Characterization of the complete mitochondrial genomes of Coronocyclus labiatus and Cylicodontophorus bicoronatus: Comparison with Strongylidae species and phylogenetic implication.

Coronocyclus labiatus and Cylicodontophorus bicoronatus are two significant horse parasitic nematodes which are classified into subfamily Cyathostominae, family Strongylidae, however, the classification of these nematodes has been controversial for more than a century. Mitochondrial (mt) genomes are considered valuable sources for parasite taxonomy, population genetics, and systematics studies. In the present study, the mt genomes of Co. labiatus and Cd. bicoronatus (type species) were determined and subsequently compared with those from closely related species by phylogenetic analysis based on concatenated datasets of amino acid sequences predicted from mt protein-coding genes. The complete mt genomes of Co. labiatus and Cd. bicoronatus were circular with 13,827 bp and 13,753 bp in size, respectively. Both mt genomes consisted of a total of 12 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes and two non-coding regions. All protein coding genes were transcribed in the same direction, and the gene order in both mt genomes belonged to the gene arrangement type 3 (GA3). There were 19 intergenic spacers with 1 bp to 35 bp and one overlap with 4 bp in mt genome of Co. labiatus, and 22 intergenic spacers with 1-29 bp in size but no overlap in the mt genome of Cd. bicoronatus. The A + T content of Co. labiatus and Cd. bicoronatus mt genomes were 75.87 % and 75.16 %, respectively. Similar to mt genones of other Strongylidae species published in GenBank, they also exhibited a strong A + T bias not only in the nucleotide composition but also in codon usage. Comparative analyses of mt genomes nucleotide sequence showed that mt genomes of Co. labiatus and Cd. bicoronatus had higher identities to that of Cylicostephanus goldi (90.3 % and 86.9 %, respectively), followed by those of two Cyathostomum species (89.9∼90.0 %; 86.4 %), respectively. Phylogenetic analyses using mt genomes of 26 Strongyloidea nematodes revealed that Co. labiatus was closely related to Cyathostomum species, and Cd. bicoronatus formed a distinct branch with Cyathostominae species, which was closer to Triodontophorus than Poteriostomum imparidentatum. We concluded Coronocyclus might be closely related with Cyathostomum but represent a distinct genus based on comparative mt genome sequences and phylogenetic analyses. The availability of complete mt genome sequences of Co. labiatus and Cd. bicoronatus provides new and useful genetic markers for further studies on Strongylidae nematodes.