The "triangle of U" diagram, showing the genetic relationships among six species of the genus Brassica. Chromosomes from each of the genomes A, B and C are represented by different colours.
The triangle of U (/uː/OO) is a theory about the evolution and relationships among the six most commonly known members of the plant genus Brassica. The theory states that the genomes of three ancestral diploid species of Brassica combined to create three common tetraploidvegetables and oilseed crop species.[1] It has since been confirmed by studies of DNA and proteins.[2]
The theory is summarized by a triangular diagram that shows the three ancestral genomes, denoted by AA, BB, and CC, at the corners of the triangle, and the three derived ones, denoted by AABB, AACC, and BBCC, along its sides.
The theory was first published in 1935 by Woo Jang-choon,[3] a Korean-Japanesebotanist (writing under the Japanized name "U Nagaharu").[4] Woo made synthetic hybrids between the diploid and tetraploid species and examined how the chromosomes paired in the resulting triploids.
The code in the "Chr.count" column specifies the total number of chromosomes in each somatic cell, and how it relates to the number n of chromosomes in each full genome set (which is also the number found in the pollen or ovule), and the number x of chromosomes in each component genome. For example, each somatic cell of the tetraploid species Brassica napus, with letter tags AACC and count "2n=4x=38", contains two copies of the A genome, each with 10 chromosomes, and two copies of the C genome, each with 9 chromosomes, which is 38 chromosomes in total. That is two full genome sets (one A and one C), hence "2n=38" which means "n=19" (the number of chromosomes in each gamete). It is also four component genomes (two A and two C), hence "4x=38".[2]
The three diploid species exist in nature, but can easily interbreed because they are closely related. This interspecific breeding allowed for the creation of three new species of tetraploid Brassica.[3] (Critics, however, consider the geological separation too large.) These are said to be allotetraploid (containing four genomes from two or more different species); more specifically, amphidiploid (with two genomes each from two diploid species).[2]
Further relationships
The framework proposed by Woo, although backed by modern studies, leaves open questions about the time and place of hybridization and which species is the maternal or paternal parent. B. napus (AACC) is dated to have originated about 8,000[5] or 38,000–51,000[6] years ago. The homologous part of its constituent chromosomes has crossed over in many cultivars.[5]B. juncea (AABB) is estimated to have originated 39,000–55,000 years ago.[6] As of 2020, research on organellar genomes shows that B. nigra (BB) is likely the "mother" of B. carinata (BBCC) and that B. rapa (AA) likely mothered B. juncea. The situation with B. napus (AACC) is more complex: some specimens have a rapa-like organellar genome, while the rest indicate an ancient, unidentified maternal plant.[2]
Data from molecular studies indicate the three diploid species are themselves paleohexaploids.[7][8]
Allohexaploid species
In 2011 and 2018, novel allohexaploids (AABBCC) located at the "center" of the triangle of U were created by different means,[9][10][11] for example by crossing B. rapa (AA) with B. carinata (BBCC), or B. nigra (BB) with B. napus (AACC), or B. oleracea (CC) with B. juncea (AABB), followed by chromosome duplication of the triploid (ABC) offspring to generate doubled haploid (AABBCC) offspring.[11]
In addition, two stable allohexaploid (AABBSS) intergeneric hybrids between Indian mustard (B. juncea, AABB) and white mustard (Sinapis alba, SS) were created in 2020 by protoplast fusion.[12]
^ abNagaharu U (1935). "Genome analysis in Brassica with special reference to the experimental formation of B. napus and peculiar mode of fertilization". Japan. J. Bot. 7: 389–452.
