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Brassica : Harvesting the Genome, Diversity and Products

3.

Scientific status and unique features of Brassica species

3.1.

One genome, many crops

3.1.1.

The relationships between the different genomes characterising the different species in the Brassica genus are outlined in the well-known “Triangle of U”.  There are three distinct diploid ‘genomes’ or ‘cytodemes’ recognised, each represented by a type species:

Genome

Species

n=

~ genome size

A

B. rapa

10

500-550 Mbp

B

B. nigra

8

470

C

B. oleracea

9

600-650

AB

B. juncea

18

1100-1500

AC

B. napus

19

1130-1240

BC

B. carinata

17

1540

3.1.2.

Genome relationships

Within each species, a number of distinct crop types have been domesticated. A non-exhaustive list is shown below:

B. rapa

Vegetable

Turnip, turnip tops, Pak choi, turnip rape, sarson

Oil

B. nigra

Condiment

Black mustard

B. oleracea

Vegetable

Broccoli, Brussels sprout, cabbage, cauliflower, kohl rabi, collard, kale, Chinese white kale (Kailaan)

Fodder

Kale

B. juncea

Brown mustard, oil

B. napus

Oil

Oilseed rape, rapeseed, Canola

Vegetable

Swede

Fodder

B. carinata

Condiiment

Ethiopian mustard

3.1.3.

A model for chromosome evolution

Due to the distribution of a series of closely related genomes and combined apmphidiploids, the Brassica genus provides a useful system to study chromosome evolution over different time-spans. In particular, it has been possible to start to understand some of the dynamics of duplication and divergence associated with speciation and polyploidisation. In addition, the close relationship with Arabidopsis allows comparisons over longer time spans at the level of chromosome segment down to sequence.

3.1.4.

Polyploidy

Genome duplication or polyploidy is a widespread phenomenon throughout the plant kingdom, and provides a mechanism and context for genetic and evolutionary adaptations. Brassica species provide excellent experimental systems to understand the consequences of genome duplication.  There are well studied examples of whole genome duplication that result in allopolyploidy (combining two or more distinct genomes) or  autopolyploidy (duplication of a diploid genome). New allopolyploid species such as Brassica napus often combine parental phenotypes with novel traits and adaptations and the fixation of hybrid

Draft White Paper for Multinational Brassica Genome Project (MBGP);   12/01/2006

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