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 to minimize a potential ecological risk in environments where B. ml,a is growing, the insertion of transgenes into chromosome regions of B. napus with a very low probability of transfer to backcross generations with B. rapa has been proposed. Recently, the progeny of four backcross generations between transgenic herbicide-tolerant B. napus and B. rapa was studied in selection experiments (Metz et al. 1997). The rapid decrease in the frequency of herbicide-tolerant plants was explained by selection against the C-chromosomes of B. napus in favor of the homeologous ii-chromosomes. Obviously, such C-chromosomes could be potential candidates as safe integration sites for transgenes. We considered these safety aspects using a simple population genetic model. Theory and experiments, however, do not favor the chromosomes of B. napus as safe candidates with respect to the introgression of transgenes into wild populations of B. rapa.