Cryptic microsporidian parasites differentially affect invasive and native Artemia spp

High temperatures and low humidity promote the occurrence of microsporidians (Microsporidia) in mosquitoes (Culicidae)

BACKGROUND

In the context of climate change, a growing concern is that vector-pathogen or host-parasite interactions may be correlated with climatic factors, especially increasing temperatures. In the present study, we used a mosquito-microsporidian model to determine the impact of environmental factors such as temperature, humidity, wind and rainfall on the occurrence rates of opportunistic obligate microparasites (Microsporidia) in hosts from a family that includes important disease vectors (Culicidae).

METHODS

In our study, 3000 adult mosquitoes collected from the field over 3 years were analysed. Mosquitoes and microsporidia were identified using PCR and sequencing of the hypervariable V5 region of the small subunit ribosomal RNA gene and a shortened fragment of the cytochrome c oxidase subunit I gene, respectively.

RESULTS

DNA metabarcoding was used to identify nine mosquito species, all of which were hosts of 12 microsporidian species. The prevalence of microsporidian DNA across all mosquito samples was 34.6%. Microsporidian prevalence in mosquitoes was more frequent during warm months (> 19 °C; humidity < 65%), as was the co-occurrence of two or three microsporidian species in a single host individual. During warm months, microsporidian occurrence was noted 1.6-fold more often than during the cold periods. Among the microsporidians found in the mosquitoes, five (representing the genera Enterocytospora, Vairimorpha and Microsporidium) were positively correlated with an increase in temperature, whereas one (Hazardia sp.) was significantly correlated with a decrease in temperature. Threefold more microsporidian co-occurrences were recorded in the warm months than in the cold months.

CONCLUSIONS

These results suggest that the susceptibility of mosquitoes to parasite occurrence is primarily determined by environmental conditions, such as, for example, temperatures > 19 °C and humidity not exceeding 62%. Collectively, our data provide a better understanding of the effects of the environment on microsporidian-mosquito interactions.

Interactions between microsporidia and other members of the microbiome

The microbiome is the collection of microbes that are associated with a host. Microsporidia are intracellular eukaryotic parasites that can infect most types of animals. In the last decade, there has been much progress to define the relationship between microsporidia and the microbiome. In this review, we cover an increasing number of reports suggesting that microsporidia are common components of the microbiome in both invertebrates and vertebrates. These microsporidia infections can range from mutualistic to pathogenic, causing several physiological phenotypes, including death. Infection with microsporidia often causes a disruption in the normal microbiome, with both increases and decreases of bacterial, fungal, viral, and protozoan species being observed. This impact on the microbiome can occur through upregulation and downregulation of innate immunity as well as morphological changes to tissues that impact interactions with these microbes. Other microbes, particularly bacteria, can inhibit microsporidia and have been exploited to control microsporidia infections. These bacteria can function through regulating immunity, secreting anti-microsporidia compounds, and, in engineered versions, expressing double-stranded RNA targeting microsporidia genes. We end this review by discussing potential future directions to further understand the complex interactions between microsporidia and the other members of the microbiome.

Differences in the proliferation trend of 'Microsporidium' sp. PL03 in Culex pipiens and C. torrentium larvae

Our study aimed to examine whether there are differences in the proliferation trend of microsporidia in mosquito larvae of the same genus (Culex spp.). DNA-barcoding and quantitative analyses were used to determine microsporidian rDNA copies in 'early' (L1 + L2) and 'late' (L3 + L4) Culex larvae in a natural population. In the study area, C. pipiens and C. torrentium larvae were infected by 'Microsporidium' sp. PL03 at similar levels. Infection by this microsporidian species probably elicits a notable immune response in C. pipiens, whereas in C. torrentium, it may evade or suppress the host immune response.

