The prospect of a commercial release in Europe of genetically modified oilseed rape (Brassica napus L.) has intensified concern over the chances of transgene movement into wild relatives. We have found that potential transgene recruitment is likely to be slow and uncertain.Brassica rapa is the most likely species occurring as natural populations to receive transgenes by introgressive hybridization 1 . B. rapa plants growing in mixed stands with B. napus have yielded hybrid-seed levels of 9-93% in field studies 2 . Similar levels (3-60% hybrid seed) were reported from B. rapa weeds growing in flowering oilseed rape 2,3 . Gene flow into weedy populations depends on subsequent introgression, or dispersal into truly wild populations. This is made improbable by the likelihood that hybrid seeds will be removed during harvest and by the uncertainty of hybrids surviving farm-management practices. Introgression into B. rapa populations occupying their natural habitat (river and stream banks) is the most likely route for transgene spread from agriculture. To date, there are no authenticated reports of hybrids in such habitats 4 .Gene flow will be greatest where natural populations are adjacent to large fields. B. rapa growing in field margins, however, may be escapes from cultivation. We studied two natural UK populations, by the River Thames, of B. rapa growing next to commercial oilseed rape. The first (at Hambleden Lock, Berkshire) contained 50 plants in 1996 and grew within 5 m of a 15-hectare field of B. napus cultivar Apex. The second (Culham, Oxfordshire) consisted of 350 plants in 1997, 1-2 m from a 12-hectare field of cultivar Apex. Seed collected from both populations was germinated and putative hybrids identified by their distinct, intermediate morphology.The plants' hybrid status was confirmed by flow cytometry, chromosome counts and Inter-SSR (ref. 5) polymerase chain reaction (PCR) (Fig. 1). The 13,341 seeds sown from Hambleden produced 7,594 seedlings (57% germination), of which only 30 were hybrids (0.4%); 2,000 seeds sown from Culham yielded 1,053 seedlings (53% germination) and just 16 hybrids (1.5%).A random selection of offspring lacking the hybrid phenotype (788 plants from Hambleden, 583 from Culham) was confirmed as non-hybrid B. rapa by flow cytometry and Inter-SSR PCR. This sufficed to ensure that undetected hybrids did not occur at a higher frequency than detected hybrids, with a confidence of > 0.95 and > 0.99 for Hambleden and Culham, respectively 6 .We surveyed 33.4 km of the River Thames in 1997 and identified 145 populations containing 10-1,000 B. rapa plants, including the Culham population and nine others (6.2% of the 145), within 360 m of oilseed rape. The density of airborne pollen beyond 360 m falls to less than 10% of that at the field margin 7 . All other populations were > 800 m from oilseed rape. We infer that there is a real, but slight, risk of gene flow in < 7% of populations in the survey area.The Hambleden population was revisited regularly during 1997 to monitor the appearance ...

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Pollen-mediated movement of transgenes from transplastomic oilseed rape (Brassica napus) into wild relatives will be avoided if chloroplasts are maternally transmitted. We assess the probability of chloroplast exchange between conventional oilseed rape and wild Brassica rapa to model the future behavior of transplastomic cultivars. Primers specific to cpDNA were used to demonstrate maternal inheritance of chloroplasts in 47 natural hybrids between cultivated B. napus and wild B. rapa. We conclude that there will be no or negligible pollen-mediated chloroplast dispersal from oilseed rape. Transgene introgression could still occur in mixed populations, however, if B. napus acted as the recurrent female parent. Rate of transfer would then depend on the abundance of mixed populations, their persistence as mixtures, and 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, UK, in 1997 and 1998, suggests mixed stands will form only rarely. Eighteen feral populations of B. napus also showed a strong tendency toward rapid decline in plant number, seed return, and ultimately, extinction within 3 years. Conversely, hybrid production is significant in mixed stands, and the absence of control practices means that oilseed rape will have slightly greater persistence. We infer that some introgression from transplastomic B. napus into B. rapa is inevitable in mixed populations even though such populations will occur infrequently and will tend to lose B. napus plants relatively quickly. Chloroplast exchange will be extremely rare and scattered.