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.     

Identification of the A and C genomes of amphidiploid Brassica napus (oilseed rape).

A genetic linkage map consisting of 399 RFLP-defined loci was generated from a cross between resynthesized Brassica napus (an interspecific B. rapa x B. oleracea hybrid) and "natural" oilseed rape. The majority of loci exhibited disomic inheritance of parental alleles demonstrating that B. rapa chromosomes were each pairing exclusively with recognisable A-genome homologues in B. napus and that B. oleracea chromosomes were pairing similarly with C-genome homologues. This behaviour identified the 10 A genome and 9 C genome linkage groups of B. napus and demonstrated that the nuclear genomes of B. napus, B. rapa, and B. oleracea have remained essentially unaltered since the formation of the amphidiploid species, B. napus. A range of unusual marker patterns, which could be explained by aneuploidy and nonreciprocal translocations, were observed in the mapping population. These chromosome abnormalities were probably caused by associations between homoeologous chromosomes at meiosis in the resynthesized parent and the F1 plant leading to nondisjunction and homoeologous recombination.     

Frequent nonreciprocal translocations in the amphidiploid genome of oilseed rape (Brassica napus).

A RFLP map of Brassica napus, consisting of 277 loci arranged in 19 linkage groups, was produced from genetic segregation in a combined population of 174 doubled-haploid microspore-derived lines. The integration of this map with a B. napus map derived from a resynthesized B. napus x oilseed rape cross allowed the 10 linkage groups of the B. napus A genome and the 9 linkage groups of the C genome to be identified. Collinear patterns of marker loci on different linkage groups suggested potential partial homoeologues. RFLP patterns consistent with aberrant chromosomes were observed in 9 of the 174 doubled-haploid lines. At least 4 of these lines carried nonreciprocal, homoeologous translocations. These translocations were probably the result of homoeologous recombination in the amphidiploid genome of oilseed rape, suggesting that domesticated B. napus is unable to control chromosome pairing completely. Evidence for genome homogenization in oilseed rape is presented and its implications on genetic mapping in amphidiploid species is discussed. The level of polymorphism in the A genome was higher than that in the C genome and this might be a general property of oilseed rape crosses.     

Genetic analysis of phenylpropanoid metabolites associated with resistance against Verticillium longisporum in Brassica napus

Verticillium longisporum is a major threat to production of oilseed rape (Brassica napus) in Europe. The aim of the study was to develop new markers and obtain insights into putative mechanisms and pathways involved in the resistance reaction. A genetic approach was used to identify quantitative trait loci (QTL) for V. longisporum resistance and metabolic traits potentially influencing resistance in a B. napus mapping population. Resistance to V. longisporum was mapped in a doubled haploid (DH) population from a cross between the partially resistant winter oilseed rape variety Express 617 and a resistant resynthesized B. napus line, R53. One major resistance QTL contributed by R53 was identified on chromosome C5, while a further, minor QTL contributed by Express 617 was detected on chromosome C1. Markers flanking the QTL also significantly correlated with V. longisporum resistance in four further DH populations derived from crosses between elite oilseed rape cultivars and other resynthesized B. napus lines originating from genetically and geographically diverse brassica A and C genome donors. The tightly-linked markers developed enable the combination of favorable alleles for novel resistance loci from resynthesized B. napus materials with existing resistance loci from commercial breeding lines. HPLC analysis of hypocotyls from infected DH lines revealed that concentrations of a number of phenylpropanoids were correlated with V. longisporum resistance. QTL for some of these phenylpropanoids were also found to co-localize with the QTL for V. longisporum resistance. Genes from the phenylpropanoid pathway are suggested as candidates for V. longisporum resistance.     

