Self-incompatibility (SI) is an important mechanism that prevents self-fertilization and avoids inbreeding in flowering plants. The most widespread SI system utilizes S -ribonucleases ( S-RNase s) and S -locus F-boxes ( SLF s) as S -determinants. In citrus, SI is ancestral; Citrus maxima (pummelo) is self-incompatible, while Citrus reticulata (mandarin) and its hybrids are self-compatible (SC). Here we identified nine highly polymorphic pistil-specific, developmentally expressed S-RNase s from pummelo that segregate with S -haplotypes in a gametophytic manner and cluster with authentic S-RNase s. We provide evidence that these S-RNase s function as the female S -determinants in citrus. Moreover, we found that each S-RNase is linked to ~nine SLF s. Analysis of 117 citrus SLF/SLFL genes revealed clustering into 12 types and evidence that the S-RNase s and intra-haplotypic SLFs/SLFL s co-evolved. Our data are consistent with citrus having an S -locus comprising a S-RNase and several SLF s that fit the non-self-recognition model. We identified a predominant single nucleotide mutation, S m - RNase , in SC citrus, which provides a ‘natural’ loss of function. We present evidence that SI-SC transitions due to the S m -RNase, initially arose in mandarin, spreading to its hybrids and became fixed. Identification of an evolutionarily distant new genus utilizing the S -RNase-based SI system, >100 million years separated from the nearest S -RNase family, is a milestone for evolutionary comparative studies.

No abstract