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Mitigation Using a Tandem Construct Containing a Selectively Unfit Gene

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Mitigation Using a Tandem Construct Containing a Selectively Unfit Gene Plant Biotechnology Journal (2005) 3, pp. 000–000 doi: 10.1111/j.1467-7652.2005.00153.x MitigationHaniBlackwellOxford,PBIPlant1467-7644©?2Original 2005 2005 BiotechnologyAl-Ahmad UKArticleBlackwell Publishing, of transgene Publishing and Journal Ltd. Jonathan Ltdestablishment Gressel in hybrid progeny using a tandem construct containing a selectively unfit gene precludes establishment of Brassica napus transgenes in hybrids and backcrosses with weedy Brassica rapa Hani Al-Ahmad and Jonathan Gressel* Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel Received 30 March 2005; Summary revised 9 June 2005; Transgenic oilseed rape (Brassica napus) plants can interbreed with nearby weedy Brassica accepted 18 June 2005. rapa, potentially enhancing the weediness and/or invasiveness of subsequent hybrid *Correspondence (fax +972 (8)934-4181; e-mail [email protected]) offspring. We have previously demonstrated that transgenic mitigation effectively reduces the fitness of the transgenic dwarf and herbicide-resistant B. napus volunteers. We now report the efficacy of such a tandem construct, including a primary herbicide-resistant gene and a dwarfing mitigator gene, to preclude the risks of gene establishment in the related weed B. rapa and its backcross progeny. The transgenically mitigated and non-transgenic × B. rapa B. napus interspecific hybrids and the backcrosses (BC1) with B. rapa were grown alone and in competition with B. rapa weed. The reproductive fitness of hybrid offspring progressively decreased with increased B. rapa genes in the offspring, illustrating the efficacy of the concept. The fitness of F2 interspecific non-transgenic hybrids was between 50% and 80% of the competing weedy B. rapa, whereas the fitness of the comparable T2 interspecific transgenic hybrids was never more than 2%. The reproductive fitness of the transgenic T2 BC mixed with B. rapa was further severely suppressed to 0.9% of that of the competing Keywords: dwarfism, ecological 1 competition, gene flow, interspecific weed due to dwarfism. Clearly, the mitigation technology works efficiently in a rapeseed hybridization, oilseed rape, transgenic crop–weed system under biocontainment-controlled environments, but field studies should mitigation. further validate its utility for minimizing the risks of gene flow. and the backcross progeny with B. rapa have varying Introduction numbers of C chromosomes (AA + 0–9C, 2n = 20–29) (U, Outcrossing of oilseed rape (Brassica napus = canola) to 1935). In contrast, gene flow between B. napus and the weedy relatives is problematic because oilseed rape is one of genetically distant wild relative Raphanus raphanistrum the few crops that may have interfertile weeds occurring (wild radish, RrRr, 2n = 18) is extremely rare (Chevre et al., within the cultivated crop. Several studies have demonstrated 2000; Rieger et al., 2001; Warwick et al., 2003), and gene the field transfer of genes between B. napus and the genet- introgression into stabilized backcrosses could not be ically close weedy B. rapa (= B. campestris = wild turnip) achieved (Chevre et al., 1998). (Jorgensen and Andersen, 1994; Brown and Brown, 1996; Many factors influence the success of spontaneous Mikkelsen et al., 1996a; Halfhill et al., 2002; Warwick et al., hybridization between B. napus and related weeds under 2003; Hall et al., 2005). B. napus is an allotetraploid (AACC, field conditions, including the physical and genetic distances 2n = 38) with the AA genome from B. rapa (2n = 20) and the between the parents, synchrony of flowering, whether pollen CC genome from B. oleracea (2n = 18) (U, 1935). F1 hybrids dispersal is mainly by wind or by insect activity, direction of between B. napus and B. rapa are triploid (AAC, 2n = 29), the cross and environmental conditions (Scheffler and Dale, © 2005 Blackwell Publishing Ltd 1 2 Hani Al-Ahmad and Jonathan Gressel 1994). Genetic studies of B. napus-specific allozyme alleles, Transgene integration and expression in the B. rapa chromosome counts, isozymes, randomly amplified & × TM B. napus ( interspecific hybrids and their polymorphic DNA (RAPD) and amplified fragment length recurrent backcrosses with weedy B. rapa & polymorphism (AFLP) markers have shown that genes of volunteer and feral B. napus can stably introgress in The integration and expression of the tandem ahasR (aceto- weedy B. rapa in a few generations (Jorgensen and Andersen, hydroxy acid synthase) and ∆gai (gibberellic acid-insensitive) 1994; Mikkelsen et al., 1996b; Hansen et al., 2001, 2003). inserts in the dwarf TM T1 and T2 interspecific hybrids and BC1 Although F1 hybrids and most of the first backcross gener- progeny