Fasciola hepatica demonstrates high levels of genetic diversity, a lack of population structure and high gene flow: possible implications for drug resistance

Abstract

Fasciola hepatica, the liver fluke, is a trematode parasite of considerable economic importance to the livestock industry and is a re-emerging zoonosis that poses a risk to human health in F. hepatica-endemic areas worldwide. Drug resistance is a substantial threat to the current and future control of F. hepatica, yet little is known about how the biology of the parasite influences the development and spread of resistance. Given that F. hepatica can self-fertilise and therefore inbreed, there is the potential for greater population differentiation and an increased likelihood of recessive alleles, such as drug resistance genes, coming together. This could be compounded by clonal expansion within the snail intermediate host and aggregation of parasites of the same genotype on pasture. Alternatively, widespread movement of animals that typically occurs in the UK could promote high levels of gene flow and prevent population differentiation. We identified clonal parasites with identical multilocus genotypes in 61% of hosts. Despite this, 84% of 1579 adult parasites had unique multilocus genotypes, which supports high levels of genotypic diversity within F. hepatica populations. Our analyses indicate a selfing rate no greater than 2%, suggesting that this diversity is in part due to the propensity for F. hepatica to cross-fertilise. Finally, although we identified high genetic diversity within a given host, there was little evidence for differentiation between populations from different hosts, indicating a single panmictic population. This implies that, once those emerge, anthelmintic resistance genes have the potential to spread rapidly through liver fluke populations.

Keywords: Anthelmintic resistance; Diversity; Fasciola hepatica; Gene flow; Microsatellites; Population genetics; Self-fertilisation.

Graphical abstract

Fig. 1

Representation of the number of clonal Fasciola hepatica parasites (those with repeated multilocus genotypes) found within each individual (A) sheep and (B) cattle and shown as a proportion of the total number of parasites genotyped from each definitive host; numbers on the x-axis are individual animal identifiers; ^∗ indicates that more than one clone set was found in an individual host, the bar is split to distinguish the number of parasites within each clone set; ^{^} indicates that clone sets are shared between hosts. (C) Histogram displaying the genotypic richness values within each definitive host, separated into sheep and cattle. Genotypic richness (R) is a measure of genetic diversity and is calculated as R = (G − 1)/(N − 1) where G = the number of genotypes identified in each host and N = the number of parasites genotyped; each histogram bar is of width 0.05 with the bar centred over the upper limit. (D) Principal Component Analysis for pairwise F_ST values between the parasites of each definitive host. Each data point and its corresponding number represent an individual animal, and the shape and colour of the symbol represent the location and species of that animal, respectively.

Fig. 2

Results of tests for isolation by distance and population structuring within Fasciola hepatica from Great Britain. (A) Isolation by distance results for F. hepatica parasites from cattle. Each point plots the genetic difference (pairwise test statistic based on F_ST/[1 − F_ST]) against the geographical distance (on a natural logarithm (Ln) scale) between each pair of populations. Each population consists of the parasites on one farm; comparisons are not made between parasites on the same farm. The regression line is shown and has the following parameters: slope = −0.00129 (95% confidence interval = −0.00317, 0.00142); intercept = −0.434; P = 0.2968. Therefore, there is no evidence of isolation by distance as the slope is negative and the P value non-significant. (B) Structure (Pritchard et al., 2000) was used to detect population structure. K represents the number of populations assumed for each simulation and is plotted against the mean natural log probabilities. Each simulation was repeated 20 times and error bars show the SDs. (C) To determine the most appropriate value for K, ΔK (the rate of change in the log probability between successive K values; Evanno et al., 2005) was determined using Structure Harvester (Earl and vonHoldt, 2012). The results indicate a single population with no structure.