Inheritance of oilseed rape (Brassica napus) RAPD markers in a backcross progeny with Brassica campestris

Fitness and Rhizobacteria of F2, F3 Hybrids of Herbicide-Tolerant Transgenic Soybean and Wild Soybean

The introduction of herbicide-tolerant (HT) transgenic soybeans (Glycine max (L.) Merr.) into farming systems raises great concern that transgenes may flow to endemic wild soybeans (Glycine soja Sieb. et Zucc.) via pollen, which may increase the ecological risks by increasing the fitness of hybrids under certain conditions and threaten the genetic diversity of wild soybean populations. In order to demonstrate the potential risk of gene flow from the HT soybean to the wild soybean, the fitness of F2 and F3 hybrids obtained from two wild soybean populations (HLJHRB-1, JSCZ) collected from China and the HT soybean was measured under farmland and wasteland soil conditions, as well as with or without weed competition. Compared with their wild progenitors, the F2 and F3 hybrids of HLJHRB-1 displayed a higher emergence rate, higher aboveground dry biomass, more pods and filled-seed plants, as well as better composite fitness under four planting conditions. The F2 and F3 hybrids of JSCZ also displayed a higher emergence rate, higher aboveground dry biomass, more pods, and more filled seeds per plant under mixed planting, whereas these characteristics were lower under pure planting conditions in wasteland and farmland soil. Therefore, the composite fitness of JSCZ hybrids was higher or lower depending on the planting conditions. Furthermore, the soil microbial communities of the F3 of HLJHRB-1, JSCZ, and the wild soybean were investigated with 16S rDNA sequencing, which showed that low alpha diversity of rhizobacteria was relative to high fitness, and Rhizobium played an important role in promoting F3 plant growth.

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

In nature, interspecific hybridization occurs frequently and can contribute to the production of new species or the introgression of beneficial adaptive features between species. It has great potential in agricultural systems to boost the process of targeted crop improvement. In the advent of genetically modified (GM) crops, it has a disadvantage that it involves the transgene escaping to unintended plants, which could result in non-specific weedy crops. Several crop species in the Brassica genus have close kinship: canola (Brassica napus) is an ancestral hybrid of B. rapa and B. oleracea and mustard species such as B. juncea, B. carinata, and B. nigra share common genomes. Hence, intraspecific hybridization among the Brassica species is most common, especially between B. napus and B. rapa. In general, interspecific hybrids cause numerous genetic and phenotypic changes in the parental lines. Consequently, their fitness and reproductive ability are also highly varied. In this review, we discuss the interspecific hybridization and reciprocal hybridization studies of B. napus and B. rapa and their potential in the controlled environment. Further, we address the fate of transgenes (herbicide resistance) and their ability to transfer to their progenies or generations. This could help us to understand the environmental influence of interspecific hybrids and how to effectively manage their transgene escape in the future.

Rapid Creation of Interspecific Hybrid Progeny to Broaden Genetic Distance through Double Haploid (DH) Inducer in Brassica napus

Interspecific hybridization of rapeseed is an important way to innovate breeding resources. This research used Brassica napus and Brassica rapa for artificial synthesis interspecific hybridization of F₁. The F₁ self-fruiting rate was particularly low. By comparing the fertilization rate and seed setting rate of nine crosses and selfing combinations of interspecific hybrid progeny F₁ and control B. napus, the results proved that the genetic stability of egg cells was greater than that of sperm cells, so the F₁ could get seed by artificial pollination with other normal pollen. Based on these results, interspecific maternal inbred offspring (induced F₁) from egg cells was obtained by emasculation and pollination with the pollen of DH inducer Y3380. It was found through morphological analysis, flow cytometry identification, and meiotic observation of induced F₁, the plants had most normal fertile tetraploid and the meiosis was normal. The FISH results showed that the induced F₁ were B. napus (2n = 4x = 38, AACC), 20 A and 19 C chromosomes. The results of SNP chip detection and genetic cluster analysis found that the genetic variation between interspecies could be preserved or broadened in the induced F₁. The use of DH inducer created special breeding resources for interspecific hybridization and distant hybridization of rapeseed while shortening time, improving efficiency, and providing a new insight into innovate breeding resources.

