Molecular Phylogenetics Of Oithona Based On COI And 18S RDNA Gene Sequences

Zooplankton is one of the most widely distributed groups in the sea,with a large variety of species and a strong breeding capacity,which occupies an important part in Marine ecosystem and food network.They participate in the material circulation and energy flow as secondary producers.They can be sensitive to the change of environmental.The number of species and community change are the important indices for monitoring the water environment and water system.Therefore,the study of zooplankton has become one of the indispensable projects in fisheries and environmental investigation.At present,the main research of zooplankton is ecological study,and relatively few genetic evolutionary studies on the molecular level.Among the small size cyclopoid,the Oithona are admitted as abundant groups which widely distributed and occurs in all kinds of marine environments(estuarine,coastal and oceanic waters).However,the phylogenetic of species has remained confusion due to the small body size and subtle morphological difference among species.Routine identification of the species has remained challenging due to the subjective deviation and small morphological differences among species.Studies have shown that the cryptic species are widely distributed in the species group of Oithona.In unique water environments,genetic evolution continues to occur.The long-distance dispersal may be lead to geographical isolation as Oithona can't resist water flow dynamics.All this increase the challenge of species identification and genetic evolution.At present,the phylogenetic relationships of Oithona is not clear,which affects the deep study of the zooplankton.The South China Sea,with its unique ecological and hydrological environment,complex climate,numerous straits,high temperature and high salt,is one of the most abundant biodiversity sea areas.Therefore,the South China Sea has important research value.In this paper,based on mitochondrial COI gene and 18 Sr DNA gene sequence,the phylogeny of several common species of Oithona in the South China Sea was studied.These researches reveal the evolutionary relationship of the oithona genus and provide molecular data for traditional morphological identification.we performed tests on DNA taxonomy(ABGD and GMYC)and built phylogenetic tree of Bayes and maximum likelihood methods with the same gene sequence data download from GenBank.The conclusions of this study following:1.Based on COI gene sequence,the results of delimitation using ABGD and GMYC model were consistent with morphological approach.It is suggested that COI gene can be used as a marker gene to identify the species of oithona,and also provides molecular data for species identification in the future.2.The average transition/transversion ratio(si/sv)of COI gene was 1.44.Transition and transversion mainly occure at third site.The average transition/transversion ratio of 18 S rDNA gene sequence is 2.96,which is higher than COI gene sequence.18 S rDNA gene was found to be highly conserved samples.3.There are differences in base composition between 18 S rDNA and COI gene.COI gene show strong(A+T)bias with the average content of A,T,C,G are 24.5%,37.6%,19.4%,18.4%.The content of A+T(62.1%)is higher than content of G+C(37.8%).However,18 S rDNA gene sequences were diffidence from mitochondria COI gene.18 S rDNA gene sequences were conservative with low frequency of variable site and high frequency invariable site.The content of base show nearly equal with the average content of A,T,C,G are 25.2%,24.3%,23.2%,27.2% respectively.4.The distance of mtCOI gen is 17.7%-44.5%,which shows that the genetic difference has reached the range of intergeneric differentiation.It is suggested that some species of the oithona genus need to be reclassified.However,the genetic distance based on 18 S rDNA gene is 0.1%-5%,showing a close genetic relationship between oithona.This means that 18 SrDNA possesses high conservation.5.Both Bayes and Maximum Likelihood phylogenetic tree that based on mtCOI show that each species was clustered together as a monophyletic group.We find two cryptic species were found in O.similis and O.plumifera which were from different sea areas.In phylogenetic tree,O.plumifera and O.setigera are clustered together.As they has similarity in morphology,maybe they has closely relationship.So 18 S rDNA is more suitable for higher classification.