Cytology, RAPD, and seed colour of progeny plants from Brassica rapa-alboglabra aneuploids and development of monosomic addition lines

Complementary gene interaction and xenia effect controls the seed coat colour in interspecific cross between Trifolium alexandrinum and T. apertum

Trifolium alexandrinum (Egyptian clover) is a widely cultivated winter annual fodder. Present work deals with inheritance of the seed coat colour in segregating progenies of the interspecific cross between T. alexandrinum and T. apertum. Although, both the parent species possessed yellow seed coat, the F₁ seeds were black coloured in the reciprocal cross (T. apertum × T. alexandrinum). Seeds borne on individual F₂ plants and the advancing generations segregated in yellow and black seed coat colour, which confirmed xenia effect. F₂ seeds collected from individual F₁ plants exhibited nine black and seven yellow segregation ratio. The segregation of the seed coat colour recorded from F₃ to F₅ generations revealed that yellow seed coat was true breeding (i.e. non-segregating) in this interspecific cross (including the reciprocal crosses). However, the black seeded progenies were either true breeding or segregated in nine black: seven yellow ratio or three black: one yellow ratio suggesting a complementary gene interaction or duplicate recessive epistasis. It indicated that the seed coat colour is controlled by complementary gene interaction along with xenia effect in interspecific crosses between T. alexandrinum and T. apertum. Occurrence of the complementary genes across the species could suggest T. apertum to be the progenitor of T. alexandrinum. Inheritance of seed coat colour in reference to its importance in Egyptian clover breeding is also discussed.

Generation and characterization of Brassica rapa ssp. pekinensis - B. oleracea var. capitata monosomic and disomic alien addition lines

Five monosomic alien addition lines (MAALs) of Brassica rapa ssp. pekinensis - B. oleracea var. capitata were obtained by hybridization and backcrossing between B. rapa ssp. pekinensis (female parent) and B. oleracea var. capitata. The alien linkage groups were identified using 42 B. oleracea var. capitata linkage group-specific markers as B. oleracea linkage groups C2, C3, C6, C7 and C8. Based on the chromosomal karyotype of root tip cells, these five MAALs added individual chromosomes from B. oleracea var. capitata: chr 1 (the longest), chr 2 or 3, chr 5 (small locus of 25S rDNA), chr 7 (satellite-carrying) and chr 9 (the shortest). Five disomic alien addition lines were then generated by selfing their corresponding MAALs.

Generation and identification of Brassica alboglabra-Brassica campestris monosomic alien addition lines

Four monosomic alien addition lines (MAALs) for Brassica alboglabra-Brassica campestris were developed through digenomic triploid (ACC) backcrossing with the recurrent parent B. alboglabra (CC). The objectives of this study were to compare morphological traits, microsatellite markers (simple sequence repeats), chromosomal karyotypes, and meiotic behaviors. Based on the new chromosome nomenclature system established for Brassica, we preliminarily identified these MAALs as CC+A1, CC+A3, CC+A6, and CC+A7. Their alien chromosomes were transmittable through both female and male gametes at rates of 11.46%-26.53% and 4.88%-12.90%, respectively.

Assigning Brassica microsatellite markers to the nine C-genome chromosomes using Brassica rapa var. trilocularis-B. oleracea var. alboglabra monosomic alien addition lines

Brassica rapa var. trilocularis-B. oleracea var. alboglabra monosomic alien addition lines (MAALs) were used to assign simple sequence repeat (SSR) markers to the nine C-genome chromosomes. A total of 64 SSR markers specific to single C-chromosomes were identified. The number of specific markers for each chromosome varied from two (C3) to ten (C4, C7 and C9), where the designation of the chromosomes was according to Cheng et al. (Genome 38:313-319, 1995). Seventeen additional SSRs, which were duplicated on 2-5 C-chromosomes, were also identified. Using the SSR markers assigned to the previously developed eight MAALs and recently obtained aneuploid plants, a new Brassica rapa-B. oleracea var. alboglabra MAAL carrying the alien chromosome C7 was identified and developed. The application of reported genetically mapped SSR markers on the nine MAALs contributed to the determination of the correspondence between numerical C-genome cytological (Cheng et al. in Genome 38:313-319, 1995) and linkage group designations. This correspondence facilitates the integration of C-genome genetic information that has been generated based on the two designation systems and accordingly increases our knowledge about each chromosome. The present study is a significant contribution to genetic linkage analysis of SSR markers and important agronomic traits in B. oleracea and to the potential use of the MAALs in plant breeding.

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.

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

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

Development of SRAP, SNP and multiplexed SCAR molecular markers for the major seed coat color gene in Brassica rapa L

Seed coat color inheritance in B. rapa was studied in F(1), F(2), F(3), and BC(1) progenies from a cross of a Canadian brown-seeded variety 'SPAN' and a Bangladeshi yellow sarson variety 'BARI-6'. A pollen effect was found when the yellow sarson line was used as the maternal parent. Seed coat color segregated into brown, yellow-brown and bright yellow classes. Segregation was under digenic control where the brown or yellow-brown color was dominant over bright yellow seed coat color. A sequence related amplified polymorphism (SRAP) marker linked closely to a major seed coat color gene (Br1/br1) was developed. This dominant SRAP molecular marker was successfully converted into single nucleotide polymorphism (SNP) markers and sequence characterized amplification region (SCAR) markers after the extended flanking sequence of the SRAP was obtained with chromosome walking. In total, 24 SNPs were identified with more than 2-kb sequence. A 12-bp deletion allowed the development of a SCAR marker linked closely to the Br1 gene. Using the five-fluorescence dye set supplied by ABI, four labeled M13 primers were integrated with different SCAR primers to increase the throughput of SCAR marker detection. Using multiplexed SCAR markers targeting insertions and deletions in a genome shows great potential for marker assisted selection in plant breeding.

High-density Brassica oleracea linkage map: identification of useful new linkages

We constructed a 1,257-marker, high-density genetic map of Brassica oleracea spanning 703 cM in nine linkage groups, designated LG1-LG9. It was developed in an F2 segregating population of 143 individuals obtained by crossing double haploid plants of broccoli "Early-Big" and cauliflower "An-Nan Early". These markers are randomly distributed throughout the map, which includes a total of 1,062 genomic SRAP markers, 155 cDNA SRAP markers, 26 SSR markers, 3 broccoli BAC end sequences and 11 known Brassica genes: BoGSL-ALK, BoGSL-ELONG, BoGSL-PROa, BoGSL-PROb, BoCS-lyase, BoGS-OH, BoCYP79F1, BoS-GT (glucosinolate pathway), BoDM1 (resistance to downy mildew), BoCALa, BoAP1a (inflorescence architecture). BoDM1 and BoGSL-ELONG are linked on LG 2 at 0.8 cM, making it possible to use the glucosinolate gene as a marker for the disease resistance gene. By QTL analysis, we found three segments involved in curd formation in cauliflower. The map was aligned to the C genome linkage groups and chromosomes of B. oleracea and B. napus, and anchored to the physical map of A. thaliana. This map adds over 1,000 new markers to Brassica molecular tools.

Pairing and recombination at meiosis of Brassica rapa (AA) x Brassica napus (AACC) hybrids

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.

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.