Harmonisation of Regulatory Oversight in Biotechnology Safety Assessment of Transgenic Organisms in the Environment, Volume 5..

II.3. BRASSICA CROPS (BRASSICA SPP.) – 213

Osborn and Williams, 1988a; Rana et al., 2004; see Figure 3.39). From this progenitor,

the ancestral Brassicaceae form evolved with x=8 chromosomes (Lysak et al. 2006). This

was followed by the divergence about 20 Mya of the ancestral genera of Arabidopsis and

tribe Brassiceae. Genome triplication via allohexaploidy occurred about 14-16 Mya

(Lysak and Lexer, 2006), followed by diploidisation and chromosome number reduction

resulting in the evolution of the ancestral Brassiceae karyotype with x=6 chromosomes

(Lysak et al., 2005; Yang et al., 1999). It is estimated that the separation of the Nigra and

Rapa/Oleracea lineages took place about 7.9 Mya (Lysak et al., 2005). The B. oleracea

and B. rapa divergence is estimated to have occurred about 4 Mya (Inaba and Nishio,

2002), with the interspecific crosses, forming B. napus, B. juncea and B. carinata, taking

place less than 10 000 years ago (Song, Osborn and Williams, 1988a; Rana et al., 2004;

Lysak et al., 2005).

Figure 3.39. Illustration of major events in the evolution of selected Brassica species

and Arabidopsis thaliana

Note: The dotted lines indicate the species believed to be the maternal parent in the interspecific cross.

Source: Modified from Song, Osborn and Williams (1988a; 1988b); Rana et al. (2004).

A slightly different scenario of the polyploidy events in the evolution of the

Brassiceae genomes has been put forward by Mun et al. (2009), following a B. rapa and

A. thaliana genome-wide comparative analysis. They suggest that a whole genome

duplication (WGD) occurred twice, once about 55-63 Mya and again at 23-30 Mya,

between the existence of an ancient ancestral species and the evolution of the ancestral

Brassicaceous karyotype. They suggest that the second WGD resulted in the divergence

of Arabidopsis from the Brassiceae linage about 13-17 Mya. This was followed by a

whole genome triplication in the Brassiceae about 11-12 Mya with the divergence of

B. rapa from B. oleracea taking place about 8 Mya. Their data also suggest that the

allopolyploidisation that resulted in the species B. napus occurred only 0.7-1 Mya.

Ancestral species

Ancestral crucifer

Ancestral Arabidopsis Ancestral Brassiceae

B. nigra B. oleracea B. rapa

A. thaliana B. carinata B. napus B. juncea

Genome duplication

(~ 65 million years ago)

~ 20 million

years ago

Genome triplication

(~ 16 million years ago)

8-14 million

years ago

~ 4 million

years ago

<10 000

years ago

Polyploidy

Diploidisation

Polyploidy

Diploidisation

Selective chromosome

diploidisation

Allopolyploidisation