Frequency-dependent fitness of hybrids between oilseed rape (Brassica napus) and weedy B. rapa (Brassicaceae)

Fitness of interspecific hybrids is sometimes high relative to their parents, despite the conventional belief that they are mostly unfit. F(1) hybrids between oilseed rape (Brassica napus) and weedy B. rapa can be significantly more fit than their weedy parents under some conditions; however, under other conditions they are less fit. To understand the reasons, we measured the seed production of B. napus, B. rapa, and different generations of hybrid plants at three different densities and in mixtures of different frequencies (including pure stands). Brassica napus, B. rapa, and backcross plants (F(1) ♀ × B. rapa) produced many more seeds per plant in pure plots than in mixtures and more seeds in plots when each was present at high frequency. The opposite was true for F(1) plants that produced many more seeds than B. rapa in mixtures, but fewer in pure stands. Both vegetative and reproductive interactions may be responsible for these effects. Our results show that the fitness of both parents and hybrids is strongly frequency-dependent and that the likelihood of introgression of genes between the species thus may depend on the numbers and densities of parents and their various hybrid offspring in the population.     

Mitigation using a tandem construct containing a selectively unfit gene precludes establishment of Brassica napus transgenes in hybrids and backcrosses with weedy Brassica rapa

Transgenic oilseed rape ( Brassica napus ) plants can interbreed with nearby weedy Brassica rapa , potentially enhancing the weediness and/or invasiveness of subsequent hybrid 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 BC₁ mixed with B. rapa was further severely suppressed to 0.9% of that of the competing weed due to dwarfism. Clearly, the mitigation technology works efficiently in a rapeseed crop–weed system under biocontainment-controlled environments, but field studies should further validate its utility for minimizing the risks of gene flow.     

Stable Bacillus thuringiensis (Bt) toxin content in interspecific F1 and backcross populations of wild Brassica rapa after Bt gene transfer

Stable expression of a transgene may lead to increased fitness for wild plants after acquiring the transgene via crop-weed hybridization. Here, we investigate the stability of Bt toxin content in wild Brassica rapa acquiring the Bt gene from Bt Brassica napus. The Bt toxin content in nine Bt-expressing B. napus lines was 0.80-1.70 micro g/g leaf tissue throughout the growing season. These nine lines were crossed with three accessions of wild B. rapa and the Bt gene was successfully transferred to interspecific hybrids (F1) and successive backcross generations (BC1 to BC4). The Bt toxin level in F1 and BC progenies containing the Bt gene remained at 0.90-3.10 micro g/g leaf tissue. This study indicates that the Bt gene can persist and be stably expressed in wild B. rapa.     

Do escaped transgenes persist in nature? The case of an herbicide resistance transgene in a weedy Brassica rapa population

The existence of transgenic hybrids resulting from transgene escape from genetically modified (GM) crops to wild or weedy relatives is well documented but the fate of the transgene over time in recipient wild species populations is still relatively unknown. This is the first report of the persistence and apparent introgression, i.e. stable incorporation of genes from one differentiated gene pool into another, of an herbicide resistance transgene from Brassica napus into the gene pool of its weedy relative, Brassica rapa , monitored under natural commercial field conditions. Hybridization between glyphosate-resistant [herbicide resistance (HR)] B. napus and B. rapa was first observed at two Québec sites, Ste Agathe and St Henri, in 2001. B. rapa populations at these two locations were monitored in 2002, 2003 and 2005 for the presence of hybrids and transgene persistence. Hybrid numbers decreased over the 3-year period, from 85 out of ~200 plants surveyed in 2002 to only five out of 200 plants in 2005 (St Henri site). Most hybrids had the HR trait, reduced male fertility, intermediate genome structure, and presence of both species-specific amplified fragment length polymorphism markers. Both F₁ and backcross hybrid generations were detected. One introgressed individual, i.e. with the HR trait and diploid ploidy level of B. rapa, was observed in 2005. The latter had reduced pollen viability but produced ~480 seeds. Forty-eight of the 50 progeny grown from this plant were diploid with high pollen viability and 22 had the transgene (1:1 segregation). These observations confirm the persistence of the HR trait over time. Persistence occurred over a 6-year period, in the absence of herbicide selection pressure (with the exception of possible exposure to glyphosate in 2002), and in spite of the fitness cost associated with hybridization.     

