We determined the nucleotide sequence of the complete mitochondrial genome of the nematode species Anisakis simplex. The genome is circular, 13,916 bp in size and conforms to the general characteristics of nematode mitochondrial DNAs. The gene arrangement of A. simplex is the same as that of Ascaris suum and almost identical to those of rhabditid species with a minor exception concerning the relative position of the AT-rich and non-coding regions and radically different from those of spirurid species. Along with comparisons of gene arrangement, phylogenetic analyses (maximum parsimony, neighbour joining and maximum likelihood methods) based on concatenated amino acid sequences of 12 protein-coding genes from 13 nematode species provided strong support for the sister-group relationship between Ascaridida and Rhabditida. The Shimodaira-Hasegawa and Templeton's tests both rejected the alternative hypothesis of a closer relationship between Ascaridida and Spirurida. These results contradicted the traditional view of nematode classification and a recent molecular phylogenetic study of 18S rDNA data that assigned Ascaridida and Spirurida as being a sister-group. Mapping of gene arrangement across the phylogenetic tree lead to the assumption that the conserved gene arrangement found in Ascaridida-Rhabditida members might have been acquired after the most recent common ancestor of ascaridid/rhabditid members branched off from the basal stock of the rhabditid lineage.
Bibliographical noteFunding Information:
We thank Michael Cummings, a director of MBL workshop on Molecualr Evolution, for his careful reading of the manuscript. Constructive comments by Professor Nicholas Sangster and two anonymous reviewers improved the paper. Hyung-Kyu Jeon assisted in isolating the larval worms of A. simplex and the materials for morphological identification were provided by the Parasite Resource Bank of Korea National Resrarch Resource Center, Republic of Korea. This work was financially supported by the Korea Research Foundation Grant (KRF-2002-070-C00080).
- Anisakis simplex
- Mitochondrial genome
- Molecular phylogeny