An Analysis of the Expressivity of Some Bithorax Transformations

Of the homeotic mutants known in Drosophila, those of the bithorax system are by far the best known genetically, thanks to the extensive analyses by E.B. Lewis (1963, 1964, 1967, 1978). The interest of their study lies not only in their effect on the development of certain segments (see Morata and Lawrence, 1977) but also in the genetic structure of the system; it has been shown that the bithorax genes are organized in a gene complex which includes several adjacent loci: bithorax (bx), Contrabithorax (Cbx), Ultrabi-thorax (Ubx), bithoraxoid (bxd), postbithorax (pbx), Hyperabdominal (Hab), Ultraabdominal (Uab), and probably others. These genes seem to be under the same control elements. (See Lewis, 1978 for a recent review.) This gene complex is one of the few examples studied and characterized in eukaryotes. In general (but not always) each of these loci is associated with one particular type of homeotic transformation; a specific segment or half-segment is transformed into another.