Figure 1.

Comparing molecular fingerprints of skeletal cells suggests evolution of the osteoblast among vertebrate clades. A. Published collagen and transcription factor gene expression in various vertebrate clades reveals variation in the molecular fingerprint of the osteoblast (red text), whereas the chondrocyte shows a conserved molecular fingerprint. References and species cited: Teleosts = A. semicinctus, B. horae, C. aceratus, C. aeneus, D. rerio, G. aculeatus, G. aymonieri, J. floridae, M. sanctaefilomenae, O. latipes, P. antarcticum, P. pangasius, P. pungitius, P. reticulate, R. trilineata, T. ladigesi [11-14]; Mammals = H. sapiens, M. musculus, R. norvegicus [15-18]; Birds = A. platyrynchos, C. coturnix japonica, G. gallus [15,19-22]. Almost every reference cited in this table focused on cranial skeletal tissues, but published work suggests that the summarized molecular fingerprints can be applied to skeletal cells throughout the body [19]. B. Three possible evolutionary scenarios explain the expression of Col2 and Col10 in osteoblasts of Teleosts, but not Tetrapods (Mammals + Birds). Hypothesis 1 suggests neofunctionalization (ectopic expression of Col2 and Col10) in osteoblasts appeared after the teleost-specific genome duplication, or TGD. Hypothesis 2 suggests neofunctionalization in osteoblasts appeared in the ancestral Actinopterygian, or ray-finned fish. Hypothesis 3 suggests Col2 and Col10 expression was present in the common ancestor of Actinopterygians and Sarcopterygians (i.e. ancestral Osteichthyan), and was subsequently lost in the Sarcopterygian (lobe-finned fish) lineage. Abbreviations: Col1 = Collagen type 1a2; Col2 = Collagen type 2a1; Col10 = Collagen type 10a1; Col11 = Collagen type 11a2.