Mitochondrial DNA Evidence Supports the Hypothesis that Species Belong to Cyathostominae.

Equine strongyles, the significant nematode pathogens of horses, are characterized by high quantities and species abundance, but classification of this group of parasitic nematodes is debated. Mitochondrial (mt) genome DNA data are often used to address classification controversies. Thus, the objectives of this study were to determine the complete mt genomes of three Cyathostominae nematode species (, and ) of horses and reconstruct the phylogenetic relationship of Strongylidae with other nematodes in Strongyloidea to test the hypothesis that spp. belong to Cyathostominae using the mt genomes. The mt genomes of , and were 13,838, 13,826, and 13,817 bp in length, respectively. Complete mt nucleotide sequence comparison of all Strongylidae nematodes revealed that sequence identity ranged from 77.8 to 91.6%. The mt genome sequences of species had relatively high identity with Cyathostominae nematodes, rather than species of the same subfamily (Strongylinae). Comparative analyses of mt genome organization for Strongyloidea nematodes sequenced to date revealed that members of this superfamily possess identical gene arrangements. Phylogenetic analyses using mtDNA data indicated that the species clustered with Cyathostominae species instead of species. The present study first determined the complete mt genome sequences of , and , which will provide novel genetic markers for further studies of Strongylidae taxonomy, population genetics, and systematics. Importantly, sequence comparison and phylogenetic analyses based on mtDNA sequences supported the hypothesis that belongs to Cyathostominae.