A PCR based B-genome-specific marker in Brassica species

Transcriptional Interactions of Single B-Subgenome Chromosome with C-Subgenome in B. oleracea-nigra Additional Lines

Serial monosomic alien addition lines (MAALs) provide an ideal system to elucidate the transcriptomic interactions between the alien chromosomes and recipient genome under aneuploidy. Herein, five available Brassica oleracea-nigra MAALs (CCB1, CCB4, CCB5, CCB6, CCB8), their derived B. oleracea plants (non-MAALs), and two parents were analyzed for their gene expressions by using high-throughput technology. Compared to parental B. oleracea, all MAALs showed various numbers of DEGs, but CCB8 gave much higher DEGs; the number of downregulated DEGs was slightly higher than the number of upregulated ones, except for in relation to CCB8. All derived B. oleracea plants also gave certain numbers of DEGs, despite these being much lower than in the respective MAALs. Compared to B. nigra, in all five MAALs more DEGs were downregulated than upregulated. Trans-effects were likely more prevailing than cis-effects, and these DEGs were predominantly associated with material transport by dysregulating the cellular component. Meanwhile, the orthologous genes on alien chromosomes could only play a feeble compensatory role for those gene pairs in C-subgenome, and different levels of the expressed genes had a greater tendency towards downregulation. These results revealed transcriptional aneuploidy response patterns between two genomes and suggested that cis- and trans-mechanisms synergistically regulated alien gene transcriptions after distant hybridization.

Production and cytology of Brassica autoallohexaploids with two and four copies of two subgenomes

Different digenomic Brassica autoallohexaploids were produced from the crosses of three allotetraploids and ancestral diploids and characterized for the cytological behavior of two subgenomes with two and four copies. Interspecific hybridization and allopolyploidization present an important pathway for plant evolution and breeding. In this study, different types of digenomic autoallohexaploids with two or four copies of two subgenomes (AAAACC, AACCCC, AAAABB, BBBBCC, BBCCCC) were synthesized by the crosses between three Brassica allotetraploids and their diploid progenitors and the chromosome doubling, and their meiotic behaviors were analyzed by fluorescence in situ hybridization (FISH). These autoallohexaploids showed some variations in pollen fertility and seed-sets and produced both euploid and aneuploid progenies with some chromosomes lost. Two subgenomes in these autoallohexaploids showed some aberrant pairings and segregations, and the degrees of meiotic regularity were negatively associated with the genome affinities. The chromosomes of the subgenome with four copies formed few quadrivalents with the average number < 2, and mainly paired as bivalents, and majority of the chromosomes from the subgenome with two copies gave the expected bivalents. The different extents of the equal and unequal segregations corresponded to the chromosome pairings. The development and cytological investigation of these autoallohexaploids provide not only the new germplasm for genetic research and breeding but also the new clues for the genome behavior and interplay between these subgenomes with different copies.

Subgenome Discrimination in Brassica and Raphanus Allopolyploids Using Microsatellites

Intergeneric crosses between Brassica species and Raphanus sativus have produced crops with prominent shoot and root systems of Brassica and R. sativus, respectively. It is necessary to discriminate donor genomes when studying cytogenetic stability in distant crosses to identify homologous chromosome pairing, and microsatellite repeats have been used to discriminate subgenomes in allopolyploids. To identify genome-specific microsatellites, we explored the microsatellite content in three Brassica species (B. rapa, AA, B. oleracea, CC, and B. nigra, BB) and R. sativus (RR) genomes, and validated their genome specificity by fluorescence in situ hybridization. We identified three microsatellites showing A, C, and B/R genome specificity. ACBR_msat14 and ACBR_msat20 were detected in the A and C chromosomes, respectively, and ACBR_msat01 was detected in B and R genomes. However, we did not find a microsatellite that discriminated the B and R genomes. The localization of ACBR_msat20 in the 45S rDNA array in ×Brassicoraphanus 977 corroborated the association of the 45S rDNA array with genome rearrangement. Along with the rDNA and telomeric repeat probes, these microsatellites enabled the easy identification of homologous chromosomes. These data demonstrate the utility of microsatellites as probes in identifying subgenomes within closely related Brassica and Raphanus species for the analysis of genetic stability of new synthetic polyploids of these genomes.

