Seedless mechanism of a new mandarin cultivar ‘Wuzishatangju’ (Citrus reticulata Blanco)

Seedlessness is often a desired characteristic for many types of fruits. A new seedless mandarin cultivar ‘Wuzishatangju’ and its original seeded cultivar ‘Shatangju’ were used to study the mechanism of seedlessness from the beginning of fertility. Embryonic development, compatibility of self-pollinated ‘Wuzishatangju’ and cross-pollinated ‘Wuzishatangju’ (‘Wuzishatangju’ × ‘Ponkan’, ‘Wuzishatangju’ × ‘Shatangju’) were studied. Results indicated that the male gametes of ‘Wuzishatangju’ were healthy. Fertility of the ‘Wuzishatangju’ embryo sac was normal and the mature embryo sac had one egg, two synergids, three antipodal cells and one big central cell containing two polar nuclei. Embryonic development of cross-pollinated ‘Wuzishatangju’ was normal. Many globular embryos and a few heart-shaped embryos were observed 2 weeks after cross-pollination (WACP). Heart- and torpedo-shaped embryos were observed at three WACP while embryos reached the torpedo stage at four WACP. Cotyledonary embryos were observed at five WACP and they developed into seeds while funiculi were still existent at seven WACP. However, embryonic development of self-pollinated ‘Wuzishatangju’ was abnormal and most embryos, which degenerated by 2 weeks after self-pollination (WASP), disappeared and seedless fruits were produced by four WASP. Pollen tubes of cross-pollinated ‘Wuzishatangju’ could grow normally in the stigma, style and ovary, and they entered into the embryo sac resulting in successful fertilization. The pollen tubes of self-pollinated ‘Wuzishatangju’ grew well in the stigma and style; however, when they entered into the ovary, they became twisted and could not enter the embryo sac. Results indicate that the activities of the male gamete and the fertility of the embryo sac of ‘Wuzishatangju’ were functioning normally with no embryo abortion during embryonic development. Gametophytic self-incompatibility (SI) caused seedlessness in ‘Wuzishatangju’ by blocking fertilization in the ovary.