Dynamic expression of green fluorescent protein and Bacillus thuringiensis Cry1Ac endotaxin in interspecific hybrids and successive backcross generations (BC1 and BC2) between transgenic Brassica napus crop and wild Brassica juncea

The persistence and stability of a transgene encoding a Bacillus thuringiensis (Bt) Cry1Ac insecticidal protein was investigated in hybrids between crop Brassica napus and a recurrent wild Brassica juncea population. Interspecific hybrids (F₁) and backcross progenies (BC₁, BC₂) containing green fluorescent protein (GFP) and Bt genes were successfully produced in the greenhouse. Stable Bt toxin levels were found in hybrid and advanced backcross progenies formed in wild B. juncea. Bt Cry1Ac concentration was significantly lower in BC₂ plants than in transgenic B. napus, F₁, BC₁, while no significant differences were detected among the latter three plant genotypes. A GFP marker gene was used as a scorable marker and indicator of Bt transgene expression. GFP fluorescence intensity was significantly correlated with Bt Cry1Ac concentration at the flowering stage and the pod formation stage in both transgenic oilseed rape hybrids and backcrossed progenies (BC₁, BC₂). It was demonstrated that GFP was a suitable marker for Bt protein in the backcross of B. juncea, which could facilitate the detection of gene flow and is useful in biosafety management.     

Potential gene flow of two herbicide-tolerant transgenes from oilseed rape to wild B. juncea var. gracilis

Four successive reciprocal backcrosses between F₁ (obtained from wild Brassica juncea as maternal plants and transgenic glyphosate- or glufosinate-tolerant oilseed rape, B. napus, as paternal plants) or subsequent herbicide-tolerant backcross progenies and wild B. juncea were achieved by hand pollination to assess potential transgene flow. The third and forth reciprocal backcrosses produced a number of seeds per silique similar to that of self-pollinated wild B. juncea, except in plants with glufosinate-tolerant backcross progeny used as maternal plants and wild B. juncea as paternal plants, which produced fewer seeds per silique than did self-pollinated wild B. juncea. Germination percentages of reciprocal backcross progenies were high and equivalent to those of wild B. juncea. The herbicide-tolerant first reciprocal backcross progenies produced fewer siliques per plant than did wild B. juncea, but the herbicide-tolerant second or third reciprocal backcross progenies did not differ from the wild B. juncea in siliques per plant. The herbicide-tolerant second and third reciprocal backcross progenies produced an amount of seeds per silique similar to that of wild B. juncea except for with the glufosinate-tolerant first and second backcross progeny used as maternal plants and wild B. juncea as paternal plants. In the presence of herbicide selection pressure, inheritance of the glyphosate-tolerant transgene was stable across the second and third backcross generation, whereas the glufosinate-tolerant transgene was maintained, despite a lack of stabilized introgression. The occurrence of fertile, transgenic weed-like plants after only three crosses (F₁, first backcross, second backcross) suggests a potential rapid spread of transgenes from oilseed rape into its wild relative wild B. juncea. Transgene flow from glyphosate-tolerant oilseed rape might be easier than that from glufosinate-tolerant oilseed rape to wild B. juncea. The original insertion site of the transgene could affect introgression.     

Additive transgene expression and genetic introgression in multiple green-fluorescent protein transgenic crop × weed hybrid generations

