Triangle of U



The triangle of U is a theory about the evolution and relationships between members of the plant genus Brassica. The theory states that the genomes of three ancestral species of Brassica combined to create three of the common modern vegetables and oilseed crop species. It has since been confirmed by studies of DNA and proteins.

The theory was first published in 1935 by Woo Jang-choon, a Korean-Japanese botanist who was working in Japan (where his name was transliterated as "Nagaharu U", his Japanese name). Woo made synthetic hybrids between the diploid and tetraploid species and examined how the chromosomes paired in the resulting triploids.

Overview
The triangle of U is illustrated by the triangular diagram at right. It 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 &minus; n=10 (alternatively AA &minus; 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, Brussels sprouts, 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 for 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). (More specifically, they are amphidiploid, i.e., containing one diploid genome from each of the two different Brassica species). Data from molecular studies indicate 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