Mitochondrial DNA rearrangements in somatic hybrids of Solanum tuberosum and Solanum brevidens

Mitochondrial genome recombination in somatic hybrids of Solanum commersonii and S. tuberosum

Interspecific somatic hybridization has been performed in potato breeding experiments to increase plant resistance against biotic and abiotic stress conditions. We analyzed the mitochondrial and plastid genomes and 45S nuclear ribosomal DNA (45S rDNA) for the cultivated potato (S. tuberosum, St), wild potato (S. commersonii, Sc), and their somatic hybrid (StSc). Complex genome components and structure, such as the hybrid form of 45S rDNA in StSc, unique plastome in Sc, and recombinant mitogenome were identified. However, the mitogenome exhibited dynamic multipartite structures in both species as well as in the somatic hybrid. In St, the mitogenome is 756,058 bp and is composed of five subgenomes ranging from 297,014 to 49,171 bp. In Sc, it is 552,103 bp long and is composed of two sub-genomes of 338,427 and 213,676 bp length. StSc has 447,645 bp long mitogenome with two subgenomes of length 398,439 and 49,206 bp. The mitogenome structure exhibited dynamic recombination mediated by tandem repeats; however, it contained highly conserved genes in the three species. Among the 35 protein-coding genes of the StSc mitogenome, 21 were identical for all the three species, and 12 and 2 were unique in Sc and St, respectively. The recombinant mitogenome might be derived from homologous recombination between both species during somatic hybrid development.

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.

Chloroplast and mitochondrial DNA composition of triploid and tetraploid somatic hybrids between Lycopersicon esculentum and Solanum tuberosum

The chloroplast (cp) DNA type and mitochondrial (mt) DNA composition of 17 somatic hybrids between a cytoplasmic albino tomato and monoploid potato (A7-hybrids) and 18 somatic hybrids between a nitrate reductase-deficient tomato and monoploid potato (C7-hybrids) were analyzed. Thirteen A7-hybrids and 9 C7-hybrids were triploids (with one potato genome); the other hybrids were tetraploid. As expected, all A7-hybrids contained potato cpDNA. Of the C7-hybrids 7 had tomato cpDNA, 10 had potato cpDNA and 1 hybrid contained both tomato and potato cpDNA. The mtDNA composition of the hybrids was analyzed by hybridization of Southern blots with four mtDNA-specific probes. The mtDNAs in the hybrids had segregated independently from the cpDNAs. Nuclear DNA composition (i.e. one or two potato genomes) did not influence the chloroplast type in the C7-hybrids, nor the mtDNA composition of A7- or C7-hybrids. From the cosegregation of specific mtDNA fragments we inferred that both tomato and potato mtDNAs probably have a coxII gene closely linked to 18S+5S rRNA genes. In tomato, atpA, and in potato, atp6 seems to be linked to these mtDNA genes.

Interaction between cytoplasmic composition and yield parameters in somatic hybrids of S. tuberosum L

The nuclear and cytoplasmic composition of five different fusion combinations, consisting of up to 50 hybrid regenerants each, was characterized by RFLP analysis. Simultaneously, the hybrid clones of four fusion combinations were evaluated in field experiments for yield and starch content.Predominantly complete chloroplast segregation was found with a 1∶1 ratio, in all but one fusion combination. Mitochondria, in contrast revealed up to 75% recombination, as proven by the partial addition of parental banding patterns and the altered assignment of the same genotypes with different probes. Newly occuring DNA bands were also indicative of rearrangements in the mitochondrial genome. Correlations between RFLP data and field parameters were calculated. Deviating RFLP patterns of the nuclear genome did not influence yield parameters. Also the assignment of hybrids to different chloroplast genotypes did not affect yield or starch content. However, mitochondrial types could be distinguished with respect to starch content and tuber yield. The more thorough analysis of mitochondrial composition, with different probes homologous to coding regions, revealed a relationship between the homogeneity of the mt genome and the yield level.