The level of transgene expression in crop × weed hybrids and the degree to which crop-specific genes are integrated into hybrid populations are important factors in assessing the potential ecological and agricultural risks of gene flow associated with genetic engineering. The average transgene zygosity and genetic structure of transgenic hybrid populations change with the progression of generations, and the green fluorescent protein (GFP) transgene is an ideal marker to quantify transgene expression in advancing populations. The homozygous T₁ single-locus insert GFP/Bacillus thuringiensis (Bt) transgenic canola (Brassica napus, cv Westar) with two copies of the transgene fluoresced twice as much as hemizygous individuals with only one copy of the transgene. These data indicate that the expression of the GFP gene was additive, and fluorescence could be used to determine zygosity status. Several hybrid generations (BC₁F₁, BC₂F₁) were produced by backcrossing various GFP/Bt transgenic canola (B. napus, cv Westar) and birdseed rape (Brassica rapa) hybrid generations onto B. rapa. Intercrossed generations (BC₂F₂ Bulk) were generated by crossing BC₂F₁ individuals in the presence of a pollinating insect (Musca domestica L.). The ploidy of plants in the BC₂F₂ Bulk hybrid generation was identical to the weedy parental species, B. rapa. AFLP analysis was used to quantify the degree of B. napus introgression into multiple backcross hybrid generations with B. rapa. The F₁ hybrid generations contained 95–97% of the B. napus-specific AFLP markers, and each successive backcross generation demonstrated a reduction of markers resulting in the 15–29% presence in the BC₂F₂ Bulk population. Average fluorescence of each successive hybrid generation was analyzed, and homozygous canola lines and hybrid populations that contained individuals homozygous for GFP (BC₂F₂ Bulk) demonstrated significantly higher fluorescence than hemizygous hybrid generations (F₁, BC₁F₁ and BC₂F₁). These data demonstrate that the formation of homozygous individuals within hybrid populations increases the average level of transgene expression as generations progress. This phenomenon must be considered in the development of risk-management strategies.Communicated by J. Dvorak     

Production of wide hybrids and backcross progenies between Diplotaxis erucoides and crop brassicas

Intergeneric hybrids were produced between D. erucoides (), a wild species, and four cultivated species of Brassica, B. campestris, B. juncea, B. napus and B. oleracea, through embryo rescue. The hybrid nature of these plants was confirmed through morphological and cytological studies. Backcross pollinations with the pollen of the respective cultivars yielded BC progenies in the hybrids D. erucoides x B. juncea and D. erucoides x B. napus but not in D. erucoides x B. campestris and D. erucoides x B. oleracea. The hybrid D. erucoides x B. campestris was also used as a bridge species and crossed with B. juncea to raise the hybrid and backcross progenies. F₂ progenies were more amenable than f₁ hybrids for raising backcross progenies. Although D. erucoides is considered to be a close relative of B. campestris and B. oleracea, incompatibility barriers of this species with different cultivars do not reflect this relationship.     

The impact on biosafety of the phosphinothricin-tolerance transgene in inter-specific B. rapa×B. napus hybrids and their successive backcrosses

 There is strong evidence indicating that gene flow from transgenic B. napus into weedy wild relatives is inevitable following commercial release. Research should now focus on the transmission, stability, and impact of transgene expression after the initial hybridization event. The present study investigated the transfer of a phosphinothricin-tolerance transgene by inter-specific hybridization between B. rapa and two transgenic B. napus lines. The expression of the transgene was monitored in the F₁ hybrids and in subsequent backcross generations. The transgene was transmitted relatively easily into the F₁ hybrids and retained activity. Large differences in the transmission frequency of the transgene were noted between offspring of the two transgenic lines during backcrossing. The most plausible explanation of these results is that the line showing least transmission during backcrossing contains a transgene integrated into a C-genome chromosome. Approximately 10% of offspring retained the tolerant trait in the BC₃ and BC₄ generations. The implications of these findings for the stable introgression of transgenes carried on one of the chromosomes of the C-genome from B. napus and into B. rapa are briefly discussed. Received: 5 November 1996 / Accepted: 21 February 1997     

Resistance to Leptosphaeria maculans is conserved in a specific region of the Brassica B genome

 Offspring from asymmetric hybrids between Brassica napus and the three B-genome species Brassica nigra, Brassica juncea and Brassica carinata were analysed for the presence of B-genome markers and resistance to the fungus Leptosphaeria maculans, the causal agent of blackleg disease. Twenty five plants from each species combination were analysed in the first backcross (BC₁) generation, 30 plants in BC₂ and 60 plants in BC₃. The plants were analysed by 46 RFLP markers detecting 85 loci dispersed throughout the B. nigra genome. The plants with additional B. carinata DNA had a decrease in the presence of RFLP markers ranging from 59% in BC₁ to 36% in BC₂ and down to 11% in BC₃. Similar results were obtained in the lines with additional DNA from B. juncea where the 60% presence of RFLP markers in BC₁ was reduced to 33% in BC₂ and to 10% in BC₃. However presence of the markers were significantly lower in the B. nigra-derived material where BC₁ had 46%, BC₂ 25% and BC₃ 8%. Since at least two loci could be detected on each end of the eight linkage groups of the B genome, the degree of symmetry was estimated. After one back-cross between 0.5 and 1.25% intact chromosomes were retained, whereas in BC₂ this frequency was 0.21% for all three B-genome donor species. The maintenance of half-chromosomes ranged from 2.63% to 5.38% in BC₁ and between 0.73% and 1.15% in BC₂. No chromosome arms were found in any of the BC₃ plants. In total, four co-segregating markers for cotyledon and adult-leaf resistance to L. maculans were found which detected six loci located on linkage groups 2, 5 and 8. When the results from the three donor species were compared, one triplicate region in the B genome had preserved the resistance loci in all three species. Received: 19 January 1999 / Accepted: 30 January 1999     

