Cytogenetics and genome analysis in Brassica crops

A wide range of chromosome numbers result from unreduced gamete production in Brassica juncea × B. napus (AABC) interspecific hybrids

The establishment of successful interspecies hybrids requires restoration of a stable "2n" chromosome complement which can produce viable "n" gametes. This may occur (rarely) via recombination between non-homologous chromosomes, or more commonly is associated with a doubling of parental chromosome number to produce new homologous pairing partners in the hybrid. The production of unreduced "2n" gametes (gametes with the somatic chromosome number) may therefore be evolutionarily useful by serving as a key pathway for the formation of new polyploid hybrids, as might specific mechanisms permitting recombination between non-homologous chromosomes. Here, we investigated chromosome complements and fertility in third generation interspecific hybrids (AABC) resulting from a cross between allopolyploids Brassica juncea (AABB) × B. napus (AACC) followed by self-pollination for two generations. Chromosome numbers ranged from 2n = 48-74 in the experimental population (35 plants), with 9-16 B genome chromosomes and up to 4 copies of A genome chromosomes. Unreduced gamete production leading to a putative genome structure of approximately AAAABBCC was hence predicted to explain the high chromosome numbers observed. Additionally, the estimation of nuclei number in post-meiotic sporads revealed a higher frequency of unreduced gametes (0.04-5.21%) in the third generation AABC interspecific hybrids compared to the parental Brassica juncea (0.07%) and B. napus (0.13%). Our results suggest that unreduced gamete production in the subsequent generations following interspecific hybridization events may play a critical role in restoration of more stable, fertile chromosome complements.

ZF-HD gene family in rapeseed (Brassica napus L.): genome-wide identification, phylogeny, evolutionary expansion and expression analyses

BACKGROUND

The zinc finger-homeodomain (ZF-HD) transcription factor family is widely involved in regulating plant growth and fruit filling, as well as responding to various abiotic stress. Rapeseed (Brassica napus L.), the second largest oil-producing crop in the world, is an annual or biennial herb of Brassica in Cruciferae. However, there is currently no systematic study on the evolutionary relationship and stress response of ZF-HD transcription factors in rapeseed.

RESULTS

In this study, 60 ZF-HD genes in B. napus (BnZHDs) were identified and named based on the chromosomal location. The evolutionary relationships, classifications, gene structures, motif compositions, chromosome localization, and gene replication events in these BnZHD genes were systematically analyzed. These 60 BnZHD members were divided into seven groups. According to the phylogenetic tree and repetitive events, subfamilies MIF, and V may have undergone stronger expansions during the evolutionary process. Interestingly, segmental duplications may have a more important contribution, which distinguishes them from other dicotyledon plants. To further investigate the evolutionary relationship of the ZF-HD family, we constructed eleven comparative genomic maps of homologous genes between rapeseed and different representative monocotyledonous and dicotyledonous plants. Finally, the gene expression pattern of 15 BnZHD genes from different subfamilies under different tissues, fruit developmental stages, and different abiotic stress were analyzed. The expression profile from real-time quantitative PCR analysis showed different expression patterns of BnZHD gene in B. napus. We found that certain BnZHD genes are preferentially expressed in specific tissues of B. napus, while most genes are expressed in multiple tissues. For example, BnZHD37, BnZHD53, and BnZHD55 may be sensitive to different hormones. Under different stresses, the expression of BnZHD3, BnZHD4, BnZHD7, BnZHD38, BnZD45, and BnZHD53 significantly increased in roots, stems, and leaves within 24 h. These genes may play important roles in the growth, development, and environmental adaptation of rapeseed.

CONCLUSIONS

These findings provide a basis for a comprehensive understanding of the ZF-HD family in rapeseed, which will provide information for further research on the functional characteristics of the BnZHD genes.

Integrated cytological and transcriptomic analyses provide new insights into restoration of pollen viability in synthetic allotetraploid Brassica carinata

KEY MESSAGE

Upregulation of genes involved in DNA damage repair and sperm cell differentiation leads to restoration of pollen viability in synthetic allotetraploid B. carinata after chromosome doubling. Apart from the well-known contribution of polyploidy to crop improvement, polyploids can also be induced for other purposes, such as to restore the viability of sterile hybrids. The mechanism related to viability transition between the sterile allodiploid and the fertile allotetraploid after chromosome doubling are not well understood. Here, we synthesised allodiploid B. carinata (2n = 2x = 17) and allotetraploid B. carinata (2n = 4x = 34) as models to investigate the cytological and transcriptomic differences during pollen development. The results showed that after chromosome doubling, the recovery of pollen viability in allotetraploid was mainly reflected in the stabilisation of microtubule spindle morphology, normal meiotic chromosome behaviour, and normal microspore development. Interestingly, the deposition and degradation of synthetic anther tapetum were not affected by polyploidy. Transcription analysis showed that the expression of genes related to DNA repair (DMC1, RAD51, RAD17, SPO11-2), cell cycle differentiation (CYCA1;2, CYCA2;3) and ubiquitination proteasome pathway (UBC4, PIRH2, CDC53) were positively up-regulated during pollen development of synthetic allotetraploid B. carinata. In summary, these results provide some refreshing updates about the ploidy-related restoration of pollen viability in newly synthesised allotetraploid B. carinata.

Injection-based hairy root induction and plant regeneration techniques in Brassicaceae

BACKGROUND

Hairy roots constitute a valuable tissue culture system for species that are difficult to propagate through conventional seed-based methods. Moreover, the generation of transgenic plants derived from hairy roots can be facilitated by employing carefully designed hormone-containing media.

RESULTS

We initiated hairy root formation in the rare crucifer species Asperuginoides axillaris via an injection-based protocol using the Agrobacterium strain C58C1 harboring a hairy root-inducing (Ri) plasmid and successfully regenerated plants from established hairy root lines. Our study confirms the genetic stability of both hairy roots and their derived regenerants and highlights their utility as a permanent source of mitotic chromosomes for cytogenetic investigations. Additionally, we have developed an effective embryo rescue protocol to circumvent seed dormancy issues in A. axillaris seeds. By using inflorescence primary stems of Arabidopsis thaliana and Cardamine hirsuta as starting material, we also established hairy root lines that were subsequently used for regeneration studies.

CONCLUSION

We developed efficient hairy root transformation and regeneration protocols for various crucifers, namely A. axillaris, A. thaliana, and C. hirsuta. Hairy roots and derived regenerants can serve as a continuous source of plant material for molecular and cytogenetic analyses.

