II.3. BRASSICA CROPS (BRASSICA SPP.) – 227
B. napus using embryo rescue. Lefol, Seguin-Swartz and Downey (1997), using
reciprocal hand crosses, obtained one slow-growing B. napus × E. gallicum F 1 hybrid
with pollen viability of 28%. Indications were that the F 1 would not survive in
competition with a B. napus crop. No seed was produced when E. gallicum served as the
female parent. The F 1 hybrid was backcrossed in all combinations and many seeds were
obtained when E. gallicum was the male and a few when B. napus was the female.
Backcross seed from the hybrid produced plants identical to E. gallicum, suggesting that
the B. napus chromosomes were lost. A survey of 22 000 seedlings of E. gallicum from
western Canadian B. napus fields yielded no hybrids, indicating that the possibility of
hybridisation between B. napus and E. gallicum is very low (<5 × 10-5) (Warwick et al.,
2003).
B. napus – B. nigra
B. nigra is a minor weed and an occasional crop in warmer, shorter day-length
locations of oilseed rape growing regions. Interspecific hand crosses between B. napus
and B. nigra have been difficult to obtain, with some success using oilseed rape as the
female (Davey, 1959; Heyn, 1977; Diederichsen and Sacristan, 1988; Nishiyama,
Sarashima and Matsuzawa, 1991; Bing, Downey and Rakow, 1991; Bing et al., 1996;
Kerlan et al., 1992; Struss, Quiros and Röbbelen, 1992; Zhu, Struss and Röbbelen, 1993).
The F 1 hybrids were moderately to highly sterile but a few F 2 and BC seeds were obtained
(Bing, Downey and Rakow, 1991; Zhu, Struss and Röbbelen, 1993). Using controlled
crosses hybridization levels were extremely low (Raybould and Gray, 1993; Scheffler and
Dale, 1994). In the cross B. napus × B. nigra, Brown and Brown (1996) observed the
pollen tubes of B nigra were short and twisted with only a few penetrating the style. No
hybrids were found in natural crosses when B. nigra was the female (Bing, Downey and
Rakow, 1991; Leckie, Smithson and Crute, 1993; Daniels et al., 2005).
B. napus – B. oleracea and Brassica vegetables
Gene flow from oilseed rape to B. napus vegetables (Swedes, rutabaga, Siberian kale)
is possible since they are all within the same species. Similarly, gene flow to B. rapa
vegetables (e.g. turnip, Chinese cabbage, etc.) is possible since they have the A genome
in common. However, B. napus and B. rapa vegetables are not considered weedy. In
addition, they are generally harvested prior to flowering.
Hand crosses between B. napus and B. oleracea have been successful but at a very
low frequency (Chiang, Chiang and Grant, 1977) and natural crosses have only been
successful with the assistance of embryo rescue (Ayotte, Harney and Souza Machado,
1987; Takeshita, Kato and Tokumasu, 1980; Quazi, 1988; Habman et al., 2010).
However, amphidiploid F 1 hybrids were fertile and readily backcrossed to either parent
(Sundberg and Glimelius, 1991; Kerlan et al., 1992; Chèvre et al., 1996).
No spontaneous hybrids between B. napus and B. oleracea were found in two UK
surveys of wild B. oleracea populations (Scheffler and Dale, 1994; Wilkinson et al.,
2000). However, a later UK survey of two wild B. oleracea populations, growing within
25 m of B. napus fields, identified one triploid F 1 hybrid and nine introgressants based on
flow cytometry and crop-specific microsatellite markers (Ford et al., 2006). The fertility
of these plants has not been reported.