Complete mitochondrial genomes of Dermacentor silvarum and comparative analyses with another hard tick Dermacentor nitens.

Ticks are obligate blood-sucking ectoparasites of a wide range of vertebrates. They can transmit a range of pathogens that cause economic losses to livestock production as well as human disease. In the present study, the complete mitochondrial (mt) genome of Dermacentor silvarum was determined. The mt genome is 14,945 bp in length contains 37 genes, including 13 are protein-coding genes (cox1-3, nad1-6, nad4L, cytb, atp6 and atp8), two ribosomal RNA genes and 22 transfer RNA genes. The nucleotide composition of the D. silvarum mt genome was A + T biased at 78.78%; T was the most abundant nucleotide and G the least abundant. The mt genome of D. silvarum was 106 bp longer than that of Dermacentor nitens and the arrangements of two genomes were identical. For the 13 protein-coding genes, comparison between D. silvarum and D. nitens revealed sequence divergence at both the nucleotide (15.46-35.14%) and amino acid (6.05-48.98%) levels. Among them, cox1 was the most conserved gene, while atp8 was the least conserved. The lengths of the 13 protein-coding genes were the same or similar, except for cytb which was significantly longer in D. silvarum than in D. nitens. The mtDNA contained a variable repeat region consisting of a "similar to nad1" motif that was repeated three times, and the "Tick-box" motifs were also found. The overall difference between the nucleotide sequences of the two complete mt genomes was 21.4%. The mtDNA data presented in this study provide a rich resource for further studies on the phylogenetics, population genetics, and molecular epidemiology of ticks.