Molecular characterisation of the interaction between the bicoid and hunchback genes in Musca domestica : insights into the evolution of a regulatory interaction

Regulatory evolution of genes controlling development is widely considered to be the major driving force of morphological change in the evolution of animal lineages. The regulation of genes is controlled by interactions between trans-acting factors and cis-regulatory DNA. The evolution of the interaction between the bicoid and hunchback genes is studied by comparing this orthologous interaction in Drosophila melanogaster with the housefly, Musca domestica.;A genomic clone of the hunchback region from Musca domestica from an earlier work was characterised by restriction mapping and sequencing. A promoter region upstream of an early embryonically expressed transcript was mapped to the hunchback genomic sequence. Sites of specific interaction between a bacterially-derived Musca domestica BICOID homeodomain protein were mapped to the promoter by DNaseI footprinting.;The hunchback regulatory sequence of Musca domestica are unalignable with Drosophila melanogaster; nevertheless, similar patterns of repetitive sequence distribution were found by computer-aided analysis, which suggest extensive slippage-like processes may have been important in the evolution of the hunchback region.;A promoter element of Musca domestica hunchback drives a bicoid-dependent pattern of expression in the anterior of transgenic Drosophila melanogaster embryos. A later ectopic expression points to a functional divergence of promoter recognition between the two species.;The in vitro DNA-binding function of the Musca domestica BICOID homeodomain was further characterised by quantitative gel-shift assays. The data suggest that the homeodomain differences between Musca domestica and Drosophila melanogaster reflect differences in sequence recognition. The homeodomain differences may have co-evolved with the binding site sequences in the hunchback promoters. It is argued that molecular coevolution of the BICOID trans-factor and cis-binding sites in the hunchback promoter has accompanied a divergence in embryonic size between Musca domestica and Drosophila melanogaster.;.