Analysis on Unveiling the Natural Dynamics of Parthenocarpy and Self-Compatibility in Apple Trees

Apple (Malus domestica) is self-incompatible and typically requires cross-pollination for seed and fruit development. Parthenocarpy (fruit development without fertilization) and self-compatibility (fruit set without external pollen) are highly desirable traits in apple breeding, as they ensure consistent fruit production and quality without cross-pollination. However, apple parthenocarpic and self-compatible accessions have not been available for practical breeding. To identify these accessions, we analysed 436 accessions of Malus domestica and 84 accessions of wild Malus species by assessing fruit production. Flowers were bagged before opening to prevent cross-pollination. If fruit developed from the bagged flowers, it indicated the presence of self-compatibility or parthenocarpy, depending on whether the fruit contained seeds. We observed and scored a range of phenotypic expressions among accessions, from weak to strong in both parthenocarpy and potential self-compatibility. Strong parthenocarpy was observed in 5.95% of wild Malus species accessions and 3.44% of M. domestica accessions. Similarly, strong self-compatibility was exhibited in 5.95% of wild Malus species accessions and 2.75% of M. domestica accessions. Although bagged flowers showed lower fruit set rates than open-pollinated (OP) flowers, fruit size, weight, firmness, and soluble sugar and starch content showed no significant differences between fruits produced from bagged and OP flowers. Furthermore, a genome-wide association study (GWAS) was conducted with a high-throughput SNP array. This analysis identified several genes potentially associated with these traits. This research provides parthenocarpic and self-compatible apple accessions for breeding, which can generate novel cultivars that eliminate the need for cross-pollination or produce seedless fruit without pollination.