Conserved patterns of chromosome pairing and recombination in Brassica napus crosses.

The patterns of chromosome pairing and recombination in two contrasting Brassica napus F1 hybrids were deduced. One hybrid was from a winter oilseed rape (WOSR) x spring oilseed rape cross, the other from a resynthesized B. napus x WOSR cross. Segregation at 211 equivalent loci assayed in the population derived from each hybrid produced two collinear genetic maps. Alignment of the maps indicated that B. napus chromosomes behaved reproducibly as 19 homologous pairs and that the 19 distinct chromosomes of B. napus each recombined with unique chromosomes from the interspecific hybrid between Brassica rapa and Brassica oleracea. This result indicated that the genomes of the diploid progenitors of amphidiploid B. napus have remained essentially unaltered since the formation of the species and that the progenitor genomes were similar to those of modern-day B. rapa and B. oleracea. The frequency and distribution of crossovers were almost indistinguishable in the two populations, suggesting that the recombination machinery of B. napus could cope easily with different degrees of genetic divergence between homologous chromosomes. Efficient recombination in wide crosses will facilitate the introgression of novel alleles into oilseed rape from B. rapa and B. oleracea (via resynthesized B. napus) and reduce linkage drag.     

Glyphosate-drift but not herbivory alters the rate of transgene flow from single and stacked trait transgenic canola (Brassica napus) to nontransgenic B. napus and B. rapa

? Transgenic plants can offer agricultural benefits, but the escape of transgenes is an environmental concern. In this study we tested the hypothesis that glyphosate drift and herbivory selective pressures can change the rate of transgene flow between the crop Brassica napus (canola), and weedy species and contribute to the potential for increased transgene escape risk and persistence outside of cultivation. ? We constructed plant communities containing single transgenic B. napus genotypes expressing glyphosate herbicide resistance (CP4 EPSPS), lepidopteran insect resistance (Cry1Ac), or both traits ('stacked'), plus nontransgenic B. napus, Brassica rapa and Brassica nigra. Two different selective pressures, a sublethal glyphosate dose and lepidopteran herbivores (Plutella xylostella), were applied and rates of transgene flow and transgenic seed production were measured. ? Selective treatments differed in the degree in which they affected gene flow and production of transgenic hybrid seed. Most notably, glyphosate-drift increased the incidence of transgenic seeds on nontransgenic B. napus by altering flowering phenology and reproductive function. ? The findings of this study indicate that transgenic traits may be transmitted to wild populations and may increase in frequency in weedy populations through the direct and indirect effects of selection pressures on gene flow.     

Hybridization between transgenic Brassica napus L. and its wild relatives: Brassica rapa L., Raphanus raphanistrum L., Sinapis arvensis L., and Erucastrum gallicum (Willd.) O.E. Schulz

The frequency of gene flow from Brassica napus L. (canola) to four wild relatives, Brassica rapa L., Raphanus raphanistrum L., Sinapis arvensis L. and Erucastrum gallicum (Willd.) O.E. Schulz, was assessed in greenhouse and/or field experiments, and actual rates measured in commercial fields in Canada. Various marker systems were used to detect hybrid individuals: herbicide resistance traits (HR), green fluorescent protein marker (GFP), species-specific amplified fragment length polymorphisms (AFLPs) and ploidy level. Hybridization between B. rapa and B. napus occurred in two field experiments (frequency approximately 7%) and in wild populations in commercial fields (approximately 13.6%). The higher frequency in commercial fields was most likely due to greater distance between B. rapa plants. All F(1) hybrids were morphologically similar to B. rapa, had B. napus- and B. rapa-specific AFLP markers and were triploid (AAC, 2n=29 chromosomes). They had reduced pollen viability (about 55%) and segregated for both self-incompatible and self-compatible individuals (the latter being a B. napus trait). In contrast, gene flow between R. raphanistrum and B. napus was very rare. A single R. raphanistrum x B. napus F1 hybrid was detected in 32,821 seedlings from the HR B. napus field experiment. The hybrid was morphologically similar to R. raphanistrum except for the presence of valves, a B. napus trait, in the distorted seed pods. It had a genomic structure consistent with the fusion of an unreduced gamete of R. raphanistrum and a reduced gamete of B. napus (RrRrAC, 2n=37), both B. napus- and R. raphanistrum-specific AFLP markers, and had <1% pollen viability. No hybrids were detected in the greenhouse experiments (1,534 seedlings), the GFP field experiment (4,059 seedlings) or in commercial fields in Québec and Alberta (22,114 seedlings). No S. arvensis or E. gallicum x B. napus hybrids were detected (42,828 and 21,841 seedlings, respectively) from commercial fields in Saskatchewan. These findings suggest that the probability of gene flow from transgenic B. napus to R. raphanistrum, S. arvensis or E. gallicum is very low (<2-5 x 10(-5)). However, transgenes can disperse in the environment via wild B. rapa in eastern Canada and possibly via commercial B. rapa volunteers in western Canada.     

