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Segmental identity within the vertebral column: what can fossils tell us?
 
SVPCA Conference
 
Platform presentation (20 minutes)
Authors
 
*Robert K Carr
 
 
Zerina A Johanson
 
 
Alex Ritchie
 
Abstract
 
The identity of individual segments within the vertebrate axial skeleton is determined, in part, by the expression boundaries of homeotic genes. Alterations of these genes (e.g., misexpression or deletion) can alter the identity of an axial segment, mimicking either more anterior or posterior segments (anteriorization or posteriorization, respectively). The expression of Hox genes has been shown to be a critical factor in establishing the anterior-posterior axis. Misexpression and/or deletion of these genes in the cervical region of mice result in a variety of anatomical malformations (including anteriorization, posteriorization, fusion, or deletion - anatomical correlates to the underlying changes in gene expression). For example, experimental anterior misexpression of Hoxd4 in the mouse vertebral column results in the occipital region of the skull resembling the anterior cervical vertebrae (Lufkin et al., 1992). Deletion of Hoxd3 has an opposite effect, with the anterior cervical vertebrae (atlas) coming to resemble parts of the occiput (Condi and Capecchi, 1993; 1994). In the 370 million year old (Late Devonian) placoderm Cowralepis maclachlani (Ritchie, 2005), the occipital and the fused anterior region of the vertebral column, the synarcual, have a nearly identical morphology. We suggest that misexpression/deletion of Hoxd3 and/or Hoxd4 occurred in Cowralepis. The putative presence of these HoxD genes suggests the presence of all four Hox clusters in placoderms, indicating that the gene duplication generating the four clusters (HoxA-D) occurred phylogenetically prior to the evolution of crown group gnathostomes, within the stem Gnathostomata. (Condi and Cappechi, 1993). Mice homozygous for a targeted disruption of Hoxd-3 (Hox-4.1) exhibit anterior transformations of the first and second cervical vertebrae, the atlas and the axis. Development 119, 579-595.
London 2020