Gene flow, which is the successful movement of genes among populations by mating or migration of seeds or other propagules, has gained much interest in agriculture in recent years because of the widescale adoption of transgenic crops and concerns over transgene escape into the wild (James 2004; Messeguer 2003; Stewart et al. 2003). Brassica napus (rapeseed), together with maize and sugar beet, have been identified among the species for which cross-pollination and transgene escape are concerns (Treu and Emberlin 2000).