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.     

Regeneration of intergeneric somatic hybrid plants between Lycopersicon esculentum and Solanum muricatum

Summary Mesophyll protoplasts of tomato (Lycopersicon esculentum) and pepino (Solanum muricatum) were fused by using an electrofusion method and cultured in modified MS medium supplemented with naphthaleneacetic acid and kinetin, in which only pepino and somatic hybrid protoplasts could divide. Somatic hybrid plants showing intermediate characteristics in morphology were regenerated from the calli exhibiting vigorous growth in contrast with those of pepino. The hybrid nature of these plants was confirmed by cytological observation and biochemical analyses of phosphoglucomutase isozymes and the fraction-1-protein. The regenerated somatic hybrids grew to flowering stage and set fruits.     

Production of somatic hybrids by electrofusion in Solanum

Summary Conditions are described for large scale electrofusion of mesophyll protoplasts of dihaploid S. tuberosum with those of diploid S. brevidens. Overall fusion frequencies of 20%–30% were achieved, and following fusion, large numbers of protoplast-derived calli were obtained. Putative somatic hybrid plants were selected from the regenerated shoots by examining their morphological characteristics. Twenty-one somatic hybrids were confirmed by isoenzyme analysis and six somatic hybrids were further confirmed by Southern hybridization. Tetraploid hybrids were obtained, but cytogenetic studies indicated that more of the regenerated hybrids were hexaploid than had previously been found following chemical fusion of the same partners. Some advantages of electrofusion over chemical fusion are discussed.     

Production of fertile somatic hybrids of Gossypium hirsutum?+?G. bickii and G. hirsutum?+?G. stockii via protoplast fusion

Fertile somatic hybrids were obtained via symmetric electrofusion of protoplasts from two combinations of tetraploid cotton (G. hirsutum cv. Coker 201, AD genome) and diploid wild cottons G. bickii (G genome) and G. stockii (E genome), respectively. Observation by morphological, flow cytometric analysis, chromosome counting and RAPD analysis of the tested hybrids of Coker 201 + G. bickii and Coker 201 + G. stockii confirmed the regenerated plants as hybrid status. Cytological investigation of the metaphase root-tip cells revealed there were 78 chromosomes in the hybrids. Flow cytometric analysis showed the tested plants had a relative DNA contents close to the total DNA contents of the two parents. RAPD analysis revealed the hybrids contained specific genomic fragments from both fusion partners, further confirmed their hybridity. The morphology of the hybrids was intermediate between the two fusion partners. The hybrid plants were successfully transferred to the soil, and they bloomed and set bolls. It is sure that the new hexaploids developed by cell fusion would contribute to cotton breeding through backcrossing with the elite genotypes of G. hirsutum.     

Improved protoplast culture and stability of cytoplasmic traits in plants regenerated from leaf protoplasts of cauliflower (Brassica oteracea ssp.botrytis)

Nearly 1000 plants have been regenerated from leaf protoplasts of two cauliflower (Brassica oleracea ssp.botrytis) alloplasmic inbred lines. One line (7642A) carried the Ogura (R1) cms cytoplasm derived from radish; the other line (7642B) carried a normalBrassica cytoplasm and was the fertile maintainer for the cms line. The majority of regenerated plants displayed normal vegetative morphology; they formed normal cauliflower heads and retained the floral characteristics of seed-grown plants from which they were derived. We found no change in either male sterility or in the low temperature-induced chlorosis associated with the 7642A line. Mitochondrial DNA analysis by hybridization with five cloned mtDNA probes revealed no apparent alteration in 75 regenerated plants of both lines. These results indicate that cytoplasmic traits inBrassica oleracea are stable after one cycle of in vitro culture and regeneration.     

Effect of radiation dose on the production of and the extent of asymmetry in tomato asymmetric somatic hybrids

Summary Asymmetric somatic hybrids were recovered following fusion of tomato leaf mesophyll protoplasts with irradiated protoplasts isolated from Lycopersicon pennellii suspension cells. The asymmetry was determined by scoring the regenerants at between 20 and 24 loci using isozymes and restriction fragment length polymorphisms. In addition, three quantitative traits, fruit size, leaf shape, and stigma exsertion, were measured in the regenerants. The recovery of asymmetric somatic hybrids was as high as 50% of the regenerants, and there was no requirement for the transfer of a selectable marker gene from the irradiated partner. The amount of nuclear DNA transferred from the irradiated protoplast fusion partner was found to be inversely proportional to the radiation dose. It was possible to recover tomato asymmetric somatic hybrids which were self-fertile and contained limited amounts of genetic information from L. pennelli.     

