| The orb-weaving spiders are members of Arthropoda,Chelicerata,Arachnida,Araneae,Araneomorphae,Araneoidea and Araneidae,which contain more than 3 thousands described species.Araneids are important agricultural pest predators with diverse habitation and wide distribution,and play important roles in pest biocontrol.Mitogenome has the characteristics of compact genome,matrilineal inheritance,moderate substation rate,low level of gene recombination and relatively simple gene structure,and hence commonly used as genetic markers and in the phylogenetic analysis.To date,only four araneids mitogenomes are available in Gen Bank,which are extremely limit for mitogenome structure comparative analysis and araneids phylogenetic studies.In this study,we sequenced the complete mitogenomes of six araneids(N.doenitzi,N.nautica,N.theisi,Araneus ventricosus,C.argenteoalba and C.nagasakiensis).Comparative mitogenomics analysis was performed to analyze the genome features.Additionally,phylogenetic trees were constructed using Maximum likelihood and Bayesian methods based on 101 mitogenomes sequences of Arachnida species to explore the evolutionary relationship of Araneae,Limulus polyphemus(Xiphosura)was selected as an outgroup.The six complete mitogenomes sequences are 14,161 bp(N.doenitzi),14,049bp(N.nautica),14,157 bp(N.theisi),14,617 bp(A.ventricosus),14,575 bp(C.argenteoalba)and 14,402 bp(C.nagasakiensis),respectively.They have the similar nucleotide composition and structure characters:(1)All the mitogenomes contain 37 genes and a control region.Among them,22 genes and the control region were encoded on the majority strand,while the rest 15 genes were encoded on the minority strand.Mitogenomes have compact gene structures,the overlap nucleotides between genes are more than that in the interval(2)All the mitogenomes show AT preference with a high A+T content(73.4%~78.8%)and significant T-skew and G-skew.(3)All the protein-coding genes use ATT,ATA,TTA or TTG codon as a start codon.Most of the protein-coding genes utilize the canonical termination codons(TAA and TAG),but few of them have incomplete termination codons TA or T.Besidely,the codon usage of the protein-coding genes was biased and the codons in NNU and NNA form were mostly used.(4)Most of t RNAs lack the DHU arm or T ψ C arm in their secondary structures,and almost all t RNAs show mismatches and G · U weakly coordination.(5)The gene arrangement is identical for all six mitogenomes.When compared with the mitogenome of L.polyphemus,trn L2,trn N,trn S1,trn I and trn Q are found to translocate their positions,trn I and trn Q are found to inverse their positions.However,there were still some differences among the mitogenomes of six orb-weaving spiders:(1)The protein-coding genes in N.doenitzi and N.nautica did not use CGC codon,which appeared in the mitogenomes of the rest four spiders.(2)The A+T contents of the control region in C.argenteoalba and C.nagasakiensis were slightly lower than that in the rest sequences of the whole mitogenome,but this phenomenon was not observed in the rest four spider mitogenomes.(3)The repeat unions were only found in the control region of A.ventricosus mitogenome.Analysis the amino acid and nucleotide of protein-coding genes in six araneids showed that COI is the most conserved gene.Phylogenetic analysis was performed on 13 PCGs dataset and the topologies of phylogenetic trees inferred from two methods(ML and BI)were almost identical.The obvious error branches in the two phylogenetic trees are that scorpions were classified into Araneae and the suborder in Araneae could not form monophyly.Appropriate and extensive taxon sampling is one of the most important determinants of accuract phylogenetic estimation.Therefore,to achieve increased resolution of the Araneae phylogeny,additional mitogenome sequences from spiders,especially in phylogenetically distant taxa Mesothelae,are required. |
