Are We Polyploids? A Brief History of One Hypothesis

Formulation of the 2R Hypothesis

The late Susumu Ohno was one of the most prolific and influential modern biologists. “He has thought at least half of the thoughts that form the basis for the work being carried out all over the world in respect to genetic analysis.” In 1970, Ohno published his seminal Evolution by Gene Duplication (Ohno 1970). The book brought (in addition to the now widely accepted title hypothesis) another intriguing thought, namely the hypothesis of two or more full genome duplications in early stages of vertebrate evolution. Interestingly, Ohno regarded tetraploidization as a more important evolutionary mechanism than tandem gene duplication: “ … tandem duplication is meaningless unless supplemented periodically by simultaneous duplication of all gene loci by tetraploidy” (Ohno 1970). He based this suggestion solely on genome size differences in different chordates and evidence of recent tetraploidization in some fish lineages. In fact, he was not quite sure how often and when vertebrate genome duplications occurred. “I venture a guess that paedomorphosis from tunicate-like creatures to amphioxus-like creatures […] was accompanied by a two to threefold increase in the genome size […]. Whether this increase was accomplished exclusively by tandem duplication or by combination of tandem duplication and tetraploidy cannot be resolved at the moment” (Ohno 1970). Later, in the same chapter, he wrote: “It is our contention that the ancestors of reptiles, birds, and mammals have experienced at least one tetraploid evolution either at the stage of fish or at the stage of amphibians”.

Three years later, the same author proposed “at least one round of tetraploid evolution” at the stage of fish or amphibian (Ohno 1973). This time the supportive evidence was the fact that, in mammals, many paralogous genes are unlinked. Such a situation would be expected in the case of whole chromosome duplication, as opposed to tandem duplications, which …