Triangle of U


The triangle of U is a theory about the evolution and relationships among 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 since 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.

Overview

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 "n" of chromosomes in each full genome set, 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, hence "2n=38" which means "n=19". It is also four component genomes, hence "4x=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.
Data from molecular studies indicate the three diploid species are themselves paleopolyploids.

An allohexaploid species

Recently, a novel which is located at the "center" of the triangle of U has been 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.