Cytogenetics of flower modification of a cytoplasmic male-sterile tobacco

Plants combining the cytoplasm of Nicotiana debneyi and the 48 chromosomes from N. tabacum are male sterile. Early backcross generations of the amphidiploid hybrid to male N. tabacum produced a great variety of plants from which a series of phenotypes with characteristic flower forms and transmission rates have been isolated. Type 1A possesses completely feminized stamens and deeply split corollas, breeds true when backcrossed to normal males and carries 48 N. tabacum chromosomes. Other phenotypes, 2C, 3E and 4H, range toward normal morphology of corollas and stamens. Like 1A, 2C forms no anther tissue and has 48 chromosomes. This type is transmitted to 36.3% of the backcross progeny, the remainder being of type 1A; presumably 2C carries a chromosome segment from N. debneyi that is responsible for the partial restoration of flower structure. In contrast, both 3E and 4H produce anthers and possess an extra chromosome. The extra chromosomes are transmitted to only 19.9% and 7.1% of the progeny, respectively. Significantly, the extra chromosomes found in the anther-forming types are nucleolus organizing and carry a satellite from N. debneyi. On the basis of these observations, we surmise that differentiation of anthers in plants with N. debneyi cytoplasm may depend on the presence of a nucleolus-organizing chromosome from that species. This chromosome is unstable; unaltered, it conditions a highly restored phenotype (4H), but when structurally modified, it may control different phenotypic expressions. Other examples of satellited restorer chromosomes had been reported for different cytoplasmically male-sterile combinations; therefore, the phenomenon may have more general significance.