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A narrow genetic base is found in modern watermelon(Citrullus lanatus L.)cultivars due to many years of selection for desirable horticultural qualities.Wild watermelon species are supposed to be important potential sources within novel alleles for watermelon breeders because of their many perfect agronomic traits.Excavating and making full use of excellent genes of wild watermelon germplasm resources are considered to be an important way to improve watermelon qualities,however,the research on introgression into cultivars are less.The BC7 population was constructed by using A69(C.lanatus var.citroides)(which was known to have different characters compared to the recurrent parent)as a donor parent,and the elite watermelon cultivar 203Z(C.lanatus var.lanatus)as the recurrent parent.Lots of fruit characters variation were discovered in the BC7 population,such as different flesh colors,sour,bitter,firmness flesh,low sugar,and so on.Some of variant lines were chosen and marked by simple sequence repeats(SSR),the genotype was analyzed by Graphical Geno Types(GGT).A total of 178 SSR markers which distributed uniformly on 11 chromosomes of watermelon were amplified among A69,203Z and F1,99 SSR markers which showed polymorphism were used for analysing genotype in the BC7 population.There were 9 variant lines which had different flesh firmness were chosen for analysing genotype.1-2 exogenous wild segments were found in every variant lines.An interesting result is that only one exogenous wild segment on 6 chromosome of watermelon was got by GL1-1,4.6cM in length,99.5%of the recovery rate.1-2 exogenous wild segments on the 2 or 6 chromosome were revealed may associated with the fruit hardness in most flesh firmness variant lines.However,whether candidate genes existed were still unclear,the segments were too long to be confirmed the firmness-determining gene,more research is needed.