Molecular characterization of imidazolinone-resistant Brassica rapa × B. napus hybrids

Herbicide-resistant oilseed rape (Brassica napus) cultivation in our country entails the risk of gene transfer to related wild species. One of these species is the wild turnip (B. rapa), an important weed of winter crops widely distributed in the Pampas region. Despite hybridization risks, Clearfield ® oilseed rape is available in Argentina. In 2008, a B. rapa population, which was sympatric to an imidazolinone-resistant and a conventional oilseed rape cultivar, was located on a farm in the main cropping area of the country. Herbicide-resistant individuals were found in the progeny of this population in a herbicide screening test. Therefore, a molecular characterization using cleaved amplified polymorphic sequence (CAPS) and simple sequence repeat (SSR) markers was conducted on these plants to determine their hybrid nature and to establish the origin of the imidazolinone resistance trait. The results of this study, along with information of field records, confirmed that the resistant plants were first generation interspecific hybrids. Imidazolinone resistance had been effectively transferred from the herbicide-resistant oilseed rape, even in the particular situation of pollen competition. Oilseed rape resistant cultivars are becoming more common in the country. So, considering that seed loss and crop volunteers are common in these species, it is crucial to avoid the dispersion of new resistant weed biotypes as they reduce the effectiveness of chemical control technologies.

The poor lonesome A subgenome of Brassica napus var. Darmor (AACC) may not survive without its mate

Constitutive genomes of allopolyploid species evolve throughout their life span. However, the consequences of long-term alterations on the interdependency between each original genome have not been established. Here, we attempted an approach corresponding to subgenome extraction from a previously sequenced natural allotetraploid, offering a unique opportunity to evaluate plant viability and structural evolution of one of its diploid components. We employed two different strategies to extract the diploid AA component of the Brassica napus variety 'Darmor' (AACC, 2n = 4x = 38) and we assessed the genomic structure of the latest AA plants obtained (after four to five rounds of selection), using a 60K single nucleotide polymorphism Illumina array. Only one strategy was successful and the diploid AA plants that were structurally characterized presented a lower proportion of the B. napus A subgenome extracted than expected. In addition, our analyses revealed that some genes lost in a polyploid context appeared to be compensated for plant survival, either by conservation of genomic regions from B. rapa, used in the initial cross, or by some introgressions from the B. napus C subgenome. We conclude that as little as c. 7500 yr of coevolution could lead to subgenome interdependency in the allotetraploid B. napus as a result of structural modifications.

Characteristics Analysis of F1 Hybrids between Genetically Modified Brassica napus and B. rapa

A number of studies have been conducted on hybridization between transgenic Brassica napus and B. rapa or backcross of F1 hybrid to their parents. However, trait changes must be analyzed to evaluate hybrid sustainability in nature. In the present study, B. rapa and transgenic (BrAGL20) B. napus were hybridized to verify the early flowering phenomenon of F1 hybrids, and F1 hybrid traits were analyzed to predict their impact on sustainability. Flowering of F1 hybrid has been induced slightly later than that of the transgenic B. napus, but flowering was available in the greenhouse without low temperature treatment to young plant, similar to the transgenic B. napus. It is because the BrAGL20 gene has been transferred from transgenic B. napus to F1 hybrid. The size of F1 hybrid seeds was intermediate between those of B. rapa and transgenic B. napus, and ~40% of F1 pollen exhibited abnormal size and morphology. The form of the F1 stomata was also intermediate between that of B. rapa and transgenic B. napus, and the number of stomata was close to the parental mean. Among various fatty acids, the content of erucic acid exhibited the greatest change, owing to the polymorphism of parental FATTY ACID ELONGASE 1 alleles. Furthermore, F2 hybrids could not be obtained. However, BC1 progeny were obtained by hand pollination of B. rapa with F1 hybrid pollen, with an outcrossing rate of 50%.