Fitness of hybrids between rapeseed (Brassica napus) and wild Brassica rapa in natural habitats

Fitness of hybrids between genetically modified (GM) crops and wild relatives influences the likelihood of ecological harm. We measured fitness components in spontaneous (non-GM) rapeseed x Brassica rapa hybrids in natural populations. The F1 hybrids yielded 46.9% seed output of B. rapa, were 16.9% as effective as males on B. rapa and exhibited increased self-pollination. Assuming 100% GM rapeseed cultivation, we conservatively predict < 7000 second-generation transgenic hybrids annually in the United Kingdom (i.e. approximately 20% of F1 hybrids). Conversely, whilst reduced hybrid fitness improves feasibility of bio-containment, stage projection matrices suggests broad scope for some transgenes to offset this effect by enhancing fitness.     

Competitive interactions and the effect of herbivory on Bt-Brassica napus, Brassica rapa and Lolium perenne

1.  The probability of a transgenic crop establishing a feral population outside cultivated areas and possibly outcompeting naturally occurring species needs to be assessed to make an ecological risk assessment of the transgenic crop.
2.  The interaction between herbivory and competition is thought to determine the ecological success of insect-resistant plants, and this interaction was investigated in a competition experiment with transgenic insect-resistant Bt- Brassica napus , Brassica rapa , Lolium perenne , and herbivory from the large white butterfly Pieris brassicae .
3.  As expected, herbivory had a negative effect on the biomass of B. rapa at high plant densities. The competitive ability of L. perenne , when growing with B. rapa , increased significantly with the level of herbivory on B. rapa .
4.  To predict the effect of herbivory in a natural ecosystem, plant competition between the two annual Brassica species was analysed in a population ecological model. It was concluded that it is probable that transgenic Bt- B. napus plants may invade a natural habitat if herbivory is sufficiently high and the habitat is suitable for B. napus .
5.   Synthesis and applications . The results indicate that it is important to study the interaction between herbivory and competition when assessing the ecological risk of insect-resistant genetically modified crops. Furthermore, combining ecological data from manipulated experiments with population ecological modelling is a fruitful approach when conducting environmental risk assessments.     

Loss of C-genome-specific markers during transgene introgression from Brassica napus to wild Brassica juncea

Transgene flow from engineered Brassica napus to wild weed relatives could potentially have an environmental effect. To evaluate the introgression of transgenic B. napus into wild Brassica juncea, the hybrid F₁ and backcross progenies derived from B. juncea (genome constitution AABB) and transgenic B. napus (AACC) crosses were investigated. C-genome-specific simple sequence repeat (SSR) markers corresponding to linkage groups N11–N19 in B. napus were screened and used to estimate the marker frequency in hybrid F₁ and backcross progenies. C-genome-specific markers could be stably detected in hybrid F₁ and backcross BC₁ plants, but were only rarely found in the BC₂–BC₅ generations. For example, a specific SSR marker for linkage group N12 segregated in BC₂ generation but were completely lost in BC₃–BC₅, while a specific SSR marker of linkage group N15 segregated in BC₁, BC₂ and BC₃ generations and was absent in more advanced backcrossed generations (BC₄ and BC₅). The results indicate that a certain gene regions in Brassica napus plants are transmitted at a relatively lower frequency to wild relatives, and more rapidly disappeared in subsequent backcross generations. We propose that a foreign gene or transgene that is integrated in the C-chromosome of Brassica napus could reduce the risk of introgression in nature.     

