Genetic analysis of cell heredity in imaginal discs of Drosophila melanogaster.

Somatic crossing-over was used in heterozygous Drosophila melanogaster to effect clonal homozygosity for three mutants (h, Hw, ac). In these mutants, one type of process, chaetae, replaces another type, trichomes, in specific patterns on the adult fly. Heterozygous individuals were irradiated at different larval or pupal ages with x-rays and the somatic crossing-over was identified in the adult cuticle by genetically coupled mutants serving as cell markers of chaetae and trichomes. Induction of homozygosis more than 8 hr before puparium formation resulted in autonomous differentiation of the mutant pattern in the homozygous patch of tissue for the three mutants tested; homozygosis induced within 8 hr of puparium formation was not followed by expression of the corresponding genotype. 8 hr before puparium formation, each cell type still has to divide twice before metamorphosis and differentiation. Thus, the genetically conditioned decision of a specific cell to differentiate either a chaeta or a trichome is made during the growth of the wing imaginal disc and is transmitted clonally to descendant cells. Once this decision has been made, subsequent changes in the genotype of the cell or of its daughters are not material to the fate of the cell. We introduce the term "perdurance" to designate the persistence of a cellular developmental fate for several cell generations after the loss of the genetic basis for that cellular development.