The factors of seedless expression in Citrus kinokuni hort. ex Tanaka ‘Mukaku Kishu’, ‘Southern Yellow’ {‘Tanikawa Buntan’ pummelo [an uncertain hybrid with C. maxima (Burm.) Merr.] × ‘Mukaku Kishu’}, and the hybrid seedling BSY lines of ‘Southern Yellow’ × Bu1-7 pummelo [chance seedling of ‘Suisho Buntan’ pummelo (C. maxima)] were investigated histologically. ‘Mukaku Kishu’, ‘Southern Yellow’, BSY18, and BSY21 are completely seedless, whereas others are seedy. The percentage of abnormal embryo sacs and fertilization in seedless cultivars and lines showed no differences from seedy ones, indicating that the seedlessness of ‘Mukaku Kishu’ and its progenies was not involved in abnormal embryo sacs and unfertilized ovules. In seedy cultivars and lines, embryos developed to the cotyledon stage by 14 weeks after pollination. In the seedless cultivars and lines, however, embryos only developed to the early stages of zygote or embryo development. These results indicate that the seedless expression of the seedless progenies of ‘Mukaku Kishu’ coincides with ‘Mukaku Kishu’ and is caused by an arrested seed development at the early stage.

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.