QTL ANALYSES OF ORANGE COLOR AND CAROTENOID CONTENT AND MAPPING OF CAROTENOID BIOSYNTHESIS GENES IN CUCUMBER (CUCUMIS SATIVUS L.)

Carotenoids play an important role in human nutrition because of its pro-vitamin A activity. The common cucumbers (Cucumis sativus L.) always develop white fruit with lower carotenoid, 22-48 μg/100 g fresh weight. While Xishuangbanna gourd (XIS; Cucumis sativus var. xishuangbannanesis Qi et Yuan) develops orange fruit rich in carotenoid, -700 μg/100 g flesh weight, which makes this germplasm attractive to plant improvement programs interested in improving the nutrition of cucumber. Until now there is no further understanding of the carotenoid in cucumber on molecular lever. A study was designed to identify quantitative trait loci (QTL) associated with orange color fruit flesh using two F2 populations derived from parental lines with white flesh ['EC1' and 'Gy7'] crossed to orange flesh ['SWCC9' and USDA #14 derived from XIS]. Two genetic linkage maps were constructed with the markers of RAPD, SCAR, SSR, EST, SNP, AFLP and SSAP, which defined a common collinear region containing four molecular markers (3 dominant and 1 codominant) on linkage group (LG) LG6 in Map 1 and LG3 in Map 2. These regions contained QTL associated with orange mesocarp (mc)/endocarp (ec) color [mc6.1/ec6.1 (Map1) and mc3.1/ec3.1 (Map2)]. Biochemical analyses indicated that β-carotene (β) and xanthophyll (x) were the two predominant carotenoids in mc and ec tissue. QTLs controlling the content of β-carotene in endocarp (edb3.1) and xanthophyll in mesocarp (mdx3.1) mapped to the same interval as mc3.1 and ec3.1, respectively, in Map2. Moreover, one cucumber carotenoid biosynthesis gene, NCED (9-cis-Epoxycarotenoid dioxygenase), mapped to the same interval as orange flesh color QTLs (mc6.1/ec6.1 and mc3.1/ec3.1) in both maps. The QTLs identified herein should be considered for use in marker-assisted selection for introgression of β-carotene genes into commercial cucumber.