Phaseolus vulgaris mutants reveal variation in the nuclear genome

Phaseolus vulgaris L. (common bean) is an essential source of proteins in the human diet worldwide. Bean breeding programs to increase genetic diversity based on induced mutagenesis have a long tradition in Bulgaria. Common bean varieties with high productivity, wide environmental adaptability, good nutritional properties, and improved disease resistance have been successfully developed. In this study, we aimed to investigate selected nuclear genome features, such as the genome size, the number and chromosomal distribution of 5S and 35S rDNA loci by using the fluorescence in situ hybridization (FISH), as well as the level of DNA damage in some local Bulgarian accessions and mutants of P. vulgaris. Flow cytometry analyses revealed no significant differences in genome size between analyzed lines except for one of the analyzed mutants, M19. The value of genome size 2C DNA is about 1.37 pg2C -1 for all lines, whereas it is 1.42 pg2C-1 for M19. The chromosome number remains the same (2n=22) for all analyzed lines. Results of FISH analyses showed that the number of 5S rDNA was stable among accessions and mutant lines (four loci), while the number of 35S rDNA loci was shown as highly polymorphic, varying between ten and sixteen, and displaying differences in the size and location of 35S rDNA loci between analyzed genotypes. The cell cycle profile was different for the analyzed genotypes. The results revealed that wide variation in genome organization and size as well as DNA damage characterizes the analyzed genetic resources of the common bean.

Karyotyping of aneuploid and polyploid plants from low coverage whole-genome resequencing

BACKGROUND

Karyotype, as a basic characteristic of species, provides valuable information for fundamental theoretical research and germplasm resource innovation. However, traditional karyotyping techniques, including fluorescence in situ hybridization (FISH), are challenging and low in efficiency, especially when karyotyping aneuploid and polyploid plants. The use of low coverage whole-genome resequencing (lcWGR) data for karyotyping was explored, but existing methods are complicated and require control samples.

RESULTS

In this study, a new protocol for molecular karyotype analysis was provided, which proved to be a simpler, faster, and more accurate method, requiring no control. Notably, our method not only provided the copy number of each chromosome of an individual but also an accurate evaluation of the genomic contribution from its parents. Moreover, we verified the method through FISH and published resequencing data.

CONCLUSIONS

This method is of great significance for species evolution analysis, chromosome engineering, crop improvement, and breeding.

A novel visual marker to distinguish haploids from doubled haploids in rice (Oryza sativa, L) at early growth stages

Doubled haploid technology, which enables the generation of homozygous lines in a single step, is one of the modern tools being employed for accelerating breeding processes in different crops. In rice, a globally important staple food crop, doubled haploid production through androgenesis is increasingly being employed in breeding programs. Amongst the androgenic rice lines, doubled haploids are formed spontaneously at about 50-60%, while the remaining 40-50% of plants remain as haploids. As haploids cannot be easily identified, it is routine to grow all the rice androgenic lines till maturity and harvest the seeds from the fertile doubled haploids. Therefore, the methods that facilitate easy identification of haploids at an early developmental stage in rice would enable treatment of such haploid lines with colchicine, to increase the efficiency of doubled haploid production. Further, it would also help in eliminating the operational cost involved in maintaining them till maturity. In the above context, a systematic study to identify easily observable physiological and morphological differences between haploid and doubled haploid rice lines was undertaken. Rice haploids were found to be noticeably different from doubled haploids in photosynthetic rate, transpiration rate, stomatal conductance, and morphology of lodicules, stigma and style, features which have not been reported before. Most importantly, rice haploids invariably have acute leaf apex which is easily distinguishable from the doubled haploids that have attenuated leaf apex shape. Very high per cent accuracy in the prediction of ploidy level was observed when haploids were identified at an early developmental stage based on leaf apex shape, and the results verified with flow cytometry perfectly matches with leaf apex shape. The study establishes 'acute leaf apex' shape as an accurate visual marker to rapidly identify haploid rice lines at an early developmental stage in a cost-effective manner.

Anthocyanins identification and transcriptional regulation of anthocyanin biosynthesis in purple Brassica napus

The main anthocyanin components were identified, and the transcriptional regulation pattern of anthocyanin related genes in leaves and stem bark was elucidated in a purple B. napus. Brassica napus is one of the most important oil crops planted worldwide, and developing varieties of dual-purpose for oil and vegetable is beneficial to improve economic benefits. Anthocyanins are a class of secondary metabolites that not only make plants present beautiful colors, but have a variety of important physiological functions and biological activities. Therefore, increasing the accumulation of anthocyanin in vegetative organs can improve vegetable value of rapeseed. However, anthocyanin enriched varieties in vegetative organs are rare, and there are few studies on category identification and accumulation mechanism of anthocyanin, which limits the utilization of anthocyanins in B. napus. In this study, 157 anthocyanin biosynthesis related genes (ABGs) were identified in B. napus genome by homology comparison and collinearity analysis of genes related to anthocyanin synthesis and regulation in Arabidopsis. Moreover, five anthocyanins were identified in the stem bark and leaves of the purple B. napus PR01 by high performance liquid chromatography-mass spectrometry (HPLC-MS), and the expression characteristics of ABGs in the leaves and stem bark of PR01 were analyzed and compared with the green cultivar ZS11 by RNA-Seq. Combining further weighted gene co-expression network analysis (WGCNA), the up-regulation of transcript factors BnaA07. PAP2 and BnaC06. PAP2 were identified as the key to the up-regulation of most of anthocyanin synthesis genes that promoted anthocyanin accumulation in PR01. This study is helpful to understand the transcriptional regulation of anthocyanin biosynthesis in B. napus and provides the theoretical basis for breeding novel varieties of dual-purpose for oil and vegetable.

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.

Genome-Wide Characterization of DGATs and Their Expression Diversity Analysis in Response to Abiotic Stresses in Brassica napus

Triacylglycerol (TAG) is the most important storage lipid for oil plant seeds. Diacylglycerol acyltransferases (DGATs) are a key group of rate-limiting enzymes in the pathway of TAG biosynthesis. In plants, there are three types of DGATs, namely, DGAT1, DGAT2 and DGAT3. Brassica napus, an allotetraploid plant, is one of the most important oil plants in the world. Previous studies of Brassica napus DGATs (BnaDGATs) have mainly focused on BnaDGAT1s. In this study, four DGAT1s, four DGAT2s and two DGAT3s were identified and cloned from B. napus ZS11. The analyses of sequence identity, chromosomal location and collinearity, phylogenetic tree, exon/intron gene structures, conserved domains and motifs, and transmembrane domain (TMD) revealed that BnaDGAT1, BnaDGAT2 and BnaDGAT3 were derived from three different ancestors and shared little similarity in gene and protein structures. Overexpressing BnaDGATs showed that only four BnaDGAT1s can restore TAG synthesis in yeast H1246 and promote the accumulation of fatty acids in yeast H1246 and INVSc1, suggesting that the three BnaDGAT subfamilies had greater differentiation in function. Transcriptional analysis showed that the expression levels of BnaDGAT1s, BnaDGAT2s and BnaDGAT3s were different during plant development and under different stresses. In addition, analysis of fatty acid contents in roots, stems and leaves under abiotic stresses revealed that P starvation can promote the accumulation of fatty acids, but no obvious relationship was shown between the accumulation of fatty acids with the expression of BnaDGATs under P starvation. This study provides an extensive evaluation of BnaDGATs and a useful foundation for dissecting the functions of BnaDGATs in biochemical and physiological processes.

