Cytoplasmic male sterility in Nicotiana, restoration of fertility, and the nucleolus.

Plants with the cytoplasm of Nicotiana repanda and the chromosomes of N. tabacum produce nonfunctional and feminized anthers. Introduction of a satellited fragment chromosome, apparently derived from N. repanda, restores normal anthers and pollen fertility. The fragment is somatically stable and addition-homozygotes transmit it to the great majority of their offspring. Cells with the fragment exhibit amphiplasty, i.e. , in plants having one or two fragments, nucleolar organizers of N. tabacum are suppressed and the nucleoli are entirely or largely produced by the fragments. Formation of nucleoli by organizers from N. repanda in N. repanda cytoplasm may thus be a condition for male fertility. The manner in which nucleoli may influence the development of originally indeterminate sex primordia is discussed. Further cytological studies of cytoplasmically male-sterile plants and restorer chromosomes are needed to decide whether the observed relationship is of functional significance or merely accidental.