DEVELOPMENTAL CONSEQUENCES OF UNRESTRICTED EXPRESSION OF THE ABD-A GENE OF DROSOPHILA

The abdominal-A (abd-A) gene of Drosophila specifies abdominal segment s two through eight (A2-A8). We have found that the uniform expression of the abd-A protein under heat shock control transforms embryonic se gments anterior to the abd-A domain into an abdominal segment of the A 2-A6 type. Posterior abdominal segments and telson undergo little or n o transformation, that is, the abd-A product is phenotypically suppres sed posterior to its realm of expression. The comparison of wildtype e mbryos with embryos carrying the heat shock-abd-A construct but no abd -A endogenous product indicate that some elements of the pattern-like shape of denticle belts or ventral pits depend on the amount of abd-A protein. The concurrent expression of abd-A and other homeotic genes s uggest competition for common downstream genes. This competition has b een studied by looking at the expression on the visceral mesoderm of a gene downstream of abd-A, decapentaplegic, when two homeoproteins wit h opposing effects on its transcription are simultaneously induced.