Figure 7.

An evolutionary scenario for cartilage calcification in jawed vertebrates. Expression patterns and cartilage matrix calcification were mapped onto a simplified vertebrate phylogenetic tree to deduce ancestral states and polarize evolutionary change. We propose that, in the last vertebrate common ancestor, the expression of Col2a1 experienced a strong downregulation in maturing, non-calcified, cartilaginous regions. This downregulation was subsequently inherited by distinct vertebrate lineages, and is associated to hard cartilage in cyclostomes and to calcified cartilage in jawed vertebrates. The chondrichthyan and osteichthyan representatives analyzed in this study display a calcified Col2a1-negative vertebral cartilage, a likely jawed vertebrate synapomorphy. Tesserae calcification, a recent chondrichthyan innovation, occurs in the absence of Col2a1 downregulation. Perichondrium and bone have been omitted for the sake of simplicity. See text for details.