SummaryMany Prunus species, including sweet cherry and Japanese apricot, of the Rosaceae, display an S-RNase-based gametophytic self-incompatibility (GSI). The specificity of this outcrossing mechanism is determined by a minimum of two genes that are located in a multigene complex, termed the S locus, which controls the pistil and pollen specificities. SFB, a gene located in the S locus region, encodes an F-box protein that has appropriate S haplotype-specific variation to be the pollen determinant in the self-incompatibility reaction. This study characterizes SFBs of two self-compatible (SC) haplotypes, S 4¢ and S f , of Prunus. S 4¢ of sweet cherry is a pollen-part mutant (PPM) that was produced by X-ray irradiation, while S f of Japanese apricot is a naturally occurring SC haplotype that is considered to be a PPM. DNA sequence analysis revealed defects in both SFB 4¢and SFB f . A 4 bp deletion upstream from the HVa coding region of SFB 4¢ causes a frame-shift that produces transcripts of a defective SFB lacking the two hypervariable regions, HVa and HVb. Similarly, the presence of a 6.8 kbp insertion in the middle of the SFB f coding region leads to transcripts for a defective SFB lacking the C-terminal half that contains HVa and HVb. As all reported SFBs of functional S haplotypes encode intact SFB, the fact that the partial loss-of-function mutations in SFB are present in SC mutant haplotypes of Prunus provides additional evidence that SFB is the pollen S gene in GSI in Prunus.
This study demonstrates that self-compatible (SC) peach has mutant versions of S haplotypes that are present in self-incompatible (SI) Prunus species. All three peach S haplotypes, S (1), S (2), and S (2m), found in this study encode mutated pollen determinants, SFB, while only S (2m) has a mutation that affects the function of the pistil determinant S-RNase. A cysteine residue in the C5 domain of the S (2m)-RNase is substituted by a tyrosine residue, thereby reducing RNase stability. The peach SFB mutations are similar to the SFB mutations found in SC haplotypes of sweet cherry (P. avium) and Japanese apricot (P. mume). SFB (1) of the S (1) haplotype, a mutant version of almond (P. dulcis) S (k) haplotype, encodes truncated SFB due to a 155 bp insertion. SFB (2) of the S (2) and S (2m) haplotypes, both of which are mutant versions of the S (a) haplotype in Japanese plum (P. salicina), encodes a truncated SFB due to a 5 bp insertion. Thus, regardless of the functionality of the pistil determinant, all three peach S haplotypes are SC haplotypes. Our finding that peach has mutant versions of S haplotypes that function in almond and Japanese plum, which are phylogenetically close and remote species, respectively, to peach in the subfamily Prunoideae of the Roasaceae, provides insight into the SC/SI evolution in Prunus. We discuss the significance of SC pollen part mutation in peach with special reference to possible differences in the SI mechanisms between Prunus and Solanaceae.
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