Callus cultures were established from immature embryos of inbred W182BN with the S (USDA) type of cytoplasmic male sterility (cms) in 1981, 1983, and 1984; plants were regenerated from these cultures (MNL 57:53). At least some plants from 16 of 20 different cultures had fertile tassels. Male-fertility of other plants from these cultures could not be scored accurately because of abnormal plant morphology; however, seeds were obtained from many regenerates after pollination with standard W182BN. Fertile progeny were obtained from 13 of 14 cultures, including ones from different subgroups of cms-S (CA, S, Me, D, and L).

Fertile regenerated plants always gave fertile progeny, others produced either all fertile or all sterile ones. Some callus cultures produced only fertile progeny, others initially produced sterile progeny and later fertile ones, and still others produced both fertiles and steriles at the same time. Some fertile plants or progeny were recovered from cultures that were only 3 months old, indicating that at least some types of somaclonal variation in corn can occur rather rapidly.

Several lines of evidence indicate that the observed reversions to fertility are maternally inherited. These include the absence of segregation for fertility even after pollination with standard W182BN, results from reciprocal crosses, results from studies of pollen fertility, and mitochondrial DNA data (see below). Revertant plants did not restore standard cms-S lines to fertility, further evidence that they have not acquired nuclear restorer genes. Inheritance studies of the more recently isolated revertants are still in progress, but the results to date are consistent with those from earlier revertants.

Agarose gel electrophoresis of unrestricted mitochondrial DNA (mt DNA) of 37 regenerated plants and/or progeny from the cms-S cultures was carried out. All fertile lines lacked the S-1 and S-2 plasmid-like molecules characteristic of sterile cms-S lines. Fertility of mature regenerates (or their progeny) could be predicted with 100% accuracy from analysis of leaf samples from small regenerates. These results are consistent with those from studies of the first cytoplasmic cms-S revertants found in the field by Laughnan and Laughnan-Gabay; these also lacked free S-1 and S-2, portions of which were integrated into the high MW mtDNA. More recently, Escote et al. have reported that cytoplasmic revertants from inbred Wf9 retain free S-1 and S-2 (MGL 59:100). We have therefore initiated cultures from Wf9 with cms-S (subgroup D) and have begun regenerating plants. These plants will be checked for fertility and the presence of the free plasmids.

No revertants to fertility were seen among plants regenerated from cultures of W182BN with 2 other types of cms (C and T) or among their progeny. These results suggest that the mitochondrial DNA of cms-S maize is particularly unstable in vitro. The cytoplasmic revertants recovered from culture may aid in the molecular analysis of cms-S by providing large numbers of independent revertants for comparative studies.

E.D. Earle and V.E. Gracen
 
 

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