Low probability of chloroplast movement from oilseed rape (Brassica napus)into wild Brassica rapa

Pollen-mediated movement of transgenes from transplastomic oilseed rape (Br assica napus) into wild relatives will be avoided if chloroplasts are mater nally transmitted. We assess the probability of chloroplast exchange betwee n conventional oilseed rape and wild Brassica rapa to model the future beha vior of transplastomic cultivars. Primers specific to cpDNA were used to de monstrate maternal inheritance of chloroplasts in 47 natural hybrids betwee n cultivated B. napus and wild B. rapa. We conclude that there will be no o r negligible pollen-mediated chloroplast dispersal from oilseed rape. Trans gene introgression could still occur in mixed populations, however, if B. n apus acted as the recurrent female parent. Rate of transfer would then depe nd on the abundance of mixed populations, their persistence as mixtures, an d hybridization frequency within stands. A low incidence of sympatry (0.6-0 .7%) between wild B. rapa and cultivated B. napus along the river Thames, U K, in 1997 and 1998, suggests mixed stands will form only rarely. Eighteen feral populations of B. napus also showed a strong tendency toward rapid de cline in plant number, seed return, and ultimately, extinction within 3 yea rs, Conversely, hybrid production is significant in mixed stands, and the a bsence of control practices means that oilseed rape will have slightly grea ter persistence. We infer that some introgression from transplastomic B. na pus into B. rapa is inevitable in mixed populations even though such popula tions will occur infrequently and will tend to lose B. napus plants relativ ely quickly. Chloroplast exchange will be extremely rare and scattered.