Analysis of chloroplast and mitochondrial DNAs in asymmetric somatic hybrids between tobacco and carrot

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.

Detailed analyses of chloroplast and mitochondrial DNAs from the hybrid plant generated by asymmetric protoplast fusion between radish and cabbage

In a previous report, intergeneric somatic hybrids between red cabbage (Brassica oleracea L. var.capitata) and radish (Raphanus sativus L. cv. Shougoin) were produced by protoplast fusion. Plant morphology, chromosome number, isozyme patterns, andSma1 cleavage pattern of chloroplast DNA indicated that the hybrid plants have the red cabbage nucleus and the radish chloroplasts. In this report, we analyzed the organization of chloroplast and mitochondrial DNAs from this hybrid using Southern hybridization. The restriction patterns of almost all regions of the chloroplast DNA from the hybrid were similar to that of radish, except for one region near therps16 gene, which encodes the chloroplast ribosomal protein S16. In contrast to chloroplast DNA, the restriction pattern of mitochondrial DNA from the hybrid was quite different from that of the parents.

Production and analysis of asymmetric hybrid plants between monocotyledon (Oryza sativa L.) and dicotyledon (Daucus carota L.)

Asymmetric hybrid plants were obtained from fused protoplasts of a monocotyledon (Oryza sativa L.) and a dicotyledon (Daucus carota L.). X-ray-irradiated protoplasts isolated from a cytoplasmic malesterile (cms) carrot suspension culture were fused with iodoacetoamide-treated protoplasts isolated from a 5-methyltryptophan (5MT)-resistant rice suspension culture by electrofusion. The complementary recovered cells divided and formed colonies, which were then cultivated on regeneration medium supplemented with 25mg/l 5MT to eliminate any escaped carrot cells. Somatic hybrids were regenerated from 5 of the 5MT-resistant colonies. The morphologies of most of the regenerated plants closely resembled that of the parental carrot plants. A cytological analysis of callus cultures induced from these plants indicated that most of the cells possessed 20-22 chromosomes and were resistant to 5MT. An isozyme analysis revealed that several regenerated plants had the peroxidase isozyme patterns of both parents. A Southern hybridization analysis with non-radioactively labelled DNA fragments of the rgp1 gene showed that regenerated plants had hybridizing bands from both rice and carrot. Chloroplast (cp) and mitochondrial (mt) DNAs were also analyzed by Southern hybridization by using several probes. CpDNA patterns of the regenerated plants were indistinguishable from those of the carrot parent. However 1 of the regenerated plants had a novel band pattern of mtDNA that was not detected in either of the parents, indicating a possible recombination of mitochondrial genomes.

Production of somatic hybrids between Daucus carota L. and Nicotiana tabacum

Protoplasts of a kanamycin-resistant (KR, nuclear genome), streptomycin-resistant (SR, chloroplast genome) and chlorophyll-deficient (A1, nuclear genome) Nicotiana tabacum (KR-SA) cell suspension cultures or X-ray-irradiated mesophyll protoplasts of kanamycin- and streptomycin-resistant green plants (KR-SR) were fused with protoplasts of a cytoplasmic male-sterile (CMS) Daucus carota L. cell suspension cultures by electrofusion. Somatic hybrid plants were selected for kanamycin resistance and the ability to produce chlorophyll. Most of the regenerated plants had a normal D. carota morphology. Callus induced from these plants possessed 23-32 chromosomes, a number lower than the combined chromosome number (66) of the parents, and were resistant to kanamycin, but they segregated for streptomycin resistance, which indicated that N. tabacum chloroplasts had been eliminated. Genomic DNA from several regenerated plants was analyzed by Southern hybridization for the presence of the neomycin phosphotransferase gene (NPTII); all of the plants analyzed were found to contain this gene. Mitochondrial (mt) DNA was analyzed by Southern hybridization of restriction endonuclease digests of mtDNA with two DNA probes, PKT5 and coxII. The results showed that the two plants analyzed possessed the mitochondria of D. carota. These results demonstrate that the regenerated plants are interfamilial somatic hybrids.

