'Hyuganatsu' (HY; Citrus tamurana) is a late-season citrus grown in several specific prefectures in Japan. Although it is self-incompatible, its bud mutation, 'Nishiuchi Konatsu' (NK), is self-compatible, and interestinglymost of the seeds in harvested fruits are aborted. In our previous study we revealed that NK produces aproportion of unreduced 2N pollen grains, resulting in unusual seed development. In this study, we havefocused on seed and embryo development when pollen from NK was used to pollinate HY, NK, and 'Hassaku' (HS; Citrus hassaku), which is compatible with both HY and NK. When NK pollen was used to pollinate HY orNK, most of the seeds aborted, whereas all seeds developed successfully from crossing using HS pollen. Theseeds from NK-pollinated fruits were found to be significantly smaller than those from HS-pollinated fruits 10-12 weeks after pollination during periodic samplings of fruits, suggesting that the seed abortion phenotypeis expressed during this time period. Embryo development was observed using differential interferencecontrast or stereo microscopy. Embryos that had formed from NK self-pollination had aborted at 10 weeksafter pollination, whereas control embryos had reached the globular or heart-shaped stage. In the cross HS× NK, both normal and aborted seeds were present in the resulting fruit. Embryo development was found tocease at 12 weeks after pollination, and at 14 weeks after pollination, normal and aborted seeds were easilydistinguishable. In addition to the characterization of seed development, preliminary attempts at embryorescue have been made to recover triploid seedlings. After making the crosses HY × NK or NK × NK, abortingembryos were placed on MT media supplemented with 500 mg/L malt extract, 50 g/L sucrose, and 5 mMGA3. As a result, several triploid plantlets confirmed by flow cytometry were recovered from both pollinationcombinations, although the success rate was not high.
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