Simulating evolutionary responses of an introgressed insect resistance trait for ecological effect assessment of transgene flow: a model for supporting informed decision-making in environmental risk assessment

Predicting outcomes of transgene flow from arable crops requires a system perspective that considers ecological and evolutionary processes within a landscape context. In Europe, the arable weed Raphanus raphanistrum is a potential hybridization partner of oilseed rape, and the two species are ecologically linked through the common herbivores Meligethes spp. Observations in Switzerland show that high densities of Meligethes beetles maintained by oilseed rape crops can lead to considerable damage on R. raphanistrum. We asked how increased insect resistance in R. raphanistrum - as might be acquired through introgression from transgenic oilseed rape - would affect seed production under natural herbivore pressure. In simulation experiments, plants protected against Meligethes beetles produced about twice as many seeds as unprotected plants. All stages in the development of reproductive structures from buds to pods were negatively affected by the herbivore, with the transition from buds to flowers being the most vulnerable. We conclude that resistance to Meligethes beetles could confer a considerable selective advantage upon R. raphanistrum in regions where oilseed rape is widely grown.

Selection against hybrids in mixed populations of Brassica rapa and Brassica napus: model and synthesis

Pollen of the crop oilseed rape (Brassica napus, AACC) can cross-fertilize ovules of Brassica rapa (AA), which leads to an influx of unpaired C-chromosomes into wild B. rapa populations. The presence of such extra chromosomes is thought to be an indicator of introgression. Backcrosses and F(1) hybrids were found in Danish populations but, surprisingly, only F(1) hybrids were found in the UK and the Netherlands. Here, a model tests how the level of selection and biased vs unbiased transmission affect the population frequency of C-chromosomes. In the biased-transmission scenario the experimental results of the first backcross are extrapolated to estimate survival of gametes with different numbers of C-chromosomes from all crosses in the population. With biased transmission, the frequency of C-chromosomes always rapidly declines to zero. With unbiased transmission, the continued presence of plants with extra C-chromosomes depends on selection in the adult stage and we argue that this is the most realistic option for modeling populations. We suggest that selection in the field against plants with unpaired C-chromosomes is strong in Dutch and UK populations. The model highlights what we do not know and makes suggestions for further research on introgression.

Development of a population for substantial new type Brassica napus diversified at both A/C genomes

Intersubgenomic heterosis in rapeseed has been revealed in previous studies by using traditional Brassica napus (A(n)A(n)C(n)C(n)) to cross partial new type B. napus with A(r)/C(c) introgression from the genomes of B. rapa and B. carinata, respectively. To further enlarge the genetic basis of B. napus and to facilitate a sustained heterosis breeding in rapeseed, it is crucial to create a population for substantial new type B. napus diversified at both A/C genomes. In this experiment, hundreds of artificial hexaploid plants (A(r)A(r)B(c)B(c)C(c)C(c)) involving hundreds of B. carinata/B. rapa combinations were first crossed with elite lines of partial new type B. napus. The pentaploid plants (AABCC) were open-pollinated in isolated conditions, and their offspring were successively self-pollinated and intensively selected for two generations. Thereafter, a population of substantial new type B. napus mainly with a genomic composition of A(r)A(r)C(c)C(c) harbouring genetic diversity from 25 original cultivars of B. rapa and 72 accessions of B. carinata was constructed. The population was cytologically verified to have the correct chromosome constitution of AACC and differed genetically from traditional B. napus, in terms of the genome components of A(r)/C(c) and B(c) as well as the novel genetic variations induced by the interspecific hybridisation process. Synchronously, rich phenotypic variation with plenty of novel valuable traits was observed in the population. The origin of the novel variations and the value of the population are discussed.

RFLP and AFLP analysis of inter- and intraspecific variation of Brassica rapa and B. napus shows that B. rapa is an important genetic resource for B. napus improvement

Fingerprinting of 29 accessions of oilseed rape, including seven accessions of Brassica napus, and 22 accessions of B. rapa (B. campestris) from Europe, North America, and China was analyzed using RFLP and AFLP markers. In total, 1,477 polymorphic RFLP bands and 183 polymorphic AFLP bands from 166 enzyme-probe combinations and two pairs of AFLP primers, respectively, were scored for the 29 accessions. On average, RFLP analysis showed that the Arabidopsis EST probe detected more polymorphic bands in Brassica than the random genomic probe performed. More polymorphic RFLP markers were detected with the digestion of EcoR I or BamH I than HindIII. According to the number of bands amplified from each accession, the copy numbers of each gene in the genomes of B. rapa and B. napus were estimated. The average copy numbers in B. rapa of China, B. rapa of Europe, and B. napus, were 3.2, 3.1, and 2.9, respectively. Genetic distance based on the AFLP data was well correlated with that based on the RFLP data (r = 0.72, P<0.001), but 0.39 smaller on average. Genetic diversity analysis showed that Chinese B. rapa was more polymorphic than Chinese B. napus and European materials. Some European B. napus accessions were clustered into European B. rapa, which were distinctly different from Chinese B. napus. The larger variations of Chinese accessions of B. rapa suggest that they are valuable in oilseed rape breeding. Novel strategies to use intersubgenomic heterosis between genome of B. rapa (A(r)A(r)) and genome of B. napus (A(n)A(n)C(n)C(n)) were elucidated.