Somatic hybrid plants produced by electrofusion between Solanum melongena L. and Solanum torvum Sw

Summary Somatic hybrid plants between eggplant (Solanum melongena) and Solanum torvum have been produced by the electrofusion of mesophyll protoplasts in a movable multi-electrode fusion chamber. Using hair structure as a selection criteria, we identified a total of 19 somatic hybrids, which represented an overall average of 15.3% of the 124 regenerated plants obtained in the two fusion experiments. Several morphological traits were intermediate to those of the parents, including trichome density and structure, height, leaf form and inflorescence. Cytological analyses revealed that the chromosome numbers of the somatic hybrids approximated the expected tetraploid level (2n=4x=48). Fifteen hybrid plants were homogeneous and had relatively stable chromosome numbers (46–48), while four other hybrids had variable chromosome numbers (35–48) and exhibited greater morphological variation. The hybridity of these 19 somatic hybrid plants was confirmed by analyses of phosphoglucomutase (Pgm) and esterase zymograms.     

Somatic hybrid plants between the forage legumes Medicago sativa L. and Medicago arborea L.

Interspecific somatic hybrid plants were obtained by symmetrical electrofusion of mesophyll protoplasts of Medicago sativa with callus protoplasts of Medicago arborea. Somatic hybrid calli were picked manually from semi-solid culture medium after they were identified by their dual color in fluorescent light. Twelve putative hybrid calli were selected and one of them regenerated plants. The morphogenesis of the somatic hybrid calli was induced by the synthetic growth regulator 1,2 benzisoxazole-3-acetic acid. Somatic hybrid plants showed intensive genome rearrangements, as evidenced by isozyme and RFLP analysis. The morphology of somatic hybrid plants was in general intermediate between the parents. The production of hybrids by protoplast fusion between sexually incompatible Medicago species is related to the in vitro respon siveness of the parental protoplasts. The possibility of using somatic hybrid plants in alfalfa breeding is discussed.     

Production of intertribal somatic hybrids between Brassica napus L. and Lesquerella fendleri (Gray) Wats

Intertribal Brassica napus (+) Lesquerella fendleri hybrids have been produced by polyethylene glycol-induced fusions of B. napus hypocotyl and L. fendleri mesophyll protoplasts. Two series of experiments were performed. In the first, symmetric fusion experiments, protoplasts from the two materials were fused without any pretreatments. In the second, asymmetric fusion experiments, X-ray irradiation at doses of 180 and 200 Gy were used to limit the transfer of the L. fendleri genome to the hybrids. X-ray irradiation of L. fendleri mesophyll protoplasts did not suppress the proliferation rate and callus formation of the fusion products but did significantly decrease growth and differentiation of non-fused L. fendleri protoplasts. In total, 128 regenerated plants were identified as intertribal somatic hybrids on the basis of morphological criteria. Nuclear DNA analysis performed on 80 plants, using species specific sequences, demonstrated that 33 plants from the symmetric fusions and 43 plants from the asymmetric fusions were hybrids. Chloroplast and mitochondrial DNA analysis revealed a biased segregation that favoured B. napus organelles in the hybrids from the symmetric fusion experiments. The bias was even stronger in the hybrids from the asymmetric fusion experiments where no hybrids with L. fendleri organelles were found. X-ray irradiation of L. fendleri protoplasts increased the possibility of obtaining mature somatic hybrid plants with improved fertility. Five plants from the symmetric and 24 plants from the asymmetric fusion experiments were established in the greenhouse. From the symmetric fusions 2 plants could be fertilised and set seeds after cross-pollination with B. napus. From the asymmetric fusions 9 plants could be selfed as well as fertilised when backcrossed with B. napus. Chromosome analysis was performed on all of the plants but 1 that were transferred to the greenhouse. Three plants from the symmetric fusions contained 50 chromosomes, which corresponded to the sum of the parental genomes. From the asymmetric fusions, 11 hybrids contained 38 chromosomes. Among the other asymmetric hybrids, plants with 50 chromosomes and with chromosome numbers higher than the sum of the parental chromosomes were found. When different root squashes of the same plant were analysed, a total of 6 plants were found that had different chromosome numbers.     

Morphological and molecular characterization of fertile tetraploid somatic hybrids produced by protoplast electrofusion and PEG-induced fusion between Lycopersicon esculentum Mill. and Lycopersicon peruvianum Mill.

