Genetic characterization of asymmetric somatic hybrids between Bupleurum scorzonerifolium Willd and Triticum aestivum L.: potential application to the study of the wheat genome

The place of asymmetric somatic hybridization in wheat breeding

Since its first development some 40 years ago, the application of the somatic hybridization technique has generated a body of hybrid plant material involving a wide combination of parental species. Until the late 1990s, the technique was ineffective in wheat, as regeneration from protoplasts was proving difficult to achieve. Since this time, however, a successful somatic hybridization protocol for wheat has been established and used to generate a substantial number of both symmetric and asymmetric somatic hybrids and derived materials, especially involving the parental combination bread wheat and tall wheatgrass (Thinopyrum ponticum). This review describes the current state of the art for somatic hybridization in wheat and focuses on its potential application for wheat improvement.

Construction of whole genome radiation hybrid panels and map of chromosome 5A of wheat using asymmetric somatic hybridization

To explore the feasibility of constructing a whole genome radiation hybrid (WGRH) map in plant species with large genomes, asymmetric somatic hybridization between wheat (Triticum aestivum L.) and Bupleurum scorzonerifolium Willd. was performed. The protoplasts of wheat were irradiated with ultraviolet light (UV) and gamma-ray and rescued by protoplast fusion using B. scorzonerifolium as the recipient. Assessment of SSR markers showed that the radiation hybrids have the average marker retention frequency of 15.5%. Two RH panels (RHPWI and RHPWII) that contained 92 and 184 radiation hybrids, respectively, were developed and used for mapping of 68 SSR markers in chromosome 5A of wheat. A total of 1557 and 2034 breaks were detected in each panel. The RH map of chromosome 5A based on RHPWII was constructed. The distance of the comprehensive map was 2103 cR and the approximate resolution was estimated to be ∼501.6 kb/break. The RH panels evaluated in this study enabled us to order the ESTs in a single deletion bin or in the multiple bins cross the chromosome. These results demonstrated that RH mapping via protoplast fusion is feasible at the whole genome level for mapping purposes in wheat and the potential value of this mapping approach for the plant species with large genomes.

Progress of chromosome engineering mediated by asymmetric somatic hybridization

Plant somatic hybridization has progressed steadily over the past 35 years. Many hybrid plants have been generated from fusion combinations of different phylogenetic species, some of which have been utilized in crop breeding programs. Among them, asymmetric hybrid, which usually contains a fraction of alien genome, has received more attention because of its importance in crop improvement. However, few studies have dealt with the heredity of the genome of somatic hybrid for a long time, which has limited the progress of this approach. Over recent ten years, along with the development of an effective cytogenetical tool "in situ hybridization (ISH)", asymmetric fusion of common wheat (Triticum aestivum L.) with different grasses or cereals has been greatly developed. Genetics, genomes, functional genes and agricultural traits of wheat asymmetric hybrids have been subject to systematic investigations using gene cloning, genomic in situ hybridization (GISH) and molecular makers. The future goal is to fully elucidate the functional relationships among improved agronomic traits, the genes and underlying molecular mechanisms, and the genome dynamics of somatic introgression lines. This will accelerate the development of elite germplasms via somatic hybridization and the application of these materials in the molecular improvement of crop plants.

Production and cytogenetic characterization of intertribal somatic hybrids between Brassica napus and Isatis indigotica and backcross progenies

Intertribal somatic hybrids between Brassica napus (2n = 38, AACC) and a dye and medicinal plant Isatis indigotica (2n = 14, II) were obtained by fusions of mesophyll protoplasts. From a total of 237 calli, only one symmetric hybrid (S2) and five asymmetric hybrids (As1, As4, As6, As7 and As12) were established in the field. These hybrids showed some morphological variations and had very low pollen fertility. Hybrids S2 and As1 possessed 2n = 52 (AACCII), the sum of the parental chromosomes, and As12 had 2n = 66 (possibly AACCIIII). Hybrids As4, As6 and As7 were mixoploids (2n = 48-62). Genomic in situ hybridization analysis revealed that pollen mother cells at diakinesis of As1 contained 26 bivalents comprising 19 from B. napus and 7 from I. indigotica and mainly showed the segregation 26:26 at anaphase I (AI) with 7 I. indigotica chromosomes in each polar group. Four BC(1) plants from As1 after pollinated by B. napus resembled mainly B. napus in morphology but also exhibited some characteristics from I. indigotica. These plants produced some seeds on selfing or pollination by B. napus. They had 2n = 45 (AACCI) and underwent pairing among the I. indigotica chromosomes and/or between the chromosomes of two parents at diakinesis. All hybrids mainly had the AFLP banding patterns from the addition of two parents plus some alterations. B. napus contributed chloroplast genomes in majority of the hybrids but some also had from I. indigotica. Production of B. napus-I. indigotica additions would be of considerable importance for genome analysis and breeding.

Ginsenoside Rb1 in asymmetric somatic hybrid calli of Daucus carota with Panax quinquefolius

American ginseng (Panax quinquefolius L.) is one of the most valuable herbs in the world. Its major active components are ginsenosides. In order to produce ginsenoside heterogeneously, somatic hybridization, a novel approach for genetic introgression, was employed in this study. Protoplasts derived from respective calli of carrot (Daucus carota var. sativus Hoffm.) and American ginseng (P. quinquefolius L.) were used as the fusion partners. Hybrid calli derived from single cell lines containing chromatin of American ginseng were confirmed by the analyses of isozyme, Random amplified polymorphic DNA (RAPD) and genomic in situ hybridization (GISH). High performance liquid chromatography (HPLC) results showed that the ginseng monomer Rb(1) was synthesized in seven of the hybrid calli identified as well as in the parent American ginseng calli but not in the parent carrot calli. Results indicated that hybrid introgression lines could produce ginsenoside Rb(1) and the ginsenoside Rb(1) biosynthesis pathway has been introgressed into carrot cells via somatic hybridization. From the point of biosafety view concerning the consumer acceptance, the potential predominance to produce ginsenosides with somatic hybridization other than with genetic transformation is discussed.

Analysis of remote asymmetric somatic hybrids between common wheat and Arabidopsis thaliana

Callus-derived protoplasts of common wheat (Triticum aestivum L. cv. Hesheng 3) irradiated with ultraviolet light were fused by using the PEG method with cell suspension-derived protoplasts of Arabidopsis thaliana. Regenerated calli and green plants resembling that of wheat were obtained. The hybrid nature of putative calli and plants were confirmed by isozyme, random amplified polymorphic DNA and genomic in situ hybridization (GISH) analyses. GISH results indicated that 1 approximately 3 small chromosome fragments of A. thaliana were found introgression into the terminals of wheat chromosomes, forming highly asymmetric hybrids. Cytoplasmic genome tests did not show any cytoplasmic genetic materials from A. thaliana. However, variations from the normal wheat cytoplasmic genome were found, indicating recombination or rearrangement occurred during the process of somatic hybridization. The chromosome elimination in the asymmetric somatic hybridization of remote phylogenetic relationship was discussed. A miniature inverted-repeat transposable element related sequence was found by chance in the hybrids which might accompany and impact the process of somatic hybridization.