Role of nucleolar chromosomes in anther-restoration of male-sterile tobacco

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Abstract
Combining the cytoplasm from one species of Nicotiana with the nucleolus from another often results in male sterility. The damage may be reduced by addition of a chromosome from the genome of the cytoplasm donor. This was shown previously, when chromosomes from N. tabacum were substituted in the cytoplasms of N. repanda and N. debneyi, and again in the current study with cytoplasm from N. undulata. In all three combinations, the restorer chromosomes carried nucleolar organizing regions (NOR's), were consistently associated with nucleoli at meiotic prophase, and exhibited satellites in somatic metaphases. Furthermore, activity of NOR's of N. tabacum chromosomes was weakened by amphiplasty. Fragment chromosomes having the NOR from the cytoplasm donor and only a small portion of the remainder of the chromosome behaved like the intact chromosomes. One may thus surmise that an interaction between NOR's and cytoplasm from the same species is required for anther development. A theoretical model for this relationship is proposed. Close and highly conserved linkage between ribosomal DNA cis-trons of the NOR and restorer(s), or a nonspecific effect of NOR heterochromatin are other possibilities.