Analysis of B-genome chromosome introgression in interspecific hybrids of Brassica napus × B. carinata

Brassica carinata, an allotetraploid with B and C genomes, has a number of traits that would be valuable to introgress into B. napus. Interspecific hybrids were created between B. carinata (BBCC) and B. napus (AACC), using an advanced backcross approach to identify and introgress traits of agronomic interest from the B. carinata genome and to study the genetic changes that occur during the introgression process. We mapped the B and C genomes of B. carinata with SSR markers and observed their introgression into B. napus through a number of backcross generations, focusing on a BC(3) and BC(3)S(1) sibling family. There was close colinearity between the C genomes of B. carinata and B. napus and we provide evidence that B. carinata C chromosomes pair and recombine normally with those of B. napus, suggesting that similar to other Brassica allotetraploids no major chromosomal rearrangements have taken place since the formation of B. carinata. There was no evidence of introgression of the B chromosomes into the A or C chromosomes of B. napus; instead they were inherited as whole linkage groups with the occasional loss of terminal segments and several of the B-genome chromosomes were retained across generations. Several BC(3)S(1) families were analyzed using SSR markers, genomic in situ hybridization (GISH) assays, and chromosome counts to study the inheritance of the B-genome chromosome(s) and their association with morphological traits. Our work provides an analysis of the behavior of chromosomes in an interspecific cross and reinforces the challenges of introgressing novel traits into crop plants.     

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.     

大白菜与紫甘蓝种间杂种的获得与鉴定

以不同类型的大白菜与紫甘蓝为材料,运用蕾期授粉结合胚挽救技术获得大白菜与紫甘蓝的种间杂种,并对其进行细胞学鉴定.结果表明,大白菜与紫甘蓝杂交,获得了57株F1代幼苗;对根尖染色体数量进行观察和杂种花粉特性调查发现,其中47株具有预期的染色体数目,2n =19,鉴定为真杂种,其花粉败育;另有6株具有38条染色体,鉴定为种间异源双二倍体,应该是发生了染色体自然加倍,其花粉可育.可育的杂种F1代与大白菜回交,获得了大白菜-紫甘蓝BC1代材料.田间观测结果显示,杂种F1代植株综合性状均介于双亲之间,BC1代植株包球明显,综合性状偏向母本大白菜.     

Male fitness of oilseed rape (Brassica napus), weedy B. rapa and their F(1) hybrids when pollinating B. rapa seeds

The likelihood that two species hybridise and backcross may depend strongly on environmental conditions, and possibly on competitive interactions between parents and hybrids. We studied the paternity of seeds produced by weedy Brassica rapa growing in mixtures with oilseed rape (B. napus) and their F(1) hybrids at different frequencies and densities. Paternity was determined by the presence of a transgene, morphology, and AFLP markers. In addition, observations of flower and pollen production, and published data on pollen fertilisation success, zygote survival, and seed germination, allowed us to estimate an expected paternity. The frequency and density of B. napus, B. rapa, and F(1) plants had a strong influence on flower, pollen, and seed production, and on the paternity of B. rapa seeds. Hybridisation and backcrossing mostly occurred at low densities and at high frequencies of B. napus and F(1), respectively. F(1) and backcross offspring were produced mainly by a few B. rapa mother plants. The observed hybridisation and backcrossing frequencies were much lower than expected from our compilation of fitness components. Our results show that the male fitness of B. rapa, B. napus, and F(1) hybrids is strongly influenced by their local frequencies, and that male fitness of F(1)hybrids, when pollinating B. rapa seeds, is low even when their female fitness (seed set) is high.     

Synthesis of intergeneric hybrids and establishment of genomic affinity between <Emphasis Type="Italic">Diplotaxis catholica</Emphasis> and crop <Emphasis Type="Italic">Brassica</Emphasis> species