Extending the rapeseed gene pool with resynthesized <Emphasis Type="Italic">Brassica napus</Emphasis> II: Heterosis

Hybrid breeding relies on the combination of parents from two differing heterotic groups. However, the genetic diversity in adapted oilseed rape breeding material is rather limited. Therefore, the use of resynthesized Brassica napus as a distant gene pool was investigated. Hybrids were derived from crosses between 44 resynthesized lines with a diverse genetic background and two male sterile winter oilseed rape tester lines. The hybrids were evaluated together with their parents and check cultivars in 2 years and five locations in Germany. Yield, plant height, seed oil, and protein content were monitored, and genetic distances were estimated with molecular markers (127 polymorphic RFLP fragments). Resynthesized lines varied in yield between 40.9 dt/ha and 21.5 dt/ha, or between 85.1 and 44.6% of check cultivar yields. Relative to check cultivars, hybrids varied from 91.6 to 116.6% in yield and from 94.5 to 103.3% in seed oil content. Mid-parent heterosis varied from −3.5 to 47.2% for yield. The genetic distance of parental lines was not significantly correlated with heterosis or hybrid yield. Although resynthesized lines do not meet the elite rapeseed standards, they are a valuable source for hybrid breeding due to their large distance from present breeding material and their high heterosis when combined with European winter oilseed rape.     

Homoeologous loci control the accumulation of seed glucosinolates in oilseed rape (Brassica napus).

The genetic control of seed glucosinolate content in oilseed rape was investigated using two intervarietal backcross populations. Four QTLs segregating in the population derived from a Brassica napus L. 'Victor' x Brassica napus L. 'Tapidor' cross, together accounting for 76% of the phenotypic variation, were mapped. Three of these loci also appeared to control the accumulation of seed glucosinolates in a Brassica napus L. 'Bienvenu' x 'Tapidor' cross, and accounted for 86% of the phenotypic variation. The three QTLs common to both populations mapped to homoeologous regions of the B. napus genome, suggesting that seed glucosinolate accumulation is controlled by duplicate genes. It was possible to extend the comparative analysis of QTLs controlling seed glucosinolate accumulation by aligning the published genetic maps generated by several research groups. This comparative mapping demonstrated that high-glucosinolate varieties often carry low-glucosinolate alleles at one or more of the loci controlling seed glucosinolate accumulation.     

Mapping the mosaic of ancestral genotypes in a cultivar of oilseed rape (Brassica napus) selected via pedigree breeding.

Recent oilseed rape breeding has produced low glucosinolate cultivars that yield proteinaceous meal suitable for animal feed. The low glucosinolate character was introduced into modern cultivars from Brassica napus 'Bronowski', a cultivar that is agronomically inferior in most other respects. Residual segments of 'Bronowski' genotype in modern cultivars probably cause reduced yield, poorer winter hardiness, and lower oil content. The quantity and distribution of the 'Bronowski' genotype in the modern oilseed rape cultivar Brassica napus 'Tapidor' was investigated using a segregating population derived from a cross between 'Tapidor' and its high glucosinolate progenitor. This population was analyzed with 65 informative Brassica RFLP probes and a genetic linkage map, based on the segregation at 77 polymorphic loci, was constructed. The mapping identified 15 residual segments of donor genotype in 'Tapidor', which together occupy approximately 29% of the B. napus genome. Mapping the loci that control variation for the accumulation of total seed glucosinolates in the segregating population has identified three loci that together explain >90% of the variation for this character. All of these loci are in donor segments of the 'Tapidor' genome. This result shows the extent to which conventional breeding programmes have difficulty in eliminating residual segments of donor genotype from elite material.     

