II.3. BRASSICA CROPS (BRASSICA SPP.) – 225
growing in B. napus plots only produced 0.6 hybrid seeds per plant. Most F 1 plants had
reduced fitness with seedling emergence over three years being <1% (Chadoeuf,
Darmency and Maillet, 1998). However, some hybrids were at least as competitive as the
wild parent (Eber et al., 1994; Lefol, Fleury and Darmency, 1996).
When F 1 plants were backcrossed to H. incana and only HR progeny were selected
for further backcrossing, fewer seeds were produced in each generation. BC 3 produced
only one seed with no viable seeds obtained in BC 4 (Darmency and Fleury, 2000). It is
suggested that a H. incana gene inhibits homeologous pairing, resulting in an expulsion
of B. napus chromosomes (Kerlan et al., 1992; Lefol, Fleury and Darmency, 1996). Thus,
although interspecific F 1 hybrids will frequently occur in areas where H. incana is
prevalent, their persistence will be short and the possibility of gene introgression from
B. napus remote.
B. napus – B. juncea
B. juncea is primarily a crop plant grown in China, the Russian Federation and on the
Indian sub-continent as a major source of edible oil, and in Canada and a few other
countries as a condiment crop. However, it is present as a weed in parts of Europe and
Australia. Since B. juncea (AABB) and B. napus (AACC) have a common genome, the
chance of interspecific crossing is enhanced. In Canadian co-cultivation experiments,
Bing et al. (1996) identified five interspecific hybrids in seed harvested from
469 B. napus plants and 3 out of 990 plants when B. juncea was the female.
Jørgensen et al. (1998) noted that as the ratio of B. juncea to B. napus plants increased
from 1:3 to 1:15, the hybridization frequency on B. juncea plants decreased from 2.3% to
0.3%. Warwick (2007) reported gene flow from HR B. napus to neighbouring fields of
B. juncea at a rate of 0.245% at the adjacent B. juncea field border and 0.030%, 0.021%
and 0.005% at 50 m, 100 m, and 200 m, respectively.
The viability of F 1 pollen is reported to be low (18-26%) (Frello et al., 1995;
Choudhary and Joshi, 1999; GoshDastidar and Varma, 1999), but spontaneous
backcrossing with improved fertility has been reported (Alam et al., 1992; Bing, Downey
and Rakow, 1991; Bing et al., 1996; Jørgensen, 1999). Given this background of results,
the introgression of genes from B. napus could be expected to occur where these
two species are widely grown.
B. napus – Sinapis arvensis
S. arvensis is a serious weed in all oilseed rape growing countries. In a five-year study
of S. arvensis growing in and around GM B. napus crops in the United Kingdom,
Sweet et al. (1997) and Norris et al. (unpublished, cited in Eastham and Sweet, 2002)
failed to detect any hybridisation with S. arvensis. Also in the United Kingdom,
Daniels et al. (2005) tested 60 768 progeny from 818 S. arvensis plants, growing in or
close to 23 glufosinate resistant B. napus fields. No resistant plants were found in the
parents or their progeny. Similarly, Warwick et al. (2003) found no interspecific hybrids
among 43 828 S. arvensis progeny from plants growing in HR B. napus fields in western
Canada. Bing et al. (1996) also found no hybrids in Canadian co-cultivation experiments
involving the assessment of 7 500 S. arvensis seeds. Similar results were reported from
UK trials where 9 688 S. arvensis seedlings were screened (Moyes et al., 2002) and in
France, Lefol, Danielou and Darmency (1996) found no hybrids among the 2.9 million S.
arvensis seeds tested. However, when male sterile or emasculated B. napus plants were
pollinated with S. arvensis pollen, either naturally or artificially, a small number of
hybrids were obtained. Chèvre et al. (1996) found 0.18 hybrids per 100 pollinations while