New polymorphic microsatellite markers for the Korean manila clam (Ruditapes philippinarum) and their application to wild populations

Manila clam (Ruditapes philippinarum) is a valuable and intensively exploited shellfish species in Korea. Despite its importance, information on its genetic background is scarce. For the genetic characterization of R. philippinarum, expressed sequence tag-derived microsatellite markers were developed using next-generation sequencing. A total of 5879 tandem repeats containing di- to hexanucleotide repeat motifs were obtained from 236,746 reads (mean = 413 bp). Of the 62 loci screened, 24 (38.7%) were successfully amplified, and 10 were polymorphic in 144 individuals from 2 manila clam populations (Incheon and Geoje, Korea). The number of alleles ranged from 2 to 17 in the Incheon population and from 3 to 13 in the Geoje population (overall AR = 7.21). The mean observed and expected heterozygosities were estimated to be 0.402 and 0.555, respectively. Hence, there is less genetic variability in the Geoje population than in the Incheon population, although no significant reductions of genetic diversity were found between the populations (P > 0.05). However, significant genetic differentiation was detected between the populations (FST = 0.064, P < 0.001). Significant deviations from Hardy-Weinberg equilibrium and high inbreeding coefficients (mean FIS = 0.22-0.26) were detected in both populations. The 10 novel polymorphic microsatellite loci used in this study will be useful for future genetic mapping studies and for characterizing population structures, monitoring genetic diversity for successful aquaculture management, and developing conservation strategies for manila clam populations in Korea.