Do competitive conditions affect introgression of transgenes from oilseed rape (Brassica napus) to weedy Brassica rapa? AS case study with special reference to transplastomic oilseed rape

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 transgenes will 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 of transgenes 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-hybrid production 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. The proportion 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 was investigated 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 fathers depended on the density. Progenies from F₁-hybrid mother plants grown at the other two proportions were screened merely for individuals sired by B. rapa (BC_1rs). The density affected on the production of BC_1rs significantly but the effect differed among proportions with
both the highest and lowest frequencies of BC_1rs obtained at high plant density. With low abundance of B. rapa the numbers of BC_1rs/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 most likely at current field densities of B. napus and when B. rapa is an abundant weed.