International Journal of Marine Science, 2025, Vol.15, No.1, 15-27 http://www.aquapublisher.com/index.php/ijms 20 the genotype to determine the most likely population composition number K. By testing different K values and calculating likelihoods, researchers can determine the number of genetic populations supported by the data (Thongda et al., 2018). To detect the direction and extent of gene communication among populations, a mobility estimation model has also been applied. For example, using Migrate-n software based on co-ancestry processes, two-way mobility and historical population size can be estimated, or the recent gene flow ratio can be inferred by BayesAss. This type of approach is relatively rare in oyster studies, but is very helpful in understanding asymmetric current transmission (such as gene flows with stronger downstream directions). Some studies also used isolation-by-distance (IBD) analysis to examine the correlation between genetic distance and geographical distance to determine whether population structure meets a simple diffusion model. For contiguously distributed oyster coastal populations, significant IBD patterns can often be detected, meaning that gene flow decays with distance, for example, genetic differences are observed in oyster populations on the eastern coast of North America as the increase in coastline distance. 4.3 Gene flow, historical expansion and pedigree geographic modeling methods In addition to describing the current population structure, the study of oyster genetic connectivity involves reconstruction of historical gene flows and population dynamics, as well as lineage geographic analysis in combination with geographical information. Genealogical geography methods analyze population historical expansion and migration routes by linking genetic branches with geographical distribution. In oyster studies, mitochondrial DNA haplotype networks and phylogenetic trees are commonly used pedigree geoanalysis tools. Researchers can construct haplotype network maps of different geographical groups to observe whether there are geographically clustered haplotypes and their divergence depths. The mitochondrial haplotype lineage of European flat oysters showed several regional branches, one of which extended from the North Sea to France as a mosaic distribution, presumably related to the mixing caused by artificial transplants, while several branches were limited to a small range and were interpreted as remains of ice age shelters. By combining this genetic evidence with paleoclimatic geography knowledge, the diffusion path and velocity of oysters after ice can be inferred. In order to more quantitatively characterize historical gene flows and population dynamics, researchers began to use Bayesian inference methods based on the co-alescent model. For example, Approximate Bayesian Computation (ABC) can simulate different historical scenarios (such as population separation at different times, different mobility, etc.), compare the simulated genetic indicators with the measured data to calculate the likelihood, and thus select the best possible historical models. In terms of gene flow direction, migration models (such as MLE method) can estimate the size and direction of continuous gene flow between branches after ancestral population division under the shared aliascent framework. 5 Case Studies: Analysis of Oyster Population in Typical Sea Areas in Global 5.1 The genetic structure of Pacific oysters in the northwest Pacific (China, Japan, South Korea) Pacific oysters (Crassostrea gigas) are native to the northwest Pacific coast, including eastern China, the Korean Peninsula, Japan and the Russian Far East. They are the most representative oyster species in the region. Historically, due to widespread continuous coastal distribution and long-distance floating juveniles, the Northwest Pacific Pacific oysters were considered to be a highly connected single group. However, genetic research in recent years has found that there is still a certain geographical structure and differentiation within it. Early analysis based on mitochondrial and nuclear DNA markers showed that Pacific oysters (sometimes called "Portuguese oysters") in southern China were slightly genetically different from populations in northern Japan, and was speculated to be a result of Pleistocene Ice Age. This view is further supported by higher resolution genome-wide data. Qi et al. (2017) used SNP markers to compare Pacific oysters introduced in China, Japan and Europe, and found that the F_ST between the Chinese and Japanese populations was about 0.024, which was significantly higher than the F_ST indirectly close to 0 in Japan and European or North American breeding populations. This suggests that within the native distribution area, Pacific oysters along the Chinese coast have experienced mild genetic
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