Biotechnological approach for improvement of Crambe species as valuable oilseed plants for industrial purposes

Boosting technological innovation for a sustainable and circular bioeconomy encompasses the use of renewable materials and development of highly effective biotechnological approaches to improve the quality of oilseed crops and facilitate their industrial deployment. The interest in cultivating Crambe as a potential crop is steadily growing due to its low propensity to crossbreeding with other oilseed crops, valuable seed oil composition and a high yield capacity. The main focus is located on Crambe abyssinica as the most adapted into the agriculture and well-studied Crambe species. At the same time, the Crambe genus is one of the most numerous of the Brassicaceae family featuring several underestimated (orphaned) species with useful traits (abiotic stress tolerance, wide range of practical applications). This review features progress in the biotechnological improvement of well-adapted and wild Crambe species starting with aseptic culture establishment and plant propagation in vitro reinforced with the use of genetic engineering and breeding techniques. The aim of the paper is to highlight and review the existing biotechnological methods of both underestimated and well-adapted Crambe species improvment, including the establishment of aseptic culture, in vitro cultivation, plant regeneration and genetic transformation to modify seed oil content and morphological traits of valuable species.

Advances in the Molecular Cytogenetics of Bananas, Family Musaceae

The banana is a staple food crop and represents an important trade commodity for millions of people living in tropical and subtropical countries. The most important edible banana clones originated from natural crosses between diploid Musa balbisiana and various subspecies of M. acuminata. It is worth mentioning that evolution and speciation in the Musaceae family were accompanied by large-scale chromosome structural changes, indicating possible reasons for lower fertility or complete sterility of these vegetatively propagated clones. Chromosomal changes, often accompanied by changes in genome size, are one of the driving forces underlying speciation in plants. They can clarify the genomic constitution of edible bananas and shed light on their origin and on diversification processes in members of the Musaceae family. This article reviews the development of molecular cytogenetic approaches, ranging from classical fluorescence in situ hybridization (FISH) using common cytogenetic markers to oligo painting FISH. We discuss differences in genome size and chromosome number across the Musaceae family in addition to the development of new chromosome-specific cytogenetic probes and their use in genome structure and comparative karyotype analysis. The impact of these methodological advances on our knowledge of Musa genome evolution at the chromosomal level is demonstrated. In addition to citing published results, we include our own new unpublished results and outline future applications of molecular cytogenetics in banana research.

Using wild relatives and related species to build climate resilience in Brassica crops

Climate change will have major impacts on crop production: not just increasing drought and heat stress, but also increasing insect and disease loads and the chance of extreme weather events and further adverse conditions. Often, wild relatives show increased tolerances to biotic and abiotic stresses, due to reduced stringency of selection for yield and yield-related traits under optimum conditions. One possible strategy to improve resilience in our modern-day crop cultivars is to utilize wild relative germplasm in breeding, and attempt to introgress genetic factors contributing to greater environmental tolerances from these wild relatives into elite crop types. However, this approach can be difficult, as it relies on factors such as ease of hybridization and genetic distance between the source and target, crossover frequencies and distributions in the hybrid, and ability to select for desirable introgressions while minimizing linkage drag. In this review, we outline the possible effects that climate change may have on crop production, introduce the Brassica crop species and their wild relatives, and provide an index of useful traits that are known to be present in each of these species that may be exploitable through interspecific hybridization-based approaches. Subsequently, we outline how introgression breeding works, what factors affect the success of this approach, and how this approach can be optimized so as to increase the chance of recovering the desired introgression lines. Our review provides a working guide to the use of wild relatives and related crop germplasm to improve biotic and abiotic resistances in Brassica crop species.

Using wild relatives and related species to build climate resilience in Brassica crops

Climate change will have major impacts on crop production: not just increasing drought and heat stress, but also increasing insect and disease loads and the chance of extreme weather events and further adverse conditions. Often, wild relatives show increased tolerances to biotic and abiotic stresses, due to reduced stringency of selection for yield and yield-related traits under optimum conditions. One possible strategy to improve resilience in our modern-day crop cultivars is to utilize wild relative germplasm in breeding, and attempt to introgress genetic factors contributing to greater environmental tolerances from these wild relatives into elite crop types. However, this approach can be difficult, as it relies on factors such as ease of hybridization and genetic distance between the source and target, crossover frequencies and distributions in the hybrid, and ability to select for desirable introgressions while minimizing linkage drag. In this review, we outline the possible effects that climate change may have on crop production, introduce the Brassica crop species and their wild relatives, and provide an index of useful traits that are known to be present in each of these species that may be exploitable through interspecific hybridization-based approaches. Subsequently, we outline how introgression breeding works, what factors affect the success of this approach, and how this approach can be optimized so as to increase the chance of recovering the desired introgression lines. Our review provides a working guide to the use of wild relatives and related crop germplasm to improve biotic and abiotic resistances in Brassica crop species.

Chromosome inheritance and meiotic stability in allopolyploid Brassica napus

Homoeologous recombination, aneuploidy, and other genetic changes are common in resynthesized allopolyploid Brassica napus. In contrast, the chromosomes of cultivars have long been considered to be meiotically stable. To gain a better understanding of the underlying mechanisms leading to stabilization in the allopolyploid, the behavior of chromosomes during meiosis can be compared by unambiguous chromosome identification between resynthesized and natural B. napus. Compared with natural B. napus, resynthesized lines show high rates of nonhomologous centromere association, homoeologous recombination leading to translocation, homoeologous chromosome replacement, and association and breakage of 45S rDNA loci. In both natural and resynthesized B. napus, we observed low rates of univalents, A–C bivalents, and early sister chromatid separations. Reciprocal homoeologous chromosome exchanges and double reductions were photographed for the first time in meiotic telophase I. Meiotic errors were non-uniformly distributed across the genome in resynthesized B. napus, and in particular homoeologs sharing synteny along their entire length exhibited multivalents at diakinesis and polysomic inheritance at telophase I. Natural B. napus appeared to resolve meiotic errors mainly by suppressing homoeologous pairing, resolving nonhomologous centromere associations and 45S rDNA associations before diakinesis, and reducing homoeologous cross-overs.

Creation of fertility-restored materials for Ogura CMS in Brassica oleracea by introducing Rfo gene from Brassica napus via an allotriploid strategy

Ogura CMS fertility-restored materials, with 18 chromosomes, normal seed setting, stable fertility and closer genetic background to the parent Chinese kale, were successfully developed in B. oleracea via a triploid strategy for the first time. Ogura cytoplasmic male sterility (CMS) is the most widely used sterile type in seed production for commercial hybrids of Brassica oleracea vegetables. However, the natural Ogura CMS restorer line has not been found in B. oleracea crops. In this study, the triploid strategy was used with the aim to create euploid B. oleracea progenies with the Rfo gene. The allotriploid AAC hybrid YL2 was used as a male parent to backcross with Ogura CMS Chinese kale. After successive backcrosses, the BC₂ Rfo-positive individual 16CMSF2-11 and its BC₃ progenies, with 18 chromosomes, were developed, which were morphologically identical to the parent Chinese kale. Compared with F₁ and BC₁ plants, it showed stable fertility performance, and regular meiosis behavior and could produce seeds normally under natural pollination. The genomic composition analysis of Rfo-positive progenies by using molecular markers showed that more than 87% of the C-genome components of BC₃ Rfo-progenies recovered to the parent Chinese kale, while most or all of the A_n-genome segments were lost in 16CMSF2-11 and its progenies. The results suggested that the genetic background of Rfo-positive individuals was closer to that of the parent Chinese kale along with backcrossing. Hereof, the Ogura CMS fertility-restored materials of Chinese kale were successfully created via triploid strategy for the first time, providing a bridge for utilizing the Ogura CMS B. oleracea germplasm in the future. Moreover, our study indicates that the triploid strategy is effective for transferring genes from B. napus into B. oleracea.

