The triangle of U ( ) is a theory about the evolution and relationships among the six most commonly known members of the plant genus ''

Brassica ''Brassica'' () is a genus of plants in the cabbage and mustard family ( Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole c ...

''. The theory states that the

genomes In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

of three ancestral diploid species of ''Brassica'' combined to create three common

tetraploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...

vegetable Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, ...

s and

oilseed Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or f ...

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 Korean may refer to: People and culture * Koreans, ethnic group originating in the Korean Peninsula * Korean cuisine * Korean culture * Korean language **Korean alphabet, known as Hangul or Chosŏn'gŭl **Korean dialects and the Jeju language ** ...

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botanist (writing under the Japanized name "U Nagaharu"). Woo made synthetic hybrids between the diploid and

species and examined how the chromosomes paired in the resulting triploids.

U'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

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

s in each full genome set (which is also the number found in the pollen or

ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the '' integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the ...

), 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 A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

). It is also four component genomes (two A and two C), 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''. (Critics, however, consider the geological separation too large.) These are said to be

allotetraploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...

(containing four genomes from two or more different species); more specifically,

amphidiploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...

(with two genomes each from two diploid species).

Further relationships

The framework proposed by U, 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 8000 or 38000–51000 years ago. The homologous part of its constituent chromosomes has

crossed over ''Crossed Over'' is a 2002 Canadian television film directed by Bobby Roth, and starring Diane Keaton as Beverly Lowry and Jennifer Jason Leigh as Karla Faye Tucker. It is based on Lowry's memoir ''Crossed Over: A Murder, A Memoir''. Cast *Dian ...

in many cultivars. ''B. juncea'' (AABB) is estimated to have originated 39,000–55,000 years ago. 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. Data from molecular studies indicate the three diploid species are themselves paleohexaploids.

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. In addition, two stable allohexaploid (AABBSS) intergeneric hybrids between Indian mustard (''B. juncea'', AABB) and

white mustard White mustard (''Sinapis alba'') is an annual plant of the family Brassicaceae. It is sometimes also referred to as ''Brassica alba'' or ''B. hirta''. Grown for its seeds, used to make the condiment mustard, as fodder crop, or as a green manure ...

(''Sinapsis alba'', SS) were created in 2020 by protoplast fusion.

References

{{Brassica Genetics Brassica Hybrid plants

U's theory

Further relationships

Allohexaploid species

See also

References