A- or C-chromosomes, does it matter for the transfer of transgenes from Brassica napus

Introgression of genes from allotetraploid Brassica napus into its diploid wild relative B. mpa is generally considered to be inevitable. As a means t o minimize a potential ecological risk in environments where B. ml,a is gro wing, the insertion of transgenes into chromosome regions of B. napus with a very low probability of transfer to backcross generations with B. rapa ha s been proposed. Recently, the progeny of four backcross generations betwee n transgenic herbicide-tolerant B. napus and B. rapa was studied in selecti on experiments (Metz et al. 1997). The rapid decrease in the frequency of h erbicide-tolerant plants was explained by selection against the C-chromosom es of B. napus in favor of the homeologous ii-chromosomes. Obviously, such C-chromosomes could be potential candidates as safe integration sites for t ransgenes. We considered these safety aspects using a simple population gen etic model. Theory and experiments, however, do not favor the chromosomes o f B. napus as safe candidates with respect to the introgression of transgen es into wild populations of B. rapa.