Molecular coevolution between developmental genes in insect species.

Changes in the regulatory sequences of the genes involved in development are thought to be important in the evolution of morphology. However, molecular coevolution between functionally interacting genetic elements allows sequence divergence to be tolerated whilst the functional interaction is maintained. Molecular coevolution can lead to species-specificity in the sequence basis underlying molecular interactions. The concentration-dependent activation of hunchback (hb) expression in the anterior half of the Drosophila melanogaster embryo by the gradient of bicoid (bcd) protein represents a primary step in the elaboration of pattern along the anterior- posterior axis, and this interaction is conserved in the housefly, Musca domestica. In order to investigate the possibility that the molecular basis of this interaction may have coevolved, the bed and hb genes have been partially sequenced from M. domestica and compared to those of D. melanogaster. Analysis of the putative M. domestica hb regulatory region identified three candidate bed binding sites, with a consensus sequence of TTTAATCC, rather than the TCTAATCC of D. melanogaster. Comparison of the bed sequences revealed 5 changes within the 60 amino acids of the homedomain. Hence, it is possible that M. domestica bcd may have a subtly altered binding specificity, pointing towards the possibility that the coordinated changes in the binding site sequences have elicited compensatory changes in the M. domestica bed homeodomain. Preliminary analyses have been made of the functional significances of the observed differences. Although the functional significance of the observed differences in the bed and hb genes is not fully understood, the possibility remains that the molecular nature of the interaction between bed and hb has diverged between M. domestica and D. melanogaster