II.3. BRASSICA CROPS (BRASSICA SPP.) – 219
The development of male sterile B. napus parental lines, for the production of
commercial varieties, has also provided a means to investigate intraspecific, interspecific
and intergeneric crossing on a field scale, without pollen competition. The results have
shown that where male sterile plants were used, the frequency of interspecific crosses was
significantly higher as indicated in the following species cross reports below. Thus, the
presence of male sterile B. napus plants in commercial fields was seen as increasing the
incidence and/or risk of unwanted species hybrids.
Some of the first developed hybrid B. napus varieties used a seed-production system
termed “synthetic hybrids”. Commercial production fields growing such hybrids
consisted of about 80-90% male sterile hybrid plants with the remaining fully fertile
plants (10-20%) providing the pollen cloud necessary to fertilise the male sterile plants in
the rest of the field. Fortunately, this “synthetic hybrid” system has been replaced with
new systems that reverse the ratio of fully fertile to male sterile plants in commercial
hybrid fields. Today only a small percentage (15-20%) of male sterile plants may occur as
off-types in these hybrid varieties. Such plants would be saturated with pollen from the
surrounding B. napus plants, thus greatly reducing the risk of pollination by a foreign
pollen source.
Chèvre et al. (2004) identified 14 species related to B. napus to which gene
introgression from B. napus could be of concern to oilseed rape growing countries in
Europe and North America. The reports of interspecific and intergeneric sexual crossing
attempts between these species and B. napus are summarised in Table 3.13. Each species
cross is discussed in the following paragraphs.
Warwick, Francis and Gugel (2009) have compiled a complete list of reports on
interspecific and intergeneric hybridisation within the Brassicaceae that includes studies
that use sexual as well as special techniques to effect a cross.