Molecular Identification And Mitochondrial Genomes Of Six Species Of Acanthocephalans

Taxonomy and systematics are important tools for studying biodiversity and its underlying laws.The phylum Acanthocephala is an important group of endoparasites occurring in the alimentary canal of all major lineages of vertebrates worldwide.The systematic standard and classification system of acanthocephalans based on comparative morphology are still controversial.Sequences of nuclear ribosomal DNA(rDNA)and mitochondrial(mt)DNA used as genetic markers have been proven effective for species identification and phylogenetic analysis of acanthocephalans.However,the molecular data of mt genomes of acanthocephalans are still very limited.To date,only 17 acanthocephalans have their complete mt genomes decoded.Therefore,more mt genomic sequences of acanthocephalans are required to be decoded providing molecular resources for clarifying the controversial systematic and phylogenetic questions of acanthocephalans.The present study determined the complete mt genome sequences of six acanthocephalan species,including Sphaerirostris picae,S.lanceoides,Centrorhynchus milvus,C.clitorideus,Cavisoma magnum,and Neoechinorhynchus sp..These samples were collected from fishes or birds in Pakistan or China,respectively.The internal transcribe spacer(ITS)of rDNA and complete or nearly complete mt genome of six species of acanthocephalans were amplified by PCR using universal and self-designed primers.The ITS sequences were used for species identification.The mt genomes of these six acanthocephalans all contain 36 genes,including 12 protein-coding genes(PCGs),two ribosomal RNA genes(rrnL and rrnS),22 transporter RNA genes(tRNAs)and 1-2 non-coding regions(NCR).In the present study,the nucleotide composition,gene arrangement,relative synonymous codon usage(RSCU)and codon usage of 12 PCGs from mtDNA were compared and analyzed in eaeh acquired mt genome.The mt genome composition of these six species is the same as that of other reported species,missing the atp8 gene.Phylogeny was reconstructed and analyzed based on the concatenated amino acid sequences inferred from 12 PCGs of the six acanthocephalans and other acanthocephalans from GenBank,revealing the phylogenetics of all available Acanthocephalans so far.The present phylogenetic analyses supported that the Acanthocephala consist of three classes,Archiacanthocephala,Palaeacanthocephala and Eoacanthocephala,with Archiacanthocephala as the most basal group within the phylum Acanthocephala,but chanllenged the validity of Polyacanthocephala.In addition,our phylogenetic analysis revealed that Polymorphida in Palaeacanthocephala is monophyletic,whereas the Echinorhynchida is paraphyletic.Based on phylogenetic analysis results,the present study firstly proposed that the Centrorhynchidae(Polymorphida)has the sister relationship with species in Polymorphidae + Plagiorhynchidae,and the genus Sphaerirostris Golvan,1956 is the synonym of the genus Centrorhynchus Lühe,1911.The present study enriched the mt genome data of Acanthocephala,reconstructed the phylogenetic relationship and provided novel insights into the classification system.The results of the present study are of great theoretical significance for further taxonomic research of Acanthocephala that conforms to the natural evolution rules,as well as for revealing the early evolutionary history of Acanthocephalans.