|Other Abstract||Since the mid-to-late 1990s, jellyfish blooms have occurred frequently in the East China Sea, Yellow Sea and Bohai Sea. Jellyfish blooms can damage the structure and function of ecosystems, threatening local marine fisheries, coastal tourism and personal security, which arouses the extensive concern of scientists and government. Jellyfish has simple morphological characteristics, so it is difficult to determine whether the jellyfish populations collected in different waters are part of the same population or subpopulation. In order to clarify the classification and genetic characteristics of the populations, it is crucial to carry out phylogeographic research on jellyfish collected from different coastal waters. The population genetic characteristics of Aurelia coerulea based on 16S rRNA gene fragments were analyzed, and genetic analysis of Gonionemus vertens based on mitochondrial COI and 16S rRNA gene fragments were also carried out in Chinese coastal waters. The main results were as follows:
1. The results of genetic population analysis of A. coerulea based on mitochondrial 16S rRNA gene fragments were as follows:
The moon jellyfish A. coerulea is widely distributed in coastal Chinese waters and is one of the species of jellyfish which cause blooms in the Yellow Sea and Bohai Sea. Blooms of A. coerulea have been reported in both coastal marine waters and coastal aquaculture ponds. Four A. coerulea medusae populations collected from coastal waters and four A. coerulea ephyrae populations collected from aquaculture ponds were analyzed based on 16S rRNA gene fragments. 166 individuals of eight geographical populations were sequenced in the study. A blast search of the BLASTn database concluded that both the ephyrae collected in the aquaculture ponds were A. coerulea (Aurelia sp.1), and were the same subspecies as the moon jellyfish in coastal waters. The overall haplotype diversity and nucleotide diversity of A. coerulea were respectively 0.686 ± 0.032 (Hd > 0.5) and (0.329 ± 0.019)% (π < 0.5%). The Tajima's D tests and Fu’s Fs tests for all populations suggested that A. coerulea populations had experienced historical population expansion events. Pairwise fixation index (Fst) values showed that genetic difference existed between A. coerulea medusae collected in Weifang waters and the medusae collected in Qingdao waters, or between the medusae collected in Weifang waters and the ephyrae populations collected in Dongying, Laoting and Qingdao waters. There was limited gene flow among populations (Fst > 0.25, P < 0.05, Nm < 1). The genetic differentiation was not significant between A. coerulea medusae and ephyrae populations collected in Qingdao and Rongcheng waters, or between A. coerulea medusae collected in Caofeidian and ephyrae collected in Laoting. Because of the exchange of water between the coastal waters and aquaculture ponds, as well as the lack of geographical isolation, the A. coerulea collected in these two habitats belonged to the same genetic population. Phylogeographic analysis of the 16S rRNA region revealed that there was no significant correlation between the evolutionary relationship of the haplotypes and the distribution of geographic populations. There was no obvious systematic geographical structure among the populations in this study. This information will be helpful in analyzing the migration and dispersal of A. coerulea and determining whether different jellyfish blooms are the same species based on the results of genetic analysis.
2. The results of genetic analysis of G. vertens populations based on mitochondrial COI and 16S rRNA gene fragments were as follows:
G. vertens is widely distributed throughout the Atlantic and Pacific Oceans and is one of few venomous jellyfish species in China. To investigate the population genetic structure of G. vertens, we sequenced the mtDNA COI gene in 104 individuals collected from four geographic locations along the coast of the Yellow Sea and Bohai Sea, and the homologous sequences of another 182 individuals which were obtained from GenBank were also analyzed. The overall haplotype diversity and nucleotide diversity of G. vertens were respectively 0.743 ± 0.012 and (1.046 ± 0.097)%, the genetic diversity of G. vertens were comparatively high. This indicates that overall populations of G. vertens have evolved over a long period of time from a stable group. Pairwise fixation index (Fst) values showed that the largest population differentiation existed between the G. vertens population collected in Xiamen waters and the other four populations collected in Chinese coastal waters. There was limited gene flow among populations (Fst > 0.25, P < 0.05, Nm < 1). The factors leading to differentiation could include isolation of distance and the lack of gene flow among populations. A moderate level of population differentiation existed between G. vertens population collected from Dalian waters and populations collected from Dongying and Yantai waters (0.05 < Fst < 0.15, P < 0.05, Nm > 4). The complex life cycle characteristics, together with the prevailing ocean currents in this region, were proposed and discussed as the main contributing factors to this. Phylogeographic analysis of the COI region revealed two lineages. Although there was a slight crossover in the evolutionary branches of some haplotypes, most of the haplotypes were located on the same branch, indicating that the G. vertens populations had obvious systematic geographical structure.
The mtDNA COI and 16S rRNA gene fragment collected from four geographic locations along the coast of the Yellow Sea and Bohai Sea, combining with the homologous sequences of Xiamen population which were obtained from GenBank were analyzed. The haplotype diversity and nucleotide diversity of five populations were respectively 0.433 ± 0.056 and (0.209 ± 0.035)%, with a total of 15 polymorphic nucleotide sites detected among the populations based on COI. The haplotype diversity and nucleotide diversity of G. vertens populations collected in Chinese coastal waters were respectively 0.345 ± 0.054 and (0.134 ± 0.029)%, with a total of 7 polymorphic nucleotide sites detected among the populations based on 16S rRNA. The genetic differentiation of G. vertens based on 16S rRNA was slightly less than that based on COI in this study, probably because 16S rRNA is more conservative than COI. The haplotype diversity (Hd < 0.5) and nucleotide diversity (π < 0.5%) based COI and 16S rRNA were comparatively low, indicating that the genetic bottleneck and founder effect were caused by minority populations in the five G. vertens populations collected in Chinese coastal waters.
The results indicates that the complex life cycle characteristics, isolation of distance, together with the prevailing ocean currents in this region, were proposed and discussed as the main factors that determined the genetic patterns of G. vertens.|