While late-breaking partial fertility restoration of cytoplasmic male sterility is important both economically and theoretically, its inheritance is not well understood. Sublines of the maize inbred A632 differing in the level of restoration of C-cytoplasmic male sterility were identified. One line was nearly completely male sterile, whereas the others had varying levels of partial restoration; full fertility restoration was not observed. We crossed the fertile selections to the male sterile one and evaluated the progeny of the crosses to demonstrate the genetic nature of these differences. In two different experiments the percentage of plants that exhibited partial fertility was significantly different (P = .01). In experiment I, the progeny of the cross A632-Pr-LB-1 x A632-Pr-MS were 55% partially fertile while the progeny of A632-Pr-LB-2 x A632-Pr-MS were 75% partially fertile. In experiment II, 3% of the plants exhibited partial fertility in one cross while 9% did so in the other cross. All differences were significant. Progeny tests were performed over years, and the effect of years on fertility restoration was highly significant. The number of fertile individuals was strongly affected by the environment with only 1.4% of the plants shedding pollen in 1984 and 13.0% shedding pollen in 1985. However, there was no environment by inbred interaction for fertility restoration. Due to low penetrance and strong environmental effects it was not possible to estimate the number of genes involved. However, it appears that this trait is under polygenic control.

Cytoplasmic male sterility (cms) was found in plants derived from the F2 progeny of fertile, normal cytoplasm plants of the inbred R181 pollinated with a genetic stock carrying the recessive nuclear gene, iojap. The male sterile plants were maintained by back-crossing with the inbred W182BN which maintains all known sources of cytoplasmic male sterility. The new male sterile progeny were found to exhibit stable male sterility under field conditions in two environments. However, they were partially fertile in the hot, dry summer of 1983 at Aurora, NY. It was found that these lines were restored by lines that characteristically restore cms S group cytoplasms. Pollen phenotype studies indicated that the restoration was gametophytic in nature, also characteristic of the cms S group. Agarose gel electrophoresis of undigested mitochondrial DNA (mtDNA) from these steriles indicated that these lines have the S-1 and S-2 episomes characteristic of the cms S group. Restriction endonuclease digest patterns of mtDNA from these sterile lines digested with BamH I indicated that these steriles fit into the CA subgroup of the cms S group. The new source of cms has been designated cms Ij-1.

Differences in fertility restoration and mitochondrial nucleic acids permitted division of 25 accessions of S-type male sterile cytoplasm (cms-S) of maize into five subgroups: B/D, CA, LBN, ME, and S(USDA). S cytoplasm itself (USDA cytoplasm) was surprisingly not representative of cms-S, since only two other accessions, TC and I, matched its mitochondrial DNA pattern. CA was the predominant subgroup, containing 18 of the 25 accessions. The B/D and ME subgroups were the most fertile and LBN the most sterile. The exceptional sterility of LBN cytoplasm makes it the most promising of the 25 cms-S accessions for the production of hybrid seed. The most efficient means of quantifying the fertility of the subgroups was analysis of pollen morphology in plants having cms-S cytoplasm and simultaneously being heterozygous for nuclear restorer-of-fertility (Rf) genes. This method took advantage of the gametophytic nature of cms-S restoration. The inbred NY821LERf was found to contain at least two restorer genes for cms-S. Fertility differences were correlated with mitochondrial nucleic acid variation in the LBN, ME, and S (USDA) subgroups.

ABSTRACT The genetics of fertility restoration of cms-C group cytoplasm of maize was studied using crosses involving stable maintainer lines and lines that restored full pollen fertility. Pollen fertility in the sources of cms-C sterile cytoplasms studied was restored by a single dominant restorer (Rf4) gene. The fertility restoration was sporophytic. Allelism tests among five restorer lines showed that they all apparently carried the same alleles (Rf4 Rf4). Similar tests also demonstrated that seven nonrestoring maintainer lines had apparently the same genotype (rf4 rf4), although a partial "late break" of fertility was observed at low levels in some maintainer crosses. Comparative studies among different cms-C sources (C, Bb, ES, PR and RB) indicated that similar inheritance of fertility restoration was involved. The data indicated that a single, dominant Rf gene is involved in the restoration of several C-group cytoplasms, at least in the lines studied here. This is the first single-gene, sporophytic restorer system described in maize to date.