Evolution of self-compatibility by a mutant S(m)-RNase in citrus

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-RNases) and S-locus F-boxes (SLFs) 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-RNases from pummelo that segregate with S-haplotypes in a gametophytic manner and cluster with authentic S-RNases. We provide evidence that these S-RNases function as the female S-determinants in citrus. Moreover, we found that each S-RNase is linked to ~nine SLFs. Analysis of 117 citrus SLF/SLFL genes revealed clustering into 12 types and evidence that the S-RNases and intra-haplotypic SLFs/SLFLs co-evolved. Our data are consistent with citrus having an S-locus comprising a S-RNase and several SLFs 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.