Competition affects gene flow from oilseed rape (female symbol) to Brassica rapa (male symbol)

Characteristics and Fitness Analysis through Interspecific Hybrid Progenies of Transgenic Brassica napus and B. rapa L. ssp

Interspecific hybridization between transgenic crops and their wild relatives is a major concern for transgene dispersal in the environment. Under controlled conditions, artificial hand pollination experiments were performed in order to assess the hybridization potential and the fitness of interspecific hybrids between Brassica rapa and genetically modified (GM) Brassica napus. Initially, six subspecies of B. rapa were hybridized with GM B. napus through hand pollination. In the resulting F₁ hybrids, the combination of B. rapa ssp. narinosa (♀) × GM B. napus (♂) had the highest crossability index (16.9 ± 2.6). However, the F₁ selfing progenies of B. rapa ssp. rapa (♀) × GM B. napus were found to be more effective in producing viable future generations with the highest crossability index (1.6 ± 0.69) compared to other subspecies. Consequently, they were used for the generation of F₂ and F₃ progenies. The 18 different morphological characteristics among the parental cross-combinations and F₁ hybrid progenies were measured and visualized through hierarchical clustering. Different generations were found to be grouped based on their different morphological characteristics. The chromosome numbers among the interspecific hybrids ranged from 2n = 29 to 2n = 40. Furthermore, the SSR markers revealed the presence of genomic portions in the hybrids in comparison with their parental lines. There is a high possibility of transgene flow between GM B. napus and B. rapa. The study concluded that the interspecific hybrids between B. napus and B. rapa can be viable and can actively hybridize up to F₃ generations and more. This suggests that the GM B. napus can disperse the transgene into B. rapa, and that it can pass through for several generations by hand pollination in a greenhouse environment.

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

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.

Gene flow in commercial alfalfa (Medicago sativa subsp. sativa L.) seed production fields: Distance is the primary but not the sole influence on adventitious presence

In insect-pollinated crops, gene flow is affected by numerous factors including crop characteristics, mating system, life history, pollinators, and planting management practices. Previous studies have concentrated on the impact of distance between genetically engineered (GE) and conventional fields on adventitious presence (AP) which represents the unwanted presence of a GE gene. Variables other than distance, however, may affect AP. In addition, some AP is often present in the parent seed lots used to establish conventional fields. To identify variables that influence the proportion of AP in conventional alfalfa fields, we performed variable selection regression analyses. Analyses based on a sample-level and a field-level analysis gave similar, though not identical results. For the sample-level model, distance from the GE field explained 66% of the variance in AP, confirming its importance in affecting AP. The area of GE fields within the pollinator foraging range explained an additional 30% of the variation in AP in the model. The density of alfalfa leafcutting bee domiciles influenced AP in both models. To minimize AP in conventional alfalfa seed fields, management practices should focus on optimizing isolation distances while also considering the size of the GE pollen pool within the pollinator foraging range, and the foraging behavior of pollinators.

Fitness of F1 hybrids between 10 maternal wild soybean populations and transgenic soybean

The releasing of transgenic soybeans (Glycine max (L.) Merr.) into farming systems raises concerns that transgenes might escape from the soybeans via pollen into their endemic wild relatives, the wild soybean (Glycine soja Sieb. et Zucc.). The fitness of F1 hybrids obtained from 10 wild soybean populations collected from China and transgenic glyphosate-resistant soybean was measured without weed competition, as well as one JLBC-1 F1 hybrid under weed competition. All crossed seeds emerged at a lower rate from 13.33-63.33%. Compared with those of their wild progenitors, most F1 hybrids were shorter, smaller, and with decreased aboveground dry biomass, pod number, and 100-seed weight. All F1 hybrids had lower pollen viability and filled seeds per plant. Finally, the composite fitness of nine F1 hybrids was significantly lower. One exceptional F1 hybrid was IMBT F1, in which the composite fitness was 1.28, which was similar to that of its wild progenitor due to the similarities in pod number, increased aboveground dry biomass, and 100-seed weight. Under weed competition, plant height, aboveground dry biomass, pod number per plant, filled seed number per plant, and 100-seed weight of JLBC-1 F1 were lower than those of the wild progenitor JLBC-1. JLBC-1 F1 hybrids produced 60 filled seeds per plant. Therefore, F1 hybrids could emerge and produce offspring. Thus, effective measures should be taken to prevent gene flow from transgenic soybean to wild soybean to avoid the production F1 hybrids when releasing transgenic soybean in fields in the future.