Random chloroplast segregation and frequent mtDNA rearrangements in fertile somatic hybrids between Nicotiana tabacum L. and N. glutinosa L

Patterns of organelle inheritance were examined among fertile somatic hybrids between allotetraploid Nicotiana tabacum L. (2n=4x=48) and a diploid wild relative N. glutinosa L. (2n=2x=24). Seventy somatic hybrids resistant to methotrexate and kanamycin were recovered following fusion of leaf mesophyll protoplasts of transgenic methotrexate-resistant N. tabacum and kanamycin-resistant N. glutinosa. Evidence for hybridization of nuclear genomes was obtained by analysis of glutamate oxaloacetate transaminase and peroxidase isoenzymes and by restriction fragment length polymorphism (RFLP) analysis using a heterologous nuclear ribosomal DNA probe. Analysis of chloroplast genomes in a population of 41 hybrids revealed a random segregation of chloroplasts since 25 possessed N. glutinosa chloroplasts and 16 possessed N. tabacum chloroplasts. This contrasts with the markedly non-random segregation of plastids in N. tabacum (+)N. rustica and N. tabacum (+) N. debneyi somatic hybrids which we described previously and which were recovered using the same conditions for fusion and selection. The organization of the mitochondrial DNA (mtDNA) in 40 individuals was examined by RFLP analysis with a heterologous cytochrome B gene. Thirty-eight somatic hybrids possessed mitochondrial genomes which were rearranged with respect to the parental genomes, two carried mtDNA similar to N. tabacum, while none had mtDNA identical to N. glutinosa. The somatic hybrids were self-fertile and fertile in backcrosses with the tobacco parent.

A high frequency of intergenomic mitochondrial recombination and an overall biased segregation of B. campestris or recombined B. campestris mitochondria were found in somatic hybrids made within Brassicaceae

Mitochondrial segregation and rearrangements were studied in regenerated somatic hybrids from seven different species combinations produced using reproducible and uniform methods. The interspecific hybridizations were made between closely or more distantly related species within the Brassicaceae and were exemplified by three intrageneric, two intergeneric and two intertribal species combinations. The intrageneric combinations were represented by Brassica campestris (+) B. oleracea, B. napus (+) B. nigra and B. napus (+) B. juncea (tournefortii) hybrids, the intergeneric combinations by B. napus (+) Raphanus sativus and B. napus (+) Eruca sativa hybrids, and the intertribal combinations by B. napus (+) Thlaspi perfoliatum and B. napus (+) Arabidopsis thaliana hybrids. In each species combination, one of the two mitochondrial genotypes was B. campestris since the B. napus cultivar used in the fusions contained this cytoplasm. Mitochondrial DNA (mtDNA) analyses were performed using DNA hybridization with nine different mitochondrial genes as probes. Among the various species combinations, 43-95% of the hybrids demonstrated mtDNA rearrangements. All examined B. campestris mtDNA regions could undergo intergenomic recombination since hybrid-specific fragments were found for all of the mtDNA probes analysed. Furthermore, hybrids with identical hybrid-specific fragments were found for all probes except cox II and rrn18/rrn5, supporting the suggestion that intergenomic recombination can involve specific sequences. A strong bias of hybrids having new atp A-or atp9-associated fragments observed in the intra- and intergeneric combinations could imply that these regions contain sequences that have a high reiteration number, which gives them a higher probability of recombining. A biased segregation of B. campestris-or B. campestris-like mitochondria was found in all combinations. A different degree of phylogenetic relatedness between the fusion partners did not have a significant influence on mitochondrial segregation in the hybrids in this study.

The chloroplast and mitochondrial DNA type are correlated with the nuclear composition of somatic hybrid calli of Solanum tuberosum and Nicotiana plumbaginifolia

This paper describes the analysis of chloroplast (cp) DNA and mitochondrial (mt) DNA in 21 somatic hybrid calli of Solanum tuberosum and Nicotiana plumbaginifolia by means of Southern-blot hybridization. Each of these calli contained only one type of cpDNA; 14 had the N. plumbaginifolia (Np) type and seven the S. tuberosum (St) type. N. plumbaginifolia cpDNA was present in hybrids previously shown to contain predominantly N. plumbaginifolia chromosomes whereas hybrids in which S. tuberosum chromosomes predominated possessed cpDNA from potato. We have analyzed the mtDNA of these 21 somatic hybrid calli using four restriction enzyme/probe combinations. Most fusion products had only, or mostly, mtDNA fragments from the parent that predominated in the nucleus. The hybrids containing mtDNA fragments from only one parent (and new fragments) also possessed chloroplasts from the same species. The results suggest the existence of a strong nucleo-cytoplasmic incongruity which affects the genome composition of somatic hybrids between distantly related species.

