Abstract
'Sanenashi' is a landrace of Pyrus ussuriensis var. aromatica (Iwateyamanashi) with seedless fruit originating from northern Tohoku, Japan. To determine the mechanism of seedless fruit formation, we compared the number of styles in the pistil, fruit, seed set and the pollen tube growth between 'Sanenashi' and the Japanese pear cultivar 'Kosui'. Morphological variations such as short or browning pistils were observed in 64.2% of 'Sanenashi' and 5.9% of 'Kosui'. The initial fruit set rate of 'Sanenashi' was 48.3% at 4 weeks after cross-pollination with pear, and there were no fruit sets with peach pollen and non-pollinated sections at 8 weeks. Although the seed sets of 'Sanenashi' fruit were much lower than that of 'Kosui', 55.3% of viable 'Sanenashi' seeds germinated. Pollen tube growths were observed in the stigma and style of 'Sanenashi', but whether they reached the ovary could not be confirmed. Single sequence repeat (SSR) alleles of F(1) progenies between 'Sanenashi' and pear cultivars were presented by five SSR markers. These results suggest that the seedless fruit formation in 'Sanenashi' corresponds with pseudo-parthenocarpy (stenospermocarpy) because pollination by pear pollen is necessary for the fruit set. However, the results do not fully confirm this hypothesis and require further experiments.