Pollen-mediated gene flow from transgenic cotton is constrained by physical isolation measures

The public concern about pollen-mediated gene flow (PGF) from genetically modified (GM) crops to non-GM crops heats up in recent years over China. In the current study, we conducted greenhouse and field experiments to measure PGF with various physical isolation measures, including 90, 80, 60 and 40 holes/cm² separation nets and Sorghum bicolor, Zea mays and Lycopersicon esculentum separation crops between GM cotton and non-GM line (Shiyuan321) by seed DNA test during 2013 to 2015, and pollen grain dyeing was also conducted to assess the pollen flow in greenhouse during 2013. Our results revealed that (1) PGF varied depending on the physical isolation measures. PGF was the lowest with 90 holes/cm² separation net and S. bicolor separation crop, and the highest with 40 holes/cm² separation net and no isolation measure. (2) Similar to PGF results, 90 holes/cm² separation net and S. bicolor separation crop could minimize the pollen dispersal. (3) PGF declined exponentially with increasing distance between GM cotton and Shiyuan321. Because of the production mode of farm household (limited cultivated area) in China, our study is particularly important, which is not only benefit for constraining PGF, but also has potential application value in practical production and the scientific researches.

Assessing the value of imperfect biocontainment nationally: rapeseed in the United Kingdom as an exemplar

Paternal biocontainment methods (PBMs) act by preventing pollen-mediated transgene flow. They are compromised by transgene escape via the crop-maternal line. We therefore assess the efficacy of PBMs for transgenic rapeseed (Brassica napus) biocontainment across the United Kingdom by estimating crop-maternal hybridization with its two progenitor species. We used remote sensing, field surveys, agricultural statistics, and meta-analysis to determine the extent of sympatry between the crop and populations of riparian and weedy B. rapa and B. oleracea. We then estimated the incidence of crop-maternal hybridization across all settings to predict the efficacy of PBMs. Evidence of crop chloroplast capture by the progenitors was expanded to a national scale, revealing that crop-maternal gene flow occurs at widely variable rates and is dependent on both the recipient and setting. We use these data to explore the value that this kind of biocontainment can bring to genetic modification (GM) risk management in terms of reducing the impact that hybrids have on the environment rather than preventing or reducing hybrid abundance per se.

Agronomic performance of F1, F2 and F3 hybrids between weedy rice and transgenic glufosinate-resistant rice

BACKGROUND

Studies of hybrid fitness, of which agronomic performance may be an indicator, can help in evaluating the potential for introgression of a transgene from a transgenic crop to wild relatives. The objective of this study was to assess the agronomic performance of reciprocal hybrids between two transgenic glufosinate-resistant rice lines, Y0003 and 99-t, and two weedy rice accessions, WR1 and WR2, in the greenhouse.

RESULTS

F1 hybrids displayed heterosis in height, flag leaf area and number of spikelets per panicle. The agronomic performance of F1 between WR1 and Y0003 was not affected by crossing direction. The tiller and panicle numbers of F1 individuals were higher than their F2 counterparts. However, these traits did not change significantly from the F2 to the F3 generation or in hybrids with weedy rice as maternal or paternal plants. For all hybrids, the in vitro germination rates of fresh pollen were similar and significantly lower than those of their parents, seed sets were similar to or of lower value than those of weedy rice parents and seed shattering characteristics were partially suppressed, but the survival of hybrids over winter in the field was similar to that of weedy rice parents. All F1, F2 and F3 hybrids had similar composite agronomic performance to weedy rice parents.

CONCLUSION

There was no significant decrease in the composite agronomic performance of any of the hybrids compared with weedy rice. This implies that gene flow from transgenic cultivated rice to weedy rice could occur under natural conditions.

Impact of interspecific hybridization between crops and weedy relatives on the evolution of flowering time in weedy phenotypes

BACKGROUND

Like conventional crops, some GM cultivars may readily hybridize with their wild or weedy relatives. The progressive introgression of transgenes into wild or weedy populations thus appears inevitable, and we are now faced with the challenge of determining the possible evolutionary effects of these transgenes. The aim of this study was to gain insight into the impact of interspecific hybridization between transgenic plants and weedy relatives on the evolution of the weedy phenotype.

METHODOLOGY/PRINCIPAL FINDINGS

Experimental populations of weedy birdseed rape (Brassica rapa) and transgenic rapeseed (B. napus) were grown under glasshouse conditions. Hybridization opportunities with transgenic plants and phenotypic traits (including phenological, morphological and reproductive traits) were measured for each weedy individual. We show that weedy individuals that flowered later and for longer periods were more likely to receive transgenic pollen from crops and weed × crop hybrids. Because stem diameter is correlated with flowering time, plants with wider stems were also more likely to be pollinated by transgenic plants. We also show that the weedy plants with the highest probability of hybridization had the lowest fecundity.

CONCLUSION/SIGNIFICANCE

Our results suggest that weeds flowering late and for long periods are less fit because they have a higher probability of hybridizing with crops or weed × crop hybrids. This may result in counter-selection against this subset of weed phenotypes, and a shorter earlier flowering period. It is noteworthy that this potential evolution in flowering time does not depend on the presence of the transgene in the crop. Evolution in flowering time may even be counter-balanced by positive selection acting on the transgene if the latter was positively associated with maternal genes promoting late flowering and long flowering periods. Unfortunately, we could not verify this association in the present experiment.

