Identification And The Complete Mitochondrial Genomes Of Six Hard Ticks

Ticks belong to Arthropoda,Arachnida,Acari,Parasitiformes,Ixodoida,which are obligate hematophagous ectoparasites.Ticks play a significant role in the transmission of various pathogens,such as viruses,protozoa,fungi,bacteria and helminths,affecting human and animal health worldwide.The accurate identification and differentiation of ticks has important implications for studying their epidemiology,population biology and systematics.Usually,identification and differentiation of ticks have been based on morphological features,animal hosts and geographical locations.However,these criteria are sometimes insufficient for specific identification and differentiation of different ticks(such as closely-related species or sibling species).Due to their maternal inheritance,fast rate of evolutionary change and relatively conserved genome structures,mitochondrial(mt)DNA sequences have been proven useful and reliable genetic markers for the identification and differentiation of ticks.The objectives of the present study were to decode the complete mt genomes of six hard ticks,and to examine the phylogenetic relationships among some selected species of the hard ticks using a concatenated mt amino acid dataset.The complete mt genomes of Ixodes nipponensis,Ixodes rangtangensis,Amblyomma javanense,Hyalomma asiaticum,Rhipicephalus sanguineus and Haemaphysalis verticalis were 114,505 bp,14,543 bp,14,780 bp,14,722 bp,14,715 bp and 14,855 bp in length,respectively.Each of the six mt genomes contains 37 genes,including 13 protein-coding genes(cox1–3,nad1–6,nad4 L,cytb,atp6 and atp8),22 transfer RNA genes,two ribosomal RNA genes.A.javanense,H.asiaticum,R.sanguineus and H.verticalis had two non-coding regions,but I.nipponensis and I.rangtangensis had only one.The six mt genomes showed two gene rearrangement patterns.The nucleotide composition of the six hard ticks mt genomes is biased toward A + T(72.8%-80.0%).In these mt genomes,some mt genes start with ATG、ATT and ATA,and one genes(atp8)use ATC as start codon.All genes have complete termination codon except for some genes(such as nad1,nad5 and cytb)which use abbreviated stop codon TA or T.The rrn L of six mt genomes is located between t RNA-Leu CUN and t RNA-Val,and rrn S of six mt genomes is located between t RNA-Val and non-coding region.A total of 22 t RNA sequences(ranging from 56 to 81 nucleotides in length)were identified in the mt genomes of six hard ticks.Phylogenetic analyses of six hard ticks with other related hard ticks of the family Ixodidae were performed by MP,ML and BI based on concatenated mt amino acid sequences of 13 protein-coding genes.Our results indicated that the genus Amblyomma was paraphyletic with strong support in all of the three phylogenetic analyses in this study,and the genera Ixodes,Haemaphysalis,Hyalomma,Rhipicephalus,Bothriocroton and Dermacentor were monophyletic in all of the three phylogenetic analyses in this study.The determination of mt genome sequences of six hard ticks will provide novel genetic markers for studies into the molecular identification,systematics and phylogenetics of these ticks.