Pattern formation and determination in the antenna of the homoeotic mutant Antennapedia of Drosophila melanogaster

The development of the antenna in the antennal-leg homoeotic mutant Antennapedia (Antp^R) was investigated using somatic crossing-over to mark clones of cells in Antp^R antennal appendages. Antp^R antennae ranged from a nearly normal antenna to a nearly normal leg. The arrangement of clones of marked bristles and cuticle in the more antennalike antennae was similar to the wild type antenna, and that of the leglike antennae was similar to the wild-type leg. The contiguity of clones argued against extensive individual cell migration. The regions occupied by homoeotic leg varied considerably between different Antp^R antennae. Observation of Antp^R antennae in these phenotypic mosaics showed that specific leg parts replaced specific antennal parts. Even small groups of leg sensilla appeared only at precise locations in the antenna. These results suggest that homoeotic leg cells and antennal cells can both respond to the same positional information or prepattern. An analysis of clone size provided estimates for cell number in the Antp^R antenna. It was found that cell numbers in the wild-type and Antp^R antennae are about the same until the third instar, when the Antp^R cells start dividing more rapidly than wild type. Previous work had shown that clonal inheritance of a commitment for homoeotic leg also did not occur prior to the early third instar. It is suggested that determination for homoeotic leg occurs in the early third instar, and that thereafter this commitment is inherited by the progeny of the determined cells. The increase in growth rate is probably due to a faster growth rate in cells with a leg commitment than in cells with an antennal commitment. The results suggest that, once initiated, determination may be of two types—a clonally inherited determination (for example, to be homoeotic leg) and an environmental determination (for example, to be a specific part of a homoeotic leg). Clonal inheritance of determination in normal embryonic development and in sex determination in intersexes is discussed.