Intergeneric hybrids of the wild crucifer Diplotaxis catholica (2n = 18, D(C)D(C)) as female with two crop Brassica species, namely Brassica rapa (2n = 20; AA) and Brassica juncea (2n = 36; AABB) as male, were developed, using ovary and sequential culture. Reciprocal crosses were not successful, suggesting unilateral cross incompatibility. Morphologically, the hybrid plants resembled the crop brassica parents, but were nearly male- as well as female-sterile. Induction of amphiploidy helped to improve pollen fertility for the D. catholica x B. rapa cross (73%), but less so for the D. catholica x B. juncea cross (35-40%). Female fertility was also higher in both the amphiploids. Cytological analysis of the F(1) hybrids revealed aberrant meiosis with predominant occurrence of the univalents. Partial genomic homoeology between the A genome of B. rapa and the D(C) genome of D. catholica was indicated by the presence of up to five bivalents in 14.7% of the PMCs in the D. catholica x B. rapa hybrid, and 1-2 trivalents or a quadrivalent in nearly 44% of the PMCs in the derived amphiploid. In the second cross, D. catholica x B. juncea, up to six bivalents and one trivalent were observed indicating homoeology between the A/B genomes of B. juncea and the D(C) genome of D. catholica. The possibility of introgression of desirable genes from D. catholica into crop Brassica species exists in view of significant affinity between the D(C) and A/B genomes.     

甘蓝型油菜Ogura 雄性不育×白菜的回交杂种后代与亲本之间蕾期基因表达差异比较研究*

以甘蓝型油菜（Brassica napus L, AACC,2n＝38）Ogura细胞质雄性不育材料为母本,以不同白菜（B campestris ssp. chinensis Makino, AA, 2n＝20） 自交系‘新选一号’和‘矮脚黄’为父本进行杂交,获得了杂种F1、BC1、BC2代。利用cDNA-AFLP技术对两种材料的不同回交世代BC1、BC2代与其亲本在蕾期的基因表达进行分析。结果表明,两种白菜回交世代与其亲本的基因表达有明显差异,在质和量上都存在差异。基因表达模式有5类共7种：（1）单亲沉默型（2种）,（2）单亲一致型（2种）,（3）双亲共沉默型,（4）杂种特异型,（5）表达一致型。随着回交世代的增加,回交杂种和亲本的基因表达在单亲沉默型、双亲共沉默型呈增加趋势。而在母本一致型、父本一致型、杂交种特异型、表达一致型呈下降的趋势。两种白菜在F1、BC1、BC2 3个世代与回交亲本花蕾间的基因差异表达有15种类型,其中以在轮回亲本、F1、BC1、BC2中共同出现表达的带的比例最高。Abstract: Crosses between female parent of Ogura male sterility Brassica napus L. and male parents of B. campestris ssp. chinensis Makino were made and F1, BC1 and BC2 generations produced. Gene expression of two Chinese cabbage backcross hybrid BC1, BC2 and their parents at bud stage was analyzed by means of cDNA-AFLP technique. The results indicated that the patterns of gene expression differ significantly between BC1 and BC2 generations and their parents. There were many patterns of gene expression, including gene overexpression and gene silencing. Five patterns (seven kinds) of gene expression were observed, which include: (1) bands occurring in only one parent (two kinds); (2) bands observed in hybrids and one parent (two kinds); (3) bands occurring in only parents (one kind); (4) bands visualized in only hybrids (one kind); (5) bands observed in parents and hybrids (one kind). In accompany with the addition of backcross, the increase trend in backcross hybrids and their parents were described in the aspects of differential gene expression, bands expressed only in one parent and bands expressed only in both parents. The declined trend in backcross hybrids and their parents were observed in the aspects of bands expressed in both hybrids and one parent (two kinds), bands visualized in only hybrids and bands observed in parents and hybrid. Fifteen patterns of gene expression were observed in F1、BC1、BC2 and backcross parents. The percent of bands expressed in F1、BC1、BC2 and backcross was highest.     

Modelling gene flow between oilseed rape and wild radish. I. Evolution of chromosome structure

The assessment of gene flow from crop species to weeds has found a new emphasis over the last years because of the marketing of transgenic crops and the possible selective advantage that crop (trans)gene may confer to the weeds. Several studies focused on the F1 interspecific hybrid production but few data are available on the factors affecting the genetic structure of advanced generations. It depends on the genomic structure of the species concerned as well as on the degree of their genome homology that affect the occurrence of intergenomic recombination. Oilseed rape (Brassica napus, AACC, 2n = 38)-wild radish (Raphanus raphanistrum, RrRr, 2n = 18), a distantly related weed, is a good model to address such questions. From seven male sterile oilseed rape lines carrying an herbicide tolerance transgene, F1 interspecific hybrids and four advanced generations were produced under field conditions with wild radish as pollinator. Observation of hybrid chromosome numbers across four generations revealed a high variability, especially in the “BC1” generation. A regression model was fitted in order to describe the relationship between parent and offspring chromosome numbers. The effects of generation, transgenic line and selection pressure on the mean relationship were investigated. The first two factors had an influence on the rate of decrease of chromosome numbers, whereas selection pressure resulted in the presence of an additional chromosome in the herbicide treated plants. The model provided a convenient framework for analysing how chromosome numbers evolve over successive hybridization events and it may prove useful as a basis for simulation-based approaches.     