In vitro androgenesis: spontaneous vs. artificial genome doubling and characterization of regenerants

Androgenesis has become the most frequently chosen method of doubled haploid (DH) production in major crops. Theoretically, plantlets derived from in vitro cultured microspore encompass half of the normal chromosome number of donor plants and thus, considered to be haploid. However, depending on species/genotype and the method of haploid production, either via anther or isolated microspore culture, different ratios of spontaneous DHs and diploid (2n) or even polyploid plants originating from somatic tissues or unreduced gametes may also arise in the cultures. Adopting the method of haploid identification, anti-microtubular agent for restoring fertility, and discriminating spontaneous DHs from undesired heterozygote plants will substantially affect the success of androgenesis in breeding programs. The recent advances in the last 2 decades have made it possible to characterize the in vitro regenerants efficiently either prior to genome duplication or using in breeding programs. The herein described approaches and antimicotubular agents are, therefore, expected to improve the efficiency of DH-based breeding pipeline through the in vitro androgenesis.

The story of promiscuous crucifers: origin and genome evolution of an invasive species, Cardamine occulta (Brassicaceae), and its relatives

BACKGROUND AND AIMS

Cardamine occulta (Brassicaceae) is an octoploid weedy species (2n = 8x = 64) originated in Eastern Asia. It has been introduced to other continents including Europe and considered to be an invasive species. Despite its wide distribution, the polyploid origin of C. occulta remained unexplored. The feasibility of comparative chromosome painting (CCP) in crucifers allowed us to elucidate the origin and genome evolution in Cardamine species. We aimed to investigate the genome structure of C. occulta in comparison with its tetraploid (2n = 4x = 32, C. kokaiensis and C. scutata) and octoploid (2n = 8x = 64, C. dentipetala) relatives.

METHODS

Genomic in situ hybridization (GISH) and large-scale CCP were applied to uncover the parental genomes and chromosome composition of the investigated Cardamine species.

KEY RESULTS

All investigated species descended from a common ancestral Cardamine genome (n = 8), structurally resembling the Ancestral Crucifer Karyotype (n = 8), but differentiated by a translocation between chromosomes AK6 and AK8. Allotetraploid C. scutata originated by hybridization between two diploid species, C. parviflora and C. amara (2n = 2x = 16). By contrast, C. kokaiensis has an autotetraploid origin from a parental genome related to C. parviflora. Interestingly, octoploid C. occulta probably originated through hybridization between the tetraploids C. scutata and C. kokaiensis. The octoploid genome of C. dentipetala probably originated from C. scutata via autopolyploidization. Except for five species-specific centromere repositionings and one pericentric inversion post-dating the polyploidization events, the parental subgenomes remained stable in the tetra- and octoploids.

CONCLUSIONS

Comparative genome structure, origin and evolutionary history was reconstructed in C. occulta and related species. For the first time, whole-genome cytogenomic maps were established for octoploid plants. Post-polyploid evolution in Asian Cardamine polyploids has not been associated with descending dysploidy and intergenomic rearrangements. The combination of different parental (sub)genomes adapted to distinct habitats provides an evolutionary advantage to newly formed polyploids by occupying new ecological niches.

Connecting genome structural variation with complex traits in crop plants

Structural genome variation is a major determinant of useful trait diversity. We describe how genome analysis methods are enabling discovery of trait-associated structural variants and their potential impact on breeding. As our understanding of complex crop genomes continues to grow, there is growing evidence that structural genome variation plays a major role in determining traits important for breeding and agriculture. Identifying the extent and impact of structural variants in crop genomes is becoming increasingly feasible with ongoing advances in the sophistication of genome sequencing technologies, particularly as it becomes easier to generate accurate long sequence reads on a genome-wide scale. In this article, we discuss the origins of structural genome variation in crops from ancient and recent genome duplication and polyploidization events and review high-throughput methods to assay such variants in crop populations in order to find associations with phenotypic traits. There is increasing evidence from such studies that gene presence-absence and copy number variation resulting from segmental chromosome exchanges may be at the heart of adaptive variation of crops to counter abiotic and biotic stress factors. We present examples from major crops that demonstrate the potential of pangenomic diversity as a key resource for future plant breeding for resilience and sustainability.

Finding invisible quantitative trait loci with missing data

Evolutionary processes during plant polyploidization and speciation have led to extensive presence-absence variation (PAV) in crop genomes, and there is increasing evidence that PAV associates with important traits. Today, high-resolution genetic analysis in major crops frequently implements simple, cost-effective, high-throughput genotyping from single nucleotide polymorphism (SNP) hybridization arrays; however, these are normally not designed to distinguish PAV from failed SNP calls caused by hybridization artefacts. Here, we describe a strategy to recover valuable information from single nucleotide absence polymorphisms (SNaPs) by population-based quality filtering of SNP hybridization data to distinguish patterns associated with genuine deletions from those caused by technical failures. We reveal that including SNaPs in genetic analyses elucidate segregation of small to large-scale structural variants in nested association mapping populations of oilseed rape (Brassica napus), a recent polyploid crop with widespread structural variation. Including SNaP markers in genomewide association studies identified numerous quantitative trait loci, invisible using SNP markers alone, for resistance to two major fungal diseases of oilseed rape, Sclerotinia stem rot and blackleg disease. Our results indicate that PAV has a strong influence on quantitative disease resistance in B. napus and that SNaP analysis using cost-effective SNP array data can provide extensive added value from 'missing data'. This strategy might also be applicable for improving the precision of genetic mapping in many important crop species.

Detecting de Novo Homoeologous Recombination Events in Cultivated Brassica napus Using a Genome-Wide SNP Array

The heavy selection pressure due to intensive breeding of Brassica napus has created a narrow gene pool, limiting the ability to produce improved varieties through crosses between B. napus cultivars. One mechanism that has contributed to the adaptation of important agronomic traits in the allotetraploid B. napus has been chromosomal rearrangements resulting from homoeologous recombination between the constituent A and C diploid genomes. Determining the rate and distribution of such events in natural B. napus will assist efforts to understand and potentially manipulate this phenomenon. The Brassica high-density 60K SNP array, which provides genome-wide coverage for assessment of recombination events, was used to assay 254 individuals derived from 11 diverse cultivated spring type B. napus These analyses identified reciprocal allele gain and loss between the A and C genomes and allowed visualization of de novo homoeologous recombination events across the B. napus genome. The events ranged from loss/gain of 0.09 Mb to entire chromosomes, with almost 5% aneuploidy observed across all gametes. There was a bias toward sub-telomeric exchanges leading to genome homogenization at chromosome termini. The A genome replaced the C genome in 66% of events, and also featured more dominantly in gain of whole chromosomes. These analyses indicate de novo homoeologous recombination is a continuous source of variation in established Brassica napus and the rate of observed events appears to vary with genetic background. The Brassica 60K SNP array will be a useful tool in further study and manipulation of this phenomenon.

