222 – II.3. BRASSICA CROPS (BRASSICA SPP.)
B. napus – Raphanus raphanistrum
R. raphanistrum is an economically damaging weed with a worldwide distribution but
its range is limited to areas with acid soils. Hand crosses between B. napus and
R. raphanistrum have produced reciprocal hybrids with a higher number of hybrids
obtained with B. napus as the female (Kerlan et al., 1992: Chèvre et al., 1996). In France,
when R. raphanistrum served as the female, only three hybrids have been identified, even
though tens of thousands of seeds were examined (Eber et al., 1994; Baranger et al.,
1995; Chèvre et al., 2000, 1998, 1997b; Darmency, Lefol and Fleury, 1998; Darmency
and Fleury, 2000). Chèvre et al. (2000) estimated the hybridisation frequency to be 10-7
to 10-5 while Australian and Canadian studies reported respective frequencies of 4 × 10-8
(Rieger et al., 2001) and 3 × 10-5 (Warwick et al., 2003).
Guéritaine, Bazot and Darmency (2003) found that under field conditions the F 1
hybrid emergence was lower and slower and seedling survival significantly less than both
parents. A six-year UK monitoring programme of natural populations of R. raphanistrum
growing near fields of HR B. napus showed no evidence of intergeneric crossing
(Eastham and Sweet, 2002). Similarly in the United Kingdom, Daniels et al. (2005) found
no R. raphanistrum × B. napus plants or progeny when they sampled R. raphanistrum
plants growing in or near four fields sown to glufosinate resistant B. napus. Further, no
hybrids were found in a Swiss survey (Thalmann, Guadagnuolo and Felber, 2001). When
R. raphanistrum was the female, no hybrids were found in any of these studies. The
frequency of hybridisation can vary depending on the B. napus parental variety and the
population source of R. raphanistrum. When B. napus male sterile plants were used as
females, the frequency of hybrids was greatly increased, ranging from <0.2% (Chèvre et
al., 2000; 1996) to as high as 90% in Danish and French field trials (Eber et al., 1994;
Baranger et al., 1995; Ammitzbøll and Jørgensen, 2006). These findings would be of
concern if the use of synthetic hybrids became standard, as the vast majority of plants in
commercial oilseed rape fields would be male sterile. However, as indicated earlier, this
hybrid system has now been phased out.
In the B. napus by R. raphanistrum cross, the majority of the F 1 hybrids had half the
chromosomes of each species (ACRr, 2n=28) while one hybrid had all the chromosomes
of R. raphanistrum and half the B. napus chromosomes (RrRrAC, 2n=37) (Chèvre et al.,
2000). Thus, the fertility of the hybrids is very low (Baranger et al., 1995; Chèvre et al.,
1998, 1996; Darmency, Lefol and Fleury, 1998; Pinder et al., 1999; Thalmann,
Guadagnuolo and Felber, 2001; Warwick et al., 2003). However, Rieger et al. (2001)
reported two fertile amphidiploids hybrids with a genome complement of AACCRrRr,
2n=56. Chèvre et al. (2000) also reported four fertile amphidiploids but questioned their
genetic stability due to the presence of univalents and multi/quadrivalents at meiosis.
The fitness of F 1 hybrids produced on B. napus male sterile plants was assessed in the
field by Guéritaine, Bazot and Darmency (2003). They found that the hybrids were
slower to emerge and less likely to survive than either parent, particularly when subjected
to crop competition. The hybrids also flowered later than either parent, which limited the
opportunities for backcrossing to R. raphanistrum. It should also be noted that if crossing
between these species were to occur, it would most likely take place in a field of oilseed
rape. Thus, most of the crossed seed would be harvested and only a very small proportion
of the original hybrid seed would remain (Rieger et al., 2001). The few surviving hybrids
would germinate among B. napus volunteers with backcrosses to B. napus much more
likely than with wild radish.
