Gracen and Grogan in their 1974 paper comparing the restoration patterns of male-sterile cytoplasms left B and D cytoplasms unclassified because of their unusual restoration patterns (Agron. J. 65:654). In 1980, two British groups, using assays for specific DNA species and in vitro translation products of mitochondrial DNA, grouped B and D with the S-type cytoplasms (Kemble et al., Genetics 95:451, and Forde et al., Genetics 95:443). We have confirmed the report that B and D cytoplasms have the "S-bands" which are found in all other S-types. In addition, studies of pollen restoration in sterile x restorer crosses have shown that the restoration pattern of B and D is gametophytic, another characteristic of the S group.
Nevertheless, B and D are unusually fertile in several inbred backgrounds. For example, in inbred CrS4HLA in Cornell's cytoplasm bank, all S types except B and D have no fertile pollen, whereas B and D are fertile enough to be selfed. Even in those inbreds in which B and D are sterile--Ay191-71, C0150, NYD410, and W64A--differences in restoration capacity exist. When these four inbreds in the sterile cytoplasms are crossed as females with restorers, the F1 plants in B and D cytoplasm have a greater percentage of fertile pollen than do the F1 plants in the other sterile cytoplasms. The difference is statistically significant (P < .05). It is theoretically possible that the unusual restoration pattern of B and D is due to residual nuclear restorer genes, but this is unlikely. B and D are different in every inbred line we have tested for restoration, and up to 12 backcrosses have been made to each of these lines. We therefore suspect that some part of the cytoplasmic genome differentiates these two cytoplasms from other members of the S group.
P. H. Sisco and V. E. Gracen
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