QTLs Conferring to Fertility Restoration Against Owen Cytoplasmic Male Sterility in Sugar Beet

Hybrid seed production in sugar beet relies on cytoplasmic male sterility (CMS). As time-consuming and laborious test crosses with a CMS tester are necessary to identify maintainer lines, development of a reliable marker-assisted selection (MAS) method for the rf gene (the non-restoring allele of restorer-of-fertility locus) is highly desirable for sugar-beet breeding. Owen (1945) proposed that the sterility depends on the interaction between at least two recessive chromosomal genes and a ‘sterile cytoplasm’ (S). According to him, fully male sterile plants have the genotype (S) xxzz, while the remaining eight genotypes usually show varying degree of pollen fertility. The expression of CMS involves cytoplasmic factor S and non-restoring allele of nuclear Rf locus. Plant with the same nuclear genotype but devoid of S is male fertile, which is called maintainer and used for propagation of CMS plant. We have cloned X as the Rf1, a sugar beet Rf with major effect on fertility restoration (Matsuhira et al. 2012), via positional cloning. Two PCR markers targeting a downstream intergenic sequence and the first intron of Rf1, respectively, were developed. The electrophoretic patterns of both markers revealed multiple Rf1 alleles in beet plants with various genotypes. The sequences of the rf1 coding regions of these 22 maintainer lines were found to be identical, confirming that they shared the same rf1 allele (Moritani et al, 2013). On the other hand, little is known about the other Rf gene, Z, such as its precise location on the linkage map. As far as we experienced, fertility restoration of CMS in many breeding lines expressed in quantitative manner and was also strongly influenced by environmental condition, making us to infer that it is a complex trait, hence hindering genetic analysis of Z. Given that fertility restoration of Owen CMS in sugar beet is a complex trait, QTL analysis is suitable to solve and dissect this complex trait. We conducted QTL analysis by composite interval mapping methods for two segregation materials in sugar beet to clarify QTLs involved in the fertility restoration of Owen CMS.