Production and characterization of somatic hybrids between Brassica napus and Raphanus sativus

Intergeneric somatic hybridization between Brassica napus and Raphanus sativus was carried out to enrich gene pool of B. napus. Twelve somatic hybrids were produced via PEG-mediated protoplast fusion between B. napus and R. sativus. The hybridity was confirmed by morphological observation and molecular marker analysis. Hybrid progenies (BC₁) were obtained via backcrosses with B. napus. Behaviour of R. sativus chromosomes in a B. napus background in the F₁ and BC₁ plants was revealed by genomic in situ hybridization (GISH). The potential of somatic hybridization to enrich the suitable gene pool for rapeseed breeding is discussed.     

Development and characterisation of Brassica napus-Sinapis arvensis addition lines exhibiting resistance to Leptosphaeria maculans

Blackleg caused by Leptosphaeria maculans is one of the most important diseases affecting oilseed rape worldwide. Sinapis arvensis is valuable for the transfer of blackleg resistance to oilseed rape (Brassica napus) because this species contains high resistance against various aggressive isolates of the blackleg fungus. These include at least one Australian isolate which has been found to overcome resistance originating from species with the Brassica B genome, until now the major source for interspecific transfer of blackleg resistance. Backcross offspring from intergeneric crosses between Brassica napus and S. arvensis were subjected to phytopathological studies and molecular cytogenetic analysis with genomic in situ hybridisation (GISH). The BC₃S progenies included fertile plants exhibiting high seedling (cotyledon) and adult plant resistance associated with the presence of an acrocentric addition chromosome from S. arvensis. In addition, some individuals with adult plant resistance but cotyledon susceptibility were observed to have a normal B. napus karyotype with no visible GISH signals, indicating possible resistant introgression lines. Phytopathological analysis of selfing progenies from 3 different highly resistant BC₃ plants showed that seedling and adult plant resistance are probably conferred by different loci. Received: 20 September 1999 / Accepted: 25 March 2000     

Within-population variation in hybridisation and transgene transfer between wild Brassica rapa and Brassica napus in the UK

Several studies in Europe and North America have shown that cultivated Brassica napus will readily hybridise with wild Brassica rapa but at widely different frequencies. To understand the implications of this phenomenon with regard to transgene flow, we examined the rate at which cultivated B. napus cv. Westar containing a capsid (coat protein, CP)‐coding sequence from Turnip mosaic virus (Potyvirus) hybridised under glasshouse conditions with wild B. rapa from Culham, in Oxfordshire, UK. We found that the hybridisation rate, as judged using simple sequence repeat (SSR)‐PCR and primer oligonucleotides specific for either the C or the A genomes in progeny from individual crosses varied from 5% to 100%. In hybrids (F₁ progeny), transgene transfer was always observed (inferred by SSR‐PCR) when hybrids were detected. Our observations revealed a hitherto unrecorded source of variability in transgene flow to wild UK B. rapa.     

A RFLP-based linkage map of mustard [Brassica juncea (L.) Czern. and Coss.]

 A genetic linkage map of Brassica juncea was constructed based on restriction fragment length polymorphism (RFLP) detected by anonymous cDNA markers from B. napus, using a segregating F₁-derived doubled haploid (DH) progeny from a cross between a canola-quality mustard line (J90-4317) and a high-oil-content mustard line (J90-2733). The RFLP probes consisted of 229 cDNA probes from B. napus and a B. napus tandem repeat sequence, RDA2. The map consisted of 343 marker loci arranged in 18 major linkage groups plus five small segments with two to five marker loci, covering a total map distance of 2073 cM. Twenty-four percent of the markers were dominant in nature. Sixty-two percent of the marker loci were duplicated, and the majority were involved in inter-linkage group duplications, illustrating that complex duplications and subsequent rearrangements occurred after allopolyploidy. Deviation from the Mendelian segregation ratio for a DH population was observed for 27% of the markers. Two-thirds of these markers with a skewed segregation were clustered in 6 linkage groups and two unassigned segments. The overall average marker interval of the B. juncea map reported here was 6.6 cM, which would provide a marker density satisfactory for efficient use of the map in breeding applications, such as tagging of important agronomic traits and marker-assisted selection. Received: 14 May 1996 / Accepted: 11 October 1996     