Glyphosate drift promotes changes in fitness and transgene gene flow in canola (Brassica napus) and hybrids

BACKGROUND AND AIMS

With the advent of transgenic crops, genetically modified, herbicide-resistant Brassica napus has become a model system for examining the risks and potential ecological consequences of escape of transgenes from cultivation into wild compatible species. Escaped transgenic feral B. napus and hybrids with compatible weedy species have been identified outside of agriculture and without the apparent selection for herbicide resistance. However, herbicide (glyphosate) exposure can extend beyond crop field boundaries, and a drift-level of herbicide could function as a selective agent contributing to increased persistence of transgenes in the environment.

METHODS

The effects of a drift level (0·1 × the field application rate) of glyphosate herbicide and varied levels of plant competition were examined on plant fitness-associated traits and gene flow in a simulated field plot, common garden experiment. Plants included transgenic, glyphosate-resistant B. napus, its weedy ancestor B. rapa, and hybrid and advanced generations derived from them.

KEY RESULTS

The results of this experiment demonstrate reductions in reproductive fitness for non-transgenic genotypes and a contrasting increase in plant fitness for transgenic genotypes as a result of glyphosate-drift treatments. Results also suggest that a drift level of glyphosate spray may influence the movement of transgenes among transgenic crops and weeds and alter the processes of hybridization and introgression in non-agronomic habitats by impacting flowering phenology and pollen availability within the community.

CONCLUSIONS

The results of this study demonstrate the potential for persistence of glyphosate resistance transgenes in weedy plant communities due to the effect of glyphosate spray drift on plant fitness. Additionally, glyphosate drift has the potential to change the gene-flow dynamics between compatible transgenic crops and weeds, simultaneously reducing direct introgression into weedy species while contributing to an increase in the transgenic seed bank.

The variability of processes involved in transgene dispersal-case studies from Brassica and related genera

BACKGROUND, AIM, AND SCOPE

We strive to predict consequences of genetically modified plants (GMPs) being cultivated openly in the environment, as human and animal health, biodiversity, agricultural practise and farmers' economy could be affected. Therefore, it is unfortunate that the risk assessment of GMPs is burdened by uncertainty. One of the reasons for the uncertainty is that the GMPs are interacting with the ecosystems at the release site thereby creating variability. This variability, e.g. in gene flow, makes consequence analysis difficult. The review illustrates the great uncertainty of results from gene-flow analysis.

MAIN FEATURES

Many independent experiments were performed on the individual processes in gene flow. The results comprise information both from laboratory, growth chambers and field trials, and they were generated using molecular or phenotypic markers and analysis of fitness parameters. Monitoring of the extent of spontaneous introgression in natural populations was also performed. Modelling was used as an additional tool to identify key parameters in gene flow.

RESULTS

The GM plant may affect the environment directly or indirectly by dispersal of the transgene. Magnitude of the transgene dispersal will depend on the GM crop, the agricultural practise and the environment of the release site. From case-to-case these three factors provide a variability that is reflected in widely different likelihoods of transgene dispersal and fitness of introgressed plants. In the present review, this is illustrated through a bunch of examples mostly from our own research on oilseed rape, Brassica napus. In the Brassica cases, the variability affected all five main steps in the process of gene dispersal. The modelling performed suggests that in Brassica, differences in fitness among plant genome classes could be a dominant factor in the establishment and survival of introgressed populations.

DISCUSSION

Up to now, experimental analyses have mainly focused on studying the many individual processes of gene flow. This can be criticised, as these experiments are normally carried out in widely different environments and with different genotypes, and thus providing bits and pieces difficult to assemble. Only few gene-flow studies have been performed in natural populations and over several plant generations, though this could give a more coherent and holistic view.

CONCLUSION

The variability inherent in the processes of gene flow in Brassica is apparent and remedies are wished for. One possibility is to expose the study species to additional experiments and monitoring, but this is costly and will likely not cover all possible scenarios. Another remedy is modelling gene flow. Modelling is a valuable tool in identifying key factors in the gene-flow process for which more knowledge is needed, and identifying parameters and processes which are relatively insensitive to change and therefore require less attention in future collections of data. But the interdependence between models and experimental data is extensive, as models depend on experimental data for their development or testing.

RECOMMENDATIONS

More and more transgenic varieties are being grown worldwide harbouring genes that might potentially affect the environment (e.g. drought tolerance, salt tolerance, disease tolerance, pharmaceutical genes). This calls for a thorough risk assessment. However, in Brassica, the limited and uncertain knowledge on gene flow is an obstacle to this. Modelling of gene flow should be optimised, and modelling outputs verified in targeted field studies and at the landscape level. Last but not least, it is important to remember that transgene flow in itself is not necessarily a thread, but it is the consequences of gene flow that may jeopardise the ecosystems and the agricultural production. This emphasises the importance of consequence analysis of genetically modified plants.