Polyploidy and Genome Evolution pp 309-339
Two Rounds of Whole-Genome Duplication: Evidence and Impact on the Evolution of Vertebrate Innovations
The origin and evolution of the vertebrates have been linked to the study of genome duplications since Susumo Ohno ventured the 2R-hypothesis, suggesting that the successful diversification of complex vertebrates was facilitated by polyploidization in the stem vertebrate ancestor due to two rounds of whole-genome duplication (2R-WGD). This chapter first reviews evidence supporting Ohno’s 2R-hypothesis and gathers information about the timing and mechanisms underlying the 2R-WGD. Second, this chapter describes the impact of the 2R-WGD on the evolution of the vertebrate genome structure, gene number, and the evolutionary dynamics of the functional fate of vertebrate ohnologs (paralogous genes that originated by WGD) in comparison with non-vertebrate chordate gene homologs. Finally, this review discusses the functional consequences of the 2R-WGD on the origin and evolution of vertebrate innovations compared with urochordates and cephalochordates, paying special attention to the origin of neural crest cells, placodes, and the big complex brain, key features that probably facilitated the transition from ancestral filter-feeding non-vertebrate chordates to voracious vertebrate predators. Currently available data, however, seem to suggest that these putative key features were present to at least some extent in stem Olfactores; hence, the impact of the 2R-WGD may not be related to the immediate origin of vertebrate innovations, but to the subsequent diversification of a wide variety of complex structures that facilitated the successful radiation of vertebrates.