214 – II.3. BRASSICA CROPS (BRASSICA SPP.)
Genome mapping of B. rapa and B. oleracea has shown the gross organisation of
their genomes to be highly collinear (Lagercrantz and Lydiate, 1996) but their genome
size and complexity differ. The genome size of B. rapa is ca. 500 Mb compared to the
much larger and more complex genome of B. oleracea at ca. 600 Mb (Arumuganathan
and Earle, 1991). Comparative studies have shown that within the amphidiploids species,
B. napus, B. juncea and B. carinata, the chromosomes within the respective putative
diploid genomes have remained more or less intact (Parkin et al., 1995; Sharpe et al.,
1995; Axelsson et al., 2000). DNA sequence data indicate that the A genome of B. rapa
and the C genome of B. oleracea are very closely related while B. nigra, with its
B genome, is from an earlier divergent lineage (Mizushima, 1972; Song, Osborn and
Williams, 1988b; Prakash and Chopra, 1991). Song et al. (1995) reported there was rapid
genome change after polyploidisation in B. napus and B. juncea, which suggests that the
micro-structural changes observed in the Brassica lineage happened shortly after genome
duplication, followed by a slow but ongoing rate of change (Rana et al., 2004).
The techniques of fluorescence in situ hybridisation (FISH) facilitates the integration
of genetic and physical chromosome maps as it allows chromosomal location of labelled
DNA probes to be directly determined (Snowdon et al., 2007). Since molecular markers
can now be ordered and physical distances measured, it is possible to construct molecular
karyotypes and distinguish individual chromosomes of the A and B genomes that make
up B. napus (Fukui et al., 1998; Armstrong et al., 1998; Snowdon et al., 2002). Snowdon,
Lühs and Friedt (2007) provides a consensus genetic linkage map of molecular markers
for B. napus where linkage groups (LGs) N1-N10 correspond to the B. rapa A genome
LGs of A1-A10, and LGs N11-N19 correspond to B. oleracea C genome LGs of C1-C9.
Nuclear genome size
The genome size of the Brassica diploids (approximately 500-700 Mbp) are more
than four times that of the related Brassicaceous species A. thaliana (approximately
157 Mbp; see Table 3.12). The gene content of A. thaliana is believed to be very similar
to Brassica diploids with more than 87% sequence identity in the coding regions
(Parkin et al., 2005). Although it is believed that the diploid Brassica evolved through a
common hexaploid ancestor (Parkin et al., 2005), the necessary genome triplication
would be insufficient to explain the differences in genome size. Therefore, this important
difference in genome size is likely to reflect a different rate of non-coding DNA
accumulation.
Possible extent of repetitive or non-coding DNA sequences
Transposable elements (TEs) constitute a major fraction of non-coding DNA in plant
species. Good estimates of TE distribution and density are presently only available for the
B. oleracea genome, based on a partial draft genome sequence (Zhang and Wessler,
2004). Class 1 (retro) elements were the most abundant TE class with long terminal
repeat (LTR) and non-LTR elements comprising the largest fraction of the genome.
However, several families of class 2 (DNA) elements have amplified to very high copy
numbers in B. oleracea compared to A. thaliana and have contributed significantly to
genome expansion. Approximately 20% of the B. oleracea genome was estimated to be
composed of class 1 and class 2 TEs.
