thumb|right|360px|alt=Triangle of U|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 ( ) 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 tetraploid vegetables and oilseed crop species. It has been confirmed by studies of DNA and proteins.
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, a Korean-Japanese botanist (writing under the Japanized name "U Nagaharu"). Woo made synthetic hybrids between the diploid and tetraploid species and examined how the chromosomes paired in the resulting triploids.
Woo's theory
The six species are
{| class = wikitable
! Genomes !! Chr. count !! Species !! Description
|-
| colspan=4 style="text-align:center;" | Diploid
|-
| AA || 2=2=20 || Brassica rapa || (syn. B. campestris) turnip, napa cabbage, bok choi
|-
| BB || 2=2=16 || Brassica nigra || black mustard
|-
| CC || 2=2=18 || Brassica oleracea || cabbage, kale, broccoli, Brussels sprouts, cauliflower, kohlrabi
|-
| colspan=4 style="text-align:center;" | Tetraploid
|-
| AABB || 2=4=36 || Brassica juncea || Brown mustard
|-
| AACC || 2=4=38 || Brassica napus || rapeseed, rutabaga
|-
| BBCC || 2=4=34 || Brassica carinata || Ethiopian mustard
|}
The code in the "Chr.count" column specifies the total number of chromosomes in each somatic cell, and how it relates to the number of chromosomes in each full genome set (which is also the number found in the pollen or ovule), and the number of chromosomes in each component genome. For example, each somatic cell of the tetraploid species Brassica napus, with letter tags AACC and count "2=4=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 "2=38" which means "=19" (the number of chromosomes in each gamete). It is also four component genomes (two A and two C), hence "4=38". years ago. The homologous part of its constituent chromosomes has crossed over in many cultivars. B. juncea (AABB) is estimated to have originated 39,000–55,000 years ago.
Allohexaploid species
In 2011 and 2018, novel allohexaploids (AABBCC) located at the "center" of the triangle of U were created by different means, 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.
See also
- Cultivar
- Hybridisation