| Objectives:Oncomelania hupensis is the only intermediate host of Schistosoma japonicum with wide distributions.Significant genetic differentiation and variation have occurred among different snail habitats of Oncomelania hupensis.In recent years,there are still many current snail habitats.Meanwhile,new or re-emerged snail habitats are ever-increasing,providing a potential for the transmission and spread of schistosomiasis.Therefore,in this study,based on fluorescent-labelled microsatellite markers,the population genetics of Oncomelania hupensis snails from new,re-emerged and current snail habitats was studied and the genetic diversity of Oncomelania hupensis from different snail habitats was compared.Then the historical invasion pathways of Oncomelania hupensis in new snail habitats from 2014 to 2017 were studied combined with Approximate Bayesian Computations(ABC)method,in order to provide bases for the formulation of targeted preventive and control measures.Methods:1.Samples from current snail habitats of Shitai,Anhui(ST),re-emerged snail habitats of Hengtang,Jiangsu(HT)and new snail habitats of Taicang,Jiangsu(TC)were collected in 2015 and DNA was extracted.Then nine pairs of fluorescent-labelled microsatellite markers were used for PCR amplification and the amplified products were sent out to be sequenced in order to determine alleles per locus.The GenAIEx,Structure,BOTTLENECK,NeEstimator,GeneClass and other softwares were used to analyze the microsatellite polymorphism,genetic diversity in populations,sub-population structure,bottleneck effect,effective population size and genetic differentiation among populations of Oncomelania hupensis from different snail habitats.2.Samples of 2014 to 2017 from new snail habitats of Taicang,Jiangsu were collected and DNA was extracted.Nine pairs of STR were used for PCR amplification.Combined with data of 2015 from Taicang,the GenAIEx,NeEstimator,Structure,MEGA,DIYABC and other softwares were used to analyze the historical invasion pathways and ancestral origins of Oncomelania hupensis.Results:1.The microsatellite polymorphism186 alleles and 74.839 effective alleles were detected in three snail populations based on the nine microsatellite loci.A high level of polymorphism was observed in terms of number of different alleles,ranging from 12 alleles at locus T6-17 to 35 alleles at locus T4-25,and of number of effective alleles,varied between 5.420 at locus T4-22 and 11.354 at locus T4-25.No consistent pattern of Linkage Disequilibrium(LD)between any particular pair of loci was observed,indicating the independence of the loci used.2.The population genetics of Oncomelania hupensis snails from three different snail habitats(1)The genetic diversity in populations:Number of alleles(Na)of ST,HT and TC was 14.333,9.444 and 5.444.Number of effective alleles(NeA)of ST,HT and TC was 7.028,3.392 and 2.553.Generally,the genetic diversity was much higher in ST snails,when compared to in HT or TC.(2)The sub-population structure in populations:The most likely number of genetic sub-populations could be two for both HT and ST,and three for TC population,indicating significant genetic variations in all three populations.(3)The bottleneck effect:The distinct heterozygosity excess was observed in ST population under IAM model(P=0.00488),indicating a recent bottleneck event occurred in the population.A heterozygosity deficiency was observed in HT population under TPM model(P=0.00977),indicating a recent population expansion tendency occurred in the population.Neither bottleneck effect nor expansion tendency was detected in TC population under both IAM and TPM models(P>0.05).(4)The effective population size:For each of three populations,the Ne estimates using the heterozygote excess method by NeEstimator were infinite.Baesd on LD method,both NeEstimator and LDNe showed the consistent results,namely the largest Ne estimates in TC(NeEst:infinite;LDNe:-78.0),ST next(NeEst:179.9;LDNe:284.9),the smallest Ne estimates in HT(NeEst:17.2;LDNe:46.1).(5)The genetic differentiation among populations:All the snails could be divided into three sub-populations according to their geographical origins and no migration was found between them.3.The genetic evolution analysis of Oncomelania hupensis snails in new snail habitats from 2014 to 2017(1)The genetic diversity in populations:Number of alleles(Na)of snails from 2014 to 2017 was 3.333,4.556,4.889 and 3.667.Number of effective alleles(NeA)of snails from 2014 to 2017 was 2.568,2.682,2.654 and 2.172.(2)The effective population size:For each of four populations,the Ne estimates using the heterozygote excess method by NeEstimator were infinite.Baesd on LD method,both NeEstimator and LDNe showed the consistent results.The largest Ne estimates were found in 2015(NeEst:>5000;LDNe:-268.1)and the Ne estimates of the rest populations were all about 50.(3)The genetic differentiation among populations:All the snails of 2014 could be divided into a single sub-population and the others could be divided into another sub-population.(4)Coalescent study:Five best scenarios,namely Scenario 5,12,24,31 and 33,were choosen through the five-group scenario comparisons of First Round and then formed the sixth group to conduct scenario comparisons of Second Round.Scenario 24 was found to have the highest posterior probability.Its posterior probability and the corresponding 95%CI were 0.5345(0.5210,0.5480).Therefore,the most likely evolution model of the four populations in this study could be Scenario 24,that is,the snail population of 2014 migrated to a new area and formed the snail population of 2015,and then the snail population of 2015 migrated and spread,leading to the formations of snail populations of 2016 and 2017.Conclusion:1.The population genetics of Oncomelania hupensis snails from three different snail habitats:1)Although the genetic diversity of snails from current snail habitats of Shitai,Anhui(ST)was relatively high,a recent bottleneck event occurred in the population and the evolutionary potential was very low,which may be related to the effective preventive and control measures conducted in the local area.2)The genetic diversity of snails from re-emerged snail habitats of Hengtang,Jiangsu(HT)was relatively low.Besides,the population was not stable and the probability of inbreeding depression was very high,indicating the relatively low threat of this population.3)Although the genetic diversity of snails from new snail habitats of Taicang,Jiangsu(TC)was very low,the population was stable and the adaptability to environmental changes was very strong,leading to the potentials for the transmission of schistosomiasis.2.The genetic evolution analysis of Oncomelania hupensis snails from new snail habitats of Taicang,Jiangsu(TC)from 2014 to 2017:1)All the snails could be divided into two sub-populations,one of which was the snails of 2014 and the other one was the snails from 2015 to 2017.It was likely that snails from 2015 to 2017 had common ancestral origins,while the snails of 2014 originated from other ancestors.2)The snail population of 2015 showed the strongest adaptability to environmental changes and the potential to evolve.It was possible for this population to migrate and spread,and form the snail populations of 2016 and 2017 respectively. |
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