Molecular and genetic analyses of the S-haplotype of the self-compatible Japanese plum (Prunus salicina Lindl.) ‘Methley’

Abstract

SummaryThe mechanisms of self-compatibility in the Japanese plum cultivar, ‘Methley’ were investigated. Using PCR with S-RNase gene-specific primers, the S-genotype (S-haplotype combination) of ‘Methley’ was determined to be SbSg. This S-genotype was confirmed by cloning of the partial genomic DNA for both the S-RNase genes. After self-pollination of ‘Methley’, the resultant progenies segregated into three S-genotypes, SbSb, SbSg, SgSg, suggesting that both the Sb- and Sg-haplotypes were inherited in the progenies from the male parent. Thus, both S-haplotypes seemed to be non-functional and responsible for self-compatibility. Expression analyses in ‘Methley’ showed that both the Sb- and Sg-RNase genes were transcribed in the style. Therefore, it appeared that inhibition of transcription of the S-RNase genes was not responsible for self-compatibility in the Sb- and Sg-haplotypes of this cultivar. When flowers of the self-incompatible cultivars, ‘Sordum’ (SaSb) and ‘White Plum’ (SfSg) were fertilised with pollen from ‘Methley’, the progenies segregated into four S-genotypes, SaSb, SaSg, SbSb, SbSg and SbSf, SbSg, SfSg, SgSg, respectively. This result indicated that growth of the Sb- or Sg-pollen tubes of ‘Methley’ was not arrested in styles with functional Sb- or Sg-RNase alleles. Thus, the pollen components in both the Sb- and Sg-haplotypes of ‘Methley’ may be dysfunctional and involved in the breakdown of self-incompatibility. However, no alterations in the SFBb or SFBg gene (Shaplotype–specific F-box gene) sequences of ‘Methley’ were detected, and both the SFBb and SFBg genes were transcribed in pollen of this cultivar. Therefore, neither a mutation in the SFB gene sequences, nor any inhibition of transcription of the SFB genes appeared to be the cause of the loss function in the pollen component in Japanese plum Japanese plum (Prunus salicina Lindl.), which belongs to the family Rosaceae, exhibits the S-RNase-based gametophytic self-incompatibility system (GSI; Yamane et al., 1999), as do other Prunus fruit tree species such as sweet cherry (Tao et al., 1999), almond (Tao et al., 1997; Ushijima et al., 1998), and Japanese apricot (Tao et al., 2000; Yaegaki et al., 2001). In members of the Rosaceae, GSI is controlled by a single polymorphic locus called the S-locus, which contains the pistil-S and pollen-S genes (Yamane and Tao, 2009). In Prunus, the former is an S-RNase gene (Tao et al., 1997; 1999; Ushijima et al., 1998) and the latter is an SFB/SLF gene (i.e., an Shaplotype–specific F-box gene/S locus F-box gene; Entani et al., 2003; Romero et al., 2004; Ushijima et al., 2003; Yamane et al., 2003). Recently, we demonstrated that the S-RNase genes of Japanese plum could be amplified effectively by S-RNase gene-specific primer pairs designed from sweet cherry S-RNase gene sequences, and we identified 14 different S-RNase genes corresponding to the Sa- to Sn-haplotypes (Beppu et al., 2002; 2003).