Spontaneous capture of oilseed rape (Brassica napus) chloroplasts by wild B. rapa: implications for the use of chloroplast transformation for biocontainment

Environmental concerns over the cultivation of Genetically Modified (GM) crops largely centre on the ecological consequences following gene flow to wild relatives. One attractive solution is to deploy biocontainment measures that prevent hybridization. Chloroplast transformation is the most advanced biocontainment method but is compromised by chloroplast capture (hybridization through the maternal lineage). To date, however, there is a paucity of information on the frequency of chloroplast capture in the wild. Oilseed rape (Brassica napus, AACC) frequently hybridises with wild Brassica rapa (AA, as paternal parent) and yields B. rapa-like introgressed individuals after only two generations. In this study we used chloroplast CAPS markers that differentiate between the two species to survey wild and weedy populations of B. rapa for the capture of B. napus chloroplasts. A total of 464 B. rapa plants belonging to 14 populations growing either in close proximity to B. napus (i.e. sympatric <5 m) or else were allopatric from the crop (>1 km) were assessed for chloroplast capture using PCR (trnL-F) and CAPS (trnT-L-Xba I) markers. The screen revealed that two sympatric B. rapa populations included 53 plants that possessed the chloroplast of B. napus. In order to discount these B. rapa plants as F(1) crop-wild hybrids, we used a C-genome-specific marker and found that 45 out of 53 plants lacked the C-genome and so were at least second generation introgressants. The most plausible explanation is that these individuals represent multiple cases of chloroplast capture following introgressive hybridisation through the female germ line from the crop. The abundance of such plants in sympatric sites thereby questions whether the use of chloroplast transformation would provide a sufficient biocontainment for GM oilseed rape in the United Kingdom.

Quantifying the introgressive hybridisation propensity between transgenic oilseed rape and its wild/weedy relatives

In order to estimate the introgressive hybridisation propensity (IHP) between genetically modified (GM) oilseed rape (Brassica napus) and certain of its cross-compatible wild/weedy relatives at the landscape level, a conceptual approach was developed. A gene flow index was established enclosing the successive steps to successfully achieve introgressive hybridisation: wild/weedy relatives and oilseed rape should co-occur, have overlapping flowering periods, be compatible, produce viable and fertile progeny, and the transgenes should persist in natural/weedy populations. Each step was described and scored, resulting in an IHP value for each cross-compatible oilseed rape wild/weedy relative. The gene flow index revealed that Brassica rapa has the highest introgressive hybridisation propensity (IHP value = 11.5), followed by Hirschfeldia incana and Raphanus raphanistrum (IHP = 6.7), Brassica juncea (IHP = 5.1), Diplotaxis tenuifolia and Sinapis arvensis (IHP = 4.5) in Flanders. Based on the IHP values, monitoring priorities can be defined within the pool of cross-compatible wild/weedy oilseed rape relatives. Moreover, the developed approach enables to select areas where case-specific monitoring of GM oilseed rape could be done in order to detect potential adverse effects on cross-compatible wild/weedy relatives resulting from vertical gene flow. The implementation of the proposed oilseed rape-wild relative gene flow index revealed that the survey design of existing botanical survey networks does not suit general surveillance needs of GM crops in Belgium. The encountered hurdles to implement the gene flow index and proposals to acquire the missing data are discussed.