Factors That Determine Microsporidia Infection and Host Specificity

Microsporidia are a large phylum of obligate intracellular parasites that infect an extremely diverse range of animals and protists. In this chapter, we review what is currently known about microsporidia host specificity and what factors influence microsporidia infection. Extensive sampling in nature from related hosts has provided insight into the host range of many microsporidia species. These field studies have been supported by experiments conducted in controlled laboratory environments which have helped to demonstrate host specificity. Together, these approaches have revealed that, while examples of generalist species exist, microsporidia specificity is often narrow, and species typically infect one or several closely related hosts. For microsporidia to successfully infect and complete their life cycle within a compatible host, several steps must occur, including spore germination, host cell invasion, and proliferation of the parasite within the host tissue. Many factors influence infection, including temperature, seasonality, nutrient availability, and the presence or absence of microbes, as well as the developmental stage, sex, and genetics of the host. Several studies have identified host genomic regions that influence resistance to microsporidia, and future work is likely to uncover molecular mechanisms of microsporidia host specificity in more detail.

Microsporidian infections in the species complex Gammarus roeselii (Amphipoda) over its geographical range: evidence for both host-parasite co-diversification and recent host shifts

BACKGROUND

Microsporidians are obligate endoparasites infecting taxonomically diverse hosts. Both vertical (from mother to eggs) and horizontal (between conspecifics or between species) transmission routes are known. While the former may promote co-speciation and host-specificity, the latter may promote shifts between host species. Among aquatic arthropods, freshwater amphipod crustaceans are hosts for many microsporidian species. However, despite numerous studies, no general pattern emerged about host specificity and co-diversification. In south-eastern Europe, the gammarid Gammarus roeselii is composed of 13 cryptic lineages of Miocene to Pleistocene age but few genotypes of one lineage have spread postglacially throughout north-western Europe. Based on nearly 100 sampling sites covering its entire range, we aim to: (i) explore the microsporidian diversity present in G. roeselii and their phylogenetic relationships, especially in relation to the parasites infecting other Gammaridae; (ii) test if the host phylogeographical history might have impacted host-parasite association (e.g. co-diversifications or recent host shifts from local fauna).

METHODS

We used part of the small subunit rRNA gene as source of sequences to identify and determine the phylogenetic position of the microsporidian taxa infecting G. roeselii.

RESULTS

Microsporidian diversity was high in G. roeselii with 24 detected haplogroups, clustered into 18 species-level taxa. Ten microsporidian species were rare, infecting a few individual hosts in a few populations, and were mostly phylogenetically related to parasites from other amphipods or various crustaceans. Other microsporidians were represented by widespread genera with high prevalence: Nosema, Cucumispora and Dictyocoela. Two contrasting host association patterns could be observed. First, two vertically transmitted microsporidian species, Nosema granulosis and Dictyocoela roeselum, share the pattern of infecting G. roeselii over most of its range and are specific to this host suggesting the co-diversification scenario. This pattern contrasted with that of Dictyocoela muelleri, the three species of Cucumispora, and the rare parasites, present only in the recently colonised region by the host. These patterns suggest recent acquisitions from local host species, after the recent spread of G. roeselii.

CONCLUSIONS

Microsporidians infecting G. roeselii revealed two scenarios of host-parasite associations: (i) ancient associations with vertically transmitted parasites that probably co-diversified with their hosts, and (ii) host shifts from local host species, after the postglacial spread of G. roeselii.

Long-term prevalence data reveals spillover dynamics in a multi-host (Artemia), multi-parasite (Microsporidia) community

