Genetic Diversity And Genetic Structure Of Paraoncidium Reevesii And Platevindex Mortoni

Onchidiidae is a kind of shellfish inhabiting mud flat and mangrove forest ofsubtidal and low-tidal zones and belongs to Onchidiidae, Mollusca, Pulmonata andSystellommatophora. Onchidiidae occurrs throughout the world except the PolarRegions, and comprises over100species. Genetic diversity, genetic structure ofParaoncidium reevesii and Platevindex mortoni were analyzed. The cause of geneticdifferentiation and genetic structure among populations were also inferred.The main results are as follows:1.187individuals of Paraoncidium reevesii from coastal area of China and Phuketin Thailand were collected, including populations ZJ (30), DX (28), BH (30), CN (31),ND (28), HN (18), WC (23) and TH (17). Their16S rDNA genes were splicingcorrectted into homologous sequence of372bp, including40singleton variable sites and16parsimony informative sites. The mean base content of populations was30.7%(T),16.2%(C),30.0%(A) and23.1%(G) respectively. The base content of A+T (60.7%)was more than that of C+G (39.3%).16S rDNA indicated high haplotype diversity(0.873±0.017) and nucleotide diversity (0.0069±0.0005) existed in Paraoncidiumreevesii populations. The highest haplotype diversity (h) and nucleotide diversity (π)occurred in DX population, however, h-value and π-value in TH polulation were thelowest. The genetic differentiation index (Fst), gene flow (Nm), genetic distance andnetwork of haplotype showed that significant differentiation existed in TH populationand populations from coastal areas of China. The differentiation also occurred in BH andDX populations in some degree. Whereas, BH and DX were random mating populations(Nm>4). IBD ananlysis indicated that genetic distance was not proportional togeographical distance. TH population and pupulations from coastal areas of Chinalived in different areas significantly. Therefore, geographic isolation for long time wasinferred to lead to genetic differentiation between TH population and pupulations fromcoastal areas of China. BH and DX polulations inhabited in the environment belongedto Beibu Gulf. Beibu Gulf are semi-enclosed seas which has special hydrologicalregime and environment. Free-swimming veliger stage of P. reevesii is short (~10d)whose larvas and adults have limited migration capacity. The two aspects were inferred to lead to genetic differentiation between BH and DX populations.2. Individuals of Paraoncidium reevesii from coastal area of China and Phuket inThailand were collected, including populations ZJ (30), DX (25), BH (28), CN (31),ND (10), HN (12), WC (15) and TH (20). The genetic diversity and population geneticstructure of P. reevesii were investigated using mitochondrial cytochrome c oxidasesubunit I (CO I) gene sequences. A total of171individuals representing8collectionsites were included in the analysis. Overall,101haplotypes were defned and117polymorphic sites were observed. The P. reevesii populations had high haplotypediversity (0.974±0.005) and nucleotide diversity (0.03218±0.00132). The AMOVA testof P. reevesii populations based on haplotype frequencies revealed that49.01%of thegenetic variation occurred within the populations, whereas50.99%of the geneticvariation occurred among populations. Pairwise fxation index (Fst), gene flow (Nm) andgenetic distance analysis indicated signifcant genetic structure had appeared in P.reevesii populations. IBD ananlysis indicated that genetic distance was not proportionalto geographical distance. Demographic analyses indicated populations ZJ, CN, HN, WCand TH might experience population expansion. The time of expansion of Chinesepopulations could be0.725Ma BP~0.781Ma BP, whereas the time of expansion ofpopulation TH could be0.035Ma BP. The phenomenon might occur in Pleistoceneperiod with melting of glaciers, changes of climate warming and rising of sea level.3. Individuals of Platevindex mortoni from coastal area of China were collected,including populations ZJ (16), GX (17), CN (20), HN (11). A total of84individualsrepresenting5collection sites were included in the analysis.16S rDNA and CO Iindicated high haplotype diversity and nucleotide diversity existed in Platevindexmortoni populations. The two genes defined13haplotypes,40polymorphic sites,41haplotypes and117polymorphic sites respectively. CO I showed more polymorphismthan that of16S rDNA respectively. In total, h-value and π-value in CN population wererelatively high. Two indices in HN population were second. Fst-value and geneticdistance indicated obvious genetic divergence occurred in HN population and CNpopulation based on16S rDNA and CO I genes. No obvious genetic differentiationoccurred between GX population and the rest population compared with Paraoncidiumreevesii. Different survival capacities of P. mortoni and P. reevesii to environment wereinferred to lead to the phenomenon. P. reevesii need to live in mud flat, however, P.mortoni can live in mangrove and rock. Network of haplotype and phylogenetic treeindicated HN and CN populations shared the same haplotype. Two significantlydifferent haplotypes existed in HN population. The Mantel test failed to detect anysignifcant correlation between geographic distance and genetic distance. Neutrality test and dynamic populations ananlysis indicated population expansion was notoccurred in P. mortoni populations. Therefore, we inferred two different geneticpedigrees occurred in HN population. One of genetic pedigrees only belonged to HNpopulation, which had significantly different from genetic pedigrees of the rest fourpopulations. The other genetic pedigree had close genetic relationship with one of CNpopulation. HN population was inferred to be taked to CN area by artificialtransplantation and genetic differentation occerred between HN and CN populationswith the long-term evolution gradually. ZJ and GX populations might experiencehistorical bottleneck effect. AMOVA analysis agreed with HN population was a group,CN population was a second group, the rest populations were the third group.Population expansion was inferred to occur in XM population based on CO I gene. Thetime of population expansion was about1200000a BP. The phenomenon might occurin Pleistocene period by changes of climate of glacial-interglacial cycle.