Fluorescence-based AFLPs occur as the most suitable marker system for oilseed rape cultivar identification

Three different types of molecular markers, RAPD, SSR and fluorescence-based AFLP, were evaluated and compared for their ability to identify oilseed rape cultivars. The direct comparison of RAPD, SSR and AFLP approaches in cultivar identification showed that the AFLP methodology detected polymorphisms more efficiently than either RAPD or SSR methods. For the characterisation of six oilseed rape cultivars, 60 RAPD primers were tested and only eight of them (14%) detected sufficient levels of polymorphism. Five microsatellites out of fifteen tested were polymorphic, but in all loci, except one, only two different alleles were detected. This result indicated the limited degree of polymorphism found in Brassica napus. Each of the six tested AFLP combinations detected polymorphisms, the best combination (M-CAA/E-ACT) had 26% polymorphic peaks from a total of 90 peaks and could distinguish the analysed cultivars and 4 out of 5 core lines of cultivars. The results presented show that florescence-based AFLP is, for the purposes of oilseed rape cultivar fingerprinting, a more suitable approach than either RAPD or SSR.     

Identification of FAD2 and FAD3 genes in Brassica napus genome and development of allele-specific markers for high oleic and low linolenic acid contents

Modification of oleic acid (C18:1) and linolenic acid (C18:3) contents in seeds is one of the major goals for quality breeding after removal of erucic acid in oilseed rape (Brassica napus). The fatty acid desaturase genes FAD2 and FAD3 have been shown as the major genes for the control of C18:1 and C18:3 contents. However, the genome structure and locus distributions of the two gene families in amphidiploid B. napus are still not completely understood to date. In the present study, all copies of FAD2 and FAD3 genes in the A- and C-genome of B. napus and its two diploid progenitor species, Brassica rapa and Brassica oleracea, were identified through bioinformatic analysis and extensive molecular cloning. Two FAD2 genes exist in B. rapa and B. oleracea, and four copies of FAD2 genes exist in B. napus. Three and six copies of FAD3 genes were identified in diploid species and amphidiploid species, respectively. The genetic control of high C18:1 and low C18:3 contents in a double haploid population was investigated through mapping of the quantitative trait loci (QTL) for the traits and the molecular cloning of the underlying genes. One major QTL of BnaA.FAD2.a located on A5 chromosome was responsible for the high C18:1 content. A deleted mutation in the BnaA.FAD2.a locus was uncovered, which represented a previously unidentified allele for the high oleic variation in B. napus species. Two major QTLs on A4 and C4 chromosomes were found to be responsible for the low C18:3 content in the DH population as well as in SW Hickory. Furthermore, several single base pair changes in BnaA.FAD3.b and BnaC.FAD3.b were identified to cause the phenotype of low C18:3 content. Based on the results of genetic mapping and identified sequences, allele-specific markers were developed for FAD2 and FAD3 genes. Particularly, single-nucleotide amplified polymorphisms markers for FAD3 alleles were demonstrated to be a reliable type of SNP markers for unambiguous identification of genotypes with different content of C18:3 in amphidiploid B. napus.     

Indistinguishable patterns of recombination resulting from male and female meioses in Brassica napus (oilseed rape).

An F1 individual derived from a cross between two distinct lines of spring oilseed rape (Brassica napus) was used to produce a pair of complementary backcross populations, each consisting of 90 individuals. The F1 donated male gametes to the Male population and female gametes to the Female population. Genetic maps were generated from both populations and aligned using 117 common loci to form an integrated genome map of B. napus with 243 RFLP-defined loci. A comparison of the frequency and distribution of crossovers in the two populations of F1 gametes (assayed in the Male and Female populations) detected no differences. The genetic maps derived from the Male and Female populations each consisted of 19 linkage groups spanning 1544 and 1577 cM, respectively. The maps were aligned with other B. napus maps, and all 19 equivalent linkage groups were unambiguously assigned. The genetic size and general organisation of the new maps were comparable with those of pre-existing B. napus maps in most respects, except that the levels of polymorphism in the constituent A and C genomes were unusually similar in the new cross.     

Screening of oilseed rape and other brassicaceous genotypes for susceptibility to Ceutorhynchus pallidactylus (Mrsh.)

