浙闽沿海葡萄牙牡蛎群体遗传结构及种群历史分析

Genetic structure and population history of Crassostrea angulata from the coast of Zhejiang and Fujian Provinces

利用线粒体细胞色素氧化酶亚基因（mt COI）对浙闽沿岸5个葡萄牙牡蛎（Crassostrea angulata）自然群体的遗传结构及其种群历史进行分析。结果表明,葡萄牙牡蛎群体的遗传多样性处于较高水平,在183条序列中检测到44个多态性位点,共定义了39个单倍型;平均单倍型多样度和平均核苷酸多样度分别为0.8524和0.00406,平均核苷酸差异数在不同群体内差异较小。AMOVA分析显示,绝大多数的遗传变异都来自于群体内个体间（91.94%）,组间和组内群体间均不存在明显的遗传分化。两两群体间的ΦST值较低（–0.01193~0.11486）,但宁德群体与其他群体间出现了显著的低程度遗传差异。单倍型网络关系图整体上呈星状拓扑结构,不同地理来源的单倍型无明显分支。贝叶斯系统发生树上,各单倍型交错分布,没有表现出明显的地理分化。中性检验和错配分析均表明葡萄牙牡蛎群体经历了历史扩张,扩张时间在更新世末期的25万到21万年前。

The Portuguese oyster, Crassostrea angulata, is one of the most important shellfish species in China and occurs mainly in the intertidal zone of the south Yangtze River. It is widely cultured in Zhejiang and Fujian provinces and many of its seeds are collected from natural areas. Molecular genetic data can provide valuable insights into C. angulata management and conservation. In the present study, we investigated the population genetic structure and demographic history of C. angulata from the coast of Zhejiang（Yueqing） and Fujian（including Ningde, Langqiao, Fuqing, and Putian） provinces by analyzing the partial mitochondrial cytochrome c oxidase subunit 1（COI） sequences in 183 samples. All populations were characterized by high genetic diversity. Forty-four polymorphic sites were identified, and 39 haplotypes were found. The mean genetic and mean nucleotide diversity values were 0.8524 and 0.00406, respectively. Despite high levels of genetic diversity within the populations sampled, no significant genealogical branches corresponded to the sampling locality. Analysis of molecular variance indicated that most of the genetic variation was attributed to within-group variation（91.94%, P=0.0003）. Population pairwise ΦST values were low（ΦST =-0.01193~0.11486）, suggesting frequent gene flow among the populations in this area. However, low but significant genetic differentiation was observed between the Ningde samples and the other four samples from four sites, which was presumably due to the relatively closed geophysicallocation of Ningde. Ningde is surrounded by many bays, islands, and gyres, which act as natural barriers that may have hindered dispersal of oyster larvae. The haplotype network of COI sequences with stellate radiation structure suggests no clear geographical differentiation. A Bayesian phylogenetic tree was characterized by a staggered distribution of haplotypes and some small branches supported with moderately high bootstrap values. Significant expansion of C. angulata populations is suggested based on results from neutral tests and the mismatched distribution; the actual time that expansion likely started in 2.5×105–2.1×105 years ago. Historical recolonization（though population range and demographic expansion during the late Pleistocene） and the current gene flow（larval dispersal interacting with oceanographic processes, such as ocean currents） of adult sedentary C. angulatamay be responsible for the poor genetic differentiation. Our results will help in understanding the historical processes of intertidal species and help disentangle the complex interactions among various factors that generate the phylogeographical patterns and genetic diversity of C. angulata in the East China Sea.