The Mitochondrial Genome of <i>Cylicocyclus elongatus</i> (Strongylida: Strongylidae) and Its Comparative Analysis with Other <i>Cylicocyclus</i> Species

<i>Cylicocyclus elongatus</i> (<i>C. elongatus</i>) is one of the species in <i>Cylicocyclus</i>, subfamily Cyathostominae, but its taxonomic status in <i>Cylicocyclus</i> is controversial. Mitochondrial (mt) genome is an excellent gene marker which could be used to address the taxonomy controversy. In the present study, the complete mt genome of <i>C. elongatus</i> was determined, and sequence and phylogenetic analyses were performed based on mtDNA data to determine the classification of <i>C. elongatus</i>. The circular complete mt genome of <i>C.</i><i>e</i><i>longatus</i> was 13875 bp in size, containing 12 protein-coding genes (12 PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and 2 non-coding regions (NCRs). The A + T content of <i>C. elongatus</i> complete mt genome was 76.64%. There were 19 intergenic spacers with lengths of 2–53 bp and 2 overlaps with lengths of 1–2 bp in the impact complete mt genome. ATT and TAA were the most common start and termination codons of 12 PCGs, respectively. Comparative analyses of mt genomes nucleotide sequence and amino acid sequence showed that there were higher identities between <i>C. elongatus</i> and five other <i>Cylicocyclus</i>, rather than with <i>P. imparidentatum</i>. Phylogenetic analyses based on concatenated nucleotide sequences of 12 PCGs of 23 species in the family Strongylidae showed that <i>C. elongatus</i> was closely related to <i>Cylicocyclus</i> species, rather than <i>P. imparidentatum</i>. We concluded that <i>C. elongatus</i> was a member in <i>Cylicocyclus</i> based on comparative and phylogenetic analyses of mt genome sequences. The data of the complete mt genome sequence of <i>C. elongatus</i> provide a new and useful genetic marker for further research on Cyathostominae nematodes.