2R or not 2R: testing hypotheses of genome duplication in early vertebrates.

The widely popular hypothesis that there were two rounds of genome duplication by polyploidization early in vertebrate history (the 2R hypothesis) has been difficult to test until recently. Among the lines of evidence adduced in support of this hypothesis are relative genome size, relative gene number, and the existence of genomic regions putatively duplicated during polyploidization. The availability of sequence for a substantial portion of the human genome makes possible the first rigorous tests of this hypothesis. Comparison of gene family size in the human genome and in invertebrate genomes shows no evidence of a 4:1 ratio between vertebrates and invertebrates. Furthermore, explicit phylogenetic tests for the topology expected from two rounds of polyploidization have revealed alternative topologies in a substantial majority of human gene families. Likewise, phylogenetic analyses have shown that putatively duplicated genomic regions often include genes duplicated at widely different times over the evolution of life. The 2R hypothesis thus can be decisively rejected. Rather, current evidence favors a model of genome evolution in which tandem duplication, whether of genomic segments or of individual genes, predominates.