Genetic Diversity Of Two Marine Fishes And Phylogeny Of Sciaenidae Based On Mitochondrial Genome

Sciaenid fishes (Osterichthyes:Percitormes:Sciaenidae) are widely distributed in the coastal waters of the world. The coastal area of China is one of the main distribution areas, which possesses about 30 species in 17 genera. Black bream (Acanthopagrus schlegelii) is widely distributed in the Northwest Pacific Ocean, who lives in water areas with rocks or mud materials on bottom and never perform a habit of long distance migration. Johnius grypotus is a kind of offshore lower-middle-class fish and fond of perching on the bottom of the sediment and rocks nearby sea area. It mainly distributed along the region of Indian-Western Pacific Ocean. Both of them are produced along the coast of China, and also the important marine economic fishes in our country,Ii this study, three mitochondrial genomes of croakers were sequenced and analyzed. The phylogenetic relationship of sciaenid fishes were discussed based on mitochondrial genome with the related sequence from GenBank. Morphological and mitochondrial DNA markers were adopted comprehensively to analyze phylogeographic patterns of A. schlegelii and J. grypotus and the results revealed the genetic diversity, population genetic structure and historical dynamic condition of them. The main results are as follows:1. Mitochondrial genome structural characteristics and phylogenetic analysis of sciaenid fishes(1) This study sequenced the complete mitochondrial genomes of three sciaenid fishes Bahaba taipingensis, Nibea coibor and J. grypotus, the sequence length of them were 16,500 bp,16,509 bp, and 18,523 bp respectively. Comparative analysis of them reveal that B. taipingensis and N. coibor had the identical mitochondrial genome characteristics, and in accordance with most of the bony fish mitochondrial genome structure, which included 13 protein-coding genes,22 tRNA genes,2 rRNA genes and 2 non-coding regions. There was rearrangement phenomenon in the mitochondrial genome of J. grypotus, including 3 inserts of non-coding region and a replacement of tRNA gene, which had been reported in mitochondrial genome structure of J. belengerii. According to the reported mechanism of mitochondrial genome rearrangement, the inserts of non-coding region might be created by incomplete cutting after continuous replication of genome, and the replacement of tRNA was caused by the incorrect copying of tRNA gene around the start replication point of control region.(2) Phylogenetic tree based on mitochondrial DNA genome of 16 kinds of sciaenid fishes supported monophyletic origin of Sciaenidae. Johniinae and Sciaeninae located at the outermost end of the phylogenetic tree, which is consistent with the morphological classification results. Argyrosominae was not monophyletic origin anddistributed in different branches, a further study is necessary for its classification status. This study supported the view of putting Nibeajaponica and Nibea miichthioides into Argyrosomus, but did not support the point that Nibea genus belonged to Chrysochir. Miichthys miiuy did not clustered with Larimichthys and Collichthys, which was inconsistant with traditional classification results, so Miichthys taxonomic status was questionable.2. Population morphology and genetics study of A. schlegelii(1) In this study, the research methods of traditional morphology and otolith morphology were adopted to anlyse 8 Acanthopagrus schlegelii populaitons along coast of China. The results of principal component analysis, discriminant analysis, single factor analysis of variance and cluster analysis showed that morphological differences were existed between different populations. These morphology differences were mainly caused by genetic and environmental factors (such as gene, physicochemical properties of marine water, habitats, current effect etc.).(2) The analysis of mitochondrial DNA control region showed that genetic diversity of wild populations was higher than cultured populations. Genetically-differentiated geographical clade was not observed according to phylogenetic tree and structure of haplotype network. Significant difference of genetic structure was detected between south group and north group of wild populations. The coefficient of genetic differentiation (Fst) is small during groups. These might be related to their living and breeding habits. In addition, the demographic history examined by mismatch distribution analyses and Bayesian skyline (BSP) analyses suggested that a sudden population expansion occurred during 51 to 103 thousand years before.3 Population morphology and genetics study of J. grypotus(1) In this study, the methods of traditional morphology and otolith morphology were used to analyse Qingdao and Dongying populaitons. The results of principal component analysis, discriminant analysis, single factor analysis of variance and cluster analysis showed that there’s no morphological difference between these two populations, that is to say there was gene flow between the two populations.(2) The result of mitochondrial DNA control region analysis showed that there were significant genetic differences between the North Yellow Sea population and other populations, this phenomenon was universal in marine organisms of Northwest Pacific Ocean. There was no genentic differention between these populations except the North Yellow Sea population, which might caused by three reasons:late population expansion time, eggs or larvae are carried by coastal current, the homogeneity of living environment. Besides, the demographic history examined by mismatch distribution analyses and Bayesian skyline (BSP) analyses suggested that a sudden population expansion occured almost 20 to 40 thousand years before.