Analysis of nuclear and organellar DNA of somatic hybrid calli and plants between Lycopersicon spp. and Nicotiana spp

Protoplast fusion experiments between Lycopersicon esculentum or L. peruvianum and Nicotiana tabacum or N. plumbaginifolia were performed to investigate the possibility of producing symmetric and asymmetric somatic hybrids between these genera. These fusions, which involved 1.7 x 10(8) protoplasts, yielded 35 viable hybrid calli. Plant regeneration was successful with two calli. One of these regenerants flowered, but developed no fruits. Analysis of the nuclear DNA by means of dot blot hybridization with species-specific repetitive DNA probes combined with flow cytometry, revealed that the nuclei of most hybrid calli contained the same absolute amount of Nicotiana DNA as the Nicotiana parent or (much) less, whereas the amount of Lycopersicon DNA per nucleus was 2-5 times that of the parental genotype. Eighteen of the 34 hybrids analyzed possessed Lycopersicon chloroplast DNA (cpDNA), whereas the other 16 had DNA from Nicotiana chloroplasts. The cpDNA type was correlated with the nuclear DNA composition; hybrids with more than 2C Nicotiana nuclear DNA possessed Nicotiana chloroplasts, whereas hybrids with 2C or less Nicotiana nuclear DNA contained Lycopersicon chloroplasts. Mitochondrial DNA (mtDNA) composition was correlated with both nuclear DNA constitution and chloroplast type. Hybrids possessed only or mainly species-specific mtDNA fragments from the parent predominating in the nucleus and often providing the chloroplasts. The data are discussed in relation to somatic incompatibility which could explain the low frequency at which hybrids between Lycopersicon and Nicotiana species are obtained and the limited morphogenetic potential of such hybrids.