Genetic Diversity Of Penis Fish(Urechis Unicinctus) Based On Mitochondrial And Nuclear Gene Sequences

Penis Fish Urechis unicinctus belonging to Echiuroidea,Xenopneusta,Urechidae,Urechis,is a kind of unsegmented,coelomate,bilaterally symmetrical and soft-bodied marine worms with high nutritional and medical values,distributes around Bohai Gulf of China and Korean and Japanese coast.In order Xenopneusta,U.unicinctus is the only species that distributes along Chinese coast.The wild populations of this species have sharply declined due to overexploitation and ocean pollution,which led to the gradual habitat reduction of this species.However,population genetic structure and diversity of U.unicinctus has not been investigated systematically.In this study,we collected 105 samples from six locations of Bohai Gulf and Korean coast,and investigated genetic diversity and population genetic structure with mitochondrial COI?457 bp?,16S-rRNA?451 bp?and nuclear 28S-rRNA gene fragments?680 bp?,combining nine COI sequences and one 28S-rRNA sequence from Japan samples available in GenBank.Based on COI sequences,genetic diversity of U.unicinctus was still high?haplotype diversity:0.9595,nucleotide diversity:0.0101?.Among three markers,genetic diversity detected by 28S-rRNA gene sequences was the lowest?haplotype diversity:0.4084,nucleotide diversity:0.0007?.In addition to the differences in mutation rates and inheritance patterns between mitochondrial and nuclear genes,gene flows between geographic localities would also lead to the low diversity of nuclear sequences.F_ST statistics and AMOVA analysis based on COI sequences indicated unconspicuous differentiation among different localities(F_STT values between populations ranged from–0.00204 to 0.05210),and 99.12%genetic diversity was contributed by different individuals within population.Both phylogenetic trees?Bayesian trees based on COI,16S-rRNA and 28S-rRNA respectively,ML tree based on COI?and median-joining network based on COI did not show clear geographic cluster,and haplotypes from different populations were mixed.Our results indicated low level of genetic divergence among different localities of U.unicinctus,and this species should be treated as a whole population during wild resource conservation.The high dispersal capabilities of U.unicinctus larvae coupled with the limited physical barriers along its distribution coast may be responsible for the high degree of genetic exchange.In addition,significantly negative Tajima's D value?–1.9468?for all samples and star-like pattern in network based on COI sequences suggested that explosion event of U.unicinctus population would have occurred in history.Our study provided solid data for genetic assessment of U.unicinctus,which were valuable reference for species and genetic resource protection.