Non-random inheritance of organellar genomes in symmetric and asymmetric somatic hybrids between Lycopersicon esculentum and L. pennellii

Molecular and cytogenetic description of somatic hybrids between Gentiana cruciata L. and G. tibetica King

Somatic hybridization provides an opportunity to create cells with new genetic constitution. Here, the interspecific somatic hybrid plants regenerated in vitro following fusion of cell suspension-derived protoplasts of tetraploid Cross Gentian (Gentiana cruciata L., 2n = 52) with protoplasts released from mesophyll tissue of another tetraploid species, Tibetan Gentian (G. tibetica King, 2n = 52), were studied. According to the results of genome analyses with AFLP, ISSR, and CAPS markers, all somatic hybrids were genetically closer to "suspension" fusion partner G. cruciata than to "mesophyll" partner G. tibetica, but they got G. tibetica chloroplasts. Chromosome counting revealed little variation in the number of chromosomes in hybrid's cells (2n = 88 or 2n = 90), although all plants possessed similar nuclear DNA content which remained stable even after 2 years of in vitro culture. Fluorescence in situ hybridization (FISH) showed that hybrids possessed 4 to 7 chromosomes bearing 5S rDNA sites and 6 or 7 chromosomes with 35S rDNA sites. A part of FISH signals was smaller than those observed in the parental species, which could indicate the loss of rDNA sequences. Genomic in situ hybridization (GISH) showed the predominance of the number of G. cruciata chromosomes over chromosomes of G. tibetica. However, a significant level of cross-hybridization was observed for about one-third of hybrid chromosomes, indicating a high degree of homeology between the genomes of G. cruciata and G. tibetica.

Rare symmetric and asymmetric Nicotiana tabacum (+) N. megalosiphon somatic hybrids recovered by selection for nuclear-encoded resistance genes and in the absence of genome inactivation

Following protoplast fusion between Nicotiana tabacum (dhfr) and N. megalosiphon (nptII) somatic hybrids were selected on the basis of dual resistance to kanamycin and methotrexate. Despite strong selection for parental nuclear-encoded resistances, only nine N. tabacum (+) N. megalosiphon somatic hybrids were obtained. A preferential loss of the parental N. tabacum nuclear and organelle genome was apparent in some plants in spite of the lack of genomic inactivation by the irradiation or chemical treatment of the parental protoplasts. Only six of the nine hybrids recovered possessed both parental profiles of nuclear RFLPs and isoenzymes. The remaining three hybrids were highly asymmetric with two being identical to N. megalosiphon except for minor morphological differences and rearranged or recombined mitochondrial DNAs (mtDNA), while the other one was distinguishable only by the presence of a rearranged or recombined mtDNA, and was therefore possibly a cybrid. Overall, eight somatic hybrids possessed rearranged or recombined mtDNAs and chloroplast inheritance was non-random since eight possessed N. megalosiphon-type chloroplasts and only one had N. tabacum chloroplasts. In contrast, using the same selection approach, numerous morphologically similar symmetric somatic hybrids with nuclear RFLPs and isozymes of both the parental species were recovered from control fusions between N. tabacum and the more closely related N. sylvestris. In spite of the low frequency of recovery of symmetric N. tabacum (+) N. megalosiphon hybrids in this study, one of these hybrids displayed a significant degree of self-fertility allowing for back-crosses to transfer N. megalosiphon disease-resistance traits to N. tabacum.

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.

Distribution patterns of number of nuclei in conidia from heterokaryons of Heterobasidion annosum (Fr.) Bref. and their interpretation in terms of genomic conflict

Ratios of nuclear genotypes observed in conidia from heterokaryotic strains of Heterobasidion annosum (Fr.) Bref., obtained from pairings between sympatrically derived, sib-related and non-sib-related homokaryons, commonly deviated from 1:1. Ratios were temporally stable, and the genotypes examined could be ranked in a strict dominance hierarchy, linked both to the relatedness of the association partners and to the growth rates of the parental homokaryons. Parental homokaryons and sibrelated heterokaryons produced conidia with a mean number of nuclei of about two, whereas non-sib-related-d heterokaryons produced conidia that were predominantly uninucleate. Moreover, whereas conidia containing more than one nucleus germinated most rapidly when derived from homokaryons or sib-related heterokaryons, uninucleate conidia germinated more readily if derived from non-sib-related heterokaryons. In a study of naturally occurring heterokaryons, distribution patterns of the number of nuclei in conidia were found to be similar to those of the homokaryons. The possible interpretation of these findings in terms of inter-nuclear genomic conflict is discussed.

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.

Chloroplast segregation in somatic hybrids of Nicotiana plumbaginifolia and N. sylvestris having different ratios of parental nuclear genomes

Fusion of mesophyll protoplasts of haploid Nicotiana plumbaginifolia (P) and N. sylvestris (S) resulted in the production of somatic hybrid plants of various ploidy levels. Analysis of the restriction fragment patterns of chloroplast DNA from 118 plants belonging to genome constitutions PS, PPS, PSS, and PPSS revealed that two had a pattern corresponding to a mixture of parental DNA while all the others had the pattern of either N. plumbaginifolia or N. sylvestris. In the latter case, the ratio of the two parental types fits 1∶1 in all the four genome constitutions studied. Since the protoplasts used in the fusion experiment were physiologically similar and the hybrid cells were not deliberately selected, these results suggest that chloroplast segregation in the somatic hybrids is independent of the chloroplast input of the fusion partners and the nuclear background of the fusion products.

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.

Sexual and somatic hybridization in the genusLycopersicon

In recent years, a large number of reports have been published on the recovery of somatic hybrids in the genusLycopersicon and their potential use as a tool in plant breeding programs. Somatic hybridization as a way of enabling the incompatibility barriers which exist within the genusLycopersicon to be bypassed has attracted great interest. WildLycopersicon species harbor numerous interesting agronomic characteristics, which could be transferred to tomato by somatic hybridization. In particular, the production of asymmetric hybrids is explored as an approach to obtain the transfer of only a part of the nuclear genome of wildLycopersicon species. Considerable information is available on the fate of chloroplasts and mitochondria in fusion products inLycopersicon, and unfortunately, cybridization (transfer of chloroplasts and/or mitochondria) seems often difficult to achieve.