Conversion of fertility restoration of the sorghum IS1112C (A3) male-sterile cytoplasm from two genes to one gene

The restoration of male fertility in sorghum [Sorghum bicolor (L.) Moench] lines carrying the IS1112C male-sterile cytoplasm is characterized by limited seed set. Restoration requires complementary gametophytic action of two restoring alleles, Rf3 and Rf4, for individual gamete viability, and an F, heterozygous for the two restoring loci is expected to exhibit 25% viable pollen. The objective of this study was to demonstrate the feasibility of converting restoration to single gene behavior. Single-seed descent B3Tx398/IS1112C F-5:6 lines were used to generate homozygosity at the rf3 and rf4 loci, and critical segregants were identified by progeny tests, assays for action of the Rf3 allele, and genomic markers for the f4 locus. Using these criteria, we constructed the genotypes Rf3Rf:3rf4rf4 and rf3rf3f4Rf4 in normal, male-fertile cytoplasm lines, and in IS1112C male-sterile cytoplasm lines. Pollination of either male-sterile line with a matching male-fertile line resulted in about 25% pollen staining in the F-1, demonstrating complementation of the two restoring loci. Pollination of either male-sterile line with IS1I112C, Rf,3Rf3Rf4Rf4, resulted in about 50% pollen staining. These characteristics substantiate complementary action of the restoring alleles, and are consistent with successful conversion to a single gene fertility restoration system.