The mitochondrial genomes of petunia somatic hybrid plants, which were derived from the fusion of male fertile P. hybrida protoplasts with cytoplasmic male sterile P. parodii protoplasts, were analyzed by endonuclease restriction and Southern blot hybridization analyses. We studied sterile and fertile somatic hybrids to address two main questions. First, is there any correlation between the mitochondrial DNA restriction banding patterns of the sterile and fertile parents and the banding patterns of the respective sterile and fertile somatic hybrids? Second, does the structure of somatic hybrid mitochondrial genomes differ from the parental mitochondrial genomes?
We identified no clear sterile-specific correlation between the mitochondrial DNA restriction patterns of cytoplasmic male sterile somatic hybrids and those of the male sterile parent. Similarly, we found no clear relationship between the mitochondrial DNA restriction patterns of male fertile somatic hybrids and those of the male fertile parent. In view of this finding and the evidence that cytoplasmic male sterility and fertility segregate in somatic hybrids of petunia (Izhar et al. 1983) the nature of cytoplasmic male sterility in petunia differs significantly from cytoplasmic male sterility in tobacco (Gerstel 1980).
Restriction fragment patterns show that the somatic hybrid mitochondrial genomes differ from each other and from both parents. Somatic hybrid mitochondrial genomes consist of DNA fragments derived from both parents in novel combinations. Hybridization data revealed the fates of parental restriction fragments in the somatic hybrids. Parental fragments may be present or absent in all somatic hybrids analyzed, or they may be present in some somatic hybrids and absent in others. These data are consistent with two non-exclusive possibilities. Separate DNA molecules might assort following protoplast fusion or intermolecular recombination might occur following protoplast fusion.