Further studies of the engrailed phenotype in Drosophila.

Although most mutations at the engrailed locus of Drosophila cause embryonic death when homozygous, they are viable in clones of cells. We describe the phenotype of such clones in the eye-antenna, proboscis, humerus, wing, legs, and terminalia. When in anterior compartments the clones are normal, but in most posterior compartments they are abnormal and fail to respect the anteroposterior compartment boundary. We find that the yield of engrailed-lethal clones in posterior compartments is often significantly lower than expected, indicating that these clones are lost during development. Mutant clones are abnormal in the analia and rare in the humerus, suggesting that both structures are of posterior provenance. These results support the hypothesis that the engrailed+ gene is required exclusively in cells of posterior compartments to specify their characteristic cell affinities and pattern.