Figure 6.

An evolutionary scenario for bone formation and perichondral calcification in jawed vertebrates. Bone/perichondrium histology and gene expression patterns were mapped onto a simplified vertebrate phylogenetic tree to deduce ancestral states and polarize evolutionary change. We propose that the ancestral Clade A fibrillar collagen gene (i.e., before the duplications that produced the distinct member of this family) was expressed in the non-calcified perichondrium. This expression pattern was inherited by the unique cyclostome fibrillar collagen gene which is more closely related to the Col2a1 subgroup. In jawed vertebrates, perichondral cells and osteoblasts maintained high levels of Col1a1 and Col1a2 while the Col2a1 osteoblastic expression was dramatically reduced in most (but not all) lineages. The presence of bone in placoderms and tetrapods supports the idea that the calcified fibrous perichondrium observed in some chondrichthyan species either represents bone evolutionary remnants (Hypothesis 1) or a secondary gain of calcification (Hypothesis 2). Osteocytes have been omitted for the sake of simplicity. See text for details.