In the study of multi-host parasites, it is often found that host species contribute asymmetrically to parasite transmission. Yet in natural populations, identifying which hosts contribute to parasite transmission and maintenance is a recurring challenge. Here, we approach this issue by taking advantage of natural variation in the composition of a host community. We studied the brine shrimps Artemia franciscana and Artemia parthenogenetica and their microsporidian parasites Anostracospora rigaudi and Enterocytospora artemiae. Previous laboratory experiments had shown that each host can transmit both parasites, but could not predict their actual contributions to the parasites' maintenance in the field. To resolve this, we gathered long-term prevalence data from a metacommunity of these species. Metacommunity patches could contain either or both of the Artemia host species, so that the presence of the hosts could be linked directly to the persistence of the parasites. First, we show that the microsporidian A. rigaudi is a spillover parasite: it was unable to persist in the absence of its maintenance host A. parthenogenetica. This result was particularly striking, as A. rigaudi displayed both high prevalence (in the field) and high infectivity (when tested in the laboratory) in both hosts. Moreover, the seasonal presence of A. parthenogenetica imposed seasonality on the rate of spillover, causing cyclical pseudo-endemics in the spillover host A. franciscana. Second, while our prevalence data was sufficient to identify E. artemiae as either a spillover or a facultative multi-host parasite, we could not distinguish between the two possibilities. This study supports the importance of studying the community context of multi-host parasites, and demonstrates that in appropriate multi-host systems, sampling across a range of conditions and host communities can lead to clear conclusions about the drivers of parasite persistence.

Genetic diversity of Microsporidia in the circulatory system of endemic amphipods from different locations and depths of ancient Lake Baikal

Endemic amphipods (Amphipoda, Crustacea) of the most ancient and large freshwater Lake Baikal (Siberia, Russia) are a highly diverse group comprising >15% of all known species of continental amphipods. The extensive endemic biodiversity of Baikal amphipods provides the unique opportunity to study interactions and possible coevolution of this group and their parasites, such as Microsporidia. In this study, we investigated microsporidian diversity in the circulatory system of 22 endemic species of amphipods inhabiting littoral, sublittoral and deep-water zones in all three basins of Lake Baikal. Using molecular genetic techniques, we found microsporidian DNA in two littoral (Eulimnogammarus verrucosus, Eulimnogammarus cyaneus), two littoral/sublittoral (Pallasea cancellus, Eulimnogammarus marituji) and two sublittoral/deep-water (Acanthogammarus lappaceus longispinus, Acanthogammarus victorii maculosus) endemic species. Twenty sequences of the small subunit ribosomal (SSU) rDNA were obtained from the haemolymph of the six endemic amphipod species sampled from 0–60 m depths at the Southern Lake Baikal’s basin (only the Western shore) and at the Central Baikal. They form clusters with similarity to Enterocytospora, Cucumispora, Dictyocoela, and several unassigned Microsporidia sequences, respectively. Our sequence data show similarity to previously identified microsporidian DNA from inhabitants of both Lake Baikal and other water reservoirs. The results of our study suggest that the genetic diversity of Microsporidia in haemolymph of endemic amphipods from Lake Baikal does not correlate with host species, geographic location or depth factors but is homogeneously diverse.

Decomposing parasite fitness reveals the basis of specialization in a two-host, two-parasite system

The ecological specialization of parasites–whether they can obtain high fitness on very few or very many different host species–is a determining feature of their ecology. In order to properly assess specialization, it is imperative to measure parasite fitness across host species; to understand its origins, fitness must be decomposed into the underlying traits. Despite the omnipresence of parasites with multiple hosts, very few studies assess and decompose their specialization in this way. To bridge this gap, we quantified the infectivity, virulence, and transmission rate of two parasites, the horizontally transmitted microsporidians Anostracospora rigaudi and Enterocytospora artemiae, in their natural hosts, the brine shrimp Artemia parthenogenetica and Artemia franciscana. Our results demonstrate that each parasite performs well on one of the two host species (A. rigaudi on A. parthenogenetica, and E. artemiae on A. franciscana), and poorly on the other. This partial specialization is driven by high infectivity and transmission rates in the preferred host, and is associated with maladaptive virulence and large costs of resistance in the other. Our study represents a rare empirical contribution to the study of parasite evolution in multihost systems, highlighting the negative effects of under‐ and overexploitation when adapting to multiple hosts.

