Complete mitochondrial genome of parasitic nematode Cylicocyclus nassatus and comparative analyses with Cylicocyclus insigne.

Cylicocyclus nassatus is a common and important parasite in the large intestine of equine. In this study, the complete mitochondrial (mt) genome sequence of C. nassatus was determined and comparatively analyzed with Cylicocyclus insigne. The mt genome size of C. nassatus was 13,846 bp, 18 bp longer than that of C. insigne. The circular mt genome includes 12 protein-coding genes, two rRNA genes, 22 tRNA genes and two non-coding regions. All the genes are transcribed in the same direction and gene arrangement is consistent with that of gene arrangement 3 (GA3). The overall sequence difference between the two complete mt genomes was 10.7%. For the 12 protein-coding genes, the comparison between C. nassatus and C. insigne revealed sequence divergence at both the nucleotide (6.3-13.0%) and amino acid (0.8-6.6%) levels. The nucleotide lengths of the 12 protein-coding genes were the same, except for cox3 which was longer in C. insigne. Phylogenetic analysis based on the concatenated amino acid sequence of the 12 protein-coding genes was performed using all the Strongylidae nematodes of the horse available in the GenBank. Phylogenetic analysis showed that C. nassatus and C. insigne clustered together with very high nodal support and the genus Cylicocyclus was closer to the genus Triodontophorus than to genus Strongylus. The mtDNA data determined in this study provides novel genetic markers for further studies on the identification, population genetics and molecular epidemiology of the genus Cylicocyclus.