The evolution of bicoid interactions in the higher diptera

Development can be described as a network of genetic interactions. These interactions can be highly conserved over large evolutionary distances or they can vary between closely related species resulting in morphological differences. What are the forces promoting change in such interactions and how do they evolve The interaction between the transcription factor Bicoid (Bed) and the hunchback (hb) promoter was compared between M. domestica and D. melanogaster (Bonneton et al., 1997 Shaw etal., 2002). This interaction is conserved in function despite differences in both the Bed homeodomain and the hb promoter sequences. Functional tests of the components of this interaction suggest that they are co-evolving in each species to maintain function in spite of sequence divergence. In this thesis, two further Bed regulated genes, tailless (til) and caudal (cad), were studied to provide a comparison to the Bcd-hb promoter interaction and to investigate the consequences of regulatory sequence change within a network of interactions. The til promoter sequences are unalignable between M. domestica and D. melanogaster yet are similar in function. As with the Bed-hb promoter interaction functional tests indicate that the interaction is diverging between the species at the molecular level. The interaction between Bed and the cad mRNA was also investigated. cad sequence and expression data indicate that the function and regulation of cad is conserved between M. domestica and D. melanogaster. However, the M. domestica cad 3' regulatory sequence is unalignable with that of D. melanogaster. In conclusion, the Bed-dependent regulatory sequences are evolving relatively quickly but all indications are that function is conserved. This raises questions about the structure of regulatory sequences, the flexibility of non-coding regulatory sequences to change and the evolution of interactions and of non-coding regions in general.