Summary Mesophyl protoplasts of two genotypes of cultivated tomato (Lycopersicon esculentum Mill.) and one of its wild relative species (Lycopersicon peruvianum Mill.) were fused by using electrofusion and polyethyleneglycol-induced fusion. Forty-three fertile tetraploid somatic hybrid plants, each deriving from separate calli, were recovered from both fusion procedures. Electrofusion appeared more efficient than chemical fusion for the production of somatic hybrids. These plants appeared morphologically similar, whatever the fusion procedure and tomato genotype. They had intermediate leaf, inflorescence, and flower morphology. After self-pollination, the hybrids set fruit of intermediate size and color. The hybrid nature of these plants was confirmed by isoelectric focusing of the Rubisco small subunits used as nuclear markers. L. esculentum and L. peruvianum were distinguished by means of two chloroplast markers: CF1-ATPase subunit as analyzed by isoelectro-focusing and ct DNA restriction patterns. All hybrids displayed both ct markers of only one parent with no biased transmission. Mitochondrial (mt) DNAs were prepared from flower buds by using miniaturized CsCl gradients. Preliminary analysis indicated that mt genomes from the hybrids all differed from those of both parents. mt DNA Sall restriction enzyme analysis revealed that all but two hybrids contained one novel fragment of 13.5 kb. Gene mapping experiments showed that the mt apocytochrome b and ATPase subunit 9 homologies in the somatic hybrid mt DNA resembled L. esculentum and L. peruvianum, respectively; the mt nad5 probe distinguished at least four distinct patterns in the hybrids. These results indicated that mt DNA rearrangements involving intergenomic recombinations occurred through protoplast fusion. A greater mt DNA polymorphism was induced with chemical fusion than with electrofusion.     

Atrazine-resistant cauliflower obtained by somatic hybridization between Brassica oleracea and ATR-B. napus

Summary Somatic hybridization between Brassica oleracea ssp. botrytis (cauliflower, 2n=18), carrying the Ogura (R1) male-sterile cytoplasm and B. napus (2n= 38), carrying a male-fertile, atrazine-resistant (ATR) cytoplasm, yielded three hybrids (2n=56) and six cauliflower cybrids (2n=18), which were selected for resistance to the herbicide in vitro. The hybrids and cybrids were male fertile and self-compatible. They contained both chloroplasts and mitochondria from the ATR cytoplasm. We found no evidence for mtDNA recombination in any of the regenerated plants. Selfed progeny of the B. oleracea atrazine-resistant cybrids were evaluated for tolerance to the herbicide in the field. Resistant plants exposed to 0.56–4.48 kg/ha (0.5–4.0 pounds/acre) atrazine in the soil showed no damage at any herbicide level, whereas plants of a susceptible alloplasmic line were severely damaged at the lowest level of herbicide application and killed at all higher levels. These atrazine-resistant cauliflower may have potential horticultural use, especially in fields where atrazine carry over is a serious problem.     

Somatic hybrids between<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis> and cytoplasmic male-sterile radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis>)

Somatic hybrids were produced by protoplast fusion between Arabidopsis thaliana ecotype Columbia and a male-sterile radish line MS-Gensuke (Raphanus sativus) with the Ogura cytoplasm. Forty-one shoots were differentiated from the regenerated calli and established as shoot cultures in vitro. About 20 of these shoots were judged to be hybrids based on growth characteristics and morphology. Molecular analyses of 11 shoots were performed, confirming the hybrid features. Of these 11 shoots, eight were established as rooted plants in the greenhouse. Polymerase chain reaction and randomly amplified polymorphic DNA analyses of the nuclear genomes of all analyzed shoots and plants confirmed that they contained hybrid DNA patterns. Their chromosome numbers also supported the hybrid nature of the plants. Investigations of the organelles in the hybrids revealed that the chloroplast (cp) genome was exclusively represented by radish cpDNA, while the mitochondrial DNA configuration showed a combination of both parental genomes as well as fragments unique to the hybrids. Hybrid plants that flowered were male-sterile independent of the presence of the Ogura CMS-gene orf138.Abbreviations CMS Cytoplasmic male sterilityCommunicated by M.R. Davey     

Intertribal somatic hybrids between Brassica napus and Thlaspi perfoliatum with high content of the T. perfoliatum-specific nervonic acid

