Characteristics of Arrested Seeds in Mukaku Kishu-type Seedless Citrus

The expression of seedlessness derived from ‘Mukaku Kishu’ (Citrus kinokuni hort. ex Tanaka) was investigated histologically. Neither perfect nor imperfect seeds found in seedy cultivars/offspring were formed in ‘Mukaku Kishu’ and its seedless descendants. Among ‘Mukaku Kishu’ and its seedless descendants, the fertilized seed type (termed “type A seeds”) with an immature, soft seed coat was specifically observed. In most seedless cultivars/offspring, all embryos of type A seeds were arrested at the zygote or proembryo stage, and embryo development was arrested 10 weeks after pollination. These results indicate that the expression of Mukaku Kishu-type seedlessness is characteristic of a forming type A seed with an immature seed coat and an embryo arrested at an early stage. Endosperm abortion in type A seeds was not observed. Therefore, we demonstrated that an arrested embryo development in the type A seed is not caused by endosperm abortion. Because the expression of Mukaku Kishu-type seedlessness is not controlled by the embryo genotype, type A seed production would not be caused by gene expression in embryos but by that in maternal tissues, more likely in the immature seed coat tissue.