Triangle of U

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The Triangle of U is a theory which describes the evolution and relationships between members of the plant genus Brassica. The theory centers on the idea that initially there were three different ancestral genomes that combined, in various configurations, to create many of the common vegetables and oilseeds we know today.

The theory is named after N. U, who published it in 1935.

The triangle contends that many Brassica species were derived from three ancestral genomes, denoted by the letters AA, BB, or CC. Alone, each of these diploid genomes produces a common Brassica variety. The letter n denotes the number of chromosomes in each genome. For example Brassica rapa has an AA - n=10 designation. That means each cell contains two complete genome copies, (diploid) and each genome has ten chromosomes. Thus each cell will contain 20 chromosomes.

Initially, these three species would have existed as isolated relatives. But because they are so closely related it was possible for them to interbreed. This interspecific breeding allowed the creation of three new species of tetraploid Brassica. Because they are derived from the genomes of two different species, these hybrid plants are said to be allotetraploid (contain four genomes, derived from different parent species).

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References

  • N. U. Genome analysis in Brassica with special reference to the experimental formation of B. napus and peculiar mode of fertilization. Japanese Journal of Botany 7: 389-452 (1935).