Population Analysis Of Thamnaconus Hypargyreus In South China Sea

Lesser-spotted leatherjacket Thamnaconus hypargyreus (Cope,1871)(Tetraodontiformes: Monacanthidae) is an economically important fish species in theSouth China Sea, its products in ahead of all the populations in the South China Sea.Knowledge of population structure is particularly important for longterm fisheriesmanagement and conservation. The molecular systematics study of Tetraodontiformesis a hot spot. The part or complete mitochondrial genome sequences were usuallyused to analyze the molecular phylogenetics in Tetraodontiformes from species toorder levels. In this study, we designed15pairs of primers for amplification of themitochondrial fragments of T. hypargyreus by PCR. After cloning, sequencing, andoverlapping, we obtained the complete mitochondrial genome sequence in the firsttime. This sequence was used to analyze the mtDNA genome structural characteristicsand the phylogenetic relationship with some Thamnaconus species.The sequence of T. hypargyreus mitochondrial genome is16438base pairs (bps)in length with53.5%A+T content, and encodes22tRNA genes,2rRNA genes,13protein-coding genes, as well as a control region (D-loop) of786bp. Mostprotein-coding genes start with a typical ATG codon, except for COⅠ which startswith GTG. The usual TAA or TAG termination codons were found for most of theprotein-coding genes. However, the other3genes (COⅡ, ND4and Cyt b) have anincomplete termination codons (T or TA). It was confirmed that the gene arrangementof T. hypargyreus could be used to identify the other Tetrodontiformes fish. TransferRNA (tRNA) genes range from67to75bp in length. Construction of phylogenetictrees based on the complete mitochondrial genome sequence of Tetrodontiformesspecies constructed by NJ and MP method. The result indicated that the T.hypargyreus has closer relationship to T. tessellatus and T. modestus, whichconstituted a sister group. The results also showed that the mtDNA genome of T.hypargyreus is similar to most of the vertebrates in gene components, order,orientation, tRNA structures, skews of base GC and AT without rearrangements or pseudogenes. The order relationship of the phylogenetic trees based on completemitochondrial genome sequence of Tetraodontiformes is in accordance with those ofthe morphological classification. It suggested that the genus Thamnaconus(Tetraodontiformes, Monacanthidae) can be treated as a monophyletic group. The datawill provide the molecular basis to the genetic characteristics of T. hypargyreusmitochondrial genome and the species diversity.In order to estimate the population of T. hypargyreus which are distributed in thenorth of the South China Sea and the southwestern Nansha Islands, we amplifiedmitochondrial cytochrome b (Cyt b) gene fragments of5different geographicpopulations. Fish specimens (totally156individuals) were used in this study. Theresults were as follows:The Cyt b sequences of the fish were amplified by polymerase chain reaction(PCR) technology. Both56mutations of nucleotide acids without inserting or deletingone and58haplotypes were found among the examined779base-pair nucleotidesequences of Cyt b fragment. The ranges and the level of average genetic distancewere0.00295~0.00415for T. hypargyreus between groups. All5groups werecharacterized with high haplotype diversity (0.8201~0.9804) and relatively lownucleotide diversity (0.00262~0.00469). Analysis of molecular variance (AMOVA)and the fixation indices (Fst) of5groups showed that the genetic variance mainlycame from individuals within groups， and there was no genetic differentiationbetween groups. With high gene flow, this group could be developed and managed asan evolutional significant unit. The median-joining (MJ) network and phylogenetictree proved no phylogeographic differentiation structure in5groups. According to thecriteria distinguishing different populations, species and genera given by scholarsKartavtsev, the5groups should be merged to the same population, which implied T.hypargyreus in the North and South-West Continental Shelf of the South China Seabelongs to one population. The finding would provide a scientific basis for the studyof migration of T. hypargyreus between the two Continental Shelves and managementof its stock.Furthermore, mitochondrial D-loop (Control region, CR) gene fragments of5different geographic populations were also studied in this research. Fish specimens(totally158individuals) used in this study were also collected from5geographicallocations in the north of the South China Sea and the southwestern Nansha Islands.The results were as follows: A total of636nucleotides of the mitochondrial DNA (mtDNA) CR of the T.hypargyreus were amplified by polymerase chain reaction (PCR) technology. Both103mutations of nucleotide acids without inserting or deleting one and91haplotypeswere found among the examined CR fragment. High haplotype diversity(0.9419±0.0151) and nucleotide diversity (0.0095±0.00506) relatively togetherwith a recent and sudden population expansion which characterizes the geneticpopulation structure of this species. Analysis of molecular variance (AMOVA) and thefixation indices (Fst) of5groups showed that the genetic variance mainly came fromindividuals within groups, and there was no genetic differentiation between groups.The phylogenetic trees including maximum likelihood (ML) and bayesian inference(BI) proved no phylogeographic differentiation structure in5groups. The mtDNAmarker suggested the5groups should be genetic homogeneity, which implied T.hypargyreus in the north and southwest continental shelf of the South China Seabelongs to one population.