Population Genetic Structure And Complete Mitochondrial Genome Of Semisulcospira Libertina Of Poyang Lake Basin

Semisulcospira libertina belongs to Semisulcospiridae of Cerithioidea, is anobligate freshwater snail species, widely distributed in the small streams of Poyanglake basin. The freshwater mollusk biodiversity in Poyang Lake basin is facingnumerous threats due to dam construction, overexploitation and sand excavation. AsS.libertina specializes on the unique habitat conditions typical of headwater streamreaches with a limited capacity of dispersal, it is more sensitive to the environmentchange in Poyang Lake basin. The aim of this study was to analyze the geneticdiversity and genetic structure of S.libertina populations in Poyang Lake basin usingmitochondrial Cytochrome Oxidase Subunit I (COI) gene sequence as molecularmarker. Besides, we sequenced and analyzed the complete nucleotide sequence ofS.libertina, which could provide extensive molecular markers for genetic studies ofS.libertina and complete mitochondrial genome studies of Cerithioidea. At the sametime, this mitogenome should contribute to resolving phylogenetic position andinterrelationships of Cerithioidea and evolution study of Caenogastropodamitochondrial genome arrangements.The main results of this study are as follows:1. Genetic diversity and genetic structure of S.libertina populations in PoyangLake basin.A total of134Specimens of5S.libertina populations were collected from fiverivers of Poyang Lake basin. The mitochondrial COI fragments were amplified andsequenced using LCO1492and HCO2198primers. Sequence alignment of the COIgene portions yields625bp homological fragments,92sites are polymorphic. And24different COI haplotypes with3haplotypes shared were found in5populations, theother21Haplotypes were all unique for each population, which accounted for86.36%of the total Haplotypes. Among the134COI sequences, the overall nucleotidediversity was0.02379±0.00677, and haplotype diversity was0.729±0.041. The resultof haplotypic and nucleotide diversity analysis displayed a middle genetic diversity.However, Genetic diversity was low in GanJiang and FuHe populations (GJ:π=0.00358±0.00305; FH: π=0.00510±0.00203). The biomodel mismatch distributionof pairwise haplotype difference and the neutrality tests showed that populationexpansion has been occurred in GanJiang and FuHe population, which may result in the low genetic diversity of GanJiang and FuHe populations.The AMOVA of five river populations revealed that significant differentiationbetween5populations (FST=0.3171, P<0.01). Besides, pairwise FSTvalues showedgenetic differentiation between all populations except GanJiang and FuHe populations.The lowest genetic distance was0.005between GanJiang and FuHe populations, thegenetic distance between other populations ranged from0.024to0.046. Maximumparsimony phylogenetic tree based on the24haplotypes showed four well-supportedclades, GanJiang and FuHe populations formed one clade and mainly belonged toclade A. All those results suggested that significant differentiation between5populations except GanJiang and FuHe populations, which could be explained by thewater-depth of Poyang Lake. The water-depth of Poyang Lake was unfavor for thegene flow between populations of different river of Poyang Lake Basin. However,FuHe river was a tributary of GanJiang river before1955and flowed into the PoyangLake together, which resulted in no differentiation between GanJiang and FuHepopulations.In conclusion, to protect S.libertina populations more effectively in Poyang LakeBasin, we suggested that protection of XinJiang, RaoHe and XiuHe populationsshould first consider. Besides, as the water-depth of Poyang Lake was unfavor for thegene flow between populations of different river of Poyang Lake Basin, We suggestedthat be careful of planning building dam in the Poyang Lake Basin.2. The characterization and gene order of complete nucleotide sequence ofS.libertina.The first complete mitochondrial genome (mitogenome) of S.libertina(Cerithioidea: Semisulcospiridae) was determined using thirteen pairs of primers andthis was also the first complete mitochondrial genome of Cerithioidea. The completemitogenome of S.libertina was15,432bp in length (GenBank accession number.KF736848). The overall base composition was31.4%for A,17.8%for C,34.8%for Tand16.0%for G with a A+T bias. The mitogenome of S.libertina contained2ribosomal RNA genes,13protein-coding genes and22transfer RNA genes. Nineprotein-coding genes as well as seven tRNA genes were encoded on the heavy strain,and all remaining genes were encoded on the light strain. All protein-coding genesused ATG as start codon. Except for ND6with an incomplete stop codon “TA-”, theremaining protein-coding genes stop with TAG or TAA. The mitogenome ofS.libertina displayed extensive novel gene order arrangement compared with published Caenogastropoda mitogenomes to date. This mitogenome should contributeto resolving phylogenetic position and interrelationships of Cerithioidea and evolutionstudy of Caenogastropoda mitochondrial genome arrangements.