Recent Transposition of an LTR-Retrotransposon in the Gene Coding for S Receptor Kinase is Responsible for a Novel Self-Compatible Phenotype of Radish (Raphanus Sativus L.)
Abstract Self-incompatibility (SI) responses of radish (Raphanus sativus L.) are determined by two tightly linked genes encoding an S receptor kinase (SRK) and an S-locus cysteine-rich protein/S locus protein 11 (SCR/SP11), respectively. A radish showing an almost self-compatible (SC) phenotype was identified in this study. Inheritance patterns showed that this SC phenotype was dominant over an SI phenotype. In addition, this SC phenotype co-segregated with an S haplotype in an F2 population. This SC radish contained an RsS-26 haplotype in which duplicate SRK-like genes were previously identified. Full-length sequences of two SRK-like genes of 18,133-bp and 6,200-bp in length were obtained from radish with the RsS-26 haplotype (designated as RsSRK-26-1 and RsSRK-26-2, respectively). Duplicate SCR/SP11-like genes were also identified in the radish with the RsS-26 haplotype. Phylogenetic analyses indicated that both duplicate SRK-like and SCR/SP11-like genes were closely related to other known SRK and SCR/SP11 genes, respectively. No critical mutation was found in the coding region of SRK-like or SCR/SP11-like gene. However, a 4,146-bp intact LTR-retrotransposon was identified in the third intron of RsSRK-26-1 of the SC radish. Interestingly, this LTR-retrotransposon was not detected in three other breeding lines containing the same RsS-26 haplotype. Except for this LTR-retrotransposon, only two single nucleotide polymorphisms (SNPs) were identified in intronic regions between normal and mutant RsSRK-26-1 alleles. While normal transcription was observed for radish showing RsSRK-26-1 and SI phenotypes in these three breeding lines, no transcript of RsSRK-26-1 was detected in the SC radish, suggesting that recent transposition of an LTR-retrotransposon in the RsSRK-26-1 gene might be responsible for the SC phenotype of radish.