Molecular mapping of restorer-of-fertility 2 gene identified from a sugar beet (Beta vulgaris L. ssp. vulgaris) homozygous for the non-restoring restorer-of-fertility 1 allele

Abstract

By genetically eliminating the major restorer - of - fertility gene ( Rf ), a weak Rf gene was unveiled. It is an allele of Z , long known as an elusive Rf gene in sugar beet. In the hybrid breeding of sugar beet, maintainer-genotype selection is a laborious process because of the dependence on test crossing, despite the very low occurrence of this genotype. Marker-assisted selection (MAS) of the maintainer genotype is highly desired by sugar beet breeders. The major restorer-of-fertility gene (Rf) was identified as Rf1, and its non-restoring allele (rf1) was discriminated at the DNA level; however, some of the rf1rf1 selections retained an as yet unidentified Rf, another target locus for MAS. The objective of this study was to identify this Rf. An rfrf1 plant was crossed to a cytoplasmic male-sterile sugar beet and then backcrossed to obtain progeny segregating the unidentified Rf. The progeny exhibited partial male-fertility restoration that was unstable in single plants. The segregation ratio of restored vs. non-restored plants suggested the involvement of a single Rf in this male-fertility restoration, designated as Rf2. We confirmed the feasibility of molecular tagging of Rf2 by identifying four shared amplified fragment length polymorphism (AFLP) fragments specific to 17 restored plants. Bulked segregant analysis also was performed to screen the Rf2-linked AFLP markers, which were subsequently converted into 17 sequence-tagged site markers. All the markers, as well two additional chromosome-IV-assigned markers, were linked to each other to form a single linkage map, on which Rf2 was located. Our data suggested that Rf2 is likely an allele of Z, long known as an elusive Rf gene in sugar beet. We also discuss the importance of Rf2 for sugar beet breeding.

Fig. 1

Images of the AFLP pattern using two restored bulks (lanes indicated by A), three non-restored bulks (B), ten fertility restored plants of BC₁F₁ (C), and 15 fully male-sterile plants of BC₁F₁ (D). Selective primers were EcoRI-CCC and MseI-GAC. An arrow indicates an AFLP fragment associated with fertility restoration. Size markers are shown on the right (bp)

Fig. 2

Linkage map of the chromosomal region around Rf2 in BC₁F₁ (a) and BC₁F₃ (b) populations. Two chromosome-IV-assigned markers are connected by dotted lines. The confidence intervals of QTL for fertility restoration are indicated by vertical bars labeled with ‘Rf2’. Details of the markers are shown in Table S1. Map distances are shown in the left of each maps (cM). In the linkage map shown in b, repulsion of two regions (nir-ca2 and sc11-ant) was assumed in the linkage analysis