156 – II.3. BRASSICA CROPS (BRASSICA SPP.)
molecular analysis of nuclear and chloroplast DNA and fluorescence in situ hybridisation
(Snowdon et al., 2003; Snowdon, 2007; Warwick and Sauder, 2005; Lysak et al., 2005).
To further establish the true relationships among the genus and species of the
subtribe, Harberd (1976; 1972) proposed grouping them into “cytodemes” based on the
crossability of related subspecies with the same chromosome number. Harberd (1976)
defined cytodemes as follows: “If two populations have a common chromosome number
and are easily crossed to form a hybrid, which is neither obviously weak in vigour nor of
low fertility, then they belong in the same cytodeme. By contrast, different cytodemes
(which sometimes have the same chromosome number) are (a) difficult to cross, or
(b) give a weak hybrid, or (c) have a sterile hybrid, and frequently exhibit all
three criteria.” Harberd (1976; 1972) also defined the Brassica coenospecies as
“the group of wild species sufficiently related to the six cultivated species of Brassica to
be potentially capable of experimental hybridisation with them”. On this basis and
their chromosome number the coenospecies have been classified into 43 diploid and
13 tetraploid cytodemes (Warwick and Black, 1993: Table 3). This grouping, with the
inclusion of Raphanus and Enarthrocarpus in the subtribe, is supported by
both chloroplast and nuclear DNA analysis (Warwick and Black, 1993; Warwick and
Hall, 2009).
Cytological analyses of chromosome pairing in interspecific crosses among some of
the more important Brassica cytodemes by Mizushima (1980) provided information on
the maximum possible number of autosyndetic^2 chromosome pairs (Figure 3.1). Harberd
and McArthur (1980) extended the study of meiotic chromosome pairing to more than
50 species hybrids. These distant crosses were facilitated using embryo culture.
A chromosome analysis of the monogenomic Brassica species by Röbbelen (1960),
established that only six chromosomes were distinctly different, the remaining being
homologous with one or the other of the basic six. This evidence pointed to the presence,
in the evolutionary pathway of the Brassica species, of a now-extinct, ancient progenitor
with a basic chromosome number of × = 6. The long-standing hypothesis, that the
cultivated diploid Brassica species are ancient polyploids, has been strongly supported by
modern genomic investigations.
The genomes of Brassica species are extensively triploid (Lysak et al., 2007, 2005;
Rana et al., 2004). In B. nigra Lagercrantz and Lydiate (1996) reported that every
chromosome region appeared to be present in triplicate and the genomes of B. oleracea
and B. rapa also exhibit tripling (Rana et al., 2004; Mun et al., 2009; Wang, 2010).
High density comparative mapping of Arabidopsis and B. napus also supported the
hypothesis of a hexaploid ancestor (Parkin et al., 2005). Indeed, chromosome tripling has
been documented for the entire Brassiceae tribe (Lysak et al., 2005). Linkage maps and
genome size data (Lysak et al., 2009) indicate that the B. oleracea genome, and probably
the other monogenomic species which exhibit a range in chromosome number from 7 to
12, increased or reduced their chromosome number through duplication, translocations
(Quiros, Ochoa and Douches, 1988; Hosaka et al., 1990; McGrath et al.,1990; Truco and
Quiros, 1994), transposition of elements (Zhang and Wessler, 2004) as well as deletions
(Hu and Quiros, 1991) and fusions (Lysak et al., 2006).
