Gene Flow Risks From Transgenic Herbicide-Tolerant Crops to Their Wild Relatives Can Be Mitigated by Utilizing Alien Chromosomes

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.

Interspecific Hybridization of Transgenic Brassica napus and Brassica rapa—An Overview

“…Aside from that, GM crops and their transgenes spread via seed spillage during transportation and pollen-mediated gene transfers, resulting in feral populations [ 1 , 15 ]. If this occurs, weeds with GM traits may provide new and substantial weed control challenges [ 16 ]. The risk of crop genes transferring to weedy relatives is determined by their genetic and structural similarities as well as the strength of the transgenic selection in the weedy relative.…”

“…The risk of crop genes transferring to weedy relatives is determined by their genetic and structural similarities as well as the strength of the transgenic selection in the weedy relative. The transfer of transgenes through introgression also depends on the fitness of the first and successive generations of hybrids [ 16 , 17 , 18 ]. However, the level of hybridization and introgression among the Brassica species is highly varied.…”

Transgene Was Silenced in Hybrids between Transgenic Herbicide-Resistant Crops and Their Wild Relatives Utilizing Alien Chromosomes

“…As the typical relative of Brassica napus , wild Brassica juncea (AABB, 2n = 36), which was wildly distributed in the wastelands and farmlands of the Yangtze River Basin, Yellow River Basin and northwest China, represents a high gene homology with transgenic Brassica napus [ 7 ]. As a matter of fact, Brassica napus really hybridized with wild Brassica juncea to produce F1 under natural conditions [ 8 , 9 , 10 , 11 ], and the fitness of the progenies displayed increased fecundity and fitness progressively across the self-pollination of backcross generations, which indicated a potential ecological risk of transgene flow [ 12 , 13 , 14 ]. However, potential ecological risk depends on not only fitness of the hybrids but also genetic stability of the transgene, including its transmission and expression.…”

“…However, there is no empirical evidence to describe this internal mechanism. Moreover, it was found that the survival rate of backcross progenies of HR transgenic Brassica napus ( PAT gene insertion in the C-genome) and wild Brassica juncea under herbicide selection was significantly lower than that of backcross progenies of resistant transgenic Brassica napus (EPSPS gene insertion in the A-genome) and wild Brassica juncea [ 14 ]. PAT is the type of N-acetyltransferase gene which has the detoxification effect of glufosinate.…”

“…Bayer Crop Science cloned the genes Bar and PAT that can encode N-acetyltransferase from Streptomyces hygroscopicus and Streptomyces viridochromo , respectively [ 11 ]. This research provided evidence which was in favor of the C-genome as a safer candidate for transformation [ 14 ]. However, we do not know whether the transgene cannot be normally transmitted to the progenies or that the transgene cannot be normally expressed after transmission, i.e., gene silencing.…”