Production of oilseed rape, Brassica napus L., is affected by various insect pests. The cabbage stem weevil, Ceutorhynchus pallidactylus (Mrsh.) (Col.: Curculionidae), is one of the most damaging pests in Northern and Central Europe that requires regular control measures. Host plant resistance is a key factor in integrated pest management systems. To evaluate a large number of genotypes for their susceptibility to infestation by C. pallidactylus, new screening techniques were developed for testing both, the amount of feeding and the number of eggs deposited by adult C. pallidactylus on accessions of Brassicaceae under controlled conditions. In no‐choice screening tests, the leaf area consumed by adult cabbage stem weevil was quantified on a wide spectrum of 107 brassicaceous genotypes (B. napus, Brassica rapa L. and Brassica oleracea L. cultivars, breeding lines, resynthesized rapeseed lines and wild Brassicaceae). In comparison to feeding on the standard cultivar ‘Express’, the average leaf area consumed by C. pallidactylus on nine oilseed rape cultivars, four resynthesized rapeseed lines and five other accessions [B. oleracea, Camelina alyssum (Mill.) and Lunaria annua L.] was significantly reduced by 44–90%. In dual‐choice screening tests for the evaluation of oviposition preferences on 42 genotypes, female C. pallidactylus laid significantly fewer eggs into plants of two oilseed rape cultivars, five resynthesized rapeseeds and three accessions of B. oleracea and Brassica fruticulosa Cyrillo, respectively, than into plants of the standard cv ‘Express’. Results of both laboratory screening tests were confirmed by results of additional field testing.     

Detection and effects of a homeologous reciprocal transposition in Brassica napus

A reciprocal chromosomal transposition was identified in several annual oilseed Brassica napus genotypes used as parents in crosses to biennial genotypes for genetic mapping studies. The transposition involved an exchange of interstitial homeologous regions on linkage groups N7 and N16, and its detection was made possible by the use of segregating populations of doubled haploid lines and codominant RFLP markers. RFLP probes detected pairs of homeologous loci on N7 and N16 for which the annual and biennial parents had identical alleles in regions expected to be homeologous. The existence of an interstitial reciprocal transposition was confirmed by cytological analysis of synaptonemal complexes of annual x biennial F1 hybrids. Although it included approximately one-third of the physical length of the N7 and N16 chromosomes, few recombination events within the region were recovered in the progenies of the hybrids. Significantly higher seed yields were associated with the parental configurations of the rearrangement in segregating progenies. These progenies contained complete complements of homeologous chromosomes from the diploid progenitors of B. napus, and thus their higher seed yields provide evidence for the selective advantage of allopolyploidy through the fixation of intergenomic heterozygosity.     

Comparison of rapeseed cultivars and resynthesized lines based on allozyme and RFLP markers

It has frequently been suggested to use the resynthesis of rapeseed (Brassica napus) from B. campestris and B. oleracea to broaden its genetic base. The objective of the present study is twofold: (1) to compare the genetic variation within resynthesized rapeseed with a world-wide collection of oilseed rape cultivars, and (2) to compare genetic distances estimated from RFLP markers with distances estimated from a relatively small number of allozyme markers. We investigated 17 resynthesized lines and 24 rapeseed cultivars. Genetic distances were estimated either based on the electrophoresis of seven allozymes, with a total of 38 different bands, or based on RFLP data of 51 probe/enzyme combinations, with a total of 355 different bands. The results of allozyme and RFLP analyses agreed reasonably well. Genetic distances, estimated from two independent sets of RFLP data with 25 and 26 probe/enzyme combinations respectively, were highly correlated; hence about 50 RFLP markers are sufficient to characterize rapeseed material with a large genetic diversity. The cultivars were clustered into three groups: (1) spring rapeseed of European and Northern American origin, (2) winter rapeseed of European and Northern American origin, and (3) rapeseed of Asian origin. Several of the resynthesized rapeseed lines were similar to European winter rapeseed cultivars, whereas others had quite unique patterns. It is concluded, that resynthesized rapeseed is a valuable source for broadening the genetic variation in present breeding material of Brassica napus. However, different lines differ widely in their suitability for this purpose.     

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.     

