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Geographic Variations and Genetic Distance of Three Geographic Cyclina Clam (Cyclina sinensis Gmelin) Populations from the Yellow Sea

The gDNA isolated from Cyclina sinensis from Gochang (GOCHANG), Incheon (INCHEON) and a Chinese site (CHINESE), were amplified by PCR. Here, the seven oligonucleotide decamer primers (BION-66, BION-68, BION-72, BION-73, BION-74, BION-76, and BION-80) were used to generate the unique shared loci to each population and shared loci by the three cyclina clam populations. As regards multiple comparisons of average bandsharing value results, cyclina clam population from Chinese (0.763) exhibited higher bandsharing values than did clam from Incheon (0.681). In this study, the dendrogram obtained by the seven decamer primers indicates three genetic clusters: cluster 1 (GOCHANG 01~ GOCHANG 07), cluster 2 (INCHEON 08~INCHEON 14), cluster 3 (CHINESE 15~CHINESE 21). The shortest genetic distance that displayed significant molecular differences was between individuals 15 and 17 from the Chinese cyclina clam (0.049), while the longest genetic distance among the twenty-one cyclina clams that displayed significant molecular differences was between individuals GOCHANG no. 03 and INCHEON no. 12 (0.575). Individuals of Incheon cyclina clam population was somewhat closely related to that of Chinese cyclina clam population. In conclusion, our PCR analysis revealed a significant genetic distance among the three cyclina clam populations.


INTRODUCTION
Korean cyclina clam (Cyclina sinensis Gmelin) is one species of an economically important bivalve species, belonging to the family Veneridae, and the order Veneroida.
The clams inhabit in the estuary flats consisting of a lot of sand, mud and slime in the coastal tidal wetland.
Cyclina clam is widely distributed in the field of reeds and seawater habitats of the East Sea, the Yellow Sea and the southern sea such as the coasts of Incheon, Taean, Boryeong, Seocheon, Gunsan, Yeosu, Suncheon, and Namhae in the Korean Peninsula. The clams are silvery white and coarse in the shell surface under natural conditions. The ribs of the shell surface are compact and yellowish brown or light gray. In general, the size, color and type of this bivalve vary according to environmental factors, such as geography, water depth, water temperature, nutrition, growth period, and other common factors. However, the environmental necessities and forbearances of clam from different geographic areas remain unknown, as does its population structure.
As the clam culture industry grows, so doe's interest into the genetics of this shellfish species. However, little information currently exists regarding the genetics of cyclina clam. Until now, the gonadal development and reproductive cycle and the age, rickettsia-like organisms, cytochemical characteristics, and morphological characters in clams have been assessed by manifold ecological and biochemical researches, as reported earlier by many workers (Yoon & Park, 2002;Jung et al., 2004).

J-M Yoon
Dev. Reprod. Vol. 16, No. 4 (2012) 316 To analyze the genetics of organisms, a number of analytical and molecular techniques have been applied, including morphological standards (Orozco-Castillo et al., 1994), allozyme variation (Smith et al., 1997), and various PCR-based molecular techniques. Polymorphisms are determined from specific positions in the banding patterns of the amplified products (Tassanakajon et al., 1998;. Suitable loci with high reproducibility allow the identification of unambiguously different species (Greig et al., 2005). So, this technique has been used for identification and detection of genetic diversity in various shellfish (Tassanakajon et al., 1998;McCormack et al., 2000;Yoon and Kim, 2003;Park et al., 2005) and teleost species (Callejas & Ochando, 1998;. Author carried out clustering analyses to clarify the genetic distances in the Gochang population (Fig. 1A). Entertainingly, the decamer primer BION-68 generated 35 unique loci to each population, which were identifying each population, approximately 150 bp, 200 bp, 250 bp, 400 bp and 500 bp, in Gochang population (Fig. 1B). Particularly, this primer detected 21 shared loci by the three populations, major and/or minor fragments of sizes 500 bp, which were identical in all samples, as illustrated in Table 1.  (Callejas & Ochando, 1998). However, the average bandsharing value recorded in our study is also higher than the average value between the two oyster populations (0.282 ±0.008)  abalone, cluster analysis of the pairwise population matrix generated from RAPD data showed that geographically close populations tended to cluster together (Huang et al., 2000).
In this study, PCR analysis revealed a significant genetic distance among three cyclina clam populations.
The existence of population differentiation and DNA polymorphisms among three cyclina clam populations were detected by PCR analysis. This shows that the method is one of the adequate tools for comparing the DNA of individuals, species and/or populations of clam.
As mentioned above, the potential of this analysis to identify diagnostic markers for the identification of three cyclina clam populations has also been demonstrated (Tassanakajon et al., 1998;McCormack et al., 2000;Yoon & Park, 2002;Jung et al., 2004;Yoon, 2008). It was reported that the species relationships revealed by the PCR approach should be consistent with previously obtained data using morphological portrait (Nebauer et al., 2000). From what has been said above, a dendrogram revealed close relationships between individual identities within three geographical populations. Thus, this PCR analysis revealed a significant genetic distance among the three cyclina clam populations. High levels of a significant genetic distance among three cyclina clam populations showed this PCR approach is one of the most suitable tools for individuals and/or population biological DNA studies. Therefore, this method can also be applied to other species of Veneridae clam and make technically-convenient the analysis of many samples in a short time.

ACKNOWLEDGEMENT
Particular thanks go to the anonymous reviewers who assisted him with thorough and far-reaching criticisms.
The author thanks our laboratory colleague, Mr. D. H.
Kim, Y. J. Song, H. Oh and other undergraduate students for their assistance with sample collection, PCR techniques, and statistical analysis treatments.