Genomic insights into the origin, domestication and diversification of Brassica juncea

Despite early domestication around 3000 BC, the evolutionary history of the ancient allotetraploid species Brassica juncea (L.) Czern & Coss remains uncertain. Here, we report a chromosome-scale de novo assembly of a yellow-seeded B. juncea genome by integrating long-read and short-read sequencing, optical mapping and Hi-C technologies. Nuclear and organelle phylogenies of 480 accessions worldwide supported that B. juncea is most likely a single origin in West Asia, 8,000-14,000 years ago, via natural interspecific hybridization. Subsequently, new crop types evolved through spontaneous gene mutations and introgressions along three independent routes of eastward expansion. Selective sweeps, genome-wide trait associations and tissue-specific RNA-sequencing analysis shed light on the domestication history of flowering time and seed weight, and on human selection for morphological diversification in this versatile species. Our data provide a comprehensive insight into the origin and domestication and a foundation for genomics-based breeding of B. juncea.

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

Integration of a transgene into chromosomes of the C-genomes of oilseed rape (AACC, 2n = 38) may affect their gene flow to wild relatives, particularly Brassica juncea (AABB, 2n = 36). However, no empiric evidence exists in favor of the C-genome as a safer candidate for transformation. In the presence of herbicide selections, the first- to fourth-generation progenies of a B. juncea × glyphosate-tolerant oilseed rape cross [EPSPS gene insertion in the A-genome (Roundup Ready, event RT73)] showed more fitness than a B. juncea × glufosinate-tolerant oilseed rape cross [PAT gene insertion in the C-genome (Liberty Link, event HCN28)]. Karyotyping and fluorescence in situ hybridization-bacterial artificial chromosome (BAC-FISH) analyses showed that crossed progenies from the cultivars with transgenes located on either A- or C- chromosome were mixoploids, and their genomes converged over four generations to 2n = 36 (AABB) and 2n = 37 (AABB + C), respectively. Chromosome pairing of pollen mother cells was more irregular in the progenies from cultivar whose transgene located on C- than on A-chromosome, and the latter lost their C-genome-specific markers faster. Thus, transgene insertion into the different genomes of B. napus affects introgression under herbicide selection. This suggests that gene flow from transgenic crops to wild relatives could be mitigated by breeding transgenic allopolyploid crops, where the transgene is inserted into an alien chromosome.

Investigating genetic relationship of Brassica juncea with B. nigra via virtual allopolyploidy and hexaploidy strategy

Brassica juncea is an important economic crop of the world; however, the narrow genetic base of this crop has tremendously decreased its crop productivity. As an ancestral species of B. juncea, B. nigra is of great importance in widening the genetic diversity of B. juncea. In the present study, 42 SSR markers were employed to screen the genetic diversity among 83 B. nigra, 16 B. juncea, and other Brassica accessions. The molecular characteristics of 498 virtual B. juncea lines were deduced based on the bands of B. nigra and B. rapa via a virtual allopolyploid strategy, and then compared with natural B. juncea accessions. It was found that B. nigra had rich genetic diversity and could be classified into four subgroups, of which subgroup B-III and subgroup B-IV exhibited the closest and the most distant genetic relationship with B. juncea, respectively. To verify this, a hexaploidy strategy was applied to generated synthetic B. juncea from 20 B. nigra accessions, resulting in 45 new-type B. juncea genotypes. The genetic analyses detected that synthetic B. juncea derived from B. nigra in subgroup B-III was close to natural B. juncea, while B. juncea synthesized with B. nigra from subgroup B-IV exhibited wide genetic diversity and was most distant with current B. juncea. This study revealed a great potential of B. nigra in widening genetic diversity of B. juncea particularly using B. nigra in subgroup B-IV, and is helpful in better understanding of the genetic relationship between B. nigra and B. juncea.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-020-01197-7.