Production and characterization of asymmetric somatic hybrids between Arabidopsis thaliana and Brassica napus

Cell suspension-derived protoplasts of a chlorsulfuron-resistant (GH50) strain of Arabidopsis thaliana cv Columbia were X-irradiated at 60 or 90 krad, to facilitate the elimination of GH50 donor chromosomes in fusion products. Irradiated GH50 protoplasts were fused, with polyethylene glycol, to protoplasts derived from stem epidermal strips of Brassica napus cv Westar. Chlorsulfuron-resistant colonies were selected in vitro and then transferred to shoot and root regeneration medium. Seventeen hybrid lines were regenerated in vitro, and eight were successfully established in the greenhouse, where they flowered. These eight asymmetric hybrids were intermediate in vegetative morphology between Arabidopsis and Brassica. The flowers from these hybrids were male-sterile with abnormal petal and pistil structures. Zymograms for phosphoglucomutase, esterase, and peroxidase showed the presence of all parental isozymes in each of the hybrids tested. Nuclear hybridity was also confirmed for the ribosomal RNA genes using a wheat rDNA probe; however, the chloroplast genome in each of the hybrids was derived solely from the Brassica parent. All selected somatic hybrids were capable of rooting at levels of chlorsulfuron which were inhibitory to unfused Brassica plantlets. The degree of herbicide resistance in the hybrid shoots is presently being evaluated.

Highly asymmetric intergeneric nuclear hybrids between Nicotiana and Petunia: evidence for recombinogenic and translocation events in somatic hybrid plants after "gamma"-fusion

Extremely asymmetric nuclear hybrids have been obtained via protoplast fusion in an intergeneric combination. Irradiated (cobalt(60),100 krad) kanamycinresistant Petunia hybrida mesophyll protoplasts were chemically fused with wild-type mesophyll protoplasts of Nicotiana plumbaginifolia. Eighty-six hybrid colonies were selected on kanamycin-containing medium, and twenty-four of these could be induced to regenerate numerous shoots. Cytological analysis of the regenerants showed the presence of a few chromosome fragments in some lines, and even a metacentric chromosome in yet another line. Besides additional chromosome fragments some lines only possessed typical Nicotiana chromosomes, and this at the diploid (2n = 2X = 20) as well as the tetraploid (2n = 2X = 40) level. Biochemical analysis showed that all regenerants had neomycin phosphotransferase activity (NPTII), which suggests that intergenomic recombination and or translocation events took place at least in those lines where no additional chromosome fragments could be detected. The presence of the NPTII gene was shown by Southern hybridization. All regenerants tested were fertile, and the segregation ratios for the kanamycin gene (for self and backcross pollinations to the recipient partner) for some of the regenerants correspond with Mendelian rules for a monogenic dominant marker. Most of the regenerants showed abnormal segregation ratios; in this case, no correlation could be made between segregation ratio and chromosome composition.Our results demonstrate the existence of intergenomic recombination and translocations evens in nuclear somatic hybrid plants obtained via "gamma"-fusion.

Species-specific sequences in the genus Solanum: identification, characterization, and application to study somatic hybrids of S. brevidens and S. tuberosum

To aid in the identification and analysis of somatic hybrids between potato (Solanum tuberosum, dihaploid line PDH 40) and the non tuber-bearing wild species S. brevidens, a series of species-specific repetitive DNA sequences have been isolated. This was accomplished by making libraries of HaeIII-digested total DNA of S. tuberosum and S. brevidens, by cloning fragments into the SmaI site of plasmid pUC18 and transforming them into E. coli (JM83). The S. brevidens library consisted of 1,000 recombinant clones, and that of S. tuberosum, 700. Nitrocellulose filters with recombinant clones were hybridised to nick-translated total DNA of S. brevidens and also S. tuberosum, and, following autoradiography, clones that hybridised strongly to the DNA of only one of the species were chosen. Two highly repeated S. brevidens clones (pSB1, 400 bp and pSB7,210 bp), one highcopy-number s. tuberosum clone (pST10, 200 bp) and one low-copy-number sequence of S. tuberosum (pST3, 1.5 kbp) were selected for further analysis by Southern hybridisation to digested total DNA. Clone pSB7 gave a ladder pattern on hybridisation to EcoR1-digested total DNA of S. brevidens, with signals at multiples of 200 bp DNA. Using these probes it was possible to verify the hybridity of putative hybrids of dihaploid S. tuberosum and S.brevidens, and to confirm by Southern analysis and by slot blots the parental genome dosage of hexaploid hybrids (two s. brevidens: one S. tuberosum, and vice-versa). The S. tuberosum-specific probe, pSTIO, hybridised with DNA of three other tuber-bearing wild species (S. hjertingii, S. capsicibaccatum and S. berthaultii). A squash-blot procedure was developed using the probes that would allow early identification of somatic hybrid callus. There are a number of useful applications of such species-specific probes in the identification and analysis of somatic hybrids.