult male tail. Expression was also seen in nonepidermal organs known to contain syncytia, including the pharynx and uterus. Interestingly, a few cells that express
eff-1p::gfp have never been observed to fuse, such as some ventral epidermal precursors in the embryo and several neurons.