人工合成甘蓝型油菜中花色与芥酸含量的遗传连锁分析

利用人工合成的甘蓝型油菜品系．No．2127-17(白花、有芥酸)与加拿大双低甘蓝型油菜品种Quantum(黄花、低芥酸)配制杂交组合。对亲本、F1、BC1、F2和DH(doubled haploid)5个世代的花色及芥酸含量进行分析,结果表明：花色受单基因控制,且白花对黄花为显性;芥酸含量仅表现出一对基因的差异且具有加性效应的遗传模式。花色和芥酸含量的连锁分析表明：白花与高芥酸紧密连锁．在DH群体中重组频率为5．8％。采用集团分离分析法(Bulked Segregant Analysis,BSA),从685条10个碱基的随机引物筛选到一个与黄花和低芥酸含量紧密连锁的RAPD标记S92-1400。在遗传图谱上黄花基因和低芥酸基因距离S92-1400标记的图距分别为2．2cM和5．4cM。     

Resynthesized lines and cultivars of Brassica napus L. provide sources of resistance to the cabbage stem weevil ( Ceutorhynchus pallidactylus (Mrsh.))

The cabbage stem weevil (Ceutorhynchus pallidactylus (Mrsh.)) (Col., Curculionidae) is a serious pest of winter oilseed rape (Brassica napus L. var. oleifera Metzg.) in central and northern Europe. Although host-plant resistance is a key tool in integrated pest management systems, resistant genotypes are not yet available for this species. Resynthesized rapeseed lines (B. oleracea L.×B. rapa L.) are broadening the genetic diversity and might have potential as sources of resistance to pest insects. The host quality, of nine resynthesized rapeseed lines and six genotypes of B. napus to cabbage stem weevil, was evaluated in laboratory screening tests and in a semi-field experiment. In dual-choice oviposition tests, female C. pallidactylus laid significantly fewer eggs on five resyntheses and on swede cv 'Devon Champion' than on the moderately susceptible oilseed rape cv 'Express', indicating a lower host quality of these genotypes. Results of laboratory screenings were confirmed in a semi-field experiment, in which twelve genotypes were exposed to C. pallidactylus females. The number of larvae was significantly lower in two resyntheses and in cv 'Devon Champion' than in oilseed rape cv WVB 9. The total, as well as individual, glucosinolate (GSL) content in the leaves differed substantially among the genotypes tested. The amount of feeding by larvae of C. pallidactylus, as measured by a stem-injury coefficient, was positively correlated with the indolyl GSL compounds 3-indolylmethyl and 4-methoxy-3-indolylmethyl, and with the aromatic GSL 2-phenylethyl, whereas it was negatively correlated with 4-hydroxy-3-indolylmethyl. Thus, the composition and concentration of GSL compounds within the plant tissue might be a key factor in breeding for pest resistance in oilseed rape.     

Progressive introgression between Brassica napus (oilseed rape) and B. rapa

We have earlier shown extensive introgression between oilseed rape (Brassica napus) and B. rapa in a weedy population using AFLP markers specific for the nuclear genomes. In order to describe the progress of this introgression, we examined 117 offspring from 12 maternal plants from the introgressed population with the same AFLP-markers; AFLP data were supported by chromosome counting. We also analysed the offspring with a species-specific chloroplast marker and finally evaluated the reproductive system in selected maternal plants. Our results indicated a high outcrossing rate of the introgressed maternal plants. It seemed that B. rapa most often functioned as the maternal plant in the introgression process and that the amount of oilseed rape DNA was highly diminished in the offspring compared to their introgressed maternal plants. However, our analysis of plants from the weedy population indicated that introgression can lead to both (1) exchange of chloroplast DNA between species producing B. rapa-like plants with B. napus chloroplasts and (2) incorporation of B. napus C-genome DNA into the B. rapa genome. Therefore, we question whether it can be regarded as containment to position transgenes in the chloroplast or in specific parts of the nuclear genome of B. napus.     