A high-contiguity Brassica nigra genome localizes active centromeres and defines the ancestral Brassica genome

It is only recently, with the advent of long-read sequencing technologies, that we are beginning to uncover previously uncharted regions of complex and inherently recursive plant genomes. To comprehensively study and exploit the genome of the neglected oilseed Brassica nigra, we generated two high-quality nanopore de novo genome assemblies. The N50 contig lengths for the two assemblies were 17.1 Mb (12 contigs), one of the best among 324 sequenced plant genomes, and 0.29 Mb (424 contigs), respectively, reflecting recent improvements in the technology. Comparison with a de novo short-read assembly corroborated genome integrity and quantified sequence-related error rates (0.2%). The contiguity and coverage allowed unprecedented access to low-complexity regions of the genome. Pericentromeric regions and coincidence of hypomethylation enabled localization of active centromeres and identified centromere-associated ALE family retro-elements that appear to have proliferated through relatively recent nested transposition events (<1 Ma). Genomic distances calculated based on synteny relationships were used to define a post-triplication Brassica-specific ancestral genome, and to calculate the extensive rearrangements that define the evolutionary distance separating B. nigra from its diploid relatives.

Development of a DNA Barcoding-Like Approach to Detect Mustard Allergens in Wheat Flours

The spread of food allergens is a topic of global importance due to its impact on public health. National and International regulations ask food producers and manufacturers to declare product compositions on the label, especially in case of processed raw materials. Wheat flour (Triticum aestivum) can be contaminated by a wide range of species belonging to the Brassicaceae in the field or during grain harvests, storage, and processing. Among them, mustards (Brassica nigra, Brassica juncea and Sinapis alba) are well known allergenic species. Often, food quality laboratories adopt an ELISA approach to detect the presence of mustard species. However, this approach shows cross-reactivity with other non-allergenic species such as Brassica napus (rapeseed). In the last few years, DNA barcoding was proposed as a valid identification method, and it is now commonly used in the authentication of food products. This study aims to set up an easy and rapid DNA-based tool to detect mustard allergenic species. DNA barcoding (matK and ITS2) and chromosome markers (A6, B, C1 genome regions) were selected, and specific primers were validated on incurred reference food matrices. The developed test was proven to be able to distinguish mustard from rapeseed and wheat, overcoming cross-reactivity with Brassica napus.

Development of Brassica oleracea-nigra monosomic alien addition lines: genotypic, cytological and morphological analyses

We report the development and characterization of Brassica oleracea - nigra monosomic alien addition lines (MAALs) to dissect the Brassica B genome. Brassica nigra (2n = 16, BB) represents the diploid Brassica B genome which carries many useful genes and traits for breeding but received limited studies. To dissect the B genome from B. nigra, the triploid F₁ hybrid (2n = 26, CCB) obtained previously from the cross B. oleracea var. alboglabra (2n = 18, CC) × B. nigra was used as the maternal parent and backcrossed successively to parental B. oleracea. The progenies in BC₁ to BC₃ generations were analyzed by the methods of FISH and SSR markers to screen the monosomic alien addition lines (MAALs) with each of eight different B-genome chromosomes added to C genome (2n = 19, CC + 1B_1-8), and seven different MAALs were established, except for the one with chromosome B2 which existed in one triple addition. Most of these MAALs were distinguishable morphologically from each other, as they expressed the characters from B. nigra differently and at variable extents. The alien chromosome remained unpaired as a univalent in 86.24% pollen mother cells at diakinesis or metaphase I, and formed a trivalent with two C-genome chromosomes in 13.76% cells. Transmission frequency of all the added chromosomes was far higher through the ovules (averagely 14.40%) than the pollen (2.64%). The B1, B4 and B5 chromosomes were transmitted by female at much higher rates (22.38-30.00%) than the other four (B3, B6, B7, B8) (5.04-8.42%). The MAALs should be valuable for exploiting the genome structure and evolution of B. nigra.

A multiplex PCR for rapid identification of Brassica species in the triangle of U

BACKGROUND

Within the Brassicaceae, six species from the genus Brassica are widely cultivated throughout the world as oilseed, condiment, fodder or vegetable crops. The genetic relationships among the six Brassica species are described by U's triangle model. Extensive shared traits and diverse morphotypes among Brassica species make identification and classification based on phenotypic data alone challenging and unreliable, especially when dealing with large germplasm collections. Consequently, a major issue for genebank collections is ensuring the correct identification of species. Molecular genotyping based on simple sequence repeat (SSR) marker sequencing or the Illumina Infinium Brassica napus 60K single nucleotide polymorphism (SNP) array has been used to identify species and assess genetic diversity of Brassica collections. However, these methods are technically challenging, expensive and time-consuming, making them unsuitable for routine or rapid screening of Brassica accessions for germplasm management. A cheaper, faster and simpler method for Brassica species identification is described here.

