Evolution of invertebrate deuterostomes and Hox/ParaHox genes

Abstract

Transcription factors encoded by Antennapedia-class homeobox genes play crucial roles in controlling development of animals, and are often found clustered in animal genomes. The Hox and ParaHox gene clusters have been regarded as evolutionary sisters and evolved from a putative common ancestral gene complex, the ProtoHox cluster, prior to the divergence of the Cnidaria and Bilateria (bilaterally symmetrical animals). The Deuterostomia is a monophyletic group of animals that belongs to the Bilateria, and a sister group to the Protostomia. The deuterostomes include the vertebrates (to which we belong), invertebrate chordates, hemichordates, echinoderms and possibly xenoturbellids, as well as acoelomorphs. The studies of Hox and ParaHox genes provide insights into the origin and subsequent evolution of the bilaterian animals. Recently, it becomes apparent that among the Hox and ParaHox genes, there are significant variations in organization on the chromosome, expression pattern, and function. In this review, focusing on invertebrate deuterostomes, I first summarize recent findings about Hox and ParaHox genes. Next, citing unsolved issues, I try to provide clues that might allow us to reconstruct the common ancestor of deuterostomes, as well as understand the roles of Hox and ParaHox genes in the development and evolution of deuterostomes.

Figure 1

Deuterostome phylogeny with schematics of the genomic organization of Hox and ParaHox genes. Colored rectangles and ovals indicate Hox and ParaHox genes, respectively. Anterior Hox and Gsx are indicated in red; group3 and Xlox in yellow; central Hox in green; and posterior Hox and Cdx in blue, according to the nomenclature in a previous study . Lines passing under rectangles or ovals indicate clustered gene linkage on a chromosome. Slashes on the line represent a large gap between Hox or ParaHox genes, indicating disorganization of the cluster. Unconnected lines also indicate an unlinked situation. Rectangles and ovals without horizontal lines passing under them indicate the genes whose linkage has not been investigated. Information about vertebrate and xenoturbella (or xenacoelomorph) Hox and ParaHox genes is omitted.