Triangle of U

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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 (

oilseed crop species.[1] It has since been confirmed by studies of DNA and proteins.[2]

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

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

Woo's theory

The six species are

Genomes Chr. count Species Description
Diploid
AA 2n=2x=20 Brassica rapa (syn. B. campestris)
bok choi
BB 2n=2x=16
Brassica nigra
black mustard
CC 2n=2x=18 Brassica oleracea
Brussels sprouts, cauliflower, kohlrabi
Tetraploid
AABB 2n=4x=36 Brassica juncea
Brown mustard
AACC 2n=4x=38
Brassica napus
 rapeseed, rutabaga
BBCC 2n=4x=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 n of chromosomes in each full genome set (which is also the number found in the pollen or ovule), 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 (one A and one C), hence "2n=38" which means "n=19" (the number of chromosomes in each gamete). It is also four component genomes (two A and two C), hence "4x=38".[2]

The three diploid species exist in nature, but can easily interbreed because they are closely related. This

amphidiploid (with two genomes each from two diploid species).[2]

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 (AACC) is dated to have originated about 8,000[5] or 38,000–51,000[6] years ago. The homologous part of its constituent chromosomes has crossed over in many cultivars.[5] B. juncea (AABB) is estimated to have originated 39,000–55,000 years ago.[6] 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.[2]

Data from molecular studies indicate the three diploid species are themselves

paleohexaploids.[7][8]

Allohexaploid species

In 2011 and 2018, novel

doubled haploid (AABBCC) offspring.[11]

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

protoplast fusion.[12]

See also

References

  1. ISBN 978-0-470-38762-7
    .
  2. ^
    PMID 32595682
    . Comparative genomic analyses can assign the subgenomes of the allotetraploids, B. juncea and B. napus, with their diploid parental taxa, and the results were in agreement with U's triangle (Chalhoub et al., 2014; Yang et al., 2016a). [...]
  3. ^ a b Nagaharu U (1935). "Genome analysis in Brassica with special reference to the experimental formation of B. napus and peculiar mode of fertilization". Japan. J. Bot. 7: 389–452.
  4. ^ "인터넷 과학신문 사이언스 타임즈" (in Korean). Archived from the original on 2007-09-27.
  5. ^
    S2CID 206556986
    .
  6. ^
    PMID 27595476
    .
  7. PMID 17720758
    . Retrieved 2010-08-22.
  8. PMID 26653025
    .
  9. S2CID 84504896
    .
  10. PMID 30210508
    .
  11. ^
    S2CID 91439428
    .
  12. PMID 33329636.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
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