224 – II.3. BRASSICA CROPS (BRASSICA SPP.)
for the experiment of 0.46% (Jenkins, Conner and Frampton, 2001). A study of B. rapa
populations growing outside B. napus fields in the United Kingdom found few hybrids
(0.4-4.5%) in 7% of the populations, and no hybrids in the remaining 93% (Scott and
Wilkinson, 1998).
Hybridisation also occurs with B. napus as the female; however, most of the hybrid
seed that is formed will be removed from the field at harvest. Any hybrids that volunteer
the following year are almost certain to be surrounded by B. napus volunteers. Thus, any
backcrosses will quickly revert to B. napus form and chromosome number.
Compared to the parent species, natural interspecific hybrids have reduced fertility
and poor seed set, averaging two to five seeds per pod (Jørgensen and Andersen, 1994).
The survival rate of hybrid seedlings is also low, with <2% survival (Scott and
Wilkinson, 1998), reducing the rate of introgression (Jørgensen et al., 1996; Sweet et al.,
1999b). Interspecific vegetative and reproductive competition strongly impacts the
relative and absolute fitness of the hybrids (Hauser et al., 2001). When Mikkelsen, Jensen
and Jørgensen (1996) sowed interspecific hybrids within a B. napus population, no
B. rapa × hybrid BC progeny were found among 2 000 offspring raised from 30 B. rapa
plants. Further, the hybrids lacked primary seed dormancy (Linder, 1998). This may
explain why Landbo and Jørgensen (1997) found interspecific hybrids in feral B. rapa
populations, but no hybrid seed in the seed banks at those sites. Introgression of HR
transgenes from B. napus to B. rapa has occurred in Europe (Jørgensen, 1999;
Hansen et al., 2001; Norris and Sweet, 2002). However, no evidence of introgression was
found in seed samples taken from B. rapa plants in the field, indicating there may be
selection pressure against backcross individuals (Norris and Sweet, 2002).
The rate of introgression of a B. napus trait into the B. rapa genome will greatly
depend on the selection pressure exerted on the gene (Scott and Wilkinson, 1998;
Sweet et al., 1999a; Snow and Jørgensen, 1999). The introgression of a gene into the
B. rapa genome might be slowed by positioning it in the C genome of B. napus but the
findings of Stewart, Halfhill and Warwick (2002), where 12 independent B. napus
transformations distributed across both the A and C genomes all generated backcrosses at
similar rates, suggests this theory may not be valid. Leflon et al. (2006) found that the
transmission rate of the C chromosomes depended on which C chromosome was
involved, and that a gene carried on a C chromosome is less likely to be transferred in a
B. rapa background than if it was on an A chromosome. The presence of an introgressed
HR gene in B. rapa did not increase its fitness or weediness relative to conventional
non-GM B. rapa including glufosinate resistant BC 3 hybrids (Snow, Andersen and
Jørgensen, 1999) or BC 2 F 2 glyphosate hybrids (Warwick, 2007). It should be kept in mind
that if introgression of an R gene does occur, the resulting HR B. rapa plant(s) can be
controlled with other herbicides or cultivation. In Canada, with 16 years of experience
growing millions of hectares of HR B. napus each year, no significant agronomic
problems with HR B. rapa have been encountered (Beckie et al., 2006).
B. napus – Hirschfeldia incana
H. incana is an important weed in some European countries and eastern Australia, but
not in Canada or the Indian sub-continent. Hand crosses between B. napus and H. incana
produced 1.3 and 3.1 hybrids per 100 pollinations when H. incana and B. napus,
respectively, were used as the female (Kerlan et al., 1992). In the field, when male sterile
B. napus was used as the female, 1.9 hybrids were recorded per pollinated flower
(Eber et al., 1994). However, in three years of field trials, isolated H. incana plants