Highly preserved roles of Brassica MIR172 in polyploid Brassicas: ectopic expression of variants of Brassica MIR172 accelerates floral transition

Functional characterization of regulatory genes governing flowering time is a research priority for breeding earliness in crop Brassicas. Highly polyploid genomes of Brassicas pose challenges in unraveling homeolog gene function. In Arabidopsis, five MIR172 paralogs control flowering time and floral organ identity by down-regulating AP2 and AP2-like genes. The impact of homeolog diversification on MIR172 loci, however, needs to be examined in morphologically diverse Brassicas. Herein, we analyze fractionation status and phylogeny of MIR172 and target AP2 from Brassicas and compare functionality of MIR172 variants representing distinct sub-genomes and progenitor genomes. Copy number analysis revealed higher retention of MIR172 loci relative to AP2 in diploid and amphi-diploid Brassica species. Dendrogram of 87 MIR172 sequences from Brassicaceae showed five major clusters corresponding to MIR172a-MIR172e which further separated into sub-genome and progenitor genome specific clades. Similar groupings were observed in the phylogeny of 11 Brassica AP2 and AP2-like genes. Over-expression of a pair of natural variants for each of MIR172b, MIR172d and MIR172e representing sub-genomes, progenitor genomes and species of Brassicas displayed floral acceleration in all transgenic lines indicating a strong selection pressure on MIR172. All gain-of-function lines, except 35S::MIR172e and 35S::MIR172e' displayed floral organ defects implying altered target spectrum of MIR172e relative to MIR172b and MIR172d. Expression of MIR172e caused marginal earliness in flowering time in B. juncea. In conclusion, this study demonstrates tightly preserved role of homeologs and natural variants of MIR172 family in mediating flowering in Brassicas and suggests their deployment for introgression of early flowering trait.

High-density ddRAD linkage and yield-related QTL mapping delimits a chromosomal region responsible for oil content in rapeseed (Brassica napus L.)

Rapeseed (Brassica napus L.) is one of the most important oil crops almost all over the world. Seed-related traits, including oil content (OC), silique length (SL), seeds per silique (SS), and seed weight (SW), are primary targets for oil yield improvement. To dissect the genetic basis of these traits, 192 recombinant inbred lines (RILs) were derived from two parents with distinct oil content and silique length. High-density linkage map with a total length of 1610.4 cM were constructed using 1,329 double-digestion restriction site associated DNA (ddRAD) markers, 107 insertion/deletions (INDELs), and 90 well-distributed simple sequence repeats (SSRs) markers. A total of 37 consensus quantitative trait loci (QTLs) were detected for the four traits, with individual QTL explained 3.1-12.8% of the phenotypic variations. Interestingly, one OC consensus QTL (cqOCA10b) on chromosome A10 was consistently detected in all three environments, and explained 9.8% to 12.8% of the OC variation. The locus was further delimited into an approximately 614 kb genomic region, in which the flanking markers could be further evaluated for marker-assisted selection in rapeseed OC improvement and the candidate genes targeted for map-based cloning and genetic manipulation.

Cytogenetic Diversity of Simple Sequences Repeats in Morphotypes of Brassica rapa ssp. chinensis

A significant fraction of the nuclear DNA of all eukaryotes is comprised of simple sequence repeats (SSRs). Although these sequences are widely used for studying genetic variation, linkage mapping and evolution, little attention had been paid to the chromosomal distribution and cytogenetic diversity of these sequences. In this paper, we report the distribution characterization of mono-, di-, and tri-nucleotide SSRs in Brassica rapa ssp. chinensis. Fluorescence in situ hybridization was used to characterize the cytogenetic diversity of SSRs among morphotypes of B. rapa ssp. chinensis. The proportion of different SSR motifs varied among morphotypes of B. rapa ssp. chinensis, with tri-nucleotide SSRs being more prevalent in the genome of B. rapa ssp. chinensis. We determined the chromosomal locations of mono-, di-, and tri-nucleotide repeat loci. The results showed that the chromosomal distribution of SSRs in the different morphotypes is non-random and motif-dependent, and allowed us to characterize the relative variability in terms of SSR numbers and similar chromosomal distributions in centromeric/peri-centromeric heterochromatin. The differences between SSR repeats with respect to abundance and distribution indicate that SSRs are a driving force in the genomic evolution of B. rapa species. Our results provide a comprehensive view of the SSR sequence distribution and evolution for comparison among morphotypes B. rapa ssp. chinensis.

Effects of genome structure variation, homeologous genes and repetitive DNA on polyploid crop research in the age of genomics

Compared to diploid species, allopolyploid crop species possess more complex genomes, higher productivity, and greater adaptability to changing environments. Next generation sequencing techniques have produced high-density genetic maps, whole genome sequences, transcriptomes and epigenomes for important polyploid crops. However, several problems interfere with the full application of next generation sequencing techniques to these crops. Firstly, different types of genomic variation affect sequence assembly and QTL mapping. Secondly, duplicated or homoeologous genes can diverge in function and then lead to emergence of many minor QTL, which increases difficulties in fine mapping, cloning and marker assisted selection. Thirdly, repetitive DNA sequences arising in polyploid crop genomes also impact sequence assembly, and are increasingly being shown to produce small RNAs to regulate gene expression and hence phenotypic traits. We propose that these three key features should be considered together when analyzing polyploid crop genomes. It is apparent that dissection of genomic structural variation, elucidation of the function and mechanism of interaction of homoeologous genes, and investigation of the de novo roles of repeat sequences in agronomic traits are necessary for genomics-based crop breeding in polyploids.

Microspore culture reveals complex meiotic behaviour in a trigenomic Brassica hybrid

BACKGROUND

Development of synthetic allohexaploid Brassica (2n = AABBCC) would be beneficial for agriculture, as allelic contributions from three genomes could increase hybrid vigour and broaden adaptation. Microspore culture of a near-allohexaploid hybrid derived from the cross (B. napus × B. carinata) × B. juncea was undertaken in order to assess the frequency and distribution of homologous and homoeologous crossovers in this trigenomic hybrid. SNP and SSR molecular markers were used to detect inheritance of A, B and C genome alleles in microspore-derived (MD) progeny. SNP allele copy number was also assessed. The MD progeny were also compared to progeny derived by self-pollination and open-pollination for fertility (estimated by self-pollinated seed set and pollen viability) and DNA ploidy (measured by flow cytometry).

RESULTS

In the trigenomic hybrid, homologous chromosome pairs A(j)-A(n), B(j)-B(c) and C(n)-C(c) had similar meiotic crossover frequencies and segregation to that previously observed in established Brassica species, as demonstrated by marker haplotype analysis of the MD population. Homoeologous pairing between chromosomes A1-C1, A2-C2 and A7-C6 was detected at frequencies of 12-18 %, with other homoeologous chromosome regions associating from 8 % (A3-C3) to 0-1 % (A8-C8, A8-C9) of the time. Copy number analysis revealed eight instances of additional chromosomes and 20 instances of chromosomes present in one copy in somatically doubled MD progeny. Presence of chromosome A6 was positively correlated with self-pollinated seed set and pollen viability in the MD population. Many MD progeny were unable to produce self-pollinated seed (76 %) or viable pollen (53 %), although one MD plant produced 198 self-pollinated seeds. Average fertility was significantly lower in progeny obtained by microspore culture than progeny obtained by self-pollination or open-pollination, after excluding MD progeny which had not undergone chromosome doubling.

