Studies On Molecular Phylogenetics Of Genus Mytilus And Population Morphology And Genetics Of Three Mussels

Mytilus spp., belongs to the family Mytilidae (Rafinesque, 1815), widely distributed in different climate, latitude and sea conditions. In China, marine mussels are a commercially important species group. Among the mussels, Mytilus galloprovincialis, M. coruscus and Perna viridis are the main cultured species in China.We use traditional morphology and mtDNA COI sequences to study the genetic diversity level of three mussels. In order to study the divergent level of M and F type genome, we examined the genetic diversity and genetic differentiation of six M. galloprovincialis populations using sequence analysis from the cytochrome oxidase subunit I (COI) of the mitochondrial DNA (mtDNA). The divergent time of M and F type genome was also calculated. The partial sequences of the mitochondrial cytochrome oxidase I and 16S ribosomal RNA of the five Mytilus species were compared to determine their phylogenetic relationships.1. Analysis of six traditional morphological characters (shell length, shell height, shell width, whole body weight, wet shell weight and wet weight of soft tissues) among 9 populations of M. galloprovincialis were carried out with the method of clustering and one-way ANOVA. All above analysis indicated that there were significant differences among the nine populations in two morphological characteristics. The results of cluster analysis revealed that there were certain degree of differentiations among nine populations. Exponential growth relationship was found between wet weight of soft tissues, whole body weight and shell length. Linear growth relationship was found between shell height, shell width and shell length.Four morphological characters of populations of P. viridis were analysed by clustering and one-way ANOVA. The multivariate analysis and one-way ANOVA results showed that there were significant differences among the seven populations in two morphological characteristics. The results of cluster analysis revealed that there were differentiations among seven populations. Exponential growth relationship was found between whole body weight and shell length. Linear growth relationship was found between shell height, shell width and shell length. Three traditional morphological characters (shell length, shell height, shell width) among three populations of M. coruscus were analysed by clustering and one-way ANOVA. All above analysis indicated significant genetic divergency existed between Fuding population and other two populations.2. To assess the roles that Pleistocene ice ages have played in the evolutionary history of the marine organisms and ecological connectivity among marine populations in the Northwestern Pacific, we explored the intraspecific phylogeographic pattern and dispersal patterns in three mussels.Sequence variation of the COI region was analyzed among 11 populations of M. galloprovincialis, defining 11 haplotypes. Three distinct lineages were found, which might be isolated and diverged in different marginal seas of the Northwestern Pacific during Pleistocene low sea level stands. There were little geographical differences in haplotype frequencies of the three lineages. Hierarchical molecular variance analysis (AMOVA) and conventional population FST comparisons revealed no significant genetic structure throughout the examined range, which is inconsistent with previous findings based on the morphological and ecological studies.Seven populations of P. viridis were studied, defining 17 haplotypes. Two distinct lineages were found, which might be isolated and diverged in different marginal seas of the Northwestern Pacific during Pleistocene low sea level stands. However, these clades did not appear to have geographic structure. Molecular variation analyses and the conventional population statistic FST revealed significant genetic structure between Chinese populations and Japanese population (Kochi).Five populations of M. coruscus were studied, defining 27 haplotypes. Two distinct lineages were found, which might be isolated and diverged in South China Sea and East China Sea during Pleistocene low sea level stands. Lineage A was the main lineage in all the populations. Molecular variation analyses and the conventional population statistic FST revealed no significant genetic structure. Altogether, the results indicate that M. coruscus is panmictic throughout the examined range.These results indicated that pelagic larvae can potentially interconnect distant populations through dispersal on China Coastal Current and Kuroshio Current, leading to high gene flow among different populations.3. Doubly uniparental inheritance (DUI) phenomenon was discovered in mollusk mitochondrial DNA. In order to detect the genetic divergent between F and M genome, six populations of M. galloprovincialis were collected and two types of genomes were detected (F and M type). A total of 162 out of 661 sites revealed as nucleotide substitution. The genetic differences within the M, F type and between the two types were 0.006, 0.002 and 0.202, respectively. Significant differences were found between two types of the genome based on COI region and the M genome evolves faster than the F genome. It is suggested that a subtle balance between relaxed selection and a higher mutation rate explains the faster evolutionary rate of the M genome. Based on the analysis of COI region, the divergence of F and M-type genome occurred about 4.39-5.75 MYA, indicating Early Pliocene divergence. 4. In this study the partial sequences of the mitochondrial cytochrome oxidase I and 16S ribosomal RNA of the five Mytilus species were compared to determine their phylogenetic relationships. The phylogenetic trees were reconstructed based on neighbor-joining, maximum parsimony, maximum likelihood, Bayesian Inference and combined Bayesian analyses methods. P. viridis and Septifer virgatus were used as outgroups. The phylogenetic analysis revealed that the five species, M. edulis, M. galloprovincialis, M. trossulus, M. coruscus, M. californianus formed into two clades. M. californianus is the most basal among all species of genus Mytilus, and the secondary is M. coruscus. M. galloprovincialis and M. edulis are the most similar among the three closely related species. M. coruscus is closer to M. californianus than to M. edulis, M. galloprovincialis and M. trossulus. The molecular phylogeny analysis provides the theoretic foundation for future comparative studies of species evolution, immigration and breeding strategies in Mytilus.