Low probability of chloroplast movement from oilseed rape (Brassica napus) into wild Brassica rapa

Pollen-mediated movement of transgenes from transplastomic oilseed rape (Brassica napus) into wild relatives will be avoided if chloroplasts are maternally transmitted. We assess the probability of chloroplast exchange between conventional oilseed rape and wild Brassica rapa to model the future behavior of transplastomic cultivars. Primers specific to cpDNA were used to demonstrate maternal inheritance of chloroplasts in 47 natural hybrids between cultivated B. napus and wild B. rapa. We conclude that there will be no or negligible pollen-mediated chloroplast dispersal from oilseed rape. Transgene introgression could still occur in mixed populations, however, if B. napus acted as the recurrent female parent. Rate of transfer would then depend on the abundance of mixed populations, their persistence as mixtures, and hybridization frequency within stands. A low incidence of sympatry (0.6-0.7%) between wild B. rapa and cultivated B. napus along the river Thames, UK, in 1997 and 1998, suggests mixed stands will form only rarely. Eighteen feral populations of B. napus also showed a strong tendency toward rapid decline in plant number, seed return, and ultimately, extinction within 3 years. Conversely, hybrid production is significant in mixed stands, and the absence of control practices means that oilseed rape will have slightly greater persistence. We infer that some introgression from transplastomic B. napus into B. rapa is inevitable in mixed populations even though such populations will occur infrequently and will tend to lose B. napus plants relatively quickly. Chloroplast exchange will be extremely rare and scattered.     

Genome discrimination in progeny of interspecific hybrids between Brassica napus and Raphanus raphanistrum.

Genomic in situ hybridization (GISH) applied to the F1 interspecific hybrid between oilseed rape (Brassica napus, AACC, 2n = 38) and wild radish (Raphanus raphanistrum, RrRr, 2n = 18) showed the predicted 19 chromosomes from B. napus and 9 chromosomes from R. raphanistrum. The very low female fertility of these interspecific hybrids when backcrossed to R. raphanistrum led to only two descendants. Their chromosome number varied between 45 and 48. Both of these progenies showed only 9 chromosomes from R. raphanistrum and 36-39 chromosomes from B. napus. These results indicate the efficiency and limits of GISH as a suitable tool to assess and interpret the behavior of chromosomes after such interspecific crosses. The unexpected chromosome combination is discussed.     

Genomic changes in resynthesized Brassica napus and their effect on gene expression and phenotype

Many previous studies have provided evidence for genome changes in polyploids, but there are little data on the overall population dynamics of genome change and whether it causes phenotypic variability. We analyzed genetic, epigenetic, gene expression, and phenotypic changes in approximately 50 resynthesized Brassica napus lines independently derived by hybridizing double haploids of Brassica oleracea and Brassica rapa. A previous analysis of the first generation (S0) found that genetic changes were rare, and cytosine methylation changes were frequent. Our analysis of a later generation found that most S0 methylation changes remained fixed in their S5 progeny, although there were some reversions and new methylation changes. Genetic changes were much more frequent in the S5 generation, occurring in every line with lines normally distributed for number of changes. Genetic changes were detected on 36 of the 38 chromosomes of the S5 allopolyploids and were not random across the genome. DNA fragment losses within lines often occurred at linked marker loci, and most fragment losses co-occurred with intensification of signal from homoeologous markers, indicating that the changes were due to homoeologous nonreciprocal transpositions (HNRTs). HNRTs between chromosomes A1 and C1 initiated in early generations, occurred in successive generations, and segregated, consistent with a recombination mechanism. HNRTs and deletions were correlated with qualitative changes in the expression of specific homoeologous genes and anonymous cDNA amplified fragment length polymorphisms and with phenotypic variation among S5 polyploids. Our data indicate that exchanges among homoeologous chromosomes are a major mechanism creating novel allele combinations and phenotypic variation in newly formed B. napus polyploids.