CONCLUSIONS

Based on SNP data analysis of the microspore-derived progeny, crossover frequency per chromosome in the allohexaploid hybrid was found to be similar to that in established Brassica species, suggesting that the higher chromosome number did not significantly disrupt cellular regulation of meiosis. SNP allele copy number analysis revealed the occurrence not only of homoeologous duplication/deletion events but also other cryptic duplications and deletions that may have been the result of mitotic instability. Microspore culture simplified the assessment of chromosome behaviour in the allohexaploid hybrid but yielded progeny with lower fertility and a greater range of ploidy levels compared to progeny obtained by self- or open-pollination.

Creating new interspecific hybrid and polyploid crops

Agricultural selection of desirable traits in domesticated plant and animal species mimics natural evolutionary selection for ability of species to survive, thrive, and reproduce in the wild. However, one evolutionary process is currently underutilised for human agricultural purposes: speciation through interspecific hybridisation and polyploid formation. Despite promising successes in creation of new hybrid and or polyploid species in many genera, few geneticists and breeders deliberately take advantage of polyploidy and interspecific hybridisation for crop improvement. We outline the possible benefits as well as potential problems and criticisms with this approach, and address how modern advances in technology and knowledge can help to create new crop species for agriculture.

Constructing a dense genetic linkage map and mapping QTL for the traits of flower development in Brassica carinata

An integrated dense genetic linkage map was constructed in a B. carinata population and used for comparative genome analysis and QTL identification for flowering time. An integrated dense linkage map of Brassica carinata (BBCC) was constructed in a doubled haploid population based on DArT-Seq(TM) markers. A total of 4,031 markers corresponding to 1,366 unique loci were mapped including 639 bins, covering a genetic distance of 2,048 cM. We identified 136 blocks and islands conserved in Brassicaceae, which showed a feature of hexaploidisation representing the suggested ancestral crucifer karyotype. The B and C genome of B. carinata shared 85 % of commonly conserved blocks with the B genome of B. nigra/B. juncea and 80 % of commonly conserved blocks with the C genome of B. napus, and shown frequent structural rearrangements such as insertions and inversions. Up to 24 quantitative trait loci (QTL) for flowering and budding time were identified in the DH population. Of these QTL, one consistent QTL (qFT.B4-2) for flowering time was identified in all of the environments in the J block of the B4 linkage group, where a group of genes for flowering time were aligned in A. thaliana. Another major QTL for flowering time under a winter-cropped environment was detected in the E block of C6, where the BnFT-C6 gene was previously localised in B. napus. This high-density map would be useful not only to reveal the genetic variation in the species with QTL analysis and genome sequencing, but also for other applications such as marker-assisted selection and genomic selection, for the African mustard improvement.

Global analysis of cis-natural antisense transcripts and their heat-responsive nat-siRNAs in Brassica rapa

BACKGROUND

Brassica rapa includes several important leaf vegetable crops whose production is often damaged by high temperature. Cis-natural antisense transcripts (cis-NATs) and cis-NATs-derived small interfering RNAs (nat-siRNAs) play important roles in plant development and stress responses. However, genome-wide cis-NATs in B. rapa are not known. The NATs and nat-siRNAs that respond to heat stress have never been well studied in B. rapa. Here, we took advantage of RNA-seq and small RNA (sRNA) deep sequencing technology to identify cis-NATs and heat responsive nat-siRNAs in B. rapa.

RESULTS

Analyses of four RNA sequencing datasets revealed 1031 cis-NATs B. rapa ssp. chinensis cv Wut and B. rapa ssp. pekinensis cv. Bre. Based on sequence homology between Arabidopsis thaliana and B. rapa, 303 conserved cis-NATs in B. rapa were found to correspond to 280 cis-NATs in Arabidopsis; the remaining 728 novel cis-NATs were identified as Brassica-specific ones. Using six sRNA libraries, 4846 nat-siRNAs derived from 150 cis-NATs were detected. Differential expression analysis revealed that nat-siRNAs derived from 12 cis-NATs were responsive to heat stress, and most of them showed strand bias. Real-time PCR indicated that most of the transcripts generating heat-responsive nat-siRNAs were upregulated under heat stress, while the transcripts from the opposite strands of the same loci were downregulated.

CONCLUSIONS

Our results provide the first subsets of genome-wide cis-NATs and heat-responsive nat-siRNAs in B. rapa; these sRNAs are potentially useful for the genetic improvement of heat tolerance in B. rapa and other crops.

Karyotype analysis of Panax ginseng C.A.Meyer, 1843 (Araliaceae) based on rDNA loci and DAPI band distribution

Ginseng has long been considered a valuable plant owing to its medicinal properties; however, genomic information based on chromosome characterization and physical mapping of cytogenetic markers has been very limited. Dual-color FISH karyotype and DAPI banding analyses of Panax ginseng C. A. Meyer, 1843 were conducted using 5S and 45S rDNA probes. The somatic chromosome complement was 2n=48 with lengths from 3.3 μm to 6.3 μm. The karyotype was composed of 12 metacentric, 9 submetacentric, and 3 subtelocentric pairs. The 5S rDNA probe localized to the intercalary region of the short arm of pair 11, while the 45S rDNA was located at the secondary constriction of the subtelocentric satellited chromosome 14. DAPI bands were clearly observed for most chromosomes, with various signal intensities and chromosomal distributions that consequently improved chromosome identification. As a result, all 24 chromosomes could be distinguished and numbers were assigned to each chromosome for the first time. The results presented here will be useful for the on-going ginseng genome sequencing and further molecular-cytogenetic studies and breeding programs of ginseng.

Seed colour loci, homoeology and linkage groups of the C genome chromosomes revealed in Brassica rapa-B. oleracea monosomic alien addition lines

BACKGROUND AND AIMS

Brassica rapa and B. oleracea are the progenitors of oilseed rape B. napus. The addition of each chromosome of B. oleracea to the chromosome complement of B. rapa results in a series of monosomic alien addition lines (MAALs). Analysis of MAALs determines which B. oleracea chromosomes carry genes controlling specific phenotypic traits, such as seed colour. Yellow-seeded oilseed rape is a desirable breeding goal both for food and livestock feed end-uses that relate to oil, protein and fibre contents. The aims of this study included developing a missing MAAL to complement an available series, for studies on seed colour control, chromosome homoeology and assignment of linkage groups to B. oleracea chromosomes.

METHODS

A new batch of B. rapa-B. oleracea aneuploids was produced to generate the missing MAAL. Seed colour and other plant morphological features relevant to differentiation of MAALs were recorded. For chromosome characterization, Snow's carmine, fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) were used.

