Abstract
Integration of a transgene into chromosomes of the C-genomes of oilseed rape (AACC, 2n = 38) may affect their gene flow to wild relatives, particularly Brassica juncea (AABB, 2n = 36). However, no empiric evidence exists in favor of the C-genome as a safer candidate for transformation. In the presence of herbicide selections, the first- to fourth-generation progenies of a B. juncea × glyphosate-tolerant oilseed rape cross [EPSPS gene insertion in the A-genome (Roundup Ready, event RT73)] showed more fitness than a B. juncea × glufosinate-tolerant oilseed rape cross [PAT gene insertion in the C-genome (Liberty Link, event HCN28)]. Karyotyping and fluorescence in situ hybridization–bacterial artificial chromosome (BAC-FISH) analyses showed that crossed progenies from the cultivars with transgenes located on either A- or C- chromosome were mixoploids, and their genomes converged over four generations to 2n = 36 (AABB) and 2n = 37 (AABB + C), respectively. Chromosome pairing of pollen mother cells was more irregular in the progenies from cultivar whose transgene located on C- than on A-chromosome, and the latter lost their C-genome-specific markers faster. Thus, transgene insertion into the different genomes of B. napus affects introgression under herbicide selection. This suggests that gene flow from transgenic crops to wild relatives could be mitigated by breeding transgenic allopolyploid crops, where the transgene is inserted into an alien chromosome.
Highlights
One of the concerns about growing transgenic herbicide-tolerant (HT) oilseed rape (Brassica napus, AACC = 38) is that HT transgenes may spread to their weedy relatives via spontaneous hybridization and introgression
We aim to reveal the different mechanisms involved in the introgression of transgenes located on A- and C-chromosomes under herbicide selection by addressing the following three questions: (1) How do chromosomes change across four selfpollination generations of BC1? (2) How does transgene insertion on the chromosomes of the A or C genomes affect the processes of introgression from oilseed rape (AACC) into its wild relative B. juncea (AABB)? (3) What is the fate of C- chromosomes of oilseed rape introgressed into B. juncea over four generations?
Successful gene flow from transgenic species to their wild relatives depends on their introgression ability, including the inheritance stability of the transgenes and fitness among hybrid progenies; these may be determined by genetic factors related to chromosome structures and transgene insertion locations
Summary
One of the concerns about growing transgenic herbicide-tolerant (HT) oilseed rape (Brassica napus, AACC = 38) is that HT transgenes may spread to their weedy relatives via spontaneous hybridization and introgression. If this were to happen, the weeds with HT traits could create new and serious weed control problems (Kling, 1996; Chèvre et al, 1997; Dale et al, 2002; Chapman and Burke, 2006; Rizwan et al, 2019; Clark and Maselko, 2020). Besides the initial crop-weed hybridization, gene transmission during successive generations, the production of fertile and fit offspring through successive backcrosses or selfing, and effective gene recombination between genomes are important determinants of introgression (Chèvre et al, 1998, 2007; Jenczewski et al, 2003; Jørgensen et al, 2009)
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