关键词: 西瓜, 果肉, 柠檬黄色, 类胡萝卜素, 基因定位

Abstract:

【Objective】In order to find out the formation mechanism of watermelon canary yellow flesh color, the pigment composition, pigment contents, and the inheritance of white and canary yellow flesh watermelon color were studied in this study, and the candidate genes related to canary yellow flesh were predicted by BSA-seq. 【Method】Two six generations populations were constructed by crossing four materials, including Bingtangcui (white flesh) and Xihua (canary yellow flesh), as well as Sashengnaiyougua (white flesh) and Xinjinlanxuan (canary yellow flesh). High performance liquid chromatography (HPLC) was used to determine the carotenoid composition and content in flesh of four parents at four different development stages. Two BSA-seq populations (BSA-seq I and BSA-seq II) were initially located by using the bulked sergeant analysis (BSA), and then the candidate genes were screened according to the annotation information of watermelon reference genome ‘97103’ V2, and verified by real-time quantitative polymerase chain reaction (qRT-PCR).【Result】During the development of watermelon fruit, violaxanthin and lutein accumulated differently in both parents, and the content of violaxanthin was higher in canary yellow flesh than in white flesh. The content of violaxanthin in white and canary yellow flesh watermelon fruits were (10.96±4) μg·g ^-1DW and (22.84±2) μg·g^-1DW, respectively. The content of lutein in white and canary yellow flesh watermelon fruits were (2.23±1) μg·g^-1DW and (3.97±1) μg·g^-1DW, respectively. The content of violaxanthin was about 7 times of lutein. Two six generations segregation populations were analyzed, and the results showed that the flesh color of F₁ and Ⅰ BC₁P₁ and Ⅱ BC₁P₁ populations were non-canary yellow, the segregation ratio of non-canary yellow and canary yellow in F₂ population were consistent with the Mendelian segregation ratio of 3:1, and the segregation ratio of non-canary yellow and canary yellow inⅠ F₁, Ⅱ F₁, Ⅰ BC₁P₂ and Ⅱ BC₁P₂ backcross populations were 1:1, so it could be concluded canary yellow was recessive to white in watermelon flesh. Through SNP and InDel association analysis of BSA-seq Ⅰ and BSA-seq Ⅱ data, the major locus which regulated canary yellow flesh color in watermelon fruit was mapped on chromosome 6 within a physical distance between 24.00 to 24.61 Mb, and there were 70 genes in this region. Combined with reference genomic annotation and qRT-PCR experiment, five genes that maybe related to canary yellow flesh color trait in watermelon fruit were obtained. Among them, Cla97C06G121680, Cla97C06G121700 and Cla97C06G121890 were all related to chloroplast biogenesis and chloroplast structure size. These three genes might affect fruit flesh color by affecting the formation of chromoplast; Cla97C06G121910 was an AP2 transcription factor in response to ethylene synthesis which related to watermelon fruit ripening, which might affect the accumulation of carotenoids in flesh by affecting fruit ripening and Cla97C06G122090 was described as a transmembrane transport effect, which played a role in the transmembrane transport of carotenoids.【Conclusion】The main pigments of white and canary yellow flesh watermelon were violaxanthin and lutein, and the pigment accumulation in canary yellow flesh was significantly higher than white flesh watermelon. Genetic analysis showed that canary yellow was recessive to white in watermelon flesh color. Based on BSA-seq analysis, a major locus was located in the region of 24.00 Mb to 24.61 Mb on chromosome 6, while Cla97C06G121680, Cla97C06G121700, Cla97C06G121890, Cla97C06G122090 and Cla97C06G121910 were predicted as candidate genes which related to canary yellow formation in watermelon flesh.

Key words: watermelon, flesh, canary yellow, carotenoids, gene mapping