Comparative analysis of leg and antenna development in wild-type and homeotic<Emphasis Type="Italic"> Drosophila melanogaster</Emphasis>

The insect leg and antenna are thought to be homologous structures, evolved from a common ancestral appendage. The homeotic transformations of antenna to leg in Drosophila produced by mutation of the Hox gene Antennapedia are position-specific, such that every particular antenna structure is transformed into a specific leg counterpart. This has been taken to suggest that the developmental programmes of these two appendages are still similar. In particular, the mechanisms for the specification of a cell's position within the appendage would be identical, only their interpretation would be different and subject to homeotic gene control. Here we explore the degree of conservation between the developmental programmes of leg and antenna in Drosophila and other dipterans, in wild-type and homeotic conditions. Most of the appendage pattern-forming genes are active in both appendages, and their expression domains are partially conserved. However, the regulatory relationships and interactions between these genes are different, and in fact cells change their expression while undergoing homeotic transformation. Thus, the positional information, and the mechanisms which generate it, are not strictly conserved between leg and antenna; and homeotic genes alter the establishment of positional clues, not only their interpretation. The partial conservation of pattern-forming genes in both appendages ensures a predictable re-specification of positional clues, producing the observed positional specificity of homeotic transformations.