Mutational dissection of gene expression in the abdominal region of the bithorax complex of Drosophila in imaginal tissue.

The phenotypic effects in imaginal hypodermal tissue of a number of Abdominal-B mutations of the bithorax complex are described. Evidence is given from complementation analysis that the phenotypic heterogeneity in both the spatial limits and the nature of the homeotic transformations produced is not an arbitrary classification of allelic differences that we find. We have used genetic mosaic analysis to support the interpretation that the Abdominal-B genetic unit can exist in a number of alternative functional states of expression during development and that individual Abdominal-B mutations may abolish some states whilst leaving others unaffected.

Brista: a gene involved in the specification and differentiation of distal cephalic and thoracic structures in Drosophila melanogaster.

A gene Brista has been identified in chromosome 2R, in the region 60D11-E4, in which mutations cause homoeotic transformation of distal antennal structures to distal leg derivatives, and in which certain alleles also lead to upsets in the formation of distal elements of the legs. This gene is haploinsufficient for the homoeotic phenotype. Several putative null and two hypomorphic alleles have been recovered. The effects of exposure to the non-permissive temperature of a temperature-sensitive allele are cummulative and depend upon the length of the exposure during the period of antennal cell proliferation. It is suggested that this gene contributes to the stability of the state of determination in distal domain of the antennal and leg discs, and its relationship to other genes with similar mutant phenotype is discussed.