Dysplasia of male organs induces apomixis in Malus crabapples

Apomixis has high value in fruit crop propagation and breeding applications, particularly for the transgenerational fixation of heterosis. In this study, we found that flowers of the triploid Malus cultivar 'Royalty' that were bagged after emasculation (EB) and pollinated with pollen from the diploid cultivar 'Flame' (EFB) produced high fruit set and increased seed numbers per fruit. The offspring exhibited superior growth vigour compared to those of the diploid sister cultivar 'Flame'. The fruit setting rate for 'Royalty' EFB was 98.44%, which was much higher than the fruit setting rate for 'Royalty' BA (Bagging without emasculation) at 19.7%. The fruit setting rate for 'Royalty' EB was 47.46% compared to 19.7% for 'Royalty' BA. Furthermore, offspring derived from either 'Royalty' EB or EFB exhibited superior growth vigour compared to those of the diploid 'Flame'. These results suggested that 'Royalty' exhibits facultative apomixis and has strong sexual reproductive ability under the stimulation of foreign pollen. Additionally, 'Royalty' anthers did not dehisce after flowering and had malformed pollen grains and a low pollen germination rate. Therefore, we proposed that dysplasia of the male organ could be an important driver factor of apomixis in 'Royalty'. Our study provides new insight into the mechanism of apomixis in Malus crabapples.