Population Genetics Research Of Chiloscyllium Plagiosum And Scoliodon Laticaudus In China's Southern Coast

Sharks and their relatives,the rays(subclass Elasmobranchii) are a group of about 1100 species of mostly marine fishes.They can affect population structure of many other marine lifes because of their top predators in marine food chain and an important component of marine ecosystems.Therefore,the composition of biological communities and the ecological environment in marine ecosystems will be seriously affected if they are over-fished.Most sharks and rays that have been studied have slow growth,late maturity and low fecundity compared to bony fishes.These attributes result in low intrinsic rates of increase and low resilience to fishing mortality.Thus,most shark and ray populations can withstand only modest levels of fishing without depletion and stock collapse,and decline more rapidly and are not able rebound as quickly as other bony fishes to population reductions.At present shark fisheries have developed around the world,the global shark resources have been seriously damaged,rapid stock decline and collapse with decades,so fisheries will become commercially extinct before the targets of those fisheries become biologically extirpated.In these circumstances,studies of population genetics are important to protection of shark resources.But it is difficult to study population genetics of sharks using traditional tagging observation and morphological measurement methods because they exhibits relatively low levels of morphological features.Since the development of isozyme electrophoresis in 1960s, molecular markers have been increasingly used to study population genetics of sharks.It could directly reveal the genetic differences among individuals and populations without being affected by environment,developmental phase,apparatus and tissue differences.Currently,there are a large number of research reports about elasmobranchs population genetics in abroad,but domestic research in this field has not been reported.Thereby we studied population genetics of two representative sharks(Chiloscyllium plagiosum and Scoliodon laticaudus) by molecular marker technique in China's southern coast,enrich the basic theory of elasmobranchs population genetics and provide a scientific information for our protection and rational exploitation of elasmobranchs resources.1 Random amplified polymorphic DNA(RAPD) technique was used to assess genetic diversity and population structure within and among three whitespotted bamboosharks (C.plagiosum) stocks from Mindong(MN),Minnan(MN) and Yuexi(YX) of China's southern coast.Thirty-four primers produced 316 loci in all samples.There were 308,308 and 303 loci in three stocks respectively,in which 49(15.90%),52(16.88%) and 42(13.86%) were polymorphic.Shannon genetic diversity indices of three stocks(0.0998,0.1055,0.0936) indicates that genetic diversity of C.plagiosum is low.In addition,the results of UPGMA cluster analysis and Nei's genetic distances showed that gene exchange between C.plagiosum stocks fits Isolation-By-Distance Model and genetic differences among stocks correlates to their geographical distance.Using Analasis of Molecular Variance(AMOVA),weak but significant population differentiation was detected with a globle fixation index(Fst= 0.04404,P＜0.05).It meaning that the greater variance occurred among populations(95.60%) and lesser variance within populations(4.402%).2 The entire mitochondrial control region sequences(1094～1096 nucleotides) were amplified from five whitespotted bamboosharks(C.plagiosum) stocks in coastal waters of southern China then 8 haplotypes were defined by 6 polymorphic sites after sequence alignment.Gene diversity(h=0.5425～0.7448),mean number of pairwise differences(K= 0.625287～1.073563) and nucleotide diversity(π=0.000571～0.000980) indicates that genetic diversity of C.plagiosum is low.Using Analasis of Molecular Variance(AMOVA), significant population differentiation was detected with a globle fixation index(Fst= 0.21626,P＜0.0001).It meaning that the greater variance occurred among populations(78.4%) and lesser variance within populations(21.6%).The results of genetic distances analysis showed that genetic differences among stocks correlates to their geographical distance. Statistical phylogeography analysis indicates that MN stock is more ancient than other stocks.3 Random amplified polymorphic DNA(RAPD) technique was used to assess genetic diversity and population structure within and among four spadenose shark(S.laticaudus) stocks from MD,MN,YX and BBW of China's southern coast.Twenty-nine primers produced 282 loci in all samples.There were 273,274,272 and 276 loci in four stocks respectively,in which 54(19.78%),57(28.80%),52(19.12%)and 45(16.30%)were polymorphic.Shannon genetic diversity indices of four stocks(0.104,0.107,0.103,0.090) indicates that genetic diversity of S.laticaudus is low.In addition,the results of UPGMA cluster analysis and Nei's genetic distances showed that gene exchange between S.laticaudus stocks fits Isolation-By-Distance Model and genetic differences among stocks correlates to their geographical distance.Using Analasis of Molecular Variance(AMOVA),weak but significant population differentiation was detected with a globle fixation index(Fst=0.019,P＜0.05).It meaning that the greater variance occurred among populations(98.06%) and lesser variance within populations(1.94%). 4 The entire mitochondrial control region sequences(1064 nueleotides) were amplified from four spadenose shark(S.laticaudus) stocks in coastal waters of southern China then 19 haplotypes were defined by 12 polymorphic sites after sequence alignment.Gene diversity(h =0.7829～0.8301),mean number of pairwise differences(K=1.314943～1.492473) and nucleotide diversity(π=0.001236～0.001403) indicates that genetic diversity of S.laticaudus is low.Using Analasis of Molecular Variance(AMOVA),significant population differentiation was detected with a globle fixation index(Fst=0.02078,P＜0.05).It meaning that the greater variance occurred among populations(97.92%) and lesser variance within populations(2.08%).The results of genetic distances analysis showed that genetic differences among stocks correlates to their geographical distance.Statistical phylogeography analysis indicates that frequency of ancient haplotype in all stocks is similar,but all stocks possess their distinct haplotyes clade.The haplotyes clade of BBW stock is most distinct,it may related to more closed geographical characteristics of Beibu Bay and philopatry in S. laticaudus.5 There 19 haplotypes were defined by 12 polymorphic sites after mitochondrial control region sequences alignment of 4 seasonal stocks in Beibu Bay.Gene diversity(h=0.7540～0.8309),mean number of pairwise differences(K=1.128736～1.503171) and nucleotide diversity(π=0.001061～0.001413) of 4 seasonal S.laticaudus stocks is low,in which summer stock is lowest and autumn stock is highest.AMOVA result indicates no significant population differentiation was detected with a globle fixation index(Fst=-0.00752,P＞0.05).6 The results of low population genetic diversity in C.plagiosum and S.laticaudus mainly due to slow rates of mitochondrial DNA evolution in elasmobranchs,but stock depletion by high intensity fisheries may also lower population genetic diversity.7 Genetic structure changes trends between stocks is different in C.plagiosum and S. laticaudus despite their sampling locations is similarity,it consistent to their behavior characters.C.plagiosum move and swimming slowly because of their benthic life,thus geographical distance strongly restricted their gene flow between stocks and resulted in significant genetic structure between MD,MN and YX stocks.But in similar geographical distance scales,S.laticaudus with stronger movement ability didn't present significant genetic structure between these three stocks.