Protoplast fusions were performed between hypocotyl protoplasts of Brassica napus and mesophyll protoplasts of Thlaspi perfoliatum. The two species are members of the Lepidieae and Brassiceae tribes, respectively, in the family of Brassicaceae. Seeds of T. perfoliatum are rich in the fatty acid C₂₄₁ (nervonic acid), an oil valuable for technical purposes. In the search for renewable oils to replace the mineral oils, plant breeders have been trying to develop oil crops with a high content of long-chain fatty acids. After fusion of B. napus protoplasts with non-irradiated as well as irradiated protoplasts of T. perfoliatum selection was carried out by flow cytometry and cell sorting. Of the shoots regenerated from different calli 27 were verified as hybrids or partial hybrids using the isoenzyme phosphoglucose isomerase (PGI) as a marker. Another 6 plants were identified as partial hybrids using a T. perfoliatum-specific repetitive DNA sequence. Slot blot experiments were performed to estimate the copy number of the repetitive DNA sequence in the parental species and in the hybrids. In T. perfoliatum there were approximately 10⁵ copies per haploid genome, and the range in the hybrids was 1–37% of the value in T. perfoliatum. When the nuclear DNA content of the regenerated shoots was analysed we found partial as well as symmetric hybrids. Even though the rooting and establishment of hybrid shoots in the greenhouse were difficult, resulting in the death of many plants, 19 plants were cultured to full maturity. Seeds obtained from 15 plants were analysed to determine whether they contained nervonic acid, and 5 of the hybrids were found to contain significantly greater amounts of nervonic acid than B. napus.     

Analysis of somatic hybrids between two sterile dihaploid Solanum tuberosum L. breeding lines. Restoration of fertility and complementation of G. pallida Pa2 and Pa3 resistance

Fourteen somatic hybrids generated by electrofusion of mesophyll protoplasts from a non-flowering dihaploid S. tuberosum clone, DHAK-11, and a male-sterile dihaploid clone S. tuberosum, DHAK-33, were grown in the greenhouse and subjected to morphological assessments and tests for fertility and resistance to the white potato cyst nematode Globodera pallida pathotypes Pa2 and Pa3. The ploidy level of the hybrids ranged from 38 to 63 chromosomes. All hybrids developed flowers with violet petals except for one, hy-56, that possessed red petals. The colour of the tuber skin was purple in all hybrids except in hy-56 where the tuber skin was red. All of the hybrids were female fertile and generated viable seeds. Near-tetraploid hybrids produced the highest number of seeds per fruit and these seeds had a normal size. Hybrids with 58 or more chromosomes produced smaller seeds and less seeds per fruit. The germination frequency of the seeds was not influenced by the chromosome number of the hybrids. Pollen viability was determined and the male fertility of three hybrids was tested. Pollination with these three hybrids gave rise to fruit development, but only one produced viable seeds. The hybrids were tested for resistance to G. pallida pathotypes Pa2 and Pa3. A high level of resistance to Pa3, inherited from one parental clone, DHAK-11, and a high level of resistance to Pa2, inherited from the other parental clone, DHAK-33, was combined in four hybrids. These results demonstrate, that protoplast fusion is an efficient method for restoring the fertility of somatic hybrids generated from sterile parent clones, and is a powerful procedure for the complementation of multigenetic disease resistance traits in potato breeding lines.     

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.     

Genome composition of asymmetric hybrids in relation to the phylogenetic distance between the parents. Nucleus-chloroplast interaction

Summary A series of fusion experiments were performed between protoplasts of a cytoplasmic albino mutant of tomato, Lycopersicon esculentum (ALRC), and gamma-irradiated protoplasts of L. hirsutum and the Solanum species S. commersonii, S. etuberosum and S. nigrum. These species were chosen for their different phylogenetic relationships to tomato. In all fusion combinations except from those between ALRC and S. nigrum, green calli were selected as putative fusion products and shoots regenerated from them. They were subsequently analyzed for their morphology, nuclear DNA composition and chloroplast DNA origin. The hybrids obtained between ALRC and L. hirsutum contained the chloroplasts of L. hirsutum and had the flower and leaf morphology of L. esculentum. After Southern blot analysis, using 13 restriction fragment length polymorphisms (RFLPs) randomly distributed over all chromosomes, all hybrids showed L. esculentum hybridization patterns. No chromosomes of L. hirsutum were found. These results indicate that these hybrids were true cybrids.The putative asymmetric hybrids, obtained with S. commersonii and S. etuberosum, showed phenotypic traits of both parents. After hybridization with species-specific repetitive nuclear DNA probes it was found that nuclear material of both parents was present in all plants. In the case of S. nigrum, which combination has the greatest phylogenetic distance between the fusion parents, no hybrid plants could be obtained. The chloroplast DNA of all hybrid plants was of the donor type suggesting that chloroplast transfer by asymmetric protoplast fusion can overcome problems associated with large phylogenetic distances between parental plants.     

