Characterization of a Novel Self-compatible S^3′ Haplotype Leads to the Development of a Universal PCR Marker for Two Distinctly Originated Self-compatible S haplotypes in Japanese Apricot (Prunus mume Sieb. et Zucc.)

Abstract

Japanese apricot (Prunus mume Sieb. et Zucc.) exhibits the S-RNase-based gametophytic self-incompatibility (SI) system, which is controlled by a single polymorphic locus called the S locus containing the pistil S (S-RNase) and pollen S genes (SFB/SLF for S haplotype-specific F-box gene/S locus F-box gene). This study was conducted to elucidate the unknown molecular basis of self-compatibility (SC) in a selection line 1K0-26, an offspring of ‘Benisashi’ × ‘Koshinoume’, to explore the possible use of this line for SC breeding in Japanese apricot. Controlled pollination and segregation analyses demonstrated that 1K0-26 has an intact SI S⁷ haplotype and a pollen-part mutant SC S^3′ haplotype. Cloning and DNA sequence analysis of the S^3′ locus revealed a 7.1 kb insertion in the pollen determinant SFB^3′, which makes a premature stop codon to produce transcripts for truncated dysfunctional SFB, just like the mode of mutation in SFB^f, another SC S haplotype in Japanese apricot. The inserted sequence appeared to be a non-autonomous retroposon with long terminal repeats, a part of which is identical to part of the inserted sequence to SFB^f. Taking advantage of the similarity of the inserted sequences to the two SC S haplotypes of distinct origin, S^3′ and S^f, we have developed a universal PCR marker for the marker-assisted selection for SC in Japanese apricot. We surveyed 86 Japanese apricot cultivars and lines using the PCR marker, and it appeared that 38, 1, and 3 appeared to have only S^f, only S^3′, and both S^f and S^3′, respectively. Marker-assisted selection for SC in Japanese apricot and the possible use of these cultivars and lines for future SC Japanese apricot breeding programs are discussed.