Characterization and phylogenetic analysis of the first complete mitochondrial genome of Cylicocyclus radiatus.

Cylicocyclus spp. are common parasites in the intestine of equine animals. Hosts infected by these strongylid nematodes might face emaciation, anemia, and even death, all of which lead to significant economic losses. In the present study, the complete mitochondrial genome (mitogenome) of Cylicocyclus radiatus, the type species of the genus Cylicocyclus, was sequenced and characterized for the first time using next-generation sequencing technology. Results show that the mitogenome of C. radiatus is a double-stranded 13,836 bp circular molecule, containing 12 protein-coding genes, 22 tRNA genes, two rRNA genes, and two non-coding regions. Moreover, it has a clear bias in nucleotide composition and codon preference towards protein-coding genes with elevated AT content. All protein-coding genes are initiated by ATT and TTG codons, and almost all of them end with complete TAA and TAG termination codons; only the COШ gene harbors the incomplete T termination codon. All tRNA genes fold into an atypical clover-leaf secondary structure. The tRNA-Ser genes possess a TΨC arm and variable loop, but lack the dihydrouridine arm and loop. Twenty tRNAs contain the dihydrouridine arm and loop, in which the variable TΨC arm and loop are replaced by a "TV-replacement loop". Most tRNA gene secondary structures contain G-U, U-U, and A-G mismatches. Phylogenetic analysis based on nucleotide sequences of the 12 protein-coding genes using maximum likelihood and Bayesian inference indicated that C. radiatus is related more closely to Cylicocyclus nassatus than to Cylicocyclus insigne. In summary, this study provides the first annotated sequence of the mitogenome of C. radiatus, paving the way for further research on species classification and the molecular evolution of strongylid nematodes.