Genetic polymorphisms of Echinococcus tapeworms in China as determined by mitochondrial and nuclear DNA sequences

Abstract

The genetic polymorphisms of Echinococcus spp. in the eastern Tibetan Plateau and the Xinjiang Uyghur Autonomous Region were evaluated by DNA sequencing analyses of genes for mitochondrial cytochrome c oxidase subunit 1 (cox1) and nuclear elongation factor-1 alpha (ef1a). We collected 68 isolates of Echinococcus granulosus sensu stricto (s.s.) from Xinjiang and 113 isolates of E. granulosus s. s., 49 isolates of Echinococcus multilocularis and 34 isolates of Echinococcus shiquicus from the Tibetan Plateau. The results of molecular identification by mitochondrial and nuclear markers were identical, suggesting the infrequency of introgressive hybridization. A considerable intraspecific variation was detected in mitochondrial cox1 sequences. The parsimonious network of cox1 haplotypes showed star-like features in E. granulosus s. s. and E. multilocularis, but a divergent feature in E. shiquicus. The cox1 neutrality indexes computed by Tajima's D and Fu's Fs tests showed high negative values in E. granulosus s. s. and E. multilocularis, indicating significant deviations from neutrality. In contrast, the low positive values of both tests were obtained in E. shiquicus. These results suggest the following hypotheses: (i) recent founder effects arose in E. granulosus and E. multilocularis after introducing particular individuals into the endemic areas by anthropogenic movement or natural migration of host mammals, and (ii) the ancestor of E. shiquicus was segregated into the Tibetan Plateau by colonising alpine mammals and its mitochondrial locus has evolved without bottleneck effects.

Fig. 1

Frequencies of mitochondrial cytochrome c oxidase subunit 1 (cox1) haplotypes in Chinese Echinococcus granulosus and their network based on statistical parsimony. In the network, the size of ovals indicates the frequency of the haplotypes. Small circles show hypothetical haplotypes. The haplotypes whose frequencies are more than 1% are connected with bold lines. Dark ovals represent the haplotypes found in Xinjiang, whereas the localities of gray ovals are Qinghai and Sichuan. A white oval shows the common haplotype.

Fig. 2

The statistical parsimony networks of mitochondrial cytochrome c oxidase subunit 1 (cox1) haplotypes in Echinococcus spp. The size of ovals indicates the frequency of the haplotypes. Small circles show hypothetical haplotypes. (A) The Peruvian population of Echinococcous granulosus sensu stricto. Gray ovals represent the haplotypes specific to Peru. (B) The Tibetan population of Echinococcus multilocularis. (C) The Tibetan population of Echinococcus shiquicus.