Fertile somatic hybrids between Petunia hybrida and a wild species,Petunia variabilis

Somatic hybrid plants were regenerated following electrofusion between leaf mesophyll protoplasts of P. hybrida (2n = 14) and a wild sexually incompatible species, P. variabilis (2n = 18). The selection of hybrids was based on the hybrid vigour, expressed both in the growth of the callus and at the shoot formation stage, resulting from the combination of parental genomes. Calli exhibiting vigorous growth were selected, and upon transfer to regeneration medium gave rise to shoots. Four regenerated plants from three calli had morphological characteristics intermediate between those of the parents. The hybrid nature of these plants was confirmed by chromosome counts as well as isozyme and DNA analyses. They had amphidiploid chromosome numbers (2n = 32) and were fertile. Following self-pollination and backcrossing with P. variabilis, large numbers of F₂ and BC₁ seedlings were obtained.     

Production and identification of somatic hybrids between Solanum tuberosum and S. papita by using the rolC gene as a morphological selectable marker

A successful hybridization of a diploid clone of Solanum tuberosum with a rolC-transgenic, diploid S. papita clone is reported. By using leaf expiants of this S. papita clone, which after transformation expressed kanamycin resistance, intact protoplasts were obtained, but these protoplasts did not develop to microcalli or regenerate to mature plants. However, protoplasts of the S. tuberosum clone showed a high capacity to regenerate plants from isolated protoplasts. On a medium containing Kanamycin only calli regenerated to plants, which revealed a rolC phenotype (reduced apical dominance with a large number of adventitious shoots and a pale green color of leaves) and later on turned out to be true hybrids. Self fusions of S. papita never developed to microcalli and those of S. tuberosum ceased to develop on the kanamycin-containing medium. Identification of somatic hybrids was done by RFLP and RAPD analysis. In the greenhouse, out of four selected hybrids only FK3.1 was successfully crossed with two standard S. tuberosum varieties (Datura, Desirée). Out of all the seeds germinated, only rolC-negative F¹ seedlings were further characterized. Within the seedling population obvious differences were evident in respect of the S. papita and S. tuberosum characteristics.     

Detection and analysis of QTLs based on RAPD markers for polygenic resistance to Plasmodiophora brassicae Woron in Brassica oleracea L.

Resistance to Plasmodiophora brassicae Woron, the causal fungus of clubroot, was examined in an F₂ population of a cross between a clubroot-resistant kale (Brassica oleracea L. var. acephala) and a susceptible cauliflower (Brassica oleracea L. var. botrytis). QTL detection was performed with RAPD markers. Two resistance notations, carried out at different times after inoculation, were used. Three markers were associated with these two notations and three were specifically linked to only one notation. QTL analysis suggests the existence of at least two genetic mechanisms implicated in the resistance phenomenon.     

Production of intergeneric somatic hybrids between round kumquat (Fortunella japonica Swingle) and ‘Morita navel’ orange (Citrus sinensis Osbeck)

Intergeneric somatic hybrids between embryogenic callus-derived protoplasts of round kumquat (Fortunella japonica Swingle) and Morita navel orange (Citrus sinensis Osbeck) were produced by electrofusion. Among the eight different fusion strains obtained, six showed normal morphology, whereas the remaining two showed malformation. All the regenerated plants were intermediate in leaf morphology and had thick and round leaves, which are typical characteristics of polyploids. Ploidy analyses by flow cytometry and chromosome counting in root-tip cells revealed that these plants are amphidiploid (2n=4×=36). Hybridity of the fusion products was confirmed by random amplified polymorphic DNA and cleaved amplified polymorphic sequence (CAPS) analyses. Furthermore, analyses of chloroplast (cp) and mitochondrial (mt) DNA by CAPS showed that these somatic hybrids contained cp- and mt-DNA of round kumquat without recombination in the regions analyzed.Abbreviations BA 6-Benzylaminopurine - CAPS Cleaved amplified polymorphic sequence - RAPD Random amplified polymorphic DNACommunicated by H. Ebinuma