212 – II.3. BRASSICA CROPS (BRASSICA SPP.)
(López-Granados and Lutman, 1998; Gulden, Thomas and Shirtliffe, 2004). In general, if
post-harvest tillage is delayed and volunteers are controlled in the intervening years, the
evidence indicates the presence of volunteers from the seed bank decline by at least 90%
by the fourth year (Lutman, Freeman and Pekrun, 2003: Gulden, Shirtliffe and Thomas,
2003b; Baker and Preston, 2008). Failure to follow the above-recommended practices can
extend the presence of seed bank volunteers by several years (Lutman et al., 2005).
Linder and Schmitt (1995) assessed the persistence, in field and greenhouse trials, of
GM B. napus lines with elevated levels of stearate and laurate fatty acids in their seed
oils. They concluded the risk of persistence of the high stearate and high laurate
genotypes, compared with their parental non-GM types, was low. No interspecific hybrid
seed could be obtained from hand-crossing GM high stearate B. napus × wild B. rapa.
Greenhouse trials using seed from the high laurate B. napus × B. rapa cross indicated that
such hybrids “will not possess seed bank dynamics promoting reproduction”.Genetics
Relevant detailed genetic informationCytology
Mitotic metaphase chromosomes of the Brassicaceae are very small. Conventional
cytological protocols condense Brassica meiotic chromosomes to tiny rods or dot-like
shapes. Their small size, lack of distinctive cytological features and the difficulties of
pachytene investigations make cytological identification of individual chromosomes
almost impossible. Although the small chromosome size of the Brassicaceae family has
limited the direct cytology approach, the sequencing of the Arabidopsis thaliana (The
Arabidopsis Genome Initiative 2000), B. rapa (Wang, 2010; The B. rapa Genome
Sequencing Project Consortium, 2011), B. oleracea and B. napus genomes (Bayer
CropScience, 2009) are providing a much clearer picture of species interrelationships.
2014 saw the culmination of a major effort worldwide to generate “reference” annotated
Brassica genome sequences, and some are available online for B. napus, B. oleracea and
B. rapa. From 2015, the focus is on a range of “re-sequencing” efforts (The Multinational
Brassica Genome projet, 2015).^5
Comparative mapping, using more than 20 linkage maps for B. oleracea, B. rapa,
B. nigra, B. napus and B. juncea, has contributed greatly to the understanding of
chromosome homology and colinearity (Lysak and Lexer, 2006). In addition, great strides
have been made in determining the extent of genome colinearity, and rates and modes of
evolution in the Brassicaceae family. Comparative cytogenetic studies now employ a
wide array of techniques including, among others, rDNA probes, nucleolus organizer
regions (NORs), variation in centromeric satellite repeats, genome in situ hybridisation
(GISH), fluorescence in situ hybridisation (FISH), combined with bacterial artificial
chromosomes (BAC FISH) and large-scale comparative chromosome painting (CCP).
Such techniques have helped to unravel the genomic evolution of A. thaliana,
B. oleracea, B. rapa, B. juncea and B. napus as well as the time frame in which the
species arose.
Research into the genome microstructure of the Brassicaceae species indicates the
family originated from an ancestral karyotype that evolved after the monocot/dicot split.
The ancestral karyotype had a basic chromosome number of x=4 and underwent a
genome duplication some 65 million years ago (Mya) followed by diploidisation (Song,