Analysis Of Complete Mitochondrial Genomes And Genetic Relationships Of Three Nematodes Of Genus Triodontophorus

Triodontophorus spp. nematodes live in the large intestine of horse, ass, mule and other equids, and can cause mild clinical symptoms including anaemia, emaciation, weight loss, and colitis, or even death with heavy infection. The parasite is highly infective, has a worldwide distribution and may exert significant economic impact on livestock industries. Previous studies on the Triodontophorus spp. nematodes have mainly focused on morphological, epidemiological investigation, diagnosis, treatment and prevention. However, complete mitochondrial genome sequence analysis and genetic relationship within Triodontophorus nematodes have not been reported to date.Adult worms were collected from the large intestine of naturally infected horses, which the worms were identified as Triodontophorus serratus, T. brevicauda and T. nipponicus based on morphological characteristics. The whole sequences of mitochondrial genome of the three Triodontophorus species were amplified by using PCR method, and were sequenced and analyzed. The results showed that the length of complete mitochondrial genome of Triodontophorus serratus, T. brevicauda and T. nipponicus were 13 794 bp, 14 305 bp, and13 701 bp, respectively. Each mitochondrial genome contains 36 genes, 12 genes for proteins, 22 genes for t RNA, 2 genes for r RNA. The amount of noncoding regions of three species were 2, 3and 2, respectively. The nucleotide homology of entire mitochondrial genome sequence of three nematodes of Triodontophorus spp. were 86.5%(T. serratus ~ T. brevicauda), 86.4%(T. brevicauda ~ T. nipponicus), 85.6%(T. nipponicus ~ T. serratus), respectively.The amino acid homology of 12 protein-coding genes sequence of three nematodes of Triodonto phorus spp. were 93.6%(T. serratus ~ T. brevicauda), 93.4%(T. brevicauda ~T. nipponicus), 92.8%(T. nipponicus ~ T. serratus), respectively. The length of 12 protein-coding genes were same of three nematodes of Triodontophorus spp., which the order was nad5 > cox1 > nad4 > cytb > nad1 > nad2 > cox3 > cox2 > atp6 > nad6 > nad3 > nad4 L. Three Triodontophorus species were belonged to GA3 type. In order to analyze the genetic relationships between three species of Triodontophorus and other Strongylidae nematodes. Using maximum parsimony(MP), maximum likelihood(ML), Bayesian inference(BI) methods,phylogenetic tree were reconstructed based on the concatenated amino acid sequence data of all12 mt protein-coding genes. Topologies of three nematodes of Triodontophorus inferred by the three methods with three different evolutionary models were quite similar. Phylogenetic analysis showed that the Strongyloidea cluster is divided into two large clades, which one clade nematodes parasitized in digestive system, while another clade worms parasitized in respiratorysystem. Three nematodes of Triodontophorus spp. were located on the branch of the digestive system, and formed a small clade with genera Strongylus and Cylicocyclus. The genus Triodontophorus was high closer related to Cylicocyclus than Strongylus. And the relationships of T. serratus and T. brevicauda were closer than T. nipponicus.In this study, the sequences of three nematodes of Triodontophorus spp. were obtained and determined for the first time. These results not only supplement mt DNA of nematodes, but also provide valuable information for further studies of the genetic evolution, systematics and population genetics of the genus Triodontophorus.