Distribution of Ultrabithorax proteins in mutants of Drosophila bithorax complex and its transregulatory genes

The phenotypic effects of mutations have classically been described in terms of their penetrance, expressivity and specificity1. These three parameters can give insights into the wild-type function of the genes affected by the mutations; for example, the topographic specificity of homoeotic mutations provides a crucial clue to our understanding of the normal function of homoeotic genes in development. The null alleles of such genes have constant specificity2, and the expectation is that these genes are expressed in all the cells of the compartments affected by their mutations. However, hypomorphic mutations of the ‘selector’ genes that specify compartment identity are also known which have only partial, and sometimes, variable, specificity. We have now studied the distribution of protein products of the Drosophila Ultrabithorax (Ubx) gene in various mutants, both of the Ubx domain of the bithorax complex and of trans-regulatory genes predicted to affect Ubx expression3. We find that the topographical specificity of the mutations correlates with the distribution of Ubx protein. In particular, in partially transformed compartments we find sharp within-compartment variation in the level of Ubx protein expression. This may correspond to alternative steady states of gene activity established by cell–cell interactions, which we suggest may reflect processes of normal development involved in embryonic compartmentalization.Peer reviewe