Impacts of crustacean invasions on parasite dynamics in aquatic ecosystems: A plea for parasite-focused studies

While there is considerable interest in, and good evidence for, the role that parasites play in biological invasions, the potential parallel effects of species introduction on parasite dynamics have clearly received less attention. Indeed, much effort has been focused on how parasites can facilitate or limit invasions, and positively or negatively impact native host species and recipient communities. Contrastingly, the potential consequences of biological invasions for the diversity and dynamics of both native and introduced parasites have been and are still mainly overlooked, although successful invasion by non-native host species may have large, contrasting and unpredictable effects on parasites. This review looks at the links between biological invasions and pathogens, and particularly at crustacean invasions in aquatic ecosystems and their potential effects on native and invasive parasites, and discusses what often remains unknown even from well-documented systems. Aquatic crustaceans are hosts to many parasites and are often invasive. Published studies show that crustacean invasion can have highly contrasting effects on parasite dynamics, even when invasive host and parasite species are phylogenetically close to their native counterparts. These effects seem to be dependent on multiple factors such as host suitability, parasite life-cycle or host-specific resistance to parasitic manipulation. Furthermore, introduced hosts can have drastically contrasting effects on parasite standing crop and transmission, two parameters that should be independently assessed before drawing any conclusion on the potential effects of novel hosts on parasites and the key processes influencing disease dynamics following biological invasions. I conclude by calling for greater recognition of biological invasions' effects on parasite dynamics, more parasite-focused studies and suggest some potential ways to assess these effects.

Resurrection ecology in Artemia

Resurrection ecology (RE) is a very powerful approach to address a wide range of question in ecology and evolution. This approach rests on using appropriate model systems, and only few are known to be available. In this study, we show that Artemia has multiple attractive features (short generation time, cyst bank and collections, well‐documented phylogeography, and ecology) for a good RE model. We show in detail with a case study how cysts can be recovered from sediments to document the history and dynamics of a biological invasion. We finally discuss with precise examples the many RE possibilities with this model system: adaptation to climate change, to pollution, to parasites, to invaders and evolution of reproductive systems.

Surveillance of Vittaforma corneae in hot springs by a small-volume procedure

Vittaforma corneae is an obligate intracellular fungus and can cause human ocular microsporidiosis. Although accumulating reports of V. corneae causing keratoconjunctivitis in both healthy and immunocompromised persons have been published, little is known about the organism's occurrence in aquatic environments. Limitations in detection sensitivity have meant a large sampling volume is required to detect the pathogen up to now, which is problematic. A recent study in Taiwan has shown that some individuals suffering from microsporidial keratitis (MK) were infected after exposure to the pathogen at a hot spring. As a consequence of this, a survey and analysis of environmental V. corneae present in hot springs became an urgent need. In this study, sixty water samples from six hot spring recreation areas around Taiwan were analyzed. One liter of water from each sample site was filtered to harvest the fungi. The positive samples were detected using a modified nested PCR approach followed by sequencing using specific SSU rRNA gene primer pairs for V. corneae. In total fifteen V. corneae-like isolates were identified (25.0% of sites). Among them, six isolates, which were collected from recreational areas B, C and D, were highly similar to known V. corneae keratitis strains from Taiwan and other countries. Furthermore, five isolates, which were collected from recreation areas A, C, E and F, were very similar to Vittaforma-like diarrhea strains isolated in Portugal. Cold spring water tubs and public foot bath pools had the highest detection rate (50%), suggesting that hot springs might be contaminated via untreated water sources. Comparing the detection rate across different regions of Taiwan, Taitung, which is in the east of the island, gave the highest positive rate (37.5%). Statistical analysis showed that outdoor/soil exposure and a high heterotrophic plate count (HPC) were risk factors for the occurrence of V. corneae. Our findings provide empirical evidence supporting the need for proper control and regulations at hot spring recreational waters in order to avoid health risks from this pathogen. Finally, we have developed a small volume procedure for detecting V. corneae in water samples and this has proved to be very useful.

