Hox gene expression in the hemichordate Saccoglossus kowalevskii and the evolution of deuterostome nervous systems

The evolutionary origins of the chordate central nervous system remain uncertain. Conclusions drawn from classical morphological comparisons and from a broad range of metazoan phyla conflict with the new molecular genetic information from developmental model systems characterized by central nervous systems. This has led to debate as to the nature of the ancestral deuterostome nervous system. Hemichordates as basal deuterostomes occupy a phylogenetically critical position for addressing hypotheses on the evolution of the chordate nervous system. Characterizing the molecular basis of the development of their diffuse nervous system offers insights into the role of conserved body patterning genes in the evolution of specific neural anatomies. We present a description of hox gene expression during the development of the direct-developing hemichordate Saccoglossus kowalevskii. The nested ectodermal expression of these genes in a hemichordate suggest that they play a general patterning role in the anterior/posterior regionalization of the ectoderm of bilaterians rather than being uniquely associated with the development and evolution of central nervous systems.