Identification of molecular variants of the nonrestoring restorer-of-fertility 1 allele in sugar beet (Beta vulgaris L.)

Abstract

Only three variants of nonrestoring alleles for sugar beet Rf1 were found from the US maintainer lines which were the selections from a broad range of genetic resources. Cytoplasmic male sterility is widely used for hybrid breeding of sugar beets. Specific genotypes with a nonsterility-inducing cytoplasm and a nonrestoring allele of restorer-of-fertility gene (rf) are called maintainers. The infrequent occurrence of the maintainer genotype evokes the need to diagnose rf alleles. Molecular analysis of Rf1, one of the sugar beet Rfs, revealed a high level of nucleotide sequence diversity, but three variants were tightly associated with maintainer selection in Japan. The question was raised whether this small number of variants would be seen in cases where a wider range of genetic resources was used for maintainer selection. Fifty-seven accessions registered as maintainers in the USDA germplasm collection were characterized in this study. Mitochondrial DNA types (mitotypes) of 551 plants were diagnosed based on minisatellite polymorphism. A mitotype associated with sterility-inducing (S) cytoplasm was identified in 58 plants, indicating S-cytoplasm contamination. The organization of rf1 was investigated by two PCR markers and DNA gel blot analysis. Eight haplotypes were found among the US maintainers, but subsequently two haplotypes were judged as restoring alleles after a test cross and another haplotype was not inherited by the progeny. Nucleotide sequences of rf1 regions in the remaining five haplotypes were compared, and despite the sequence diversity of the gene-flanking regions, the gene-coding regions were identified to be three types. Therefore, there are three rf1 variants in US maintainers, the same number as in the Japanese sugar beet germplasm collection. The implications of having a small repertoire of rf1 variants are discussed.

A fertility-restoring genotype of beet (Beta vulgaris L.) is composed of a weak restorer-of-fertility gene and a modifier gene tightly linked to the Rf1 locus

“…vulgaris (Leaf Beet Group)] accession ‘Fukkoku-ouba’, an old Japanese landrace, were obtained from Genebank, National Agriculture and Food Research Organization (NARO), Japan (accession number JP25745). Sugar beet line TA-33BB-CMS is a CMS line and TA-33BB-O is its maintainer inbred line [ 35 ]. Sugar beet line NK-198 has S mitochondria but is male fertile due to Rf1 [ 30 ].…”

“…For crossing experiments, inflorescences were paper-bag enclosed before anthesis. After anthesis, inflorescences of pollen-parents were placed in the bags [ 35 ].…”

“…Organizational comparison at the molecular level of the Rf1 locus in various sugar beets revealed multiple variants whose diversity is unknown [ 34 ]. For the sake of maintainer identification, rf1 organization in maintainers was investigated [ 34 ] [ 35 ]. The results showed that three molecular variants (and two derivatives) have been selected as non-restoring alleles by sugar beet breeders, of which one variant is predominant.…”

What Does the Molecular Genetics of Different Types of Restorer-of-Fertility Genes Imply?

“…The important question is whether RF1-Oma1 has functions other than altering the higher order structure of preSATP6 protein. Concerning this question, the organization of rf1 alleles from different origins was investigated by analyzing sugar beet lines selected for the non-restoring genotype [78]. Interestingly, all genotypes had preserved, intact RF1-Oma1 copies despite lacking the ability to bind with preSATP6 protein [60,78], strongly suggesting an unknown function of RF1-Oma1.…”

Genome-wide Identification of Chile Pepper Pentatricopeptide Repeat Domains Provides Insight into Fertility Restoration

“…Two Rf genes have been identified in rice (Hu et al, 2012;Luo et al, 2013;Wang et al, 2006). Furthermore, multiple haplotypes have been identified for an allele of an Rf gene in sugar beet [Beta vulgaris (Ohgami et al, 2016)]. Multiple haplotypes could exist for the Rf candidate genes identified here.…”