Utility of DNA Barcoding in Distinguishing Species of the Family Taeniidae

The family Taeniidae comprises many parasitic species, which cause serious zoonoses. However, effective identification of Taeniidae species is a long-standing problem, especially in samples from wild hosts with mixed infections of different Taeniidae species. DNA barcoding analysis of small fragments of the cytochrome c oxidase subunit I (COI) gene has been confirmed as an effective and useful method for identifying Taenia species. We therefore performed DNA barcoding analysis using a 351-bp region of the COI gene to identify 27 taeniid species including 9 in the genus Echinococcus, 2 in Hydatigera, 15 in Taenia, and 1 in Versteria. A total of 484 COI sequences were used to calculate genetic divergence expressed by the Kimura 2-parameter (K2P) distance. The mean intraspecific K2P distance in the family Taeniidae was 0.71 +/- 0.17% (+/- SE), while inter-specific divergences were considerably higher. We found that, generally, a 2.0% optimal barcoding threshold could be set to distinguish taeniid species. Taenia polyacantha and Hydatigera taeniaeformis were the only 2 false-positive species identification cases in this study for their intraspecific divergences above the 2.0% optimal threshold. Their high intraspecific divergences coincided with fact that cryptic divergences exist in these 2 species, to which new taxa were recommended. On the other hand, sister species T. asiatica and T. saginata showed a 2.48 +/- 0.83% interspecific divergence, which was the smallest among all the taeniid species. Although fitting the 2.0% optimal species barcoding threshold, the close genetic relationship between T. asiatica and T. saginata implies that longer mitochondrial DNA sequences like the complete COI sequence are needed to strictly distinguish them. Therefore, we concluded that the barcoding technique based on a 351-bp region of the COI gene is able to distinguish taeniid species except for cryptic T. polyacantha and H. taeniaeformis and should be carefully used in distinguishing the closely related species T. asiatica and T. saginata.