Somatic Hybrids for Cytoplasmic Male Sterility,Mesophyll Protoplasts,Morphology,Physiology,Nuclear Genotype,Hybrid Mitochondrial Genome

	Somatic Hybrids for Cytoplasmic Male Sterility - Recently methods have been developed to substitute the nucleus of one species into the cytoplasm of another species, whose mitochondria are inactivated. This type of substitution in some cases, led to generation of cytoplasmic male sterility. For this purpose, in a recent report (published in August, 1992; PNAS, USA Vol. 89: 68-32) published by G. Melchers (from Germany) and his coworkers (from Japan), the two types of protoplasts, used for the production of, somatic hybrids, were treated differently, as follows:
(i) mesophyll protoplasts of tomato (Lycopersicon esculentum) were treated with iodoacetamide (IDA) to inactivate mitochondria and (ii) mesophyll protoplasts of Solanum acaule (or S. tuberosum = potato) were irradiated with γ or x-rays to inactivate nuclei. The protoplasts were mixed in 1: 1 ratio and induced to fuse using Ca₂+ and PEG, leading to the production of heterologous hybrids. Among the fusion products, some hybrid tomato plants were indistinguishable from the original cultivars, with respect to morphology, physiology and chromosome number (2n=24), but exhibited various degrees of male sterility. The variation included
(i) complete lack of pollen or malformation of anthers; (ii) shrunken pollen, (iii) normal looking stainable pollen that could not germinate. In five tomato cultivars, male sterility induced in this manner was inherited maternally over several generations. Therefore, it was obviously cytoplasmic male sterility. The mitochondrial DNA of these CMS hybrids did not resemble mtDNA of either parent, and was instead recombinant type, representing a hybrid mitochondrial genome.
Therefore, protoplast fusion can be effectively used for production of CMS lines and has the following advantages: (i) only one step is required; (ii) the nuclear genotype of the cultivar remains unaffected, (iii) there are prospects that 100% of the progenies of somatic hybrids will be CMS. The restorer lines for these CMS lines have also been shown to be available in tomato, so that hybrid seed can be produced without manual emasculation.