Abstract：Leaf area is critical to photosynthesis efficiency and a major trait affecting crop yield. Wild cucumbers (Cucumis sativus var. hardwickii) bearing little leaves, after domestication, the leaf area of cultivated cucumber (Cucumis. sativus var. sativus) is enlarged around 2 to 3 times. Previously, a major gene ll (little leaf) was mapped on chromosome 6. In this study, F2 population of 205 individuals segregating at leaf area by crossing from 2 cucumber lines XF-24 (little leaf) and DF-32 (large leaf). The result of normal distribution test by SAS showed that the area data of mature leaves from the same nodes of 205 individuals were in normal distribution, which characterized the trait of quantitative inheritance. In order to effectively accelerate the process of study, we utilized insertion and deletion(InDel) markers for gene mapping after the whole genome sequencing of parents of F2 population. InDels of the whole genome sequencing data from parents of F2 population were found in bioinformatics analysis. Eighty and eight InDel markers were evenly designed on all chromosomes by Primer Premier 5.0. These InDels were acted as primers to amplify the two gene pools of both large and little leaves, which came from bulked segregant analysis, the polymorphic InDels screened from the previous step were used to amplify DNA of F2 population, and 7 InDel markers identified on chromosome 7 were InDel-1, InDel-2, InDel-3, InDel-4, InDel-5, InDel-6 and InDel-7, respectively. Genetic linkage map was constructed by JionMap4 .0 and QTL locus was found by MapQTL4.0, using the result of PCR and traits of F2 population. Seven InDel markers listed above were contained in the genetic linkage map, spanning 22.1 cM. This result led to the discovery of a second major QTL (gene) controlling leaf area in cucumber, here designated ll2 (little leaf 2). Further analysis delimited ll2 into a 1.24 Mb genomic interval between the 2 markers of InDel-2 and InDel-4. ll2 was the first QTL found on chromosome 7 with a narrower interval, compared with the previous studies. Two major QTLs, ll and ll2, controlling leaf area in cucumber (Cucumis sativus L.)were found on chromosome 6 and 7, respectively. It is the 2 major ones that show the complexity of the genetic and molecular mechanisms of cucumber leaf area. Genetic mapping of ll2 will pave the way for deciphering the genetic and molecular mechanisms of leaf area in cucumber as well as for developing tools for molecular-assisted breeding.
