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9.4 Intergeneric crossings

Potential gene flow between B. napus or B. juncea, and Australian Brassicaceae weed species is summarised in Table 9.

The flowering periods of many weedy Brassicaceae species overlap with those of B. napus and B. juncea. Depending on the season and region, the synchrony of flowering between species can also influence the rate of outcrossing in the field. Generally, in Australia commercially grown Brassica species flower from September to January, while many weedy Brassicaceae species begin flowering around August. However, this will vary with environmental conditions and under ideal growing conditions, some weedy species may flower at any time during the year (Rieger et al. 1999).

Significant pre-and post-fertilization barriers exist between B. napus or B. juncea and their weedy relatives in Australia (Salisbury, 2006). Gene movement between B. napus or B. juncea, and other members of the Brassicaceae family is rare, and in most cases probably never occurs. It is considered that, if such hybrids were to be produced under natural conditions, their chance of survival would be extremely low (Salisbury, 2006).

As for interspecific crosses, the use of modern breeding techniques has allowed the production of intergeneric hybrids that would otherwise have failed. Hybrids have been generated in vitro by crossing B. napus with Diplotaxis tenuifolia, Hirschfeldia incana, Raphanus raphanistrum and Sinapis arvensis (FitzJohn et al. 2007). See Warwick et al. (2009) for an extensive review of available interspecific and intergeneric hybridization data.

This section focuses mainly on Raphanus raphanistrum, Sinapis arvensis and Hirschfeldia incana. These species are recognised as major weeds of commercial Brassica crops and have been described as potentially compatible with B. napus (Eastham & Sweet 2002). Relative weediness of these three species in agricultural ecosystems is summarized in Table 10.

Table 10. Relative weediness of R. raphanistrum, S. arvensis and H. incana in Australia. Adapted from Groves et al. (2003).

	Australian rating	QLD	NSW	VIC	TAS	SA	WA	NT
R. raphanistrum	5	5	5	5	5	4	5	n/a
S. arvensis	5	2	5	3	5	1	5	n/a
H. incana	5	1	3	5	2	2	1	n/a

Raphanus raphanistrum is a major weed of canola in all canola growing States, especially in WA (Salisbury 2002b). Hybrids have been generated by co-cultivation under field conditions or in glasshouse, using a male sterile B. napus (Ammitzboll & Jorgensen 2006; Darmency et al. 1998; Gueritaine et al. 2003). Hybridisation rate observed by Darmency et al. (1998) was of 0.05%. No details were given regarding hybridisation rates for the other studies. Gueritaine et al. (2003) showed that such hybrids are less likely than both parents to emerge and survive competition with other plants, both in agronomic conditions and in disturbed habitats. There is no record of hybrids generated under natural conditions with B. juncea as the pollen donor (FitzJohn et al. 2007, and references therein; Warwick et al. 2009). Transfer of genes of B. napus or B. juncea to R. raphanistrum would be highly unlikely (Gueritaine et al. 2003).

Sinapis arvensis is an occasional weed of canola in all canola growing areas (Salisbury 2002b). Using co-cultivation with male-sterile B. napus, hybridization rates of 0.12-0.18% were observed (Chevre et al. 1996; Lefol et al. 1996). There is no record of hybrids generated under natural conditions with B. napus as the pollen donor (FitzJohn et al. 2007, and references therein; Warwick et al. 2009). B. juncea x S. arvensis hybrids were generated using co-cultivation under field conditions, at a rate of 0.0018% (Warwick & Martin 2013). Hybrids showed reduced fertility and no backcross progeny was obtained using S. arvensis. The authors suggested that the likelihood of transgene introgression from B. juncea to S. arvensis is low to negligible (Warwick & Martin 2013). Gene flow is not likely to occur between either B. napus or B. juncea, and S. arvensis (Eastham & Sweet 2002).

Hirschfeldia incana is a weed of disturbed soils in eastern Australia and an occasional weed of canola in all canola growing regions (Salisbury 2002b). Using co-cultivation under field conditions, Lefol et al. (1996) obtained 0.36-1.0 B. napus x H. incana hybrid per plant. Back-crossing the hybrids to H. incana produced only non-viable plants. Darmency and Fleury (2000) estimated frequency of hybrid descendants to be as low as 0.002%. Gene introgression was deemed as extremely unlikely (Darmency & Fleury 2000). Potential gene flow from B. juncea to H. incana under Australian conditions has also been described as extremely unlikely (Salisbury 1991; Salisbury 2006).