DNA Markers and the Molecular Mechanism of Self-(in)compatibility in Japanese Pear (Pyrus pyrifolia Nakai)

Japanese pear (Pyrus pyrifolia Nakai) is an important Rosaceous fruit tree in Japan. This species exhibits gametophytic self-incompatibility (GSI), which is controlled by a single S-locus with multiple S-haplotypes. Although GSI is a genetic mechanism to prevent inbreeding and promote outcrossing to maintain genetic diversity, it can be problematic in fruit trees because it causes unstable fruit set. Therefore, orchardists interplant Japanese pear with other lines as pollinizers or conduct artificial pollination to ensure fruit set. To achieve stable fruit production and to reduce or eliminate the need for artificial pollination, research on the GSI of Japanese pear has been conducted by pollination experiments and by characterization of self-compatible (SC) mutants. Additionally, breeding programs have progressed to produce SC cultivars with high fruit quality. Recently, molecular analyses of GSI and SC mutants in Japanese pear have provided new information that is relevant to the stable fruit production and efficient breeding of Japanese pear. This review focuses on studies of the GSI of Japanese pear, and especially on the recent development of DNA markers for S-genotyping and marker-assisted selection of SC trees. In addition, the candidate genes controlling pollen S specificity and a model of the molecular mechanism of GSI in Japanese pear are described.