Genetic analysis of Indoplanorbis and Lymnaea and their trematode parasites in Thailand

Abstract

Indoplanorbis exustus and lymnaeid snails, a group of freshwater pulmonate snails, are widely distributed in tropical and subtropical zones. They play a significant role as the first intermediate host of trematode parasites that affect both human and livestock health. A full understanding of the genetic relationship of hosts and parasites is of paramount importance for effective parasite management. The goals of this study were to investigate the current transmission status of trematode cercariae in I. exustus and lymnaeid snails in Thailand and to examine the genetic diversity, genetic structure, and demographic history of these snails. In this study, 575 Indoplanorbis and 672 lymnaeid snails were collected from 56 locations in 27 provinces in six regions of Thailand. Subsequently, cercarial infection in the snails was observed by using the shedding method. I. exustus and lymnaeid snails released 5 types of trematode cercariae, namely, xiphidiocercariae, echinostome cercariae I, echinostome cercariae II, furcocercous cercariae, and strigea cercariae. The overall infection rate of cercariae in snails was 1.76% (22/1,247). The phylogenetic analysis based on ITS2 and 28S rDNA sequences revealed 5 cercaria types assigned to four trematode families, of which two belong to the group of human intestinal flukes. By incorporating shell morphology and sequence analysis of the mitochondrial COI and 16S rDNA genes, the lymnaeid snails were classified into two species, Radix rubiginosa and Orientogalba viridis. Haplotype analysis of R. rubiginosa and O. viridis revealed only a few haplotypes were infected with cercariae. The genetic diversity and genetic structure revealed that R. rubiginosa and O. viridis experienced a bottleneck phenomenon and limited gene flow between populations. Population demographic history analyses revealed that R. rubiginosa and O. viridis experienced population reductions followed by recent population expansion. Meanwhile, phylogenetic and network analyses of I. exustus haplotypes based on sampled sequences from Thailand and a publicly accessible database of snails from other countries using the COI, 16S rDNA, and ITS1 genes demonstrated four main clades. Only snails in clade A were distributed in all regions of Thailand and harbored trematode cercariae. The level of genetic diversity of I. exustus in Thailand was relatively high, but most populations were not genetically different, thus suggesting the appearance of gene flow within the I. exustus populations. Overall, the haplotype network was star-shaped, thus suggesting the recent demographic expansion of populations. This result was also supported by the unimodal mode of the mismatch distribution graph and the large negative values of the neutrality tests. Therefore, the I. exustus snail was likely another freshwater snail of the invasive species in Thailand. These findings may improve our understanding of parasite-snails evolutionary relationships, as well as the underlying molecular genetic basis, which is information that can be used for further effective control of the spread of trematode disease.

-