Triangle of U


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. 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
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, 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, hence "2=38" which means "=19". It is also four component genomes, hence "4=38".
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. These are said to be allotetraploid ; more specifically, amphidiploid.

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 is dated to have originated about 8,000 or 38,000-51,000 years ago. The homologous part of its constituent chromosomes has crossed over in many cultivars. B. juncea is estimated to have originated 39,000-55,000 years ago. As of 2020, research on organellar genomes shows that B. nigra is likely the "mother" of B. carinata and that B. rapa likely mothered B. juncea. The situation with B. napus is more complex: some specimens have a rapa-like organellar genome, while the rest indicate an ancient, unidentified maternal plant.
Data from molecular studies indicate the three diploid species are themselves paleohexaploids.

Allohexaploid species

In 2011 and 2018, novel allohexaploids located at the "center" of the triangle of U were created by different means, for example by crossing B. rapa with B. carinata, or B. nigra with B. napus, or B. oleracea with B. juncea, followed by chromosome duplication of the triploid offspring to generate doubled haploid offspring.
In addition, two stable allohexaploid intergeneric hybrids between Indian mustard and white mustard were created in 2020 by protoplast fusion.