Figure 6

The mechanism of functional divergence of ar-C enhancers of duplicated shh genes in zebrafish. (a) Model for motif structure and interaction in ar-C enhancers involved in the regulation of midline expression of shha and shhb in zebrafish. Schemes on the top and bottom represent the structure of the ar-C enhancer of shha (blue) and shhb (red) with the position of the conserved motifs indicated in colored boxes, as in Figures 4 and 5. In the middle, schematic cross-sections of the neural tube with the floor plate (fp) and the notochord (nt) are shown (ventral to the left). Dark green indicates strong enhancer activity. Arrows indicate activator and blunt arrows indicate repression function by individual motifs. (b) Evolution of ar-C enhancers of vertebrates. Phylogenetic relationship of the genes and the motif composition of the respective ar-C enhancers are shown. Shha gene enhancers are shown in blue and shhb gene enhancers in red. On the left, a predicted pre-duplicated ancestral shha ar-C enhancer is shown. Below, the predicted activity of the ancestral shha gene is depicted in blue in a schematic cross-section of an embryonic midline. On the right, schematic cross sections of midlines in green indicate ar-C (SFPE2 [sonic floor plate enhancer 2]) enhancer activities; shades of green indicate strength of enhancer activity in the respective midline tissues. In blue the expression activity of the respective shha/shhb genes are shown.