Genetically engineered plants and foods: a scientist's analysis of the issues (part II)

Genetic engineering provides a means to introduce genes into plants via mechanisms that are different in some respects from classical breeding. A number of commercialized, genetically engineered (GE) varieties, most notably canola, cotton, maize and soybean, were created using this technology, and at present the traits introduced are herbicide and/or pest tolerance. In 2007 these GE crops were planted in developed and developing countries on more than 280 million acres (113 million hectares) worldwide, representing nearly 10% of rainfed cropland. Although the United States leads the world in acres planted with GE crops, the majority of this planting is on large acreage farms. In developing countries, adopters are mostly small and resource-poor farmers. For farmers and many consumers worldwide, planting and eating GE crops and products made from them are acceptable and even welcomed; for others GE crops raise food and environmental safety questions, as well as economic and social issues. In Part I of this review, some general and food issues related to GE crops and foods were discussed. In Part II, issues related to certain environmental and socioeconomic aspects of GE crops and foods are addressed, with responses linked to the scientific literature.

Pairing and recombination at meiosis of Brassica rapa (AA) x Brassica napus (AACC) hybrids

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.

Impact environnemental des cultures transgéniques

L’adoption à grande échelle des cultures transgéniques depuis dix ans a soulevé de nombreuses questions quant aux impacts possibles de ces nouvelles lignées végétales sur les écosystèmes agricoles et naturels. Des questions ont été soulevées, en particulier, sur le devenir des transgènes dans le milieu et sur une possible « pollution » du patrimoine génétique des organismes vivants à l’échelle des écosystèmes. Après une énumération des impacts environnementaux associés aux végétaux transgéniques, cet article de synthèse dresse un aperçu des connaissances actuelles sur le devenir – ou la migration – des transgènes dans le milieu. Les phénomènes d’hybridation et d’introgression génique en direction d’espèces ou de lignées apparentées sont d’abord abordés, après quoi sont considérés les phénomènes de transfert horizontal des transgènes en direction d’organismes non apparentés. Un article complémentaire publié dans ce même numéro traite de l’impact environnemental des protéines recombinantes encodées par les transgènes (Michaud 2005).

Intersubgenomic heterosis in seed yield potential observed in a new type of Brassica napus introgressed with partial Brassica rapa genome

This paper reports the observation on the intersubgenomic heterosis for seed yield among hybrids between natural Brassica napus (A(n)A(n)C(n)C(n)) and a new type of B. napus with introgressions of genomic components of Brassica rapa (A(r)A(r)). This B. napus was selected from the progeny of B. napus x B. rapa and (B. napus x B. rapa) x B. rapa based on extensive phenotypic and cytological observation. Among the 129 studied partial intersubgenomic hybrids, which were obtained by randomly crossing 13 lines of the new type of B. napus in F(3) or BC(1)F(3) to 27 cultivars of B. napus from different regions as tester lines, about 90% of combinations exceeded the yield of their respective tester lines, whereas about 75% and 25% of combinations surpassed two elite Chinese cultivars, respectively. This strong heterosis was further confirmed by reevaluating 2 out of the 129 combinations in a successive year and by surveying hybrids between 20 lines of the new type of B. napus in BC(1)F(5) and its parental B. napus in two locations. Some DNA segments from B. rapa were identified with significant effects on seed yield and yield components of the new type of B. napus in BC(1)F(5) and intersubgenomic hybrids in positive or negative direction. It seems that the genomic components introgressed from B. rapa contributed to improvement of seed yield of rapeseed.

Inheritance of GFP-Bt transgenes from Brassica napus in backcrosses with three wild B. rapa accessions

Transgenes from transgenic oilseed rape, Brassica napus (AACC genome), can introgress into populations of wild B. rapa (AA genome), but little is known about the long-term persistence of transgenes from different transformation events. For example, transgenes that are located on the crop's C chromosomes may be lost during the process of introgression. We investigated the genetic behavior of transgenes in backcross generations of wild B. rapa after nine GFP (green fluorescent protein)-Bt (Bacillus thuringiensis) B. napus lines, named GT lines, were hybridized with three wild B. rapa accessions, respectively. Each backcross generation involved crosses between hemizygous GT plants and non-GT B. rapa pollen recipients. In some cases, sample sizes were too small to allow the detection of major deviations from Mendelian segregation ratios, but the segregation of GT:non-GT was consistent with an expected ratio of 1:1 in all crosses in the BC1 generation. Starting with the BC2 generation, significantly different genetic behavior of the transgenes was observed among the nine GT B. napus lines. In some lines, the segregation of GT:non-GT showed a ratio of 1:1 in the BC2, BC3, and BC4 generations. However, in other GT B. napus lines the segregation ratio of GT:non-GT significantly deviated from 1:1 in the BC2 and BC3 generations, which had fewer transgenic progeny than expected, but not in the BC4 generation. Most importantly, in two GT B. napus lines the segregation of GT:non-GT did not fit into a ratio of 1:1 in the BC2, BC3 or BC4 generations due to a deficiency of transgenic progeny. For these lines, a strong reduction of transgene introgression was observed in all three B. rapa accessions. These findings imply that the genomic location of transgenes in B. napus may affect the long-term persistence of transgenes in B. rapa after hybridization has occurred.

