Hox cluster duplications and the opportunity for evolutionary novelties.

Hox genes play a key role in animal body plan development. These genes tend to occur in tightly linked clusters in the genome. Vertebrates and invertebrates differ in their Hox cluster number, with vertebrates having multiple clusters and invertebrates usually having only one. Recent evidence shows that vertebrate Hox clusters are structurally more constrained than invertebrate Hox clusters; they exclude transposable elements, do not undergo tandem duplications, and conserve their intergenic distances and gene order. These constraints are only relaxed after a cluster duplication. In contrast, invertebrate Hox clusters are structurally more plastic; tandem duplications are common, the linkage of Hox genes can change quickly, or they can lose their structural integrity completely. We propose that the constraints on vertebrate Hox cluster structure lead to an association between the retention of duplicated Hox clusters and adaptive radiations. After a duplication the constraints on Hox cluster structure are temporarily lifted, which opens a window of evolvability for the Hox clusters. If this window of evolvability coincides with an adaptive radiation, chances are that a modified Hox cluster becomes recruited in an evolutionary novelty and then both copies of duplicated Hox clusters are retained.