The Genetic Structure And Geographical Distribution Pattern Of Mytilus Unguiculatus In China

Mytilus unguiculatus is a species of mussel which widely distributed in the temperate Northwestern Pacific Ocean.It plays an important role in mussel aquaculture in China due to its characteristic and nutritive value.In this study,three different molecular tools(mitochondrial DNA markers(COI and Cyt b),microsatellite DNA and single nucleotide polymorphism)were used to estimate the phylogeography of M.unguiculatus in East China Sea and Yellow Sea.The main study results are as follows.i)Mitochondrial DNA markers analysisUsing 2 mitochondrial DNA markers(COI and Cyt b),the sequence fragments of 7 samples were amplified with 195 individuals.Results showed that the genetic diversity of each sample was at a high-level.34 haplotypes of COI genes and 29 haplotypes of Cyt b were obtained.The haplotype diversity ranged from 0.77 to 0.93(Cyt b: 0.83 to 0.91).The nuclear diversity ranged from 0.0044 to 0.0064(Cyt b: 0.0049 to 0.0063).The genetic structure was also weak.The distribution of each haplotype in samples was dispersed and uniform.It was not confirmed to the geographical distribution or sample distribution.In the selection pressure analysis,the samples were detected in a negative selection.In the historical dynamic analysis,it was no significant evidence that population events of samples had happened,including population amplification or population bottlenecks.Bayesian skyline plot analysis showed a stable state,and the evolution of each sample was relatively stable within nearly 10,000 years.It was speculated that 7 samples in current study should belong to the same group.ii)Microsatellite DNA marker analysisOne hundred and twenty pairs of microsatellite primers were designed using published M.unguiculatus transcriptome data.23 pairs of microsatellite DNA marker primers with high polymorphism were screened and developed.The most suitable 10 pairs of primers were selected as microsatellite markers to analyze 350 individuals from 7 samples.After amplification and genotyping,the results showed that the genetic diversity was at a medium level,and the genetic inbreeding index of most locus among samples was significantly positive,indicating that inbreeding might exist within samples.There was a weak variation of genetic structure.It has a weak differentiation between Qingdao samples of Yellow Sea and other samples of East China Sea.It did not detect a population bottleneck event or expansion event in the sample.M.unguiculatus in China showed a decline in genetic diversity and weaken genetic structure with the influence of inbreeding.iii)?B-RAD sequencing technique analysisApplying ?B-RAD technique and adopting the method of population genomics,the genetic characteristic of 7 samples were analyzed.After sequencing 70 individuals from 7 samples,without reference genomes,the average sequencing depth was 15×,and 20 Gb genome data was obtained.13,009 SNP markers were developed and obtained.The genetic diversity was at a medium level.Most of the genomic data of individuals were clustered in the same region,and the genetic differentiation was not obvious,which did not coincide with the geographical distribution.iv)The impact of different molecular markers on the resultsI used three different molecular markers and estimated the merits and demerits of their results.With a suitable high-throughput sequencing technique,the SNP could obtain complete and accurate genetic data of the samples.Based on the classic population genetics theories and population genome analysis methods,SNP could do well in analyzing the genetic data of individuals,samples and inter-samples,and it also could gain the genetic diversity and the genetic structure among samples.The planktonic larval stage of M.unguiculatus lasts 3 to 4 weeks,and the spawning time is in winter.The long plankton period and the winter breeding period enable the coastal current to promote larvae to spread in ocean.In winter,the diluent water from rivers, especially the Yangtze River,is in a weak state,which was unable to stop the larvae flowing between north and south.These two factors resulted in the free propagate of larvae in the sea area by ocean currents.This led to frequent gene exchange among different regions,reducing the genetic diversity and weakening the genetic structure.The artificial culture environment and natural habitat environment were mixed.It made breeding samples possible to escape from natural samples,and further weakened the genetic integrity of natural samples.In addition,most breeding samples were from single parents,which increased the inbreeding.It also led to the genetic diversity dilution of natural samples.This study will provide a comprehensive survey and data accumulation of the genetic characteristics of M.unguiculatus,filling the gaps among related studies.It provides theoretical support and material accumulation for seed selection and breeding,genetic resources protection and breeding management in the future.I believed that the results from this study can be used to provide important insights in genetic management units and sustainable utilization of M.unguiculatus resources and provide a better understand of genetic structure of marine bivalve with similar planktonic larval stage in the China Sea.