Genetic analysis of fruit traits and bacterial canker resistance in hybrid offspring of Actinidia rufa ×Actinidia chinensis

【Objective】This study aimed to breed new kiwifruit varieties with strong resistance to bacterial canker disease through hybridization between Actinidia rufa (Siebold and Zuccarini) Planchon ex Miquel and Actinidia chinensis var. chinensis C. F. Liang.【Methods】A. rufa 5- 10, characterized by small fruit size but strong tolerance to bacterial canker, was selected as the female parent, while A. chinensis var. chinensis 2-41, known for the yellow flesh and average fruit mass of approximately 80 g for its sister line, served as the male parent. The 65 female plants bearing fruits were identified from the F1 population. When the soluble solid content (SSC) reached 7.0%, 20 fruits per vine were harvested and ripened at room temperature. Fruit quality and morphological traits, including single fruit mass, SSC, longitudinal diameter and transverse diameter were measured. The vine tolerance to bacterial canker was evaluated with observation on Psa syptoms in the field. The mean value, standard deviation, coeffi-cient of variation, and other genetic variation parameters of the data were analyzed by using Excel 2016 and Origin 2022 software.【Results】The results demonstrated the complex influence of maternal and paternal inheritance on various fruit traits, and provided evidence of polygenic inheritance and heterosis as well. The key findings were as follows:1. The coefficients of variation for average fruit weight in 2023 and 2024 were 32.61% and 29.97%, respectively. The supermaternal heterosis rate of fruit mass in the hybrid progeny was notably significant, reaching 106.06% in 2023 and 83.16% in 2024.The average fruit mass of the progeny was 31.06 g, which reflected a maternal genetic tendency, suggesting that the female parent contributed significantly to the trait. The coefficients of variation for SSC in 2023 and 2024 were 15.97% and 14.83% , respectively. The supermaternal heterosis rate of SSC in the hybrid progeny was also notably significant, reaching 22.30% in 2023 and 54.74% in 2024. Meanwhile, the average SSC of the progeny was 14.21%, indicating a substantial paternal influence and underscoring the critical role of the paternal parent in determining sugar content. The dual contribution from both parents underscored the genetic intricates involved in the formation of fruit mass and SSC traits in the hybrid offspring. The two traits exhibited continuous variation and were characterized by polygenic inheritance, meaning that multiple genes were involved in controlling these traits. 2. The coefficients of variation for longitudinal diameter, transverse diameter, and fruit shape index ranged from 10.83% to 14.22%, among which transverse diameter showed the smallest variation (10.83% ) and fruit shape index showed the largest variation (14.22%). The average values of longitudinal diameter, transverse diameter, and fruit shape index were all bigger than those of the maternal parent. The longitudinal diameter exhibited a supermaternal heterosis rate of 26.38%, showing a stronger paternal genetic influence. The transverse diameter displayed a supermaternal heterosis rate of 12.68%, indicating a greater maternal genetic contribution. The fruit shape index demonstrated a genetic tendency toward larger. Both the longitudinal and transverse diameters of the fruit exhibited a continuous distribution, reflecting the genetic characteristics of quantitative traits controlled by multiple genes. 3. Fruit appearance and maturity traits including fruit shape, skin and flesh color, and maturity timing, predominantly resembled the female parent. In particular, the skin color was mostly green-brown (77.59%), the flesh color was green, and the maturity occurred in early October. All fruits were covered with short hairs, a trait influenced by the male parent, indicating a clear paternal contribution to this morphological feature. The fruit shoulder shape of the female parent was square, while the paternal sister line was round. The shoulder shapes in the hybrid progeny were predominantly square (56.9%) or round (41.38%). The fruit beak shape of the female parent was flat, while the paternal sister line was round. This trait of the hybrid progeny varied widely, including blunt convex (6.9%), slightly convex (56.9%), deep concave (12.07%), flat (30.69%), and round (3.45%). 4. The F1 population exhibited a range of taste changes, with 41.38% sweet, 31.03% lightly sweet, 17.24% sour-sweet, and 10.34% sour. Additionally, 20.69% of fruits exhibited aroma, adding to the diversity in sensory traits. 5. 68.97% of the F1 vines exhibited sensitivity to bacterial canker, indicating a paternal genetic tendency of this trait.【Conclusion】The comprehensive evaluation of hybrid progeny from A. rufa and A. chinensis var. chinensis revealed genetic variation in fruit quality traits. Fruit mass, SSC, longitudinal diameter, and transverse diameter may be quantitative traits controlled by multiple genes. The hybrids exhibited a genetic tendency toward smaller fruit size, higher SSC, and intermediate longitudinal and transverse diameters. Maternal inheritance was more evident in fruit shape, skin and flesh color, maturity timing, and flavor, while paternal influence was more obvious in fruit hairiness and tolerance to bacterial canker. The wide variation in fruit beak shape underscored the male parent's genetic contribution. These findings provide valuable insights into the development ofkiwifruit varieties with strong disease tolerance and desirable fruit characteristics.