A- or C-chromosomes, does it matter for the transfer of transgenes from Brassica napus

Introgression of genes from allotetraploid Brassica napus into its diploid wild relative B. rapa is generally considered to be inevitable. As a means to minimize a potential ecological risk in environments where B. rapa is growing, the insertion of transgenes into chromosome regions of B. napus with a very low probability of transfer to backcross generations with B. rapa has been proposed. Recently, the progeny of four backcross generations between transgenic herbicide-tolerant B. napus and B. rapa was studied in selection experiments (Metz et al. 1997). The rapid decrease in the frequency of herbicide-tolerant plants was explained by selection against the C-chromosomes of B. napus in favor of the homeologous A-chromosomes. Obviously, such C-chromosomes could be potential candidates as safe integration sites for transgenes. We considered these safety aspects using a simple population genetic model. Theory and experiments, however, do not favor the chromosomes of B. napus as safe candidates with respect to the introgression of transgenes into wild populations of B. rapa. Received: 5 July 1999 / Accepted: 29 July 1999     

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 (AⁿAⁿCⁿCⁿ) and a new type of B. napus with introgressions of genomic components of Brassica rapa (A^rA^r). This B. napus was selected from the progeny of B. napus × B. rapa and (B. napus × B. rapa) × 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 F₃ or BC₁F₃ 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 BC₁F₅ 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 BC₁F₅ 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.     

结合SSR标记和STS标记对家蚕无鳞毛翅基因的定位

家蚕突变表型无鳞毛翅(non-lepis wing, nlw)由隐性基因nlw控制。由于家蚕雌性不发生交换, 文章采用有鳞毛翅品系P50和无鳞毛翅品系U06两个品系组配F1代及BC₁回交群体, (U06×P50)×U06和U06×(U06×P50)分别记作BC₁F和BC₁M, 根据已经构建的家蚕SSR分子标记连锁图谱及已经发表的有关序列对nlw基因进行了连锁及定位分析。得到8个与nlw基因连锁的SSR(Simple sequence repeat)标记和1个STS(Sequence-tagged sites)标记。BC₁F群中的所有正常翅个体均表现出与(U06×P50)F₁相同的杂合带型; 而所有无鳞毛个体带型与亲本U06一致, 为纯合型。利用BC₁M群体构建了关于nlw基因的遗传连锁图, 连锁图的遗传距离为125.7 cM, 与nlw基因最近的引物为STS标记cash2p, 图距为11.4 cM。     

Mapping and validation of chromosome regions conferring a new boron-efficient locus in <Emphasis Type="Italic">Brassica napus</Emphasis>

Considerable genotypic variation exists in the response of different cultivars of rapeseed (Brassica napus) to B deficiency. This raises the possibility of genetic improvement of a B nutrition trait that will make the plant more tolerant to low B stress. The results of our study showed that B-efficient backcross plants had lower B concentration and more dry matter when grown at low levels of B when compared with the recurrent parent. Accordingly, we proposed that the improved B efficiency was attributed to either a high B utilization efficiency or less demand for B. The results of the genetic analysis showed that B efficiency is a dominant trait that is controlled by a single locus, namely BnBE2. By using bulked segregant analysis (BSA) in combination with amplified fragment length polymorphism (AFLP) and sequence related amplified polymorphism (SRAP) techniques, five SRAP markers and one converted single strand conformation polymorphism (SSCP) marker were identified to be linked to BnBE2 after screening 1,800 primer combinations. The six markers together with BnBE2 were mapped in a region that covered a genetic distance of 6.9 cM on a linkage group using a BC₆ population. This region was located on linkage group N14 after mapping these markers in two doubled haploid (DH) populations (TNDH and BQDH). The SRAP and AFLP markers were sequenced and found to be homologous to a BAC sequence from Brassica oleracea (CC). This finding suggested that the segment containing BnBE2 locus originated from the C genome of Brassica oleracea. Three SSR markers were identified to be linked to BnBE2 through comparative mapping. All these markers might have potential value for facilitating the pyramiding of the BnBE2 gene with other B efficient genes in order to improve the B efficiency trait and for further fine mapping of the BnBE2 gene in Brassica napus.     

