Somatic hybrids of Nicotiana knightiana (2n=2X=24) and an albino mutant of Nicotiana tabacum (2n=4X=48) were selected after polyethylene glycol induced protoplast fusion. Three lines were selected on the basis of the simultaneous expression of shoot inducibility and green pigmentation, traits originally separated in the parental species. The hybrid nature of the lines was confirmed by their characteristic isoenzyme patterns, the morphology of the regenerated plants, and by the appearance of heterochromatic blocks in the interphase nuclei. Chromosome numbers in the somatic hybrids varied greatly within individual plants. Variegation in leaf and flower colour and segregation for morphological traits in vegetatively multiplied plants are attributed to segregation of chromosomes in the somatic cells, a consequence of the numerical instability. Hybridity, caryotypic changes induced by tissue culture, and high chromosome numbers, are discussed as possible reasons for the observed genetic instability.
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