Molecular characterisation and complete mitochondrial genome of Contracaecum ovale (Linstow, 1907) (Ascaridida: Ascaridoidea) from the little grebe Tachybaptus ruficollis (Pallas) (Podicipediformes: Podicipedidae).

The genus Contracaecum is an important group of anisakid nematodes containing species of veterinary, medical, and economic importance. However, current genetic sequences and the mitogenomic database for Contracaecum spp. are limited. In the present study, the nuclear 18S, 28S, and ITS regions and the mitochondrial cox1, cox2, and 12S data for C. ovale (Linstow 1907) were sequenced based on the newly collected specimens from Tachybaptus ruficollis (Podicipediformes: Podicipedidae) in China. These sequences exhibited 100% similarity in the 18S and 28S regions and 99.8% similarity in the ITS-1 region (the presence of only a single locus of nucleotide polymorphism), compared with samples of Contracaecum ovale from Podiceps cristatus australis in New Zealand. Molecular analysis of two different individuals of C. ovale, collected from T. ruficollis in China, showed no genetic divergence with respect to the 18S, 28S, ITS, cox1, and 12S regions, but a low level of divergence in the cox2 sequences (0.73%). The complete mitogenome of C. ovale, presented for the first time, is 13,769 bp in length and includes 12 protein-coding genes, 22 tRNA genes, 2 ribosomal RNAs, and 2 non-coding regions. The mitogenome of C. ovale represents the smallest known anisakid mitogenome, and its overall A + T content (71.5%) is the highest level of A + T content of any anisakid. The mitogenomic phylogeny of those Contracaecum spp. whose mitochondrial protein-coding genes (PCGs) have been studied suggests that C. rudolphii and C. ogmorhini are more closely related to each other than either is to C. ovale. The genetic and mitogenomic data presented herein will be helpful for future studies on the population genetics and phylogenetics of Contracaecum spp.