HortScience 55(3):1-7 (Jan 2020)
Cut-style Pollination Can Effectively Overcome Prefertilization Barriers of Distant Hybridization in Loquat
Qin Yang, Yan Fu

Loquat [Eriobotrya japonica (E. japonica)], a small genus of the subtribe Malinae that consists of ≈30 species, is an evergreen rosaceous fruit tree that is native to southeastern China, and some wild species that possess novel, favorable traits have excellent breeding potential. For example, Eriobotrya bengalensis blooms in late spring and ripens in early autumn in Guizhou Province, China, which prevents cold injury in winter by breeding spring-flowering cultivars using the special characters. Therefore, in the present study, the pollination treatments of cut-style pollination were evaluated that may promote successful distant hybridization in Eriobotrya japonica ‘Dawuxing’ x Eriobotrya deflexa and E. japonica ‘Dawuxing’ x E. bengalensis. The results indicated that the impairment of the pollen tube growth in the upper third of the style after pollen germination is an important factor leading to the failure of distant hybridization between the species tested in E. japonica, and that cut-style pollination can effectively overcome prefertilization barriers of the distant hybridization combination. Furthermore, the results of allele-specific polymerase chain reaction (AS-PCR) showed that S-genotypes, in accordance with the S-RNase heredity to separate the rule completely in offspring, should be both parents' S-RNase, and that the random 50 seedlings of Eb-2 and Ed-2 are true hybrids.

Cut-style pollination. Taking the varieties of E. japonica ‘Dawuxing’ as the female parent, in the full bloom of 2016, 120 flowers at the balloon stage were emasculated and bagged to avoid self-pollination and natural pollination on the branches free from pests and diseases for each treatment, with three replicates. Pollination was carried out on the third day after emasculation by the pollen from E. deflexa (logogram "Ed-1"), and E. bengalensis (logogram "Ed-1"), respectively. Pollination was carried out on the third day after emasculation by pollen from E. deflexa (logogram "Eb-2") and E. bengalensis (logogram "Eb-2"). Before this step, the upper third of the style was cut 2 h after self-pollination, as preliminary experiments indicated that the upper third of the style isthe key position for deposition of callose along the pollen tube wall, which stops pollen tube growth. In addition, pollination was carried out on the third day after emasculation by pollen from E. deflexa (logogram "Eb-3") and E. bengalensis (logogram "Eb-3"). Similarly, before this step, the upper third of the style was cut directly.

Five days after pollination, 100 pollinated pistils were collected from 20 flowers from each treatment, fixed in formaldehyde alcohol acetic acid solution [5:5:90 (v/v/v) 38% formaldehyde: acetic acid: 70% (v/v) ethanol] and prepared for fluorescence microscopy, as described by Yang et al. (2008). In addition, the fruit sets were determined for each treatment 80 d after pollination by the remaining 100 flowers, and the number of seeds was counted when the fruit was ripe.


Conclusions

In this paper, the comprehensive analysis of the effect of cut-style pollination on pollen germination, pollen tube growth, fruit set, and seed number showed that cut-style pollination can effectively overcome the prefertilization barriers of distant hybridization combinations of E. japonica "Dawuxing" x E. bengalensis, E. japonica ‘Dawuxing’ x E.deflexa. The results of AS-PCR amplification showed that S-genotypes, in accordance with the S-RNase heredity to separate the rule completely in offspring, should representthe S-RNase of both parents, or the presence of new S-RNase genotype in offspring, and that the 50 random seedlings of Eb-2 and Ed-2, respectively, are true hybrids. Therefore, cut-style pollination is an effective method to overcome the prefertilization barrier of distant hybridization in loquats. Therefore, breeding programs should focus on using this method by including breeding partners with some of the preferable traits to prevent potential future incompatibility issues.