KURATANI, S.; SHIGETANI, Y.; KAWAKAMI, Y.; SUGAHARA, F.; MURAKAMI, Y.: Evolution of the vertebrate jaw through ontogenetic repatterning

Evolutionary novelties can be brought about through the ontogenetic repatterning that often obliterates the structural homology. The vertebrate jaw may also be one of such novelties since oral structures of lampreys and gnathostomes cannot be homologized easily. To understand the jaw evolution from developmental stand point of view, we picked up the Japanese lamprey, Lampetra japonica, and observed its embryonic development including expression patterns of some regulatory genes known to be involved in craniofacial patterning. In the lamprey larva, the upper and lower lips fringing dorsal and ventral edges of the mouth, differentiate from the premandibular and mandibular ectomesenchyme, respectively. The jaws of gnathostomes, on the other hand, are exclusively derived from the mandibular arch ectomesenchyme. Expression patterns of gene cognates (Fgf8, Bmp4, Dlx1, Msx1) involved in proximo-distal patterning of the oral structures do not fit the topographical relationships of ectomesenchymal subdomains between both the animals, but are rather consistent with functional and structural resemblance of the oral structures. This suggests that the heterotopic shift of epithelial-mesenchymal interaction involving FGF8, BMP4 signaling, may have taken place in a transition from agnathans to gnathostomes. The above signaling mechanisms are commonly functioning in lamprey lip- and gnathostome jaw formation, and can be regarded as an exaptation at the molecular level for the evolution of the jaw. Thus the gnathostome jaw may have been first obtained when the signaling was secondarily restricted to the mandibular arch, which does not fit the origin of the jaw by �heterosis� of the mandibular arch.