Triangle of U
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The Triangle of U is a theory about the evolution and relationships between members of the plant genus Brassica. It says that the genomes of three ancestral species of Brassica combined to create the three common vegetables and oilseed crop species that we know today. The theory has since been confirmed by studies of DNA and proteins.
The theory was first published in 1935 by Woo Jang-choon, a Korean botanist who was working in Japan (where his name was transliterated as "Nagaharu U" [1]). Woo made synthetic hybrids between the diploid and tetraploid species and examined how the chromosomes paired in the resulting triploids. His work was influenced by work by Kihara on the origin of bread or hexaploid wheat and its relationship to its diploid ancestors.
The triangle shows how three of the 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 species. The letter n denotes the number of chromosomes in each genome, and is the number found in the pollen or ovule. For example Brassica rapa has an A - n=10 (alternatively AA - 2n=20) designation. That means each somatic cell of the plant contains two complete genome copies (diploid) and each genome has ten chromosomes. Thus each cell will contain 20 chromosomes; since this is the diploid number it is written as 2n = 2x = 20.
- AA - 2n=2x=20 - Brassica rapa (syn. Brassica campestris) - Turnip, Chinese cabbage
- BB - 2n=2x=16 - Brassica nigra - Black mustard
- CC - 2n=2x=18 - Brassica oleracea - Cabbage, kale, broccoli, cauliflower
These three species exist as separate species. But because they are 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 two different ancestral species). (Data from molecular studies indicate that the three diploid species are themselves paleopolyploids).
- AABB - 2n=4x=36 -Brassica juncea - Indian mustard
- AACC - 2n=4x=38 -Brassica napus - Rapeseed, rutabaga
- BBCC - 2n=4x=34 -Brassica carinata - Ethiopian mustard
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[edit] Bibliography
- 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).
