INHERITANCE OF ORGANELLES IN SOMATIC CELL HYBRIDS AND CYBRIDS OF NICOTIANA

In somatic cell hybrids and cybrids of Nicotiana, research has indicated the presence of a hierarchy of organelle dominance between various species. This putative hierarchy was based on results using biochemical selection of hybrids. I tested the possibility that biochemical selection pressure biased the results. Protoplasts of various alloplasmic substitutions of Nicotiana were fused together; heterologous fusion events were isolated using fluorescence-activated cell sorting. Leaves of plants regenerated from these fusion events were analyzed to identify the types of chloroplast and mitochondrial DNA present. The results reported here indicate that the hierarchy of organelle dominance appears to be a real phenomenon, even when selective pressures are removed. Chloroplasts from Nicotiana undulata strongly outcompeted chloroplasts from N. tabacum. Chloroplasts from N. suaveolens and N. glauca equally competed with chloroplasts from N. tabacum. Analysis of mitochondrial DNA revealed that mitochondria segregated independently from chloroplasts in the N. undulata - N. tabacum and N. glauca - N. tabacum fusions, but not in the N. suaveolens - N. tabacum fusions. After heterologous fusion events, mitochondrial DNA recombination was detected in some plants, but chloroplast DNA recombination was not observed. In addition, male fertility was restored in a plant regenerated from a fusion between protoplasts of two cytoplasmic male sterile, alloplasmic substitutions. These alloplasmic substitutions contained the cytoplasm of N. suaveolens or N. plumbaginifolia, each in a N. tabacum nuclear background. Thus both nuclei were identical but each of the two protoplast types had the cytoplasm of either N. suaveolens or N. plumbaginifolia. The fertile plant contained N. plumbaginifolia chloroplast and mitochondrial DNA restriction patterns, and displayed a floral morphology intermediate between the N. plumbaginifolia alloplasmically substituted parent and fertile N. tabacum. The flowers of the fertile plant were only able to set seed when manually pollinated