Selection of Nuclear Genotypes Associated with the Thermo-inducibility of Owen-type Cytoplasmic Male Sterility in Sugar Beet (Beta vulgaris L.)

The stability of cytoplasmic male sterility expression in several genetic backgrounds was investigated in sugar beet (Beta vulgaris L.). Nine genetically heterogenous plants from old cultivars were crossed with a cytoplasmic male-sterile line to obtain 266 F1 plants. Based on marker analysis using a multiallelic DNA marker linked to restorer-of-fertility 1 (Rf1), we divided the F1 plants into 15 genotypes. We evaluated the phenotypes of the F1 plants under two environmental conditions: greenhouse rooms with or without daytime heating during the flowering season. Three phenotypic groups appeared: those consistently expressing male sterility (MS), those consistently having restored pollen fertility, and those expressing MS in a thermo-sensitive manner. All plants in the consistently male-sterile group inherited a specific Rf1 marker type named p4. We tested the potential for thermo-induced male-sterile plants to serve as seed parents for hybrid seed production, and three genotypes were selected. Open pollination by a pollen parental line with a dominant trait of red-pigmented hypocotyls and leaf veins resulted in seed setting on thermo-induced male-sterile plants, indicating that their female organs were functional. More than 99.9% of the progeny expressed the red pigmentation trait; hence, highly pure hybrids were obtained. We determined the nucleotide sequences of Rf1 from the three genotypes: one had a novel allele and two had known alleles, of which one was reported to have been selected previously as a nonrestoring allele at a single US breeding station but not at other stations in the US, or in Europe or Japan, suggesting environmental sensitivity.