Style-specific self-compatible mutation caused by deletion of the S-RNase gene in Japanese pear (Pyrus serotina)

Monofactorial gametophytic self-incompatibility in the family Rosaceae is controlled by the single multi-allelic S-locus. In Japanese pear, a naturally occurring self-compatible mutant was found as a sport of a self-incompatible cultivar with an S-genotype of S²S⁴. The S⁴ allele has been shown to be non-functional in the style of the mutant, but functional in pollen. The present study shows the S²-RNase gene to be transcribed normally in the style of the mutant, but that the S⁴-RNase gene transcript is not detectable. Furthermore, the S⁴-RNase gene could not be detected in the mutant by genomic Southern blot, indicating that the mutant lacks the gene. The extent of deletion in the mutant was estimated to be more than 4 kbp, and spans the entire length of the S⁴-RNase gene. These findings provide evidence that the S-RNase gene is essential for the self-incompatibility reaction in the style, but is not involved in controlling the self-incompatibility behavior of pollen.