Allelic frequency at the Rf3 and Rf4 loci and the genetics of A3 cytoplasmic fertility restoration in converted sorghum lines

Cytoplasmic-nuclear male sterility (CMS) in sorghum [Sorghum bicolor (L.) Moench] is integral to hybrid seed production. One such CMS, A3, is controlled by a two gene gametophytic fertility restoration system involving two restoration alleles, Rf3 and Rf4, which are required for individual male gamete viability. A survey of the allelic frequencies of Rf3 and Rf4 in the sorghum germplasm collection was undertaken using 140 lines from the Sorghum Conversion Program. These lines were hybridized to two testers, A3FL3 (Rf3Rf3rf4rf4) and A3FL4 (rf3rf3Rf4Rf4). The resulting hybrids were evaluated for I-2-KI pollen stainability and seed set to determine their fertility restoration reaction. Transcript analyses were performed on lines that contained restoration alleles to determine whether they fit the expected genetic model of fertility restoration. Three lines (SC306, SC512, and SC215) were found to restore fertility with both testers and thus contain both restoration alleles. The surveyed allelic frequency of Rf3 and Rf4 was 5.1 and 4.8%, respectively. Using transcript analyses, two exceptional lines were identified (SC306 and SC215) that carried Rf3 but did not confer enhanced transcript processing activity (TPA) of mitochondrial orf107. Enhanced TPA was thought to be tightly linked to or represent gene action of Rf3, but identification of these lines demonstrates that enhanced TPA is not required for fertility restoration and does not represent gene action of Rf3.