RESULTS

A multiplex polymerase chain reaction (MPCR) consisting of new and existing primers specific to the Brassica A, B and C genomes was able to reliably distinguish all six Brassica species in the triangle of U with 16 control samples of known species identity. Further validation against 120 Brassica accessions previously genotyped showed that the MPCR is highly accurate and comparable to more advanced techniques such as SSR marker sequencing or the Illumina Infinium B. napus 60K SNP array. In addition, the MPCR was sensitive enough to detect seed contaminations in pooled seed samples of Brassica accessions.

CONCLUSION

A cheap and fast multiplex PCR assay for identification of Brassica species in the triangle of U was developed and validated in this study. The MPCR assay can be readily implemented in any basic molecular laboratory and should prove useful for the management of Brassica germplasm collections in genebanks.

Cytogenetic and Molecular Characterization of B-Genome Introgression Lines of Brassica napus L

Brassica napus introgression lines (ILs), having B-genome segments from B. carinata, were assessed genetically for extent of introgression and phenotypically for siliqua shatter resistance. Introgression lines had 7-9% higher DNA content, were meiotically stable, and had almost normal pollen fertility/seed set. Segment introgressions were confirmed by fluorescent genomic in situ hybridization (fl-GISH), SSR analyses, and SNP studies. Genotyping with 48 B-genome specific SSRs detected substitutions from B3, B4, B6, and B7 chromosomes on 39 of the 69 ILs whereas SNP genotyping detected a total of 23 B-segments (≥3 Mb) from B4, B6, and B7 introgressed into 10 of the 19 (C1, C2, C3, C5, C6, C8, C9, A3, A9, A10) chromosomes in 17 ILs. The size of substitutions varied from 3.0 Mb on chromosome A9 (IL59) to 42.44 Mb on chromosome C2 (IL54), ranging from 7 to 83% of the recipient chromosome. Average siliqua strength in ILs was observed to be higher than that of B. napus parents (2.2-6.0 vs. 1.9-4.0 mJ) while siliqua strength in some of the lines was almost equal to that of the donor parent B. carinata (6.0 vs.7.2 mJ). These ILs, with large chunks of substituted B-genome, can prove to be a useful prebreeding resource for germplasm enhancement in B. napus, especially for siliqua shatter resistance.

Distinct subgenome stabilities in synthesized Brassica allohexaploids

Trigenomic Brassica allohexaploids synthesized from three crossing strategies showed diploidized and non-diploidized meiotic behaviors and produced both euploid and aneuploid progenies during successive generations, revealing the distinct subgenome stabilities (B > A> C). Three cultivated allotetraploid Brassica species (Brassica napus, B. juncea, B. carinata) represent the model system of speciation through interspecific hybridization and allopolyploidization, but no Brassica species at higher ploidy level exists in nature. In this study, Brassica allohexaploids (2n = 54, AABBCC) were artificially synthesized using three crossing strategies, and had combinations of the genomes from the extant allotetraploids and diploids (B. rapa, B. oleracea and B. nigra). The chromosome numbers and complements of these allohexaploids and the self-pollinated progenies of successive generations (S0-S7) were determined using multicolor fluorescent in situ hybridization that distinguished the chromosomes of three constituent genomes from each other. Both euploid and aneuploid progenies were identified. The most aneuploids maintained all B- and A-genome chromosomes and variable number of C-genome chromosomes, suggesting that genome stability was B > A > C. In the extreme case, loss of whole set of C-genome chromosomes led to the production of B. juncea-type progeny. Some aneuploid progenies had the same number of chromosomes (2n = 54) as the euploid, but the simultaneous loss and gain of A- and C-genome chromosomes. The diploidized and non-diploidized meiotic behaviors co-occurred in all allohexaploid individuals of consecutive generations. The aberrant chromosome pairing and segregation mainly involved the chromosomes of A and C genomes, which resulted in aneuploidy in self-pollinated progenies. The mechanisms for the differential stability of three genomes and the stabilization of the new allohexaploids are discussed.