芥菜型油菜和白菜型油菜种间杂种遗传分析

种间杂交是一种拓宽栽培作物遗传基础和转移优良性状的重要手段,已经广泛地用于作物品质的改良。本研究通过芥菜型油菜（Brassica juncea L.）和白菜型油菜（Brassica rapa L.）种间杂交,将芥菜型油菜的有利性状转移到白菜型油菜中,创造新型白菜型油菜,以改良白菜型油菜的农艺性状、提高抗逆性和拓宽其遗传基础。研究结果表明：以芥菜型油菜作母本、白菜型油菜作父本的杂交组合较易获得杂交种子,杂种F1植株营养生长具有较明显的杂种优势,但花粉完全不育;以白菜型油菜回交获得的BC1植株间表型差异明显,平均花粉可染率为34.8%,介于 0~84%之间,群体自交不亲和;BC1F1和BC2群体变异广泛,出现自交亲和植株和黄籽植株,平均花粉可染率分别为79.7%和79.1%。     

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

Unlike most studies on hybridisation between oilseed rape and Brassica rapa, this study focused on hybridisation with oilseed rape as the maternal parent. This is a key cross because, assuming that plastids are inherited maternally, F(1)-hybrid production with maternal oilseed rape (B. napus) is the only transgene escape route from transplastomic oilseed rape. We investigated such F(1)-hybrid production in winter oilseed rape co-cultivated with weedy B. rapa at three plant densities each with two proportions of the different species. The paternity of the progeny produced on oilseed rape was assessed, and several fitness parameters were determined in oilseed rape mother plants in order to correlate hybridisation and plant competition. At higher density, the vegetative fitness per mother plant decreased significantly, but the density only affected the frequency of F(1)-hybrids significantly (a decrease) in the treatment with equal proportions of each species. As to the proportions, at higher B. napus frequencies, there were fewer F(1)-hybrids per mother plant and a significant increase in most biomass components. Thus, B. rapa was the stronger competitor in its effect on both the vegetative and reproductive fitness in B. napus, and the hybridisation frequency. In conclusion, the relative frequency of the two species was a more influential parameter than the density. Hybridisation with B. napus as the female will be most likely at current field densities of B. napus and when B. rapa is an abundant weed.     

No effect of transgene and strong wild parent effects on seed dormancy in crop–wild hybrids of rice: implications for transgene persistence in wild populations

Soil seed banks act as a gene pool for local plant species and, as such, can buffer local populations, especially those experiencing challenging environmental conditions. Seed dormancy has important implications to dynamics of soil seed banks. Therefore, estimating the seed dormancy of transgenic crop–wild hybrids could shed light on the persistence of transgenes in wild‐plant soil seed banks. Individuals from eight populations of wild rice Oryza rufipogon were crossed with those of three insect‐resistant transgenic rice lines. Selfed (F2–F4) and backcrossed populations (BC1, BC1F2 and BC1F3) were then made from the hybrids. Seed germination was tested under three treatments: (a) normal; (b) overwintering in soil; and (c) one‐week heat‐shocking. The effects of transgene, wild parent and hybrid generation on hybrid seed germination were examined. No significant effect of insect‐resistant transgenes (Bt and CpTI) was detected on the seed dormancy of crop–wild hybrids, while a significant wild parent effect was found. The seeds of advanced generation hybrids have higher germination percentages and lower dormancy than do those of F1 and BC1 generations. The study showed that the dormancy of hybrid seeds was determined mainly by their genetic backgrounds. All hybrid seeds have higher germination percentages and lower dormancy (and, consequently, a poorer overwintering ability), compared with wild seeds, and reduce dormancy would contribute to a fitness disadvantage, compared with wild types. Therefore, such seeds might form part of naturally occurring soil seed banks, through which crop genes would persist in wild populations.