Frequent occurrence of precocious flowering in zygotic seedlings derived from crosses with a monoembryonic and male sterile acid citrus hybrid between Yuzu (C. junos Sieb, ex Tanaka) and Hanayu (C. hanaju Hort. ex Shirai)

We successfully produced a male sterile and monoembryonic acid citrus hybrid plant that generated seedlings showing precocious flowering with high rates. Of 26 crosses with 16 acid citrus cultivars, three ('Sudachi' X 'Yuzu', 'Hanayu' X 'Kabosu' and 'Hanayu' X 'Yuzu') were firstly found to generate precociously flowered seedlings with low rates. Second, 23 hybrid seedlings with 'Hanayu', 'Kabosu' and 'Sudachi' in their parents were crossed with 'Hanayu', 'Kabosu' and 'Yuzu', and then one cross (HY-16 X 'Yuzu') was found to generate precocious seedlings with a high rate of 6%. When the hybrid plant HY-16 derived from 'Hanayu' X 'Yuzu' was crossed with 'Yuzu' in different years, it constantly generated seedlings showing precocious flowering with a rate of about 6%, but it did not generate them when it was crossed with 'Hanayu'. When HY-16 was crossed with grapefruit cultivars, it generated seedlings exhibiting precocious flowering with very high rates of about 15%. The seedlings derived from 'Hanayu' X 'Yuzu' and selfing of 'Hanayu' and 'Yuzu' generated seedlings showing precocious flowering with low rates of about 1%. HY-16 was a male sterile and monoembryonic plant with fruitful and seedy characters. The precociously flowered seedlings derived from crosses with HY-16 developed 6-33 leaves until flower bud formation that occurred between February to August on the shoot tip. Three-fifths of the seedlings for HY-16 X 'Yuzu' and one-second for HY-16 X grapefruit were male sterile. These results suggested that HY-16 accumulated genes related to precocious flowering and that this hybrid was useful for rapid genetic analysis of various characters such as male sterility in Citrus.