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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 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.[1] 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,[2] a Korean-Japanese botanist (writing under the Japanized name "Nagaharu U").[3] Woo made synthetic hybrids between the diploid and tetraploid species and examined how the chromosomes paired in the resulting triploids.

Contents

OverviewEdit

The six species are

Genomes Chr.Count Species Description
Diploid
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, kohlrabi
Tetraploid
AABB 2n=4x=36 Brassica juncea Indian 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".

The three diploid species exist in nature, but can easily interbred 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 (containing four genomes from two or more different species); more specifically, amphidiploid (with two genomes each from two diploid species).

Data from molecular studies indicate the three diploid species are themselves paleopolyploids.[4]

An allohexaploid speciesEdit

Recently, a novel allohexaploid (AABBCC) which is located at the "center" of the triangle of U has been created by different means [5] [6] [7], 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.

See alsoEdit

ReferencesEdit

  1. ^ Jules, Janick (2009). Plant Breeding Reviews. 31. Wiley. p. 56. ISBN 978-0-470-38762-7.
  2. ^ 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.
  3. ^ "인터넷 과학신문 사이언스 타임즈" (in Korean). Archived from the original on 2007-09-27.
  4. ^ Martin A. Lysak; Kwok Cheung; Michaela Kitschke & Petr Bu (October 2007). "Ancestral Chromosomal Blocks Are Triplicated in Brassiceae Species with Varying Chromosome Number and Genome Size" (PDF). Plant Physiology. 145 (2): 402−10. doi:10.1104/pp.107.104380. PMC 2048728. PMID 17720758. Retrieved 2010-08-22.
  5. ^ Chen, Sheng; Nelson, Matthew N.; Chèvre, Anne-Marie; Jenczewski, Eric; Li, Zaiyun; Mason, Annaliese S.; Meng, Jinling; Plummer, Julie A.; Pradhan, Aneeta; Siddique, Kadambot H. M.; Snowdon, Rod J.; Yan, Guijun; Zhou, Weijun; Cowling, Wallace A. (2011-11-01). "Trigenomic Bridges for Brassica Improvement". Critical Reviews in Plant Sciences. 30 (6): 524−547. doi:10.1080/07352689.2011.615700. ISSN 0735-2689.
  6. ^ Yang, Su; Chen, Sheng; Zhang, Kangni; Li, Lan; Yin, Yuling; Gill, Rafaqat A.; Yan, Guijun; Meng, Jinling; Cowling, Wallace A.; Zhou, Weijun (2018-08-28). "A High-Density Genetic Map of an Allohexaploid Brassica Doubled Haploid Population Reveals Quantitative Trait Loci for Pollen Viability and Fertility". Frontiers in Plant Science. 9: 1161. doi:10.3389/fpls.2018.01161. ISSN 1664-462X. PMC 6123574. PMID 30210508.
  7. ^ Gaebelein, Roman; Mason, Annaliese S. (2018-09-03). "Allohexaploids in the Genus Brassica". Critical Reviews in Plant Sciences. 37 (5): 422−437. doi:10.1080/07352689.2018.1517143. ISSN 0735-2689.