1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
In species where chloroplast inheritance is exclusively or predominantly maternal, pollen-mediated flow of transgenes is reduced if transgenes are inserted in chloroplast DNA instead of nuclear DNA. However, transmission of chloroplast-encoded transgeneswill still occur if transgenic individuals act as the maternal parent when hybridisation and backcrossing takes place. Chloroplast DNA inheritance between F1-hybrids (B. napus (?) x B. rapa) and B. rapa; the second step in the introgression process oftransgenes from transplastomic B. napus to B. rapa was investigated. It was maternal in all 122 examined cases. Field trials with B. napus and B. rapa coexisting in different proportions and densities elucidated how these factors affect the F1-hybridproduction on B. napus. Higher plant density reduced the fitness of mother plants and the abundance of F1-hybrids (at the 1:1 proportion) significantly. As to the proportion between the species, B. rapa was a stronger competitor than B. napus. Theproportion seemed to be a more powerful factor than the density. In conclusion, hybridisation on B. napus seems to be most likely at current field densities of B. napus and when B. rapa is an abundant weed. The next step in the introgression process wasinvestigated in field trials with F1-hybrids coexisting with B. napus and B. rapa in different proportions and densities. With the highest abundance of F1-hybrids, B. napus was the predominant father and the sirering success of the three possible fathersdepended on the density. Progenies from F1-hybrid mother plants grown at the other two proportions were screened merely for individuals sired by B. rapa (BC1rs). The density affected on the production of BC1rs significantly but the effect differed amongproportions with both the highest and lowest frequencies of BC1rs obtained at high plant density. With low abundance of B. rapa the numbers of BC1rs/m2 were low and with high abundance of F1-hybrids it was comparatively high. The fitness of mother plants(F1-hybrids) decreased significantly from low to intermediate density. A further increase only affected the thousand-kernel weight significantly. It was concluded that further introgression of transgenes from transplastomic oilseed rape to B. rapa is mostlikely at current field densities of B. napus and when B. rapa is an abundant weed.