Btcrops: Predicting effects of escaped transgenes on the fitness of wild plants and their herbivores

One prominent concern about genetically modified crops is the possibility of environmental impacts from the movement of fitness-enhancing traits to wild plant populations. Decisions to deregulate Bt crops in the USA have relied strongly on arguments that these crops will not interbreed with wild relatives in the permitted growing regions. Limited attention therefore has been directed to analyses of the consequences of gene flow. To provide a transparent evaluation process for risks associated with insecticidal transgene escape, we crafted a series of questions designed to guide this aspect of the risk assessment. We then explored the current knowledge base available for answering such risk-related questions for three Bt crops (cotton, rapeseed, and rice). First, we generated a list of wild relatives of these crops. A definitive list of potential transgene recipients is not yet possible for some crops. Sufficient data are not available for some crops to eliminate certain related plant species from consideration of fertile hybrid formation, thus making lists for these crops subject to speculation. Second, we queried the HOSTS database (UK) to obtain a worldwide listing of lepidopteran species that feed on these crops and their wild relatives, and to determine the host range of the larvae. To our knowledge, this list of 502 lepidopteran species is the first such list published for these crops and wild crop relatives. Third, we used a data set maintained by the Canadian Forest Service to assess Bt toxin susceptibility for these lepidopterans. Only 3% of those species have been tested for susceptibility; and the literature suggests that generalizations about susceptibility among taxa are difficult due to the variability within families. Fourth, we consulted the literature to interpret what is known about the ability of lepidopterans to regulate plant fitness or invasiveness. We could not eliminate the possibility of ecological release due to plant resistance against lepidopterans. In fact, there is strong experimental evidence that lepidopteran herbivores do limit the distribution and/or abundances of at least some wild plant species. Neither could we eliminate the possibility that non-target lepidopterans might have important functions in the ecosystem as pollinators or alternate hosts to natural enemies of pest species. This study suggests that crucial data are lacking for the development of a credible scientific basis to confirm or deny environmental risks associated with the escape of Bt transgene constructs to wild relatives. Given the absence of information on the identity, level of susceptibility, and ecological roles of lepidopterans exploiting specific wild relatives of Bt crops, we suggest that new efforts be directed to assessing possible consequences of lepidopteran mortality on resistant wild relatives.

Molecular biology of weed control

The vast commercial effort to utilize chemical and molecular tools to solve weed control problems has had a major impact on the basic biological sciences as well as benefits to agriculture, and the first generation of transgenic products has been successful, while somewhat crude. More sophisticated products are envisaged and expected. Biotechnologically-derived herbicide-resistant crops have been a considerable benefit, yet in some cases there is a risk that the same useful transgenes may introgress into related weeds, specifically the weeds that are hardest to control without such transgenic crops. Biotechnology can also be used to mitigate the risks. Molecular tools should be considered for weed control without the use of, or with less chemicals, whether by enhancing crop competitiveness with weeds for light, nutrients and water, or via allelochemicals. Biocontrol agents may become more effective as well as more safe when rendered hypervirulent yet non-spreading by biotechnology. There might be ways to disperse deleterious transposons throughout weed populations, obviating the need to modify the crops.

Tandem constructs: preventing the rise of superweeds

Transgenic crops may interbreed with nearby weeds, increasing their competitiveness, and may themselves become a 'volunteer' weed in the following crop. The desired transgene can be coupled in tandem with genes that would render hybrid offspring or volunteer weeds less able to compete with crops, weeds and wild species. Genes that prevent seed shatter or secondary dormancy, or that dwarf the recipient could all be useful for mitigation and may have value to the crop. Many such genes have been isolated in the past few years.