GENE DUPLICATIONS AND THE ORIGINS OF VERTEBRATE DEVELOPMENT

All vertebrates possess anatomical features not seen in their closest living relatives, the protochordates (tunicates and amphioxus), Some o f these features depend on developmental processes or cellular behavio urs that are again unique to vertebrates, We are interested in the gen etic changes that may have permitted the origin of these innovations, Gene duplication, followed by functional divergence of new genes, may be one class of mutation that permits major evolutionary change, Here we examine the hypothesis that gene duplication events occurred close to the origin and early radiation of the vertebrates, Genome size comp arisons are compatible with the occurrence of duplications close to ve rtebrate origins; more precise insight comes from cloning and phylogen etic analysis of gene families from amphioxus, tunicates and vertebrat es. Comparisons of Hox gene clusters, other homeobox gene families, Wn t genes and insulin-related genes all indicate that there was a major phase of gene duplication close to vertebrate origins, after divergenc e from the amphioxus lineage; we suggest there was probably a second p hase of duplication close to jawed vertebrate origins. From amphioxus and vertebrate homeobox gene expression patterns, we suggest that ther e are multiple routes by which new genes arising from gene duplication acquire new functions and permit the evolution of developmental innov ations.