KEY RESULTS

The final MAAL was developed. Morphological traits that differentiated the MAALs comprised cotyledon number, leaf morphology, flower colour and seed colour. Seed colour was controlled by major genes on two B. oleracea chromosomes and minor genes on five other chromosomes of this species. Homoeologous pairing was largely between chromosomes with similar centromeric positions. FISH, GISH and a parallel microsatellite marker analysis defined the chromosomes in terms of their linkage groups. Conclusions A complete set of MAALs is now available for genetic, genomic, evolutionary and breeding perspectives. Defining chromosomes that carry specific genes, physical localization of DNA markers and access to established genetic linkage maps contribute to the integration of these approaches, manifested in the confirmed correspondence of linkage groups with specific chromosomes. Applications include marker-assisted selection and breeding for yellow seeds.

Size and location of radish chromosome regions carrying the fertility restorer Rfk1 gene in spring turnip rape

In spring turnip rape (Brassica rapa L. spp. oleifera), the most promising F1 hybrid system would be the Ogu-INRA CMS/Rf system. A Kosena fertility restorer gene Rfk1, homolog of the Ogura restorer gene Rfo, was successfully transferred from oilseed rape into turnip rape and that restored the fertility in female lines carrying Ogura cms. The trait was, however, unstable in subsequent generations. The physical localization of the radish chromosomal region carrying the Rfk1 gene was investigated using genomic in situ hybridization (GISH) and bacterial artificial chromosome-fluorescence in situ hybridization (BAC-FISH) methods. The metaphase chromosomes were hybridized using radish DNA as the genomic probe and BAC64 probe, which is linked with Rfo gene. Both probes showed a signal in the chromosome spreads of the restorer line 4021-2 Rfk of turnip rape but not in the negative control line 4021B. The GISH analyses clearly showed that the turnip rape restorer plants were either monosomic (2n=2x=20+1R) or disomic (2n=2x=20+2R) addition lines with one or two copies of a single alien chromosome region originating from radish. In the BAC-FISH analysis, double dot signals were detected in subterminal parts of the radish chromosome arms showing that the fertility restorer gene Rfk1 was located in this additional radish chromosome. Detected disomic addition lines were found to be unstable for turnip rape hybrid production. Using the BAC-FISH analysis, weak signals were sometimes visible in two chromosomes of turnip rape and a homologous region of Rfk1 in chromosome 9 of the B. rapa A genome was verified with BLAST analysis. In the future, this homologous area in A genome could be substituted with radish chromosome area carrying the Rfk1 gene.

Genome affinity and meiotic behaviour in trigenomic hybrids and their doubled allohexaploids between three cultivated Brassica allotetraploids and Brassica fruticulosa

The wild species Brassica fruticulosa Cyr. (FF, 2n = 16) is closely related to the cultivated Brassica species. Through interspecific reciprocal crosses between B. fruticulosa and three cultivated Brassica allotetraploids (AABB, AACC, and BBCC where A = 10, B = 8, and C = 9), four trigenomic hybrids (F.AC, 2n = 27; F.AB, 2n = 26; F.BC, 2n = 25; BC.F, 2n = 25) were produced. By chromosome doubling of respective hybrids, three allohexaploids (FF.AACC, 2n = 54; FF.AABB, 2n = 52; BBCC.FF, 2n = 50) were synthesized. In pollen mother cells (PMCs) of the trigenomic hybrids, 1–2 autosyndetic bivalents were detected within A, B, and C genomes but only one within F genome; 1–3 allosyndetic bivalents between any two genomes were observed, and a closer relationship of F and B genomes than F and A genomes or F and C genomes was revealed. The allohexaploids showed a generally low but different pollen fertilities. The chromosomes in PMCs were predominantly paired as bivalents but some univalents and multivalents at variable frequencies were observed. The bivalents of homologous pairing for each genome prevailed, but allosyndetic quadrivalents and hexavalents involving any two genomes were observed, together with autosyndetic quadrivalents for A, B, and C genomes but not the F genome. The nondiploidized cytological behaviour of these allohexaploids contributed to their low fertility. The relationships between the genome affinity and meiotic behavior in these allohexaploids were discussed.

Identification of conserved and novel microRNAs that are responsive to heat stress in Brassica rapa

The species Brassica rapa includes various vegetable crops. Production of these vegetable crops is usually impaired by heat stress. Some microRNAs (miRNAs) in Arabidopsis have been considered to mediate gene silencing in plant response to abiotic stress. However, it remains unknown whether or what miRNAs play a role in heat resistance of B. rapa. To identify genomewide conserved and novel miRNAs that are responsive to heat stress in B. rapa, we defined temperature thresholds of non-heading Chinese cabbage (B. rapa ssp. chinensis) and constructed small RNA libraries from the seedlings that had been exposed to high temperature (46 °C) for 1 h. By deep sequencing and data analysis, we selected a series of conserved and novel miRNAs that responded to heat stress. In total, Chinese cabbage shares at least 35 conserved miRNA families with Arabidopsis thaliana. Among them, five miRNA families were responsive to heat stress. Northern hybridization and real-time PCR showed that the conserved miRNAs bra-miR398a and bra-miR398b were heat-inhibitive and guided heat response of their target gene, BracCSD1; and bra-miR156h and bra-miR156g were heat-induced and its putative target BracSPL2 was down-regulated. According to the criteria of miRNA and miRNA* that form a duplex, 21 novel miRNAs belonging to 19 miRNA families were predicted. Of these, four were identified to be heat-responsive by Northern blotting and/or expression analysis of the putative targets. The two novel miRNAs bra-miR1885b.3 and bra-miR5718 negatively regulated their putative target genes. 5'-Rapid amplification of cDNA ends PCR indicated that three novel miRNAs cleaved the transcripts of their target genes where their precursors may have evolved from. These results broaden our perspective on the important role of miRNA in plant responses to heat.

Stable progeny production of the amphidiploid resynthesized Brassica napus cv. Hanakkori, a newly bred vegetable

Resynthesized Brassica napus cv. Hanakkori (AACC, 2n = 38) was produced by cross-hybridization between B. rapa (AA, 2n = 20) and B. oleracea (CC, 2n = 18) as a new vegetative crop. Many studies have provided evidences for the instability and close relationship between A and C genome in the resynthesized B. napus cultivars. In fact, seed produced to obtain progeny in Hanakkori had unstable morphological characters and generated many off-type plants. In this study, we investigated the pollen fertility, chromosome number, structure, and behavior linked to various Hanakkori phenotypes to define factors of unstable phenotypic expression in the progeny. Hanakkori phenotypes were categorized into five types. The results of pollen fertility, chromosome number, and fluorescence in situ hybridization analysis for somatic mitosis cells indicated that the off-type plants had lower pollen fertility, aberrant chromosome number, and structures with small chromosome fragments. Observation of chromosomes at meiosis showed that the meiotic division in off-type plants led to appreciably higher abnormalities than in on-type plants. However, polyvalent chromosomes were observed frequently in both on- and off-type plants in diplotene stage of meiosis. We assume that the unstable morphological characters in resynthesized progeny were the result of abnormal division in meiosis. It results as important that the plants of normal phenotype, chromosome structure and minimized abnormal meiosis are selected to stabilize progeny.

