Secretion of an Argonaute protein by a parasitic nematode and the evolution of its siRNA guides.

Extracellular RNA has been proposed to mediate communication between cells and organisms however relatively little is understood regarding how specific sequences are selected for export. Here, we describe a specific Argonaute protein (exWAGO) that is secreted in extracellular vesicles (EVs) released by the gastrointestinal nematode Heligmosomoides bakeri, at multiple copies per EV. Phylogenetic and gene expression analyses demonstrate exWAGO orthologues are highly conserved and abundantly expressed in related parasites but highly diverged in free-living genus Caenorhabditis. We show that the most abundant small RNAs released from the nematode parasite are not microRNAs as previously thought, but rather secondary small interfering RNAs (siRNAs) that are produced by RNA-dependent RNA Polymerases. The siRNAs that are released in EVs have distinct evolutionary properties compared to those resident in free-living or parasitic nematodes. Immunoprecipitation of exWAGO demonstrates that it specifically associates with siRNAs from transposons and newly evolved repetitive elements that are packaged in EVs and released into the host environment. Together this work demonstrates molecular and evolutionary selectivity in the small RNA sequences that are released in EVs into the host environment and identifies a novel Argonaute protein as the mediator of this.

Contrasting patterns of structural host specificity of two species of Heligmosomoides nematodes in sympatric rodents.

Host specificity is a fundamental property of parasites. Whereas most studies focus on measures of specificity on host range, only few studies have considered quantitative aspects such as infection intensity or prevalence. The relative importance of these quantitative aspects is still unclear, mainly because of methodological constraints, yet central to a precise assessment of host specificity. Here, we assessed simultaneously two quantitative measures of host specificity of Heligmosomoides glareoli and Heligmosomoides polygyrus polygyrus infections in sympatric rodent hosts. We used standard morphological techniques as well as real-time quantitative PCR and sequencing of the rDNA ITS2 fragment to analyse parasite infection via faecal sample remains. Although both parasite species are thought to be strictly species-specific, we found morphologically and molecularly validated co- and cross-infections. We also detected contrasting patterns within and between host species with regard to specificity for prevalence and intensity of infection. H. glareoli intensities were twofold higher in bank voles than in yellow-necked mice, but prevalence did not differ significantly between species (33 vs. 18%). We found the opposite pattern in H. polygyrus infections with similar intensity levels between host species but significantly higher prevalence in mouse hosts (56 vs. 10%). Detection rates were higher with molecular tools than morphological methods. Our results emphasize the necessity to consider quantitative aspects of specificity for a full view of a parasites' capacity to replicate and transmit in hosts and present a worked example of how modern molecular tools help to advance our understanding of selective forces in host-parasite ecology and evolution.

Heligmosomoides bakeri: a new name for an old worm?

A popular model system for exploring the host-parasite relationship of gastrointestinal nematodes is commonly known as Heligmosomoides polygyrus bakeri. Recently, this parasite was raised to full species level as H. bakeri, to distinguish it from a close relative, H. polygyrus sensu stricto, the dominant intestinal nematode of wood mice in Western Europe, which is unable to infect laboratory mice (Mus sp.) without the aid of powerful immunosuppressants. Herein, the argument is presented that it is necessary to rename this parasite, and that H. bakeri is the correct name for the species used widely throughout the world as a laboratory research model. Supporting this claim, key evidence is presented demonstrating that H. bakeri and H. polygyrus are two quite distinct species.

Geography and host biogeography matter for understanding the phylogeography of a parasite.

The co-evolution between hosts and parasites has long been recognized as a fundamental driver of macro-evolutionary patterns of diversification. The effect of co-differentiation on parasite diversification is, however, often confounded by underlying geographic patterns of host distribution. In order to disentangle the confounding effects of allopatric versus host speciation, the mitochondrial cytochrome b (cyt b) gene was sequenced in seventy individuals of the parasitic nematode genus Heligmosomoides sampled in the six Apodemus mice species common in the western Palearctic region. The nuclear internal transcribed spacers (ITS) 1 and 2 were also sequenced in fifteen parasites to confirm the mitochondrial data. All lineages differentiated according to a geographic pattern and independently from the sampled host species. This suggests that host speciation did not involve concurrent parasite speciation. However, the geographic distribution range of some parasite lineages mirrors that of A. sylvaticus lineages in SW Europe, and that of A. flavicollis lineages in the Balkans and in the Middle East. Thus, regional co-differentiation likely occurred between the parasite and the two sister Apodemus hosts in different parts of their distribution range. We suggest that differences in regional abundances of A. sylvaticus and A. flavicollis are responsible for generating this pattern of regional co-differentiation. This study highlights the importance of integrating both geography and biogeographic information from potential hosts to better understand their parasite phylogeography.

Molecular evidence that Heligmosomoides polygyrus from laboratory mice and wood mice are separate species.

The gastro-intestinal (GI) nematode Heligmosomoides polygyrus is an important experimental model in laboratory mice and a well-studied parasite of wood mice in the field. Despite an extensive literature, the taxonomy of this parasite in different hosts is confused, and it is unclear whether laboratory and field systems represent the same or different Operational Taxonomic Units (OTUs). Molecular analyses reveal high sequence divergence between H. p. bakeri (laboratory) and H. p. polygyrus (field); 3% difference in the ribosomal DNA Internal Transcribed Spacers (ITS) and 8.6% variation in the more rapidly evolving mitochondrial cytochrome c oxidase I (COI) gene. The COI sequence of U.K. H. p. polygyrus is more similar to H. glareoli from voles than to H. p. bakeri, while a single isolate of H. p. polygyrus from Guernsey confirms the extent of genetic variation between H. p. polygyrus populations. Analysis of molecular variance demonstrated that mtCOI sequence variation is associated primarily with groups with distinct ITS2 sequences, and with host identity, but is not partitioned significantly with a single combined taxon H. polygyrus incorporating European and North American isolates. We conclude therefore that the laboratory OTU should be raised to the level of a distinct species, as H. bakeri from the laboratory mouse Mus musculus, and we reject the hypothesis that H. bakeri has diverged from H. polygyrus in the recent past following introduction into America. However, we are unable to reject the hypothesis that H. polygyrus and H. bakeri are sister taxa, and it may be that H. polygyrus is polyphyletic or paraphyletic.