The Para Hox genes a sister complex to the Hox genes

The origin of the Hox complex is primarily a story of tandem gene duplication and divergence. Nevertheless, additional evidence indicates that the entire prebilaterian Hox complex of three

Figure 4.8

Evolution of deuterostome Hox genes

The relative timing of vertebrate Hox complex duplications are indicated on a phylogenetic tree of deuterostomes. Basal deuterostomes, including echinoderms and cephalochordates, have a single Hox complex. At least three complexes have been identified in the primitive jawless lamprey, indicating that Hox complex duplication occurred before the divergence of lamprey from higher vertebrates. Teleost fish have undergone an additional round of tetraploidization, creating additional Hox complexes in zebrafish and Fugu. Tetrapods, including humans, mice, and frogs, have four Hox complexes and a total of 39 Hox genes. + Sharks possibly possess more than two complexes.

to four genes was duplicated early in metazoan history. The duplication of this "Proto-Hox" complex gave rise to the ancestral Hox complex and a sister ""ParaHox" complex. The Para-Hox genes (Gsx, Xlox, and Cdx) have homeodomains with high sequence similarity to the anterior group, Hox3, and posterior group Hox homeodomains, respectively. The tight chromosomal linkage between these genes in Amphioxus (Fig. 4.9) and their colinear deployment in regions of the developing gut are reminiscent of the organization and colinearity of the Hox complex. The presence of both ParaHox and Hox genes in bilaterians and cnidarians suggests that the Proto-Hox complex duplication predates the divergence of cnidarians and the bilaterian lineage.

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