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II.3. BRASSICA CROPS (BRASSICA SPP.) – 215

A. thaliana and Brassica species of “Triangle of U 1” Table 3.12. Ploidy level, chromosome number, genome size and map length of

and Brassica species of “Triangle of U 1”

Species Ploidy level Chromosome number 1C nuclear DNA

2

content (Mb)^3

Observed map length

(cM)^4

A. thaliana 2 10 157 437 and 501

B. nigra 2 16 634-765 855

B. oleracea 2 18 696-765 820-1 738

B. rapa 2 20 528-784 1 455

B. carinata 4 34 1 280-1 548 −

B. juncea 4 36 1 070-1 500 2 073

B. napus 4 38 1 127 1 441-1 765

Notes: 1. The Triangle of U is a theory about the evolution and relationships between members of the plant
genus Brassica (Source: Wikipedia, the Free Encyclopedia). 2. The C value refers to the haploid DNA content
of the species. 3. Data adopted by Lysak and Lexer (2006) compiled from Bennett and Leitch (2004) and
Johnston et al. (2005). 1 pg = 980 Mb. 4. From Lysak and Lexer (2006), choice based on map marker coverage.

Lim et al. (2005) describe the morphology and molecular organisation of

heterochromatin domains in the interphase nuclei and mitotic and meiotic chromosomes

of the ten chromosomes of B. rapa, using DAPI staining and FISH of rDNA and

pericentromere tandem repeats. They characterised the centromeric repeat sequences,

which fell into two classes, CentBr1 and CentBr2, occupying the centromeres of eight

and two chromosomes, respectively. The centromere satellites encompassed about 30%

of the total chromosomes, particularly in the core centromere blocks of all the

chromosomes. Interestingly, centromere length was inversely correlated with

chromosome length.

Main genetic diversity or variability

Considerable genetic diversity has been found within the six cultivated Brassica

species using nuclear restriction fragment length polymorphisms (RFLP) markers (Song,

Osborn and Williams, 1988b). These results suggested that: 1) B. rapa and B. oleracea

have multiple centres of origin; 2) B. nigra originated from one evolutionary pathway

whereas B. rapa and B. oleracea came from another pathway; and 3) amphidiploid

B. napus and B. juncea arose from different combinations of diploid morphotypes,

indicating polyphyletic origins may be a common mechanism for the natural occurrence

of amphidiploids in Brassica.

The genetic diversity within B. napus is considerably less than that found within

either of the diploid ancestral species. This is probably a result of B. napus being a

relatively modern species, fixed as a product of human civilisation and with no truly wild

populations. Most of the diversity within B. napus has been introduced from its diploid

progenitors. Variation in the A genome has been increased by natural B. napus × B. rapa

crosses whereas variation in the C genome is more limited. Recent molecular marker

analysis has identified more extreme genetic variation in exotic vegetable and fodder

genotypes as well as newly resynthesised B. napus lines (Snowdon and Friedt, 2004 for a

review). In B. juncea the A genome is mostly conserved and the C genome is

significantly changed, more so than the considerably altered C genome in B. carinata.

Similar genetic information, with much duplication, is contained in all three genomes

(Slocum, 1989; Slocum et al., 1990; Chyi, Hoenecke and Sernyk, 1992; Jackson et al.,

2000; Parkin, Sharpe and Lydiate, 2003). However, the chromosomal organisation and

the genetic distribution within the genome is different (Truco et al., 1996). New high

throughput and very informative simple sequence repeat (SSR) and single nucleotide