Characterization of interspecific hybrids and backcross progenies from a cross between Oryza minuta and Oryza sativa

Oryza minuta, a tetraploid wild relative of cultivated rice, is an important source for the genetic improvement. Interspecific hybrids were obtained from the cross of O. sativa L. (IR24) and O. minuta (Acc. No. 101133) with 5.58% crossability, which ranged from 0.11% to 1.62% in the backcross generations. The chromosome numbers of the backcross progenies were 24 to 48. Seven yield-related traits of the parents, hybrid F₁, and backcross progenies were evaluated. Simple sequence repeat markers analysis showed that the polymorphism ratio of SSR bands between IR24 and Acc. No. 101133 was 93.2%. The average donor segment number, length, donor genome size, and percentage of donor genome of 92 BC₃F₁ plants (2n=24) were 24.1, 17.8 cM, 438.4 cM and 26.2%, respectively. They were complex variation and uneven among the chromosomes. These introgression lines could be used to identify the favorable genes of O. minuta and provide a new platform for the genetic improvement of cultivated rice.     

Inheritance of seed coat color genes in <Emphasis Type="Italic">Brassica napus</Emphasis> (L.) and tagging the genes using SRAP,SCAR and SNP molecular markers

Seed coat color inheritance in Brassica napus was studied in F₁, F₂, F₃ and backcross progenies from crosses of five black seeded varieties/lines to three pure breeding yellow seeded lines. Maternal inheritance was observed for seed coat color in B. napus, but a pollen effect was also found when yellow seeded lines were used as the female parent. Seed coat color segregated from black to dark brown, light brown, dark yellow, light yellow, and yellow. Seed coat color was found to be controlled by three genes, the first two genes were responsible for black/brown seed coat color and the third gene was responsible for dark/light yellow seed coat color in B. napus. All three seed coat color alleles were dominant over yellow color alleles at all three loci. Sequence related amplified polymorphism (SRAP) was used for the development of molecular markers co-segregating with the seed coat color genes. A SRAP marker (SA12BG18388) tightly linked to one of the black/brown seed coat color genes was identified in the F₂ and backcross populations. This marker was found to be anchored on linkage group A9/N9 of the A-genome of B. napus. This SRAP marker was converted into sequence-characterized amplification region (SCAR) markers using chromosome-walking technology. A second SRAP marker (SA7BG29245), very close to another black/brown seed coat color gene, was identified from a high density genetic map developed in our laboratory using primer walking from an anchoring marker. The marker was located on linkage group C3/N13 of the C-genome of B. napus. This marker also co-segregated with the black/brown seed coat color gene in B. rapa. Based on the sequence information of the flanking sequences, 24 single nucleotide polymorphisms (SNPs) were identified between the yellow seeded and black/brown seeded lines. SNP detection and genotyping clearly differentiated the black/brown seeded plants from dark/light/yellow-seeded plants and also differentiated between homozygous (Y2Y2) and heterozygous (Y2y2) black/brown seeded plants. A total of 768 SRAP primer pair combinations were screened in dark/light yellow seed coat color plants and a close marker (DC1GA27197) linked to the dark/light yellow seed coat color gene was developed. These three markers linked to the three different yellow seed coat color genes in B. napus can be used to screen for yellow seeded lines in canola/rapeseed breeding programs.     

Intergeneric hybridization between Brassica napus and Sinapis pubescens,and the cytology and crossability of their progenies

The cytological possibility of gene transfer from Sinapis pubescens to Brassica napus was investigated. Intergeneric hybrids between Brassica napus (2n = 38) and Sinapis pubescens (2n = 18) were produced through ovary culture. The F₁ hybrids were dihaploid and the chromosome configurations were (0–1) _III + (2–11) _II + (5–24) _I . One F₂ plant with 38 chromosomes was obtained from open pollination of the F₁ hybrid. Thirty-one seeds were obtained from the backcross of the F₂ plant with B. napus. Five out of seven plants had 38 chromosomes, and the pollen stainability ranged from 0% to 81.4%. In the B₂ plants obtained from the backcross of B₁ plants with B. napus, 66.7% of the plants examined had 38 chromosomes. S. pubescens may become a gene source for the improvement of B. napus.     

