Conservation and elaboration of Hox gene regulation during evolution of the vertebrate head

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Abstract
The comparison of Hox genes between vertebrates and their closest invertebrate relatives (amphioxus and ascidia) highlights two derived features of Hox genes in vertebrates: duplication of the Hox gene cluster1,2, and an elaboration of Hox expression patterns and roles compared with non-vertebrate chordates3,4,5,6,7,8. We have investigated how new expression domains and their associated developmental functions evolved, by testing the cis-regulatory activity of genomic DNA fragments from the cephalochordate amphioxus Hox cluster in transgenic mouse and chick embryos. Here we present evidence for the conservation of cis-regulatory mechanisms controlling gene expression in the neural tube for half a billion years of evolution, including a dependence on retinoic acid signalling. We also identify amphioxus Hox gene regulatory elements that drive spatially localized expression in vertebrate neural crest cells, in derivatives of neurogenic placodes and in branchial arches, despite the fact that cephalochordates lack both neural crest and neurogenic placodes. This implies an elaboration of cis-regulatory elements in the Hox gene cluster of vertebrate ancestors during the evolution of craniofacial patterning.