The fin-to-limb transition and their developmental basis. (A–C) The pectoral fins and limbs of Eusthenopteron(A), Tiktaalik(B), and Acanthostega(C). Note that Eusthenopteron and Tiktaalik possess distal fin rays with the endochondral skeletons. (D) The developmental mechanisms of fin rays and digits. In zebrafish, hox13 genes are expressed in a distal domain of the endochondral disk. The cells that experienced the late phase hox activity migrate out from the distal endochondral disk into the fin fold, that differentiate into the lepidotrichia. In mice, Hox13 genes are expressed in the autopod, which develop digits at a later stage. HOXA13 is suggested to bind the regulatory region of Bmp2 and 4 by ChIP experiments (Knosp et al., 2004). While many of the downstream genes of HOX13 have been explored in mice, genes regulated by HOX13 in fish have not been identified. Note that the ossification of lepidotrichia and digits takes place at a later stage than the expression of Hox13 genes. (E) Hypotheses for the fin-to-limb transition. The fin rays (cells shaded by light orange) have degenerated and the endochondral domain (cells shaded by light gray) expanded during the appendage evolution (top). This hypothesis supports that digits and wrists are novel domains that have been acquired as fish have evolved to tetrapods. Another hypothesis claims that the fish fin has an antecedent of digits and wrists (bottom). The cell histories between fish fin rays and tetrapod digits are comparable in terms of their hox expression pattern during the embryonic development (cells shaded by orange). In this hypothesis, the cell differentiation program of fin rays might have changed to become endochondral bones, resulting in the acquisition of digits and the wrist. dr, distal radials; hu, humerus; int, intermedium; ir, intermediate radials, le, lepidotrichia; po, postaxial process, pr, preaxial radials; pro, proximal radials; ra, radius; ul,ulna, uln, ulnae; Illustrations of (A–C) are redrawn with permissions from Andrews and Westoll (1970), Shubin et al. (2006), and Coates (1996).
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