Cloning of TTG1 gene and PCR identification of genomes A, B and C in Brassica species

Arabidopsis Transparent Testa Glabra 1 (TTG1) genes were cloned from three diploid Brassica species (B. rapa, B. nigra and B. oleracea) and two amphidiploids species (B. juncea and B. carinata) by homology cloning. TTG1 homologues identified in all the accessions of the investigated species had a coding sequence of 1,014 bp. One copy was obtained from each diploid species and two copies from each amphidiploid species. Combined analysis of the TTG1 sequences cloned in this study with those obtained from public databases demonstrated that three, forty-five and seven nucleotides were specific variations in TTG1 genes from genomes A, B and C, respectively. Primers designed with genome-specific nucleotide variations were able to distinguish among TTG1 genes originating from genomes A, B and C in Brassica. Therefore, the TTG1 gene could serve as a candidate marker gene to detect the pollen flow of Brassica and provide an alternative method for the detection of pollen drift and risk assessment of gene flow in Brassica species.

Seed coat microsculpturing is related to genomic components in wild Brassica juncea and Sinapis arvensis

It has been reported that wild Brassica and related species are widely distributed across Xinjiang, China, and there has been an argument for species identification. Seed coat microsculpturing (SCM) is known to be an excellent character for taxonomic and evolutionary studies. By identifying collections from Xinjiang, China, and combining SCM pattern, flow cytometry, and genome-specific DNA markers as well as sexual compatibility with known species, this study aimed to detect potential relationships between SCM and genomic types in wild Brassica and related species. Three wild collections were found to be tetraploid with a SCM reticulate pattern similar to B. juncea, and containing A and B genome-specific loci, indicating relatively high sexual compatibility with B. juncea. The others were diploid, carrying S-genome-specific DNA markers, and having relatively high sexual compatibility with Sinapis arvensis. Moreover, their SCM was in a rugose pattern similar to that of S. arvensis. It was suggested that SCM, as a morphological characteristic, can reflect genomic type, and be used to distinguish B-genome species such as B. juncea from the related S. arvensis. The relationship between SCM and genomic type can support taxonomic studies of the wild Brassica species and related species.

Comparative genomic in situ hybridization (cGISH) analysis of the genomic relationships among Sinapis arvensis, Brassica rapa and Brassica nigra

To further understand the relationships between the SS genome of Sinapis arvensis and the AA, BB genomes in Brassica, genomic DNA of Sinapis arvensis was hybridized to the metaphase chromosomes of Brassica nigra (BB genome), and the metaphase chromosomes and interphase nucleus of Brassica rapa (AA genome) by comparative genomic in situ hybridization (cGISH). As a result, every chromosome of B. nigra had signals along the whole chromosomal length. However, only half of the condensed heterochromatic areas in the interphase nucleus and the chromosomes showed rich signals in Brassica rapa. Interphase nucleus and the metaphase chromosomes of S. arvensis were simultaneously hybridized with digoxigenin-labeled genomic DNA of B. nigra and biotin-labeled genomic DNA of B. rapa. Signals of genomic DNA of B. nigra hybridized throughout the length of all chromosomes and all the condensed heterochromatic areas in the interphase nucleus, except chromosome 4, of which signals were weak in centromeric regions. Signals of the genomic DNA of B. rapa patterned the most areas of ten chromosomes and ten condensed heterochromatic areas, others had less signals. The results showed that the SS genome had homology with AA and BB genomes, but the homology between SS genome and AA genome was clearly lower than that between the SS genome and BB genome.

Rapid divergence of repetitive DNAs in Brassica relatives

Centromeric, subtelomeric, and telomeric repetitive DNAs were characterized in Brassica species and the related Raphanus sativus and Arabidopsis thaliana. In general, rapid divergence of the repeats was found. The centromeric tandem satellite repeats were differentially distributed in the species studied, suggesting that centromeric repeats have diverged during the evolution of the A/C and B genome lineages. Sequence analysis of centromeric repeats suggested rapid evolution. Pericentromere-associated retrotransposons were identified and showed divergence during the evolution of the lineages as centromeric repeats. A novel subtelomeric tandem repeat from B. nigra was found to be conserved across the diploid Brassica genomes; however, this sequence was not identified in the related species. In contrast to previous studies, interstitial telomere-like repeats were identified in the pericentromeres of Brassica chromosomes, and these repeats may be associated with genomic stability. These results provide insight into genome evolution during polyploidization in Brassica and divergence within the Brassicaceae.

