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Fusion, gene misexpression and homeotic transformations in vertebral development of the gnathostome stem group (Placodermi).

Development of the vertebral column is controlled by a complex of pleiotropic and polygenetic phenomena, in the mouse and chick regulating formation of different parts of individual vertebrae and morphological identity along the column (Hox code). In mouse and chick, experimental misexpression, including upstream and downstream genes, results in shifts in vertebral identity, loss of particular parts of individual vertebrae or vertebral fusion. Axial skeleton homologies across the Vertebrata allow these observations to be extended to taxa such as Homo sapiens, Chondrichthyes and Placodermi, the latter an entirely fossil group. Misexpression phenotypes among fossil taxa illuminate the phylogenetic history of these regulatory mechanisms. Phenotypes associated with genes originating via genomic duplication can determine the historical depth for these duplication events. Analysis of an ontogenetic sequence for the occipital-synarcual complex in the placoderm Cowralepis mclachlani provides the basis for comparison of this early gnathostome with other placoderms, chondrichthyans and amniotes. The occipital-synarcual patterns in placoderms parallel the phenotypic misexpression in mice and chicks (fusion and homeotic mutation) and the varying degrees of fusion in the Type I-III human Klippel-Feil syndrome. The association of these phenotypes to Hoxd regulatory complexes indicates that the gnathostome genomic duplication occurred at the base of the gnathostome stem group. Given the conservative nature of regulatory genes and the homology of vertebral elements, the presence of fusion in stem gnathostomes implies that the mechanism of fusion in mouse and chick models can be extrapolated to extant chondrichthyans (testable) and accounts for the phenotypic similarity across gnathostomes. The presence of these phenotypes in fossils indicates the antiquity of these regulatory mechanisms and of genomic duplication.