Global Mitochondrial Diversity and Distribution of Taenia hydatigena (Cysticercus tenuicollis) Lineages Across Geographic and Host Groups: A cox1-Based Analysis of Shared and Widespread Haplotypes

Taenia hydatigena is a widely distributed taeniid tapeworm of veterinary significance that circulates in transmission cycles among canid definitive hosts and various domestic and wild ungulates. Mitochondrial studies have shown significant genetic variation within this species; however, the geographical and host restrictions of these mitochondrial lineages are not yet fully understood. In this study, we analyzed the global mitochondrial diversity and phylogeographic distribution of T. hydatigena lineages in geographic and host groups based on a cytochrome c oxidase subunit 1 (cox1) sequence dataset obtained from the GenBank database. A total of 406 sequences were assembled, and were used for the population genetic analyses. Mitochondrial diversity and lineage distribution were characterized using haplotype and nucleotide diversity, neutrality statistics, country- and host-associated variation, median-joining networks, and maximum-likelihood phylogenetic reconstruction. The global dataset included 123 haplotypes, with high levels of haplotype diversity (Hd = 0.897) and comparatively low levels of nucleotide diversity (π = 0.00898). Haplotype frequencies were highly skewed, with 79 of them represented by a single sequence, whereas the most common haplotype, Hap-3, contained 124 sequences. Country-based analyses showed significant mitochondrial diversity, as well as geographically widespread and geographically restricted haplotypes, but no country-specific clustering pattern. There were eight different host categories for 364 sequences for which host information was available, and the haplotype diversity (Hd) was high (0.907), whereas the nucleotide diversity (π) was low (0.00926). Major haplotypes were common among several host species, especially sheep and goats, and mitochondrial variants within major haplotypes were shared with dogs and other wild animals. The maximum-likelihood phylogeny identified a monophyletic group of T. hydatigena and showed high levels of intraspecific mitochondrial variation but no obvious geographical structure in the largest mitochondrial subclades. All Tajima’s D, Fu’s Fs, Fu and Li’s D, and Fu and Li’s F statistics were negative for the pooled dataset. These are results that show a departure from the expectations of neutral models, but should not be interpreted as evidence for demographic expansion, as they represent a mixture of geographically and biologically heterogeneous GenBank records with unequal sampling and potentially non-independent sequences. In conclusion, the available GenBank data indicated a high mitochondrial haplotype richness, as well as the presence of widespread and geographically restricted cox1 haplotypes and haplotype sharing between recorded host and geographic categories. Unbalanced host and country representation, possible non-independence of sequence records, and host and geographic origin are each potential sources of confounding that reduce the power to make inferences concerning population connectivity and host-associated structure. These results offer a comparative model to understand the distribution of mitochondrial variation in T. hydatigena and point to the need for a sampling methodology that is both geographically distributed and standardized, as well as longer mitochondrial and nuclear sequence data, for sympatric hosts.