Conservation, duplication, and divergence of developmental genes during chordate evolution

Developmental biology has been revolutionised by the discovery that many genes involved in pattern formation, morphogenesis and differentiation have been remarkably conserved through evolution. To what extent are the roles of these genes also conserved? Gene duplication is a major factor complicating comparison of developmental genes between divergent taxa, since duplication of genes promotes functional divergence. Surveys of homeobox gene diversity in vertebrates and amphioxus reveal that many classes of homeobox genes duplicated close to vertebrate origins. This raises the question of how widespread gene duplication was during early vertebrate evolution. We evaluate four lines of evidence for gene duplication in vertebrate evolution: surveys of gene family complexity, molecular phylogeny, chromosome mapping and genome size. These suggest that widespread gene duplication occurred close to vertebrate origins and again during the early gnathostome radiation; the second event may have involved tetraploidy of the genome. We suggest that gene duplication was often followed by the acquisition of new developmental roles; in this way, duplication may have facilitated the evolution of vertebrate body plans. The data imply that unity in developmental control between vertebrates and invertebrates will be found in the general properties of each gene family, not in detailed comparisons between individual genes.