Characterisation of the mitochondrial genome and phylogenetic analysis of (Nematoda: Ascarididae).

We first sequenced and characterised the complete mitochondrial genome of then studied the evolutionary relationship of the species within Toxocaridae. The complete mitochondrial genome was amplified using PCR with 14 specific primers. The mitogenome length was 14303 bp in size, including 12 PCGs (encoding 3,423 amino acids), 22 tRNAs, 2 rRNAs, and 2 NCRs, with 68.38% A+T contents. The mt genomes of had relatively compact structures with 11 intergenic spacers and 5 overlaps. Comparative analyses of the nucleotide sequences of complete mt genomes showed that had higher identities with than other congeners. A sliding window analysis of 12 PCGs among 5 species indicated that 4 had the highest sequence divergence, and 1 was the least variable gene. Relative synonymous codon usage showed that UUG, ACU, CCU, CGU, and UCU most frequently occurred in the complete genomes of The Ka/Ks ratio showed that all mt genes were subject to purification selection. The largest genetic distance between and the other 4 congeneric species was found in 2, and the smallest was found in 2. Phylogenetic analyses based on the concatenated amino acid sequences of 12 PCGs demonstrated that formed a distinct branch and was always a sister taxon to other congeneric species. The present study determined the complete mt genome sequences of , which provide novel genetic markers for further studies of the taxonomy, population genetics, and systematics of the Toxocaridae nematodes.