Analysis of the mitochondrial DNA of the somatic hybrids of Solanum brevidens and S. tuberosum using non-radioactive digoxigenin-labelled DNA probes

Mitochondrial (mt) DNAs of somatic hybrids obtained by electrical and chemical fusion of mesophyll protoplasts of S. brevidens and a dihaploid line of S. tuberosum PDH 40 were analysed by Southern hybridization using the digoxigenin-labelled mtDNA sequences nad5 or orf25. In the Southern analysis of the hybrid mtDNA probed with nad5, most of the 19 hybrids analyzed had an RFLP pattern similar, but not identical, to one of the parents, S. tuberosum, PDH40. Nineteen percent of the hybrids had most of the S. brevidens fragments. Five of the hybrids had an identical RFLP pattern to either one of the parents while another two hybrids had novel RFLP patterns. Similar results were obtained by Southern analysis with orf25. These results clearly show that mtDNA rearrangements had occurred at a high frequency in the somatic hybrids. There were no differences in the frequencies of rearrangements observed between the hybrids regenerated from chemical and electrical fusions.

Asymmetric protoplast fusion aimed at intraspecific transfer of cytoplasmic male sterility (CMS) in Lolium perenne L

Techniques have been developed for the production of cybrids in Lolium perenne (perennial ryegrass). Gamma-irradiated protoplasts of a cytoplasmically male-sterile breeding line of perennial ryegrass (B200) were fused with iodoacetamide-treated protoplasts of a fertile breeding line (Jon 401). After fusion 25 putative cybrid calli were characterized to determine mitochondrion type and composition of the nuclear genome. Analysis of phosphoglucoisomerase isozyme profiles and determination of the ploidy level by flow cytometry indicated that all of the calli tested essentially contained the nuclear DNA of the fertile line. However, the presence of parts of the nuclear DNA from the sterile line could not be excluded. Southern blotting of total DNA isolated from the parental lines and putative cybrids combined with hybridizations using the mitochondrial probes cox1 and atp6 revealed that the mitochondria of the calli originated from the fertile line (5 calli), the sterile line (5 calli) or from both parental lines (15 calli). The hybridization patterns of the mtDNA from the cybrid calli showed extensive quantitative and qualitative variation, suggesting that fusion-induced inter- or intramolecular mitochondrial recombination had taken place.

Analysis of chloroplast and mitochondrial segregation in three different combinations of somatic hybrids produced within Brassicaceae

Mitochondrial and chloroplast DNA were characterized in three different combinations of somatic hybrids produced between different species within Brassicaceae. The fusions were made between B. campestris and B. oleracea, B. napus and B. nigra and between B. napus and Eruca sativa. The combinations represent interspecific hybridizations, but the phylogenetic distance between the species used in each instance is different. Whereas the B. campestris (+) B. oleracea and the B. napus (+)B. nigra hybrids are both examples of intrageneric hybrids, B. campestris is more closely related to B. oleracea than B. napus is to B. nigra. The fusion of B. napus and E. sativa represents an intergeneric hybridization. Since hybrids were produced with reproducible and uniform fusion and culture methods, a comparison of chloroplast and mitochondrial segregation and mitochondrial DNA (mt-DNA) rearrangements could be made between the combinations. The segregation of both chloroplasts and mitochondria was biased in the B. napus (+)B. nigra and the B. napus (+)E. sativa combination. The nonrandom segregation of chloroplasts and mitochondria could be due to the different ploidy levels of the fusion partners and/or reflect differences in organelle replication rate. Furthermore, segregation of mitochondria was correlated to the differences in phylogenetic distance between the species used in the fusions. However, mitochondrial segregation, in contrast to chloroplast segregation, could in all combinations also have been affected by the cell type used as protoplast source in the fusions. All different chloroplast types could be established within each combination. Hybrids containing chloroplast from one parent together with mitochondria from the other parent were found in two of the combinations, although the majority of the hybrids had mt-DNA that was altered compared to the parental species. The rearranged mt-DNA found in most hybrids was an effect of the heteroplasmic state following protoplast fusion rather than of the tissue culture methods, since no mt-DNA rearrangements were found in B. napus plants regenerated from protoplast culture. The mtDNA restriction patterns of the hybrids with rearranged mt-DNA indicated that specific regions of the mt-DNA were involved in the rearrangements following protoplast fusion.

Protoplast-fusion-derived Solanum cybrids: application and phylogenetic limitations

We established interspecific Solanum cybrids in order to study the intrageneric nuclear-organelle compatibility and the introgression of advantageous plasmone-coded breeding traits into potato. Cybridization was performed by the donor-recipient protoplast-fusion procedure. We found that the plastomes of S. chacoense, S. brevidens, and S. etuberosum could be transferred into the cybrids having S. tuberosum nuclear genomes; chondriome components were likewise transferred from the former species into these cybrids. The combination with S. chacoense as organelle donor and potato as recipient resulted in green fertile plants with potato morphology. By using S. etuberosum as an organelle donor and potato as recipient, male-sterile cybrid plants, most of them having pigmentation abnormalities, were obtained. The combination of S. brevidens with potato resulted in palegreen (almost albino) regenerants. The latter albino plantlets had both the chloroplast DNA and the mitochondrial DNA of the donor (S. brevidens) and did not survive the transfer into the greenhouse. An immediately applicative result of this study is the de novo establishment of male-sterile plants in a potato cultivar. Such plants should be useful as seed parents in the production of hybrid, true-potato seeds.

