Population Genetics Studieson Four Species Of Crassostrea Oysters

Current genetic patterns of marine bivalves result from the interaction of various of extrinsic and intrinsic factors on different temporal and spatial scales. Population genetics of marine bivalves are useful and helpful to elucidate its current genetic diversity and genetic structure. Crassostrea oysters are commercially important marine bivalves along the coast of China, and four species of Crassostrea oysters are studied in present study. Partial mitochondrial Cytochrome oxidase subunit (COI) was used to investigate population genetic structure and demographic history of the above four species of oysters, which will provide scientific basis for the protection and sustainable utilization of genetic resources of the oysters. The main results obtained were as follows:1. Population genetic structure of Crassostrea gigasA total of 228 samples collected from 12 sites were sequenced. Moderate haplotype diversity and low nucleotide diversity were found within populations. No significant genealogical branches were observed corresponding to sampling locality, only low but significant genetic differentiation between some of the Chinese and Japanese populations was detected, suggesting strong gene exchange among them. Historical demography showed that C. gigas had experienced significant population expansions at about 51Kya to 42Kya. The results highlighted the important role of current gene flow (by larval dispersal interact with ocean currents) and historical recolonization (by population range expansion during the interglacial in the late Pleistocene) in current genetic pattern of C. gigas in East Asia. The unique lineage of C. gigas in present study might belong to the East China Sea unit.2. Population genetic structure of C. angulataFive populations of C. angulata along the coast of Zhejiang and Fujian provinces was tested. mtCOI data indicated that all the populations were characterized by high levels of genetic diversity. No significant genetic structure was detected. However, low but significant genetic differentiation was observed between Ningde and the other four populations. Significant population expansion event was suggested during the late Pleistocene. For sedentary adult C. angulata, the long pelagic larval period and the passive transport by ocean currents flow perhaps the main reasons for its weak genetic divergence. The differentiation of Ningde population could owe to its relatively’ closed’geographical location which was surrounded by various peninsulas and bays.3. Population genetic structure of C. ariakensisWe sequenced mtCOI fragments for 230 individuals sampled from 10 populations along the northwestern Pacific. Two distinct lineages were detected, corresponding to East China Sea (ECS) and South China Sea (SCS), respectively. A pattern of isolation-by-distance (IBD) was observed among all populations. The huge outflow from Yangtzi River was most likely to be a physical barrier for larval dispersal of C. ariakensis, but its barrier effect remains to be further studied. No significant genetic differentiation was found on either side of the Leizhou peninsula, the summer upwelling and human-mediated passive dispersal via aquaculture activities were assumed to be responsible for the genetic homogeneity among the South China Sea populations. Given existence of the big genetic break, the northern and southern lineages should be managed separately to avoid the gene introgression as well as the loss of evolutionary units and genetic diversity.4. Population genetic structure of C. hongkongensisSeven populations of C. hongkongensis were studied in the southern coast of the Yangtze River. No deep divergence was detected neither between different sea region nor on either side of the Leizhou peninsula, only low level of genetic differentiation was observed among some populations. Historical demography showed that the species had experienced significant population expansions (about 48Kya to 40Kya). Frequent human-mediated passive dispersal via aquaculture activities combined with the promotion of ocean currents was most likely to drive the present genetic pattern.