Fusion, gene misexpression and homeotic transformations in vertebral development of the gnathostome stem group (Placodermi) ZERINA JOHANSON* ,1 , ROBERT CARR 2 and ALEX RITCHIE 3 1 Department of Palaeontology, Natural History Museum, London, UK, 2 Department of Biological Sciences, Irvine Hall, Ohio University, Athens, OH, USA and 3 Australian Museum, Sydney, NSW, Australia ABSTRACT 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 mecha- nisms. 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. KEY WORDS: vertebral column, Placodermi, Hox gene, misexpression The Placodermi are extinct jawed vertebrates resolved phyloge- netically to the base of the jawed vertebrate clade, basal to the crown-group Gnathostomata (Chondrichthyes + Osteichthyes; Donoghue and Purnell, 2005; Fig. 1). As such, placoderms represent the initial jawed vertebrate radiation and are crucial for determining the major evolutionary changes between jawless and jawed vertebrates. As argued below, this can include inferences on the genes or processes involved in the development of particular features, even though the Placodermi are only known from fossils (cf. Donoghue and Purnell, 2005). We focus on the craniocervical region, comprising the occipital region of the brain- case and anterior vertebral column, and genes involved in devel- Int. J. Dev. Biol. 54: 71-80 (2010) doi: 10.1387/ijdb.072508zj THE INTERNATIONAL JOURNAL OF DEVELOPMENTAL BIOLOGY www.intjdevbiol.com *Address correspondence to: Zerina Johanson. Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD UK. Fax: +44-20-7942-5546. e-mail: z.johanson@nhm.ac.uk Final author corrected PDF published online: 9 October 2009 ISSN: Online 1696-3547, Print 0214-6282 © 2009 UBC Press Printed in Spain opment of the vertebral column, including Pax-1, Gli-3, Zic, Msx1, Msx-2 as well as Hox genes and the upstream genes regulating these (Kessel et al., 1990; Lufkin et al., 1992; Condie and Cappechi, 1993, 1994; Monsoro-Burq et al., 1994; Wallin et al., 1995; Watanabe et al., 1998; Peters et al., 1999; Christ et al., 2000; Storre et al., 2002; van den Akker et al., 2002; Monsoro- Burq, 2005). These genes have been studied primarily in mice and chick- ens, but homologies in the vertebral column can be identified across the Vertebrata (Wellik, 2007; Fig. 1, mice and chickens are included in the clade Osteichthyes). The occipital region of the braincase and the vertebral column can be considered serial