Figure 5

A model for the evolution of the Runx2-VDR interaction. A schematic phylogenetic tree showing the evolutionary relationship of invertebrate (tunicates) and vertebrate (osteichthyans) chordates is shown. It is not known if the tunicate Runx (R) and VDR horthologues can interact together (question mark). As Runx3 (R3) and VDR are strongly co-expressed in the gut of many osteichthyan species, their interaction might represent an ancestral, gut-specific, regulatory module. The physical and functional interaction between Runx2 (R2) and VDR probably plays a minor role during osteichthyan skeletogenesis (i.e. only in some species or in a subset of skeletal elements). We propose here that it was particularly strengthened in amniotes, once the VDR became robustly expressed in osteoblasts.