Valuable New Resistances Ensure Improved Management of Sclerotinia Stem Rot (Sclerotinia sclerotiorum) in Horticultural and Oilseed Brassica Species

Field resistances against Sclerotinia rot (SR) (Sclerotinia sclerotiorum) were determined in 52 Chinese genotypes of Brassica oleracea var. capitata, 14 Indian Brassica juncea genotypes carrying wild weedy Brassicaceae introgression(s) and four carrying B‐genome introgression, 22 Australian commercial Brassica napus varieties, and 12 B. napus and B. juncea genotypes of known resistance. All plants were individually inoculated by securing an agar disc from a culture of S. sclerotiorum growing on a glucose‐rich medium to the stem above the second internode with Parafilm tape. Mean stem lesion length across tested genotypes ranged from <1 to >68 mm. While there was considerable diversity within the germplasm sets from each country, overall, 65% of the B. oleracea var. capitata genotypes from China showed the highest levels of stem resistance, a level comparable with the highest resistance ever recorded for oilseed B. napus or B. juncea from China or Australia. One Indian B. juncea line carrying weedy introgression displayed a significant level of both stem and leaf resistance. However, the vast majority of commercial Australian oilseed B. napus varieties fell within the most susceptible 40% of genotypes tested for stem disease. There was no correlation between expressions of stem versus leaf resistance, suggesting their independent inheritance. A few Chinese B. oleracea var. capitata genotypes that expressed combined extremely high‐level stem (≤1 mm length) and leaf (≤0.5 mean number of infections/plant) resistance will be particularly significant for developing new SR‐resistant horticultural and oilseed Brassica varieties.     

Opportunities for gene transfer from transgenic oilseed rape (Brassica napus) to related species

Before novel transgenic plant genotypes are grown outside containment facilities and evaluated under field conditions, it is necessary to complete a risk assessment to consider the possible consequences of that release. An important aspect of risk assessment is to consider the likelihood and consequences of the transgene being transferred by cross-pollination to related species, including other crops, weeds and ruderal populations. The purpose of this report is to review the literature to assess the ease with whichBrassica napus can hybridize with related species. The evidence for hybridization is considered at three levels: a) by open pollination, b) by hand pollination and c) by the use ofin vitro ovule and embryo rescue techniques; and also examines the fertility and vigour of the F₁, F₂ and backcross generations. Four species are reported to hybridize withB. napus by open pollination:B. rapa andB. juncea using fully fertile parents; andB. adpressa andR. raphanistrum using a male-sterileB. napus parent. Seventeen species are reported to form hybrids (including the four species above) withB. napus when pollination is carried out manually. At least 12 of these species were unable to form F₂ progeny, and eight were unable to produce progeny when the F₁ was backcrossed to one of the parental species. Many factors will influence the success of hybridization under field conditions, including: distance between the parents, synchrony of flowering, method of pollen spread, specific parental genotypes used, direction of the cross and the environmental conditions. Even where there is a possibility of hybridization betweenB. napus and a related species growing in the vicinity of a release, poor vigour and high sterility in the hybrids will generally mean that hybrids and their progeny will not survive in either an agricultural or natural habitat.     

Male sterility caused by cytoplasm of Brassica oxyrrhina in B. campestris and B. juncea

Summary Synthetic alloploid Brassica oxyrrhina (2n = 18, OO) x B. campestris (2n = 20, AA) was repeatedly backcrossed with B. campestris to place B. campestris nucleus in the cytoplasm of B. oxyrrhina. Alloplasmic plants, obtained in BC₅ generation, were stably male sterile but mildly chlorotic during initial development. Synthetic alloploid B. oxyrrhina-campestris was also hybridized with B. juncea to transfer B. oxyrrhina cytoplasm. Segregation for green and chlorotic plants was observed in BC₁ and BC₂ generations. By selection, however, normal green male sterile B. juncea was obtained in BC₃. Pollen abortion in both B. campestris and B. juncea is post-meiotic.