| Summary: | <p>Abstract</p> <p>Background</p> <p><it>zic </it>genes are members of the <it>gli/glis/nkl/zic </it>super-family of C2H2 zinc finger (ZF) transcription factors. Homologs of the <it>zic </it>family have been implicated in patterning neural and mesodermal tissues in bilaterians. Prior to this study, the origin of the metazoan <it>zic </it>gene family was unknown and expression of <it>zic </it>gene homologs during the development of early branching metazoans had not been investigated.</p> <p>Results</p> <p>Phylogenetic analyses of novel <it>zic </it>candidate genes identified a definitive <it>zic </it>homolog in the placozoan <it>Trichoplax adhaerens</it>, two <it>gli/glis/nkl-</it>like genes in the ctenophore <it>Mnemiopsis leidyi</it>, confirmed the presence of three <it>gli/glis/nkl</it>-like genes in Porifera, and confirmed the five previously identified <it>zic </it>genes in the cnidarian <it>Nematostella vectensis</it>. In the cnidarian <it>N. vectensis</it>, <it>zic </it>homologs are expressed in ectoderm and the gastrodermis (a bifunctional endomesoderm), in presumptive and developing tentacles, and in oral and sensory apical tuft ectoderm. The <it>Capitella teleta zic </it>homolog (<it>Ct-zic</it>) is detectable in a subset of the developing nervous system, the foregut, and the mesoderm associated with the segmentally repeated chaetae. Lastly, expression of <it>gli </it>and <it>glis </it>homologs in <it>Mnemiopsis</it>. <it>leidyi </it>is detected exclusively in neural cells in floor of the apical organ.</p> <p>Conclusions</p> <p>Based on our analyses, we propose that the <it>zic </it>gene family arose in the common ancestor of the Placozoa, Cnidaria and Bilateria from a <it>gli/glis/nkl</it>-like gene and that both ZOC and ZF-NC domains evolved prior to cnidarian-bilaterian divergence. We also conclude that <it>zic </it>expression in neural ectoderm and developing neurons is pervasive throughout the Metazoa and likely evolved from neural expression of an ancestral <it>gli/glis/nkl/zic </it>gene. <it>zic </it>expression in bilaterian mesoderm may be related to the expression in the gastrodermis of a cnidarian-bilaterian common ancestor.</p>
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