Development of genome-specific 5S rDNA markers in Brassica and related species for hybrid testing

The Brassicaceae are targets for DNA manipulation to modify oil content and composition. However, any strategy for creating novel products using genetic modification or traditional breeding must take into account the potential for hybridization with other Brassica species, many of which are important sources of edible oils. In this study we have tested Brassica carinata, a possible target for oil modification, to establish whether it can cross with other Brassica species and related genera, and we have developed molecular DNA assays to confirm hybridization.

Different genome-specific chromosome stabilities in synthetic Brassica allohexaploids revealed by wide crosses with Orychophragmus

BACKGROUND AND AIMS

In sexual hybrids between cultivated Brassica species and another crucifer, Orychophragmus violaceus (2n = 24), parental genome separation during mitosis and meiosis is under genetic control but this phenomenon varies depending upon the Brassica species. To further investigate the mechanisms involved in parental genome separation, complex hybrids between synthetic Brassica allohexaploids (2n = 54, AABBCC) from three sources and O. violaceus were obtained and characterized.

METHODS

Genomic in situ hybridization, amplified fragment length polymorphism (AFLP) and single-strand conformation polymorphism (SSCP) were used to explore chromosomal/genomic components and rRNA gene expression of the complex hybrids and their progenies.

KEY RESULTS

Complex hybrids with variable fertility exhibited phenotypes that were different from the female allohexaploids and expressed some traits from O. violaceus. These hybrids were mixoploids (2n = 34-46) and retained partial complements of allohexaploids, including whole chromosomes of the A and B genomes and some of the C genome but no intact O. violaceus chromosomes; AFLP bands specific for O. violaceus, novel for two parents and absent in hexaploids were detected. The complex hybrids produced progenies with chromosomes/genomic complements biased to B. juncea (2n = 36, AABB) and novel B. juncea lines with two genomes of different origins. The expression of rRNA genes from B. nigra was revealed in all allohexaploids and complex hybrids, showing that the hierarchy of nucleolar dominance (B. nigra, BB > B. rapa, AA > B. oleracea, CC) in Brassica allotetraploids was still valid in these plants.

CONCLUSIONS

The chromosomes of three genomes in these synthetic Brassica allohexaploids showed different genome-specific stabilities (B > A > C) under induction of alien chromosome elimination in crosses with O. violaceus, which was possibly affected by nucleolar dominance.

Enhancement of genetic diversity in canola-quality Brassica napus and B. juncea by interspecific hybridisation

Reciprocal crosses were made between Brassica napus cv. Mystic (canola) and B. juncea JN29 (near canola quality). The F1 hybrids were selfed and backcrossed in all possible combinations to parent plants. The greatest number of selfed fertile progeny were obtained when Mystic was the maternal parent, and its F1 was most successful in backcrosses to Mystic or JN29 as maternal or paternal parent. The predominant morphological type of fertile progeny was B. napus, but several B. juncea morphological types occurred in F2 and BC1-derived lines. F2 : 3 and BC1S0 : 1 progeny showed transgressive segregation for agronomic and seed quality traits in two contrasting field environments. Several of the B. juncea-type progeny had improved seed quality (lower total seed glucosinolates and higher % oleic acid) over the B. juncea parent. Selfing of interspecific hybrids between canola-quality B. napus and B. juncea has the potential to greatly enhance genetic diversity in canola-quality progeny of both species, without the loss of donor alleles that normally occurs with repeated backcrossing.

Cytogenetics and genome analysis in Brassica crops

The genus Brassica contains a wide range of diploid and amphipolyploid species including some of the most important vegetable, condiment and oilseed crops worldwide. As members of the Brassicaceae family the brassicas are the closest crop relatives to the model plant Arabidopsis thaliana, and hence are major beneficiaries from the vast array of Arabidopsis molecular genetic and genomic tools and the increasingly good annotation to major Brassica crop genomes. In this review examples are shown from recent studies that demonstrate the potential for intergenome navigation from model to crop plant and for comparisons among genetic and cytogenetic maps between the model and crop species and among different crop brassicas. The use of interspecific and intergeneric hybridization for introgression of novel traits into Brassica genomes from the secondary and tertiary crucifer genepools is described. In this context the use of the Brassica triangle of three diploid species and their corresponding amphiploids as an excellent model system for studying the mechanisms and control of homeologous recombination and polyploidization is discussed from a crop breeding perspective.

