Expansion of the Hox complex

The best-characterized history of a family of developmental regulatory genes is the evolution of the Hox genes. Hox genes have been isolated from many metazoan phyla, allowing the timing of the tandem duplication events that created the Hox complex to be determined relative to the radiation of animal phyla (Fig. 4.7). Given these data, the history of the expansion of the Hox complex can be charted through the course of animal evolution.

Cnidarians have two definitive Hox genes, which are most closely related to the "anterior" group (lab/Hoxl, pb/Hox2) and "posterior" group (AbdB/Hox9-13) genes found in bilaterians. Although some cnidarians have additional Hox genes due to cnidarian-specific duplication events, the common ancestor of Cnidaria and Bilateria most likely possessed only two Hox genes. By contrast, all bilaterians have at least two anterior group genes, multiple "central" group genes (Hox3 through Hox8, or their equivalents), and often multiple posterior group genes. Five Hox genes (lab/Hoxl, pb/Hox2, Hox3, Dfd/Hox4, and Scr/Hox5), as well as at least one more central gene and one posterior gene, can be clearly identified in all three major bilaterian lineages (the Lophotrochozoa, the arthropod + onychophora + priapulid

Expansion of posterior

Genome tetraploidization events

Vertebrates

Anterior

Central

Posterior

Expansion of central Hox genes

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