Triangle of U

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Triangle of U:Overview of genetic relationships between various members of the genus Brassica. Chromosomes from each of the genomes A, B and C are represented by different colours. The cartoon shows the origin of the AABB, AACC and BBCC species which have chromosome sets from their AA, BB and CC ancestors.

The Triangle of U describes the evolution and relationships between members of the plant genus Brassica. The data from U showed that there were three different ancestral genomes (themselves separate species in the Brassica genus) that combined to create the three combinations found in the other three common vegetables and oilseed crop species that we know today. These data have now been confirmed by molecular studies looking at DNA and proteins.

The theory is named after Woo Jang-choonhttp://junior.sciencetimes.co.kr/data/article/7000/0000006890.jsp, a Korean botanist working in Japan, who published it in 1935. He 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

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).

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