Genetic mapping of agronomic traits in false flax (Camelina sativa subsp.sativa)

The crucifer oilseed plant false flax (Camelina sativa subsp. sativa) possesses numerous valuable agronomic attributes that make it attractive as an alternative spring-sown crop for tight crop rotations. The oil of false flax is particularly rich in polyunsaturated C18-fatty acids, making it a valuable renewable feedstock for the oleochemical industry. Because of the minimal interest in the crop throughout the 20th century, breeding efforts have been limited. In this study, a genetic map for C. sativa was constructed, using amplified fragment length polymorphism (AFLP) markers, in a population of recombinant inbred lines that were developed, through single-seed descent, from a cross between ‘Lindo’ and ‘Licalla’, 2 phenotypically distinct parental varieties. Three Brassica simple sequence repeat (SSR) markers were also integrated into the map, and 1 of these shows linkage to oil-content loci in both C. sativa and Brassica napus. Fifty-five other SSR primer combinations showed monomorphic amplification products, indicating partial genome homoeology with the Brassica species. Using data from field trials with different fertilization treatments (0 and 80 kg N/ha) at multiple locations over 3 years, the map was used to localize quantitative trait loci (QTLs) for seed yield, oil content, 1000-seed mass, and plant height. Some yield QTLs were found only with the N0 treatment, and might represent loci contributing to the competitiveness of false flax in low-nutrient soils. The results represent a starting point for future marker-assisted breeding.

Tracing B-genome chromatin in Brassica napus × B. juncea interspecific progeny

We used polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) techniques to demonstrate the presence of Brassica B-genome chromosomes and putative B-genome introgressions in B. napus × B. juncea interspecific progeny. The B-genome - specific repeat sequence pBNBH35 was used to generate PCR products and FISH probes. The highest frequencies of viable progeny were obtained when B. napus was the maternal parent of the interspecific hybrid and the first backcross. B-genome - positive PCR assays were found in 34/51 fertile F2 progeny (67%), which was more than double the proportion found in fertile BC1 progeny. Four B-genome - positive F2-derived families and 1 BC1-derived family were fixed or segregating for B. juncea morphology in the F4 and BC1S2, respectively, but in only 2 of these families did B. juncea-type plants exhibit B. juncea chromosome count (2n = 36) and typical B-genome FISH signals on 16 chromosomes. The remaining B. juncea-type plants had B. napus chromosome count (2n = 38) and no B-genome FISH signals, except for 1 exceptional F4-derived line that exhibited isolated and weak B-genome FISH signals on 11 chromosomes and typical A-genome FISH signals. B. juncea morphology was associated with B-genome - positive PCR signals but not necessarily with 16 intact B-genome chromosomes as detected by FISH. B-genome chromosomes tend to be eliminated during selfing or backcrossing after crossing B. juncea with B. napus, and selection of lines containing B-genome chromatin during early generations would be promoted by use of this B-genome repetitive marker.

Molecular cytogenetic analysis of Brassica rapa-Brassica oleracea var. alboglabra monosomic addition lines

Interspecific alien chromosome addition lines can be very useful for gene mapping and studying chromosome homoeology between closely related species. In this study we demonstrate a simple but robust manner of identifying individual C-genome chromosomes (C5, C8 and C9) in the A-genome background through the simultaneous use of 5S and 25S ribosomal probes on mitotic and meiotic chromosomes of three different Brassica rapa-B. oleracea var. alboglabra monosomic addition lines. Sequential silver staining and fluorescence in situ hybridisation indicated that 18S-5.8S-25S rRNA genes on the additional chromosome C9 are expressed in the A-genome background. Meiotic behaviour of the additional chromosomes was studied in pollen mother cells at diakinesis and metaphase I. In all of the addition lines the alien chromosome was most frequently observed as a univalent. The alien chromosome C5, which carries an intercalary 5S rDNA locus, occasionally formed trivalents that involved either rDNA- or non rDNA-carrying chromosomes from the A genome. In the case of chromosomes C8 and C9, the most frequently observed intergenomic associations involved the regions occupied by 18S-5.8S-25S ribosomal RNA genes. It is possible that not all such associations represent true pairing but are remnants of nucleolar associations from the preceding interphase. Variations in the numbers and distribution of 5S and 25S rDNA sites between cultivars of B. oleracea, B. oleracea var. alboglabra and B. rapa are discussed.