Non-coding Regions of Mitochondrial DNA and the cox1 Gene Reveal Genetic Variability Among Local Belarusian Populations of the Causative Agent of Cercarial Dermatitis, Bird Schistosome Trichobilharzia szidati (Digenea: Schistosomatidae)

Abstract

Introduction: The cercariae of avian blood flukes Trichobilharzia szidati (Digenea, Schistosomatidae) are known to cause cercarial allergic dermatitis ("swimmer's itch") in humans. Global epidemics can have significant impacts on local tourism-related economies in recreational areas. Little is known about the genetic polymorphism of the parasite population, or about the variability of the non-coding regions of mitochondrial DNA (mtDNA) and the possibility of using this as a genetic marker.

Materials and methods: The T. szidati cercariae were collected over 7 years from 33 naturally infected Lymnaea stagnalis snails from five sites at two neighboring lakes in Belarus. We investigated the variability of the short (SNR) and long (LNR) non-coding regions of mt DNA and the genetic diversity within the 1125-bp sequences of the gene for subunit 1 of cytochrome c oxidase (cox1).

Results: In the SNR sequences, we found only length variability caused by changes in the number of bases in the mononucleotide tracts T6-T8. LNR demonstrates high variability in nucleotide sequence length (182-260 bp) depending on the presence of two long deletions of 59 and 78 nucleotides. Both mitochondrial loci (LNR and cox1) are characterized by high haplotype diversity (H = 0.922 and H = 1.0, respectively); the nucleotide diversity is significantly higher for LNR (π = 1.926 ± 0.443) compared to cox1 (π = 0.704 ± 0.059). Phylogenetic reconstructions based on the variability of each of the loci (LNR and cox1) and their concatenated sequences revealed their shallow structure and the absence of a correlation between the distribution of single-nucleotide polymorphisms and the geographic origin of parasites from two Belarusian lakes. We identified at last four weakly sublineages in the phylogenetic pattern of T. szidati. The carriers of each deletion have specific patterns for each of the two loci and form their own phylogeographic sublineages. An association between two fixed LNR substitutions and a fixed non-synonymous substitution in cox1 was found in four representatives of one lineage that had a short deletion in the LNR.

Conclusions: This study clarified the phylogeographic structure of the Belarusian population of T. szidati. Our data provide the basis for the use two mt markers in large-scale population studies of the parasite, as well as for studying the molecular evolution of coding and non-coding mtDNA in trematodes.

Keywords: Cox1∙population; Mitochondrial DNA; Non-coding region; Trematoda; Trichobilharzia szidati.