Tetraploid Formation from Open-Pollinated Triploid Loquat Provides a Promising Pathway for Large-Scale Seedless Breeding

Triploid loquat (Eriobotrya japonica, 2n = 3x = 51) produces seedless fruit and holds significant breeding value. However, the scarcity of tetraploids hinders large-scale triploid propagation. Notably, spontaneous tetraploidization occurs in certain triploid progenies, offering a viable approach to enrich tetraploid germplasm. In this study, open-pollinated offspring (n = 385) from white-fleshed triploid cultivars ‘Wuhe Zaoyu’ and ‘Huayu Wuhe 1’, as well as lines Q16 and Q21, were analyzed using flow cytometry for ploidy identification and molecular markers for flesh color genotyping. Ploidy analysis of open-pollinated progeny from ‘Huayu Wuhe 1’, Q16 and Q21 revealed the presence of diploids, triploids, tetraploids and aneuploids, with tetraploids being the most abundant (36–68%). In ‘Huayu Wuhe 1’ progeny, polyploids exhibited greater leaf thickness and plant height than diploids and aneuploids. Molecular marker analysis revealed that among the 172 tetraploid progeny, two flesh color genotypes were identified: heterozygous red-fleshed (24%) and white-fleshed (76%). qPCR-based allele quantification revealed that the alleles of tetraploids with heterozygous red-fleshed genotypes were Aaaa or AAaa genotypes. These findings confirm that triploid progeny can serve as an effective source of tetraploids, supporting future breeding efforts for seedless triploids. The formation of these tetraploids is possibly through 2n gametes from either diploid or triploid plants.