II.3. BRASSICA CROPS (BRASSICA SPP.) – 223
When B. napus herbicide resistant (HR) hybrids were surrounded by R. raphanistrum
plants in the field, the seed set was less than one seed per hybrid plant (Darmency, Fleury
and Lefol, 1995). Despite the low fertility and poor fitness of the hybrids, the fertility and
fitness of the backcross progeny improved with each backcross generation but the
percentage of HR plants decreased (Chèvre et al., 1999, 1998, 1997b; Darmency, Lefol
and Fleury, 1998; Benabdelmouna et al., 2003; Guéritaine, Bazot and Darmency, 2003).
In each generation the progenies were selected for herbicide tolerance and only HR plants
advanced to the next backcross (BC). None of the HR plants in the BC 3 to BC 5 had the
chromosome number of R. raphanistrum (2n=18) indicating that no genomic
introgression had occurred (Chèvre et al., 1998; Guéritaine et al., 2002). Backcrossing to
R. raphanistrum was continued up to BC 7 followed by random mating and selection
pressure in generations (G) G8 through G11 (Al Mouemar and Darmency, 2004). Root tip
cytology of HR G9 plants established that all 32 plants were either carrying extra
chromosomes or, as indicated by the non-Mendelian segregation of the progeny, did not
have the HR gene stably introgressed into the R. raphanistrum genome. The authors
concluded that “the prospect of stable introgression of herbicide tolerance to wild radish
in nature seems remote”.
B. napus – B. rapa
B. rapa, a widespread weed of cultivated and disturbed lands, is also grown as a
vegetable and oilseed crop. The weedy type differs from the cultivated oilseed form only
in the primary seed dormancy trait. Plant breeders of B. rapa and B. napus have known
for many years that these two species readily cross in nature and they were not surprised
that natural interspecies gene flow was demonstrated in several countries, including
Denmark (Landbo, Andersen and Jørgensen, 1996; Hansen et al., 2001), Canada
(Warwick et al., 2003; Beckie et al., 2003; Yoshimura et al., 2006), the United Kingdom
(Daniels et al., 2005; Allainguillaume et al., 2006), the United States (Halfhill et al.,
2002) and the Czech Republic (Bielikova and Rakousky, 2001).
Normally the highest hybrid frequencies occur when individual, self-incompatible
plants of B. rapa are present in B. napus fields (Jørgensen et al., 1996). In the field, more
hybrids are produced on B. rapa plants than on B. napus plants (Jørgensen and Andersen,
1994; Hauser, Jorgensen and Ostergard, 1997; Jørgensen et al., 1998), primarily due to
their respective self-incompatible and self-compatible breeding systems. However, in
reciprocal hand crosses, more hybrids per cross are found when B. napus is the female
(Downey, Klaasen and Stringham, 1980). Natural interspecific hybridisation between
B. rapa and B. napus varies widely, depending on the environment under which the plants
develop and the design of the experiment, particularly the ratio of B. rapa to B. napus
plants. In Danish trials, up to 95% hybrids were found in B. rapa progeny (Mikkelsen,
Jensen and Jørgensen, 1996), while in New Zealand Palmer (1962) reported a range of
10-88%. In contrast, others in Canada (Bing, Downey and Rakow, 1991) and England
(Wilkinson et al., 2000) found less than 1% hybridisation. In Canadian field experiments
(two in the east and one in the west), B. rapa plants were grown at various positions
within and alongside HR B. napus plots. Approximately 7% of the harvested B. rapa seed
was found to be triploid hybrids (AAC, 2n=29) (Warwick et al., 2003). Similarly, in
commercial B. napus fields containing sparse populations of weedy B. rapa, the hybrid
frequency was approximately 13.6%. However, the frequency of hybrids from weedy
B. rapa growing in a harvested corn field with HR B. napus volunteers was only 0.023%
(Warwick et al., 2003). In New Zealand field studies with ratios of B. rapa to B. napus
plants of 1:400 and 1:1, the hybrid frequencies ranged from 2.1% to 0.06% with the total