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. 2014 Jul 7;5:336. doi: 10.3389/fmicb.2014.00336

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Copyright © 2014 Shinzato, Mungpakdee, Satoh and Shoguchi.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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The anthozoan complement of Hox genes and the implications of the evolution of the Hox cluster. Comparing the genomic linkage of Hox genes in the sea anemone N. vectensis and the staghorn coral A. digitifera confirms that cnidarians once had a Hox cluster that contained both anterior and posterior/central class Hox genes. (A) The Hox cluster of N. vectensis includes the anterior Hox genes Anthox6 (PG1), Anthox8b (PG2), Anthox8a (PG2), and Anthox7 (PG2) as well as the Eve homeobox gene. (B) The Hox cluster of A. digitifera includes the anterior Hox genes Anthox6 (PG1) and Anthox7/8 (PG2), and the posterior/central class Hox gene Anthox1a (PG4–14), as well as the Eve homeobox gene. Another gene HlxB9 (also named MNX) is found upstream of Anthox6 in the Hox cluster of both genomes (data not shown). (C) The metazoan tree of life with inferred ancestral Hox clusters. The ancestor to protostomes and deuterostomes is thought to have had two anterior class Hox genes (Hox1 and Hox2), one paralagous group 3 gene (Hox3), three central class genes (Hox4, Hox5, and Hox6–8), one posterior class Hox gene (Hox9–14), and one Eve homeobox gene. Because of the extended cluster in A. digitifera, we can now say that the cnidarian–bilaterian ancestor had, at least, two anterior class Hox genes (Anthox6 and Anthox7/8), a central/posterior class Hox gene (Anthox1/1a), and the Eve homeobox gene. It is unclear at what point the genomic rearrangement involving the Eve homeobox gene occurred. The origin of the PG3 Hox genes also is not clear. ^*Anthox7/8 has been categorized as a PG2 Hox gene in previous publications, but it is possible, based on our current phylogenetic analysis, that Anthox7/8 descended from a Hox gene that was lost in bilaterians. Based on the genomic orientation of these genes, we also believe the ancestor likely had a fourth Hox gene potentially related to Anthox9. For more detail information, please see DuBuc et al. (2012). Abbreviations: PG, paralogous group; Ax, Anthox.