Development of SSRs to Identify Self-fertilized and Nucellar Seedlings in Common Trifoliate Orange and Flying Dragon

Most citrus rootstocks in Korea are seedlings of trifoliate orange. They are mainly known to have a nucellar origin, but at a low rate, those with self-fertilized origins are mixed in, and it is difficult to distinguish between the two types visually. If some self-fertilized seedlings are thus mixed, nonuniformity of citrus planting stocks would appear. Therefore, this study was conducted to evaluate the frequency of self-fertilized seedlings and the uniformity of seedlings using with DNA markers that identify nucellar and self-fertilized seedlings in trifoliate oranges. Twenty SSRs were selected and determined to be heterozygous in the twisted 'Flying Dragon' (Poncirus trifoliata cv. Flying Dragon) and homozygous in the straight 'Flying Dragon', originating from self-fertilization. A total of seven representative SSRs, capable of double checking in a single plant, were finally selected. As a result of an origin analysis by applying the selected SSRs to a common population of trifoliate oranges, more of those with self-fertilized origins were detected from small plants than large plants when two seedlings were grown from one seed. In particular, the largest plant did not have a self-fertilized origin when several plants were grown from one seed sown in vitro. Also, when the frequency of self-fertilized seedlings was evaluated during the first year and second year in a nursery, a self-fertilized origin was detected at a rate of 4.0% in large seedlings and 26% in small seedlings in the first year after sowing. Plants with poor growth were rogued and degenerated in the second year after sowing, and their self-fertilized origins were detected at a rate of 5.5%. The results here suggests that the selected SSR markers would be helpful to improve the uniformity of the production of citrus planting stocks and would enable the detection of nonuniform growth in citrus plants.