An EST-SSR linkage map of Raphanus sativus and comparative genomics of the Brassicaceae

Raphanus sativus (2n = 2x = 18) is a widely cultivated member of the family Brassicaceae, for which genomic resources are available only to a limited extent in comparison to many other members of the family. To promote more genetic and genomic studies and to enhance breeding programmes of R. sativus, we have prepared genetic resources such as complementary DNA libraries, expressed sequences tags (ESTs), simple sequence repeat (SSR) markers and a genetic linkage map. A total of 26 606 ESTs have been collected from seedlings, roots, leaves, and flowers, and clustered into 10 381 unigenes. Similarities were observed between the expression patterns of transcripts from R. sativus and those from representative members of the genera Arabidopsis and Brassica, indicating their functional relatedness. The EST sequence data were used to design 3800 SSR markers and consequently 630 polymorphic SSR loci and 213 reported marker loci have been mapped onto nine linkage groups, covering 1129.2 cM with an average distance of 1.3 cM between loci. Comparison of the mapped EST-SSR marker positions in R. sativus with the genome sequence of A. thaliana indicated that the Brassicaceae members have evolved from a common ancestor. It appears that genomic fragments corresponding to those of A. thaliana have been doubled and tripled in R. sativus. The genetic map developed here is expected to provide a standard map for the genetics, genomics, and molecular breeding of R. sativus as well as of related species. The resources are available at http://marker.kazusa.or.jp/Daikon.

Karyotype and identification of all homoeologous chromosomes of allopolyploid Brassica napus and its diploid progenitors

Investigating recombination of homoeologous chromosomes in allopolyploid species is central to understanding plant breeding and evolution. However, examining chromosome pairing in the allotetraploid Brassica napus has been hampered by the lack of chromosome-specific molecular probes. In this study, we establish the identification of all homoeologous chromosomes of allopolyploid B. napus by using robust molecular cytogenetic karyotypes developed for the progenitor species Brassica rapa (A genome) and Brassica oleracea (C genome). The identification of every chromosome among these three Brassica species utilized genetically mapped bacterial artificial chromosomes (BACs) from B. rapa as probes for fluorescent in situ hybridization (FISH). With this BAC-FISH data, a second karyotype was developed using two BACs that contained repetitive DNA sequences and the ubiquitous ribosomal and pericentromere repeats. Using this diagnostic probe mix and a BAC that contained a C-genome repeat in two successive hybridizations allowed for routine identification of the corresponding homoeologous chromosomes between the A and C genomes of B. napus. When applied to the B. napus cultivar Stellar, we detected one chromosomal rearrangement relative to the parental karyotypes. This robust novel chromosomal painting technique will have biological applications for the understanding of chromosome pairing, homoeologous recombination, and genome evolution in the genus Brassica and will facilitate new applied breeding technologies that rely upon identification of chromosomes.

Microspore culture preferentially selects unreduced (2n) gametes from an interspecific hybrid of Brassica napus L. x Brassica carinata Braun

We analysed the products of male meiosis in microspore-derived progeny from a Brassica napus (AAC(n)C(n)) x Brassica carinata (BBC(c)C(c)) interspecific hybrid (ABC(n)C(c)). Genotyping at 102 microsatellite marker loci and nuclear DNA contents provided strong evidence that 26 of the 28 progeny (93%) were derived from unreduced (2n) gametes. The high level of C(n)C(c) marker heterozygosity, and parallel spindles at Anaphase II in the ABC(n)C(c) hybrid, indicated that unreduced gametes were formed by first division restitution. The frequency of dyads at the tetrad stage of pollen development (2.6%) suggested that unreduced gametes were preferentially selected in microspore culture. Segregation of marker alleles in the microspore-derived progeny was consistent with homologous recombination between C(n) and C(c) chromosomes and homoeologous recombination involving A-, B- and C-genome chromosomes during meiosis in the ABC(n)C(c) hybrid. We discuss the potential for using microspore culture of unreduced gametes in interspecific hybrids to map Brassica centromeres through half-tetrad analysis.

Physical mapping of rDNA and heterochromatin in chromosomes of 16 Coffea species: a revised view of species differentiation

The chromosome organization among 15 wild diploid Coffea species and cultivated tetraploid C. arabica was determined by fluorochrome banding (CMA, DAPI) and double fluorescence in-situ hybridization (FISH) of 5S and 18S rDNA achieved on the same chromosome plates. Two to five chromosome pairs (plus one putative chromosome B) are marked. Overall, there are two SAT-chromosome pairs for East African species and one for the Malagasy and the West and Central African species. 18S rDNA loci are telomeric and strongly marked the SAT-chromosome pairs. Generally, only one pericentromeric 5S rDNA locus characterized East African species, while an additional minor locus co-localized with the 18S rDNA-SAT locus for the Malagasy species and West and Central African species. A combination of rDNA FISH plus CMA and DAPI banding patterns enables identification of almost all the species, even those for which the genetic or botanical status is still being discussed. C. arabica clearly appears to be an allotetraploid species, including one genome from East Africa and one from West and Central Africa. However, since the minor 5S rDNA-SAT locus present in West/Central African genomes is not detected, two evolutionary hypotheses could be put forward for C. arabica. Considering only the diploid species, global trends are obvious in rDNA signal patterns, genome size variations, and geographic distribution of the species, but there are no clear evolutionary trends. However, complex interactions between these factors and environmental growing conditions exist, which have resulted in loss and gain of rDNA loci and probably also in copy repeat number variations in each rDNA family.

Production and genetic analysis of partial hybrids in intertribal crosses between Brassica species (B. rapa, B. napus) and Capsella bursa-pastoris

Capsella bursa-pastoris (L.) Medic (2n = 4x = 32) is a natural double-low (erucic acid < 1%, glucosinolates < 30 micromol/g) germplasm and shows high degree of resistance to Sclerotinia sclerotiorum. Hybridizations were carried out between two Brassica species viz. B. rapa (2n = 20) and B. napus (2n = 38) as female and C. bursa-pastoris as male parent to introduce these desirable traits into cultivated Brassica species. Majority of F(1) plants resembled female parents in morphology and only a few expressed some characters of male parent, including the white petals. Based on cytological observation of somatic cells, the F(1) plants were classified into five types: two types from the cross with B. rapa, type I had 2n = 27-29; type II had 2n = 20; three types from the crosses with B. napus, type III was haploids with 2n = 19; type IV had 2n = 29; type V had 2n = 38. One to two chromosomes of C. bursa-pastoris were detected in pollen mother cells (PMCs) of type I plant by genomic in situ hybridization (GISH), together with chromosomal segments in ovary cells and PMCs of some F1 plants. Amplified fragment length polymorphism (AFLP) bands specific for the male parent, novel for two parents and absent bands in Brassica parents were generated in majority of F1 plants, even in Brassica-types and haploids, indicating the introgressions at various levels from C. bursa-pastoris and genomic alterations following hybridization. Some Brassica-type progeny plants had reduced contents of erucic acid and glucosinolates associated with improved resistance to S. sclerotiorum. The cytological and molecular mechanisms behind these results are discussed.