| It is well-known that the global trade of ornamental fish develops rapidly, thus the fishing intensity and pressure increase year by year. Besides, the wild fish resources of the Amazon basin has been seriously damaged due to the ecological environment change. It is the first time to get the complete mitochondrial genome sequences of the wild green and brown discus fish and to explore the mitochondrial DNA molecular evolution of cichlids in this paper. It aims to investigate the phylogenetic relationships of the genus Symphysodon and to provide more evolutionary genetic information of cichlid fish. In addition, the results can promote the fish resources protection in the Amazon River basin and to support effective management and the sustainable development of fisheries resources. This research includes two parts as follows:A. Sequencing of complete mitochondrial genome of two wild discus speciesThe complete mitochondrial genome sequencing was carried out by next-generation sequencing method. The mitogenome length of Symphysodon haraldi and Symphysodon aequifasciatus is 16543 bp and 16545 bp, respectively. Both genomes consist of 13 protein-coding genes, 2 rRNA gene, 22 tRNA and a D-loop region. The nucleotide composition shows a nucleotide bias against G and especially in the 12 protein-coding genes encoded by H-strand. The homologous protein-coding genes of the two groupers share the same initiation codons. Initiation codes of CO1 is GTG and the other 12 protein-coding genes are all ATG. The usage of termination codons includes TAA, TAG, TA- and T--. Among the 22 tRNAs, the other 21 tRNAs have clover second structure with conservative anticodon loop except tRNA-Ser. The D-loop district include extended termination associated sequences(ETAS), central conserved domain(CD) and conserved sequence blocks(CSB).The length difference of control region sequence between the two sequences is obvious, which is mainly due to the difference of the length of sequence repeated units. There is the largest regional mtDNA variation in non-coding region and difference in three discus fish non-coding sequence length.B. Genetic evolutionary relationship of the genus SymphysodonWe can get information about the sequence genomes and important functional genes to explain the evolutionary history of the species and understand the molecular mechanisms of growth and adaptation to the environment by genome sequences research. Through the comparison and analysis of 3 kinds of similarity among the components of the mitochondrial genome. We found that base similarity in protein coding genes was 83.69%, rRNA gene was 93.68%, tRNA gene was 94.06%, D-loop noncoding regions of 87.96%, the similarity of the amino acids in the protein coding genes was 95.34%. Combined with currently known Perciformes mitochondrial genome sequence data to black mackerel as outgroup constructed 37 kinds Perciformes mitochondrial genome phylogenetic tree.With Alepocephalus agassizii and Bajacalifornia megalops as the designated outgroup, phylogenetic tree was constructed based on the maximum-parsimony(MP) methods which include additional mitogenome sequence of other Perciformes fishes from NCBI. There are several conclusions from the DNA sequences analysis: a)Embiotocidae, Cichlidae and Pomacentridae are a monophyletic group. b)The relationship between Sparidae and Labridae is closer. S. haraldi, S. aequifasciatus and S. discus are in a monophyletic group. The relationship between S. haraldi and S. discus is closer. As to mitochondrial D-loop of three discus fish, the lowest difference exists between S. haraldi and S. discus based on the TAS area base deletion and replacement. Therefore, it can be concluded that the relationship between S. haraldi and S. discus is closer. |
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