Sequence variability in three mitochondrial genes between the two pig nodule worms Oesophagostomum dentatum and O. quadrispinulatum.

In this study, sequence variation in three mitochondrial DNA regions, namely cytochrome c oxidase subunit (cox1) and NADH dehydrogenase subunits 1 and 4 (nad1 and nad4), between Oesophagostomum dentatum and O. quadrispinulatum isolated from pigs in different geographical origins in Mainland China was examined, and their phylogenetic relationships were reconstructed. A partial of the cox1 (pcox1), nad1, and nad4 genes (pnad1 and pnad4) were amplified separately from individual nodule worms by PCR and were subjected to direct sequencing in order to define sequence variations. While the intraspecific sequence variations within each of the two species were 0.3-5.2% for pcox1, 0-4.9% for pnad1, and 0-7.1% for pnad4, the interspecific sequence differences were significantly higher, being 10.7-13.4% for pcox1, 11-14.6% for pnad1, and 14.9-18% for pnad4, respectively. There were a number of nucleotide positions in the pcox1, pnad1, and pnad4 sequences with no apparent intraspecific variation but distinct interspecific differences among those samples of Oesophagostomum spp. examined, which may be used as genetic makers for the identification and differentiation of the Oesophagostomum spp. Phylogenetic analyses using three inference methods, namely Bayesian inference, maximum likelihood, and maximum parsimony based on the combined sequences of pcox1, pnad1, and pnad4 revealed that the O. dentatum and O. quadrispinulatum form monophyletic groups, respectively. These findings demonstrated clearly the usefulness of the three mitochondrial sequences for studying systematics, population genetic structures, and the molecular ecology of Oesophagostomum spp.