was confirmed by polymerase chain reaction ation (BC1) with B. rapa have a reduced productivity (Lee (PCR) (Figure S2, available as Supplementary material), AHAS and Namai, 1992), some B. rapa-like plants with high enzyme assay (Figure S3, available as Supplementary material) fitness have been found in the first hybrid and backcross and seed selection on imazapyr and/or kanamycin agar generations (Mikkelsen et al., 1996a; Hauser et al., 1998b, medium, as described in Al-Ahmad et al. (2005a). The pattern 2003). and magnitude of resistance of the interspecific hybrids and We have previously demonstrated the effectiveness of BC1 progenies (Figure S3, available as Supplementary material) transgenic mitigation (TM) technology in tobacco (Nicotiana were similar to those of the TM B. napus plants (Figure S2a,b tabacum) as a model (Al-Ahmad et al., 2004, 2005b). In in Al-Ahmad et al. 2005a). B. napus, we demonstrated the effectiveness of the trans- genic mitigation system against the crop as a volunteer Productivity of the B. rapa & × TM B. napus ( weed (Al-Ahmad et al., 2005a) using a dwarfing gene that interspecific hybrids and their backcross progeny increased the yield of the crop when grown without com- grown in the glasshouse at wide spacing without petition from other varieties or species. These results strongly competition suggested that the Brassica crop–weed pair should be tested to ascertain how well the tandem construct will mitigate The productivity (growth without competition) of hybrids and transgene establishment in crop–weed hybrid offspring. The backcross progeny was first tested to ascertain how it com- herbicide-resistant and dwarf genes engineered in tandem pared with that of the wild-type weedy B. rapa plants (from in the same TM construct (Al-Ahmad et al., 2004) were a natural population in Quebec, Canada). Plants of the above manually crossed from TM B. napus into the closely related genotypes, as well as B. napus, were grown separately in 1- B. rapa weed. The utility of mitigation technology in preclud- L pots in the glasshouse at wide spacing between the plants. ing transgene introgression and establishment in B. rapa × B. rapa plants were taller (P ≤ 0.01) because they bolted ≤ B. napus hybrids and their recurrent backcrosses (BC1) to earlier, formed more leaves (P 0.01), grew faster and B. rapa was tested in a biocontainment screen-house thus completed their life cycle earlier than the non-transgenic ecological competition experiment, as reported below. B. napus crop and the non-transgenic interspecific hybrid (B. rapa & × B. napus () plants (Figure S4 and Table S1, avail- able as Supplementary material). The F interspecific hybrids Results and discussion 1 grew faster and were taller than the crop (P ≤ 0.01), and ≤ Non-transgenic and hemizygous TM T1 lines J7, J9 and J16 showed earlier leaf senescence than the crop (P 0.01; B. napus cv. Westar (Al-Ahmad et al., 2005a) were manually Figure S4b, available as Supplementary material). The yield crossed as pollen donors with individual non-transgenic per plant and harvest index of B. rapa and the non-transgenic weedy B. rapa plants. This is the main direction of crossing F1 and F2 interspecific hybrids were significantly lower than expected in the field, as B. napus is mostly self-fertile with those of the non-transgenic B. napus (P ≤ 0.01), demon- about 70%−80% of the seed normally arising from self- strating the results of domestication (Table S1, available as pollination (Becker et al., 1992), whereas B. rapa is highly Supplementary material). In addition, the seed quality of the self-incompatible and relies on cross-pollination, mainly by triploid hybrids was decreased, with 60% fully developed insects and wind. Thus, reciprocal hybrids were not made in seeds, compared with > 90% fully developed seeds for both this study. The TM and non-transgenic interspecific hybrids the non-transgenic crop and weed parents (Table S1, available were selfed, as well as used as pollen donors in backcrosses as Supplementary material). & × ( (BC1) to B. rapa. The phenotypic appearance of the hybrid The B. rapa TM T1 B. napus line J9 transgenic inter- and backcross progeny is described in the Supplementary specific hybrids were dwarfed at the rosette stage (P ≤ 0.01; material available online (Figure S1). Figure 1a), with shorter internodes (P ≤ 0.01; Figure 1a) and © Blackwell Publishing Ltd, Plant Biotechnology Journal (2005), 3, 000–000 Mitigation of transgene establishment in hybrid progeny 3 darker and had more leaves (P ≤ 0.01; Figure S1a, available as Supplementary material), which had shorter petioles (P ≤ 0.01; Figure S5a, available as Supplementary material). There was no significant difference in stem thickness com- pared with the non-transgenic interspecific hybrid plants (Figure 1a). Growth at the rosette stage was extended, delay-
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