Parasites and invasions: changes in gastrointestinal helminth assemblages in invasive and native rodents in Senegal

Understanding why some exotic species become widespread and abundant in their colonised range is a fundamental issue that still needs to be addressed. Among many hypotheses, newly established host populations may benefit from a parasite loss ("enemy release" hypothesis) through impoverishment of their original parasite communities or reduced infection levels. Moreover, the fitness of competing native hosts may be negatively affected by the acquisition of exotic taxa from invaders ("parasite spillover") and/or by an increased transmission risk of native parasites due to their amplification by invaders ("parasite spillback"). We focused on gastrointestinal helminth communities to determine whether these predictions could explain the ongoing invasion success of the commensal house mouse (Mus musculus domesticus) and black rat (Rattus rattus), as well as the associated decrease in native Mastomys spp., in Senegal. For both invasive species, our results were consistent with the predictions of the enemy release hypothesis. A decrease in overall gastrointestinal helminth prevalence and infracommunity species richness was observed along the invasion gradients as well as lower specific prevalence/abundance (Aspiculuris tetraptera in Mus musculus domesticus, Hymenolepis diminuta in Rattus rattus) on the invasion fronts. Conversely, we did not find strong evidence of GIH spillover or spillback in invasion fronts, where native and invasive rodents co-occurred. Further experimental research is needed to determine whether and how the loss of gastrointestinal helminths and reduced infection levels along invasion routes may result in any advantageous effects on invader fitness and competitive advantage.

Testing the enemy release hypothesis in a native insect species with an expanding range

The enemy release hypothesis (ERH) predicts that the spread of (invasive) species will be facilitated by release from their enemies as they occupy new areas. However, the ERH is rarely tested on native (non-invasive, long established) species with expanding or shifting ranges. I tested the ERH for a native damselfly (Enallagma clausum) whose range has recently expanded in western Canada, with respect to its water mite and gregarine parasites. Parasitism levels (prevalence and intensity) were also compared between E. clausum and a closely related species, Enallagma boreale, which has long been established in the study region and whose range is not shifting. A total of 1,150 damselflies were collected at three 'old' sites for E. clausum in Saskatchewan, and three 'new' sites in Alberta. A little more than a quarter of the damselflies collected were parasitized with, on average, 18 water mite individuals, and 20% were parasitized by, on average, 10 gregarine individuals. I assessed whether the differences between levels of infection (prevalence and intensity) were due to site type or host species. The ERH was not supported: Enallagma clausum has higher or the same levels of parasitism in new sites than old sites. However, E. boreale seems to be benefitting from the recent range expansion of a native, closely related species through ecological release from its parasites because the parasites may be choosing to infest the novel, potentially naïve, host instead of the well-established host.

Niche Limits of Symbiotic Gut Microbiota Constrain the Salinity Tolerance of Brine Shrimp

Symbiosis generally causes an expansion of the niche of each partner along the axis for which a service is mutually provided. However, for other axes, the niche can be restricted to the intersection of each partner's niche and can thus be constrained rather than expanded by mutualism. We explore this phenomenon using Artemia as a model system. This crustacean is able to survive at very high salinities but not at low salinities, although its hemolymph's salinity is close to freshwater. We hypothesized that this low-salinity paradox results from poor performance of its associated microbiota at low salinity. We showed that, in sterile conditions, Artemia had low survival at all salinities when algae were the only source of carbon. In contrast, survival was high at all salinities when fed with yeast. We also demonstrated that bacteria isolated from Artemia's gut reached higher densities at high salinities than at low salinities, including when grown on algae. Taken together, our results show that Artemia can survive at low salinities, but their gut microbiota, which are required for algae digestion, have reduced fitness. Widespread facultative symbiosis may thus be an important determinant of niche limits along axes not specific to the mutualistic interaction.