Cybrids of Nicotiana tabacum and Petunia hybrida have an intergeneric mixture of chloroplasts from P. hybrida and mitochondria identical or similar to N. tabacum

The mitochondrial genomes of cybrids of Nicotiana tabacum containing chloroplasts of Petunia hybrida were characterized by restriction endonuclease digestion and agarose gel electrophoresis. Cybrids that displayed normal growth and development contained mitochondrial DNA indistinguishable from N. tabacum mitochondrial DNA. Cybrids that displayed abnormal growth and development contained mitochondrial DNA that differed from N. tabacum either by possessing a few additional fragments, by lacking a few fragments, or both. In spite of these differences, the mitochondrial DNA of cybrids showing abnormal growth and development was much more similar to N. tabacum than to P. hybrida mitochondrial DNA. In those cybrids that contained P. hybrida chloroplasts and N. tabacum mitochondria, cotransfer of cytoplasmic organelles did not occur. Although P. hybrida chloroplasts can interact compatibly with the N. tabacum nucleus, no cybrids were found in which P. hybrida mitochondria coexisted with the N. tabacum nucleus.

Rearrangement, amplification, and assortment of mitochondrial DNA molecules in cultured cells of Brassica campestris

We compared Brassica campestris mitochondrial and chloroplast DNAs from whole plants and from a 2-year-old cell culture. No differences were observed in the chloroplast DNAs (cpDNAs), whereas the culture mitochondrial DNA (mtDNA) was extensively altered. Hybridization analysis revealed that the alterations are due entirely to rearrangement. At least two inversions and one large duplication are found in the culture mtDNA. The duplication element is shown to have the usual properties of a plant mtDNA high frequency "recombination repeat". The culture mtDNA exists as a complex heterogeneous population of rearranged and unrearranged molecules. Some of the culture-associated rearranged molecules are present in low levels in native plant tissue and appear to have sorted out and amplified in the culture. Other mtDNA rearrangements may have occurred de novo. In addition to alterations of the main mitochondrial genome, an 11.3 kb linear mtDNA plasmid present in whole plants is absent from the culture. Contrary to findings in cultured cells of other plants, small circular mtDNA molecules were not detected in the B. campestris cell culture.

Field assessment of dihaploid Solatium tuberosum and S. brevidens somatic hybrids

Following both chemical and electrical fusion of protoplasts of a dihaploid line of potato (Solanum tuberosum), (PDH40), with those of the wild species, Solanum brevidens, 11 and 40 somatic hybrid plants, respectively were obtained. Fifteen of these somatic hybrid genotypes and the two parents were studied further in a small field trial to assess field performance and phenotypic variability. In the UK, somatic hybrid plants are classified as genetically engineered organisms, and the UK Advisory Committee on Genetic Manipulation have imposed various restrictions on field experiments. Examination of the somatic hybrids in the field showed extensive phenotypic variability, and no two genotypes were identical. Some of the variation reflected changes in chromosome numbers, but other factors were also involved. Half the somatic hybrid genotypes produced tubers in the field, although the tubers were smaller and differed morphologically from those of PDH40. The results of the study suggest that the extent of somaclonal variation manifested in somatic hybrids is greater than that found in protoplast-derived plants of potato. The implications of this observation and the current regulations concerning field experiments of somatic hybrid plants in the UK are discussed.

Resistance to potato leaf roll virus and potato virus Y in somatic hybrids between dihaploid Solanum tuberosum and S. brevidens

Many somatic fusion hybrids have been produced between a dihaploid potato Solanum tuberosum and the sexually-incompatible wild species S. brevidens using both chemical and electrical fusion techniques. S. brevidens was resistant to both potato leaf roll virus (PLRV) and potato virus Y (PVY), the viruses being either at low (PLRV) or undetectable (PVY) concentrations as determined by enzyme-linked immunosorbent assay (ELISA). The S. tuberosum parent was susceptible to both viruses. A wide range of resistance, expressed as a decrease in virus concentration to both viruses was found amongst fusion hybrids, four of which were especially resistant. The practicality of introducing virus resistance from S. brevidens into cultivated potatoes by somatic hybridisation is discussed.