Characteristics of Fruiting and Pollen Tube Growth of Apple Autotetraploid Cultivars Showing Self-compatibility

Apples (Malus × domestica Borkh.) have gametophytic self-incompatibility (SI). Generally, the polyploidy breaks down gametophytic SI; however, there is little data on apples. In this study, we collected basic data on the self-compatibility (SC) mechanism of apple autotetraploid cultivars and polyploid mutants of original diploid cultivars. The autotetraploids showed self-fertility, and their pollen was compatible with the pistils of their original diploid cultivars, which share two S-alleles. However, the original diploid cultivars’ pollen was incompatible with the pistils of autotetraploid cultivars. Although the pollen of autotetraploid cultivars induced SC, the fruit set percentage and seed number were lower when autotetraploid cultivars were selfed or used as pollen parents in the pistils of the original diploid cultivars than in compatible diploid cross-combinations. The pollen tubes of autotetraploid cultivars grew significantly further than those of the selfed diploid cultivars, but grew significantly slower than in compatible diploid cross-combinations. These results show that the pollen of apple autotetraploid cultivars induced SC but maintained partial incompatibility.