We describe a genetic mosaic analysis procedure in which Caenorhabditis elegans mosaics are generated by spontaneous loss of an extrachromosomal array. This technique allows almost any C. elegans gene that can be used in germline transformation experiments to be used in mosaic analysis experiments. We identified a cosmid clone that rescues the mutant phenotype of
ncl-1, so that this cell-autonomous marker could be used to analyze mosaic animals. To determine the sites of action for
unc-29 and
lin-31, an extrachromosomal array was constructed containing the
ncl-1(+) cosmid linked to
lin-31(+) and
unc-29(+) cosmids. This array is mitotically unstable and can be lost to produce a clone of mutant cells. The specific cell division at which the extrachromosomal array had been lost was deduced by scoring the Ncl phenotypes of individual cells in genetic mosaics. The Unc-29 and Lin-31 phenotypes were then scored in these animals to determine in which cells these genes are required. This analysis showed that
unc-29, which encodes a subunit of the acetylcholine receptor, acts in the body muscle cells. Furthermore,
lin-31, which specifies cell fates during vulval induction and encodes a putative transcription factor similar to HNF-3/fork head, acts in the Pn.p cells.