Genetic analysis of citrus fruit-related traits in the progeny of Ehime No.28 × Kanpei

【Objective】This study was conducted to address selecting parental lines in citrus hybrid breeding for improving fruit quality traits. A cross was made between Ehime No.28, a high-quality hybrid cultivar, as the female parent, and Kanpei, a late-maturing cultivar, as the male parent. Twelve key fruit quality traits were systematically evaluated in 45 stable yielding individuals from the hybrid population. Genetic analyses included skewness analysis, calculation of transmission ability, and estimation of mid- parent heterosis rate (RHm). The study reveals the inheritance patterns of fruit morphological traits, sugar and acid components, and seedlessness, and provides reference for parental selection in citrus breeding programs.【Methods】This study systematically evaluated quality traits of ripe fruits from 45 individuals at stable yielding stages. From each plant, 9- 10 representative fruit were sampled for analysis. A total of 12 core traits encompassing fruit morphology, physiochemical quality, and color parameters were measured. Single fruit mass, peel mass, and pulp mass were determined using an electronic analytical balance with a precision of ±0.01 g. Longitudinal diameter (vertical distance from fruit apex to pedicel) and transverse diameter (maximum equatorial diameter) were measured with a vernier caliper to calculate the fruit shape index (longitudinal/transverse ratio). Peel and flesh color were quantified at three points along the fruit equator using a spectrophotometer, supplemented by visual validation. Total soluble solids (TSS) was measured with a handheld refractometer with 3 technical replicates from blended juice, while titratable acid (TA) content was analyzed by automatic potentiometric titration with 0.1 mol · L- 1 NaOH. Data were processed in Microsoft Excel 2019 to compute mean values, standard deviation, skewness, and coefficient of variation (CV). Genetic analysis included skewness assessment to classify trait distributions (e.g., normal or skewed), calculation of transmission ability (Ta) to evaluate inheritance intensity, analysis of high- parent ratio (HH) and low- parent ratio (LL) for extreme phenotypic differentiation, and mid-parent heterosis rate (RHm) to assess heterosis or depression effects. These methods collectively elucidated the genetic patterns underlying fruit morphology and physiochemical traits.【Results】The hybrid progeny exhibited significant diversity in fruit morphological traits. Fruit shapes were categorized into four types: oblate (13.33%), high oblate (37.78%), round or subround (31.11% ), and ellipsoid (17.78% ). Peel color showed a bimodal distribution between orange (42.22%) and orange-red (57.78%), while flesh color demonstrated dominant inheritance, with orange flesh present in 97.78% of offspring. Single fruit mass averaged 145.63 g with significantly rightskewed distribution (skewness = 1.08) and high variability (CV = 49.09%). The transmission ability was 63.89%, with 84.44% of progeny below the low-parent value, indicating strong environmental interaction. In contrast, the fruit shape index averaged 0.91 with quasi-normal distribution (skewness = -0.09), low variability (CV= 10.76%), and high transmission ability (107.11%), consistent with polygenic additive inheritance. Peel thickness averaged 2.97 mm with right-skewed distribution. The seed number trait showed exceptional characteristics: mean value of 0.76 seeds with extreme variability (CV = 306.11%), and 88.9% of offspring maintained seedlessness (0- 1 seeds). Flesh recovery averaged 74.00% while juice recovery reached 41.00%, both showing left-skewed distributions. TSS averaged 14.41% with significant transgressive heterosis (mid-parent heterosis rate, RHm = 18.56%). Titratable acid (TA) content averaged 0.49%, showing heterosis depression. The solid-acid ratio reached 34.80 with high variability (CV = 44.80%), indicating complex genetic and environmental regulation of sugar-acid balance.【Conclusion】The results indicated that the fruit shape index conforms to a polygenic additive inheritance pattern, exhibiting a quasi- normal distribution and demonstrating high genetic stability. Single fruit mass showed a significantly right- skewed distribution, suggesting that this trait is susceptible to both gene dosage effects and environmental factors. TSS displayed transgressive heterosis, while titratable acid content exhibited hybrid breakdown. Compared to other citrus hybrid combinations, this combination demonstrated distinct characteristics in fruit shape stability and sugar-acid balance, particularly with a favorable genetic performance in the solid-acid ratio trait. Based on the findings, the fruit shape index and solid-acid ratio can serve as stable criteria for selection in hybrid citrus breeding. In practical breeding programs, prioritizing the screening of these traits is recommended to enhance breeding efficiency. Future research should focus on elucidating the molecular mechanism of the S-locus regulating seedlessness and assessing the stability of this trait under different ecological conditions. Additionally, further validation of the parental selection strategy proposed in this study across different hybrid combinations is warranted. The results provide a foundation for parental selection in citrus hybrid breeding and offer insights for genetic improvement of fruit quality traits.