Resolution of a taxonomic conundrum: an ultrastructural and molecular description of the life cycle of Pleistophora mulleri (Pfeiffer 1895; Georgevitch 1929)

Microsporidian Pathogens of Aquatic Animals

Around 57.1% of microsporidia occupy aquatic environments, excluding a further 25.7% that utilise both terrestrial and aquatic systems. The aquatic microsporidia therefore compose the most diverse elements of the Microsporidia phylum, boasting unique structural features, variable transmission pathways, and significant ecological influence. From deep oceans to tropical rivers, these parasites are present in most aquatic environments and have been shown to infect hosts from across the Protozoa and Animalia. The consequences of infection range from mortality to intricate behavioural change, and their presence in aquatic communities often alters the overall functioning of the ecosystem.In this chapter, we explore aquatic microsporidian diversity from the perspective of aquatic animal health. Examples of microsporidian parasitism of importance to an aquacultural ('One Health') context and ecosystem context are focussed upon. These include infection of commercially important penaeid shrimp by Enterocytozoon hepatopenaei and interesting hyperparasitic microsporidians of wild host groups.Out of ~1500 suggested microsporidian species, 202 have been adequately taxonomically described using a combination of ultrastructural and genetic techniques from aquatic and semi-aquatic hosts. These species are our primary focus, and we suggest that the remaining diversity have additional genetic or morphological data collected to formalise their underlying systematics.

Wide geographic distribution of overlooked parasites: Rare Microsporidia in Gammarus balcanicus, a species complex with a high rate of endemism

Parasites and other symbionts deeply influence host organisms, and no living organism can be considered to have evolved independent of its symbionts. The first step towards understanding symbiotic influences upon host organisms is a strong supporting knowledge of parasite/symbiont diversity. Parasites of freshwater amphipods are diverse, with Microsporidia being a major group. These intracellular parasites impact gammarid fitness in different ways, ranging from reduced fitness to increased fecundity. Many Microsporidia have been recorded using molecular data, with multiple taxa pending formal taxonomic description. While some parasites are common, others are known only through sporadic records of single infections. In this study, we focus on rare/sporadic microsporidian infections within Gammarus balcanicus, a host species complex with a high level of endemism. In addition to enriching our knowledge on Microsporidia parasite diversity in amphipod hosts, we test whether these symbionts are specific to G. balcanicus or if they are the same taxa infecting other gammarid species. Of 2231 hosts from 87 sites, we catalogued 29 sequences of "rare" Microsporidia clustering into 19 haplogroups. These haplogroups cluster into 11 lineages: four pre-described taxa (Cucumispora roeselum, C. ornata, C. dikerogammari and Enterocytospora artemiae) and seven 'Molecular Operational Taxonomic Units', which are known from previously published studies to infect other European amphipod species. Our study significantly widens the geographic range of these Microsporidia and expands the known spectrum of hosts infected. Our results suggest that these parasites are ancient infections of European gammarids. For some host-parasite systems, we hypothesize that the common parasite ancestors that infected the hosts' common ancestors, diversified alongside host diversification. For others, we observe Microsporidia taxa with wide host ranges that do not follow host phylogeny.

Pathogens and other symbionts of the Amphipoda: taxonomic diversity and pathological significance

With over 10000 species of Amphipoda currently described, this order is one of the most diverse groups of freshwater and marine Crustacea. Members of this group are globally distributed, and many are keystone species and ecosystem engineers within their respective ecologies. As with most organisms, disease is a key factor that can alter population size, behaviour, survival, invasion potential and physiology of amphipod hosts. This review explores symbiont diversity and pathology in amphipods by coalescing a range of current and historical literature to provide the first full review of our understanding of amphipod disease. The review is broken into 2 parts. The first half explores amphipod microparasites, which include data pertaining to viruses, bacteria, fungi, oomycetes, microsporidians, dinoflagellates, myxozoans, ascetosporeans, mesomycetozoeans, apicomplexans and ciliophorans. The second half reports the metazoan macroparasites of Amphipoda, including rotifers, trematodes, acanthocephalans, nematodes, cestodes and parasitic Crustacea. In all cases we have endeavoured to provide a complete list of known species that cause disease in amphipods, while also exploring the effects of parasitism. Although our understanding of disease in amphipods requires greater research efforts to better define taxonomic diversity and host effects of amphipod symbionts, research to date has made huge progress in cataloguing and experimentally determining the effects of disease upon amphipods. For the future, we suggest a greater focus on developing model systems that use readily available amphipods and diseases, which can be comparable to the diseases in other Crustacea that are endangered, economically important or difficult to house.

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.

The prevalence and potential fisheries consequences of Heterosporis sutherlandae in a Minnesota lake

Heterosporis sutherlandae is an emerging microsporidian fish parasite in the Great Lakes region. H. sutherlandae forms lesions in the muscle tissue of fishes important to aquaculture and sport fishing. These lesions render the filet inedible and may have fitness consequences. We evaluated the prevalence and severity of H. sutherlandae among yellow perch (Perca flavescens) in a known-positive Minnesota lake, and used an equilibrium yield model to evaluate impacts on harvest. Twenty-eight percent of the 400 yellow perch sampled were infected with H. sutherlandae. Males were 1.5 times more likely to be infected than females and were more severely infected. The presence of the parasite did not vary with relative weight or age, but infection severity was highest among older individuals that were in better condition. These results suggest that males are more susceptible to infection, and that infection is not associated with maturity or a gape-limiting food source. These results also suggest that heterosporosis increases in severity with time or by increased exposure. Our equilibrium yield model found that a 10% increase in mortality due to H. sutherlandae could result in 30% and 10% reductions in yield and mean catch weight, respectively. The results of this study direct future field sampling and laboratory experiments to further understand and predict the impacts of this parasite.

Europe-wide reassessment of Dictyocoela (Microsporidia) infecting native and invasive amphipods (Crustacea): molecular versus ultrastructural traits

Microsporidia are common parasites infecting animals and protists. They are specifically common pathogens of amphipods (Crustacea, Malacostraca), with Dictyocoela spp. being particularly frequent and highly prevalent, exhibiting a range of phenotypic and ecological effects. Until now, seven species of Dictyocoela were defined, predominantly based on the genetic distance. However, neither the taxonomic status of this provisionally erected genus (based on eight novel sequences and one micrograph of the spore), nor its internal phylogenetic relationships have been clearly revealed. The formal description of the genus and of most of the putative species are still lacking. Here we aimed to fill this gap and performed both ultrastructural and molecular studies (based on SSU, ITS and partial LSU) using different species delimitation methods. As a consensus of these results and following conservative data interpretation, we propose to distinguish five species infecting gammarid hosts, and to keep the names introduced by the authors of the type sequences: Dictyocoela duebenum, D. muelleri, D. berillonum and D. roeselum. We provide full descriptions of these species. Moreover, thanks to our extensive sampling, we extend the known host and geographic range of these Microsporidia.

Distribution, prevalence, and pathology of a microsporidian infecting freshwater sculpins

Microsporidian infections are common in many fish species, yet detailed studies of these parasites in ecologically important wild populations are rare. Phylogenetic analysis using rDNA sequence data and parasite morphology indicate that mottled sculpin Cottus bairdii and slimy sculpin C. cognatus are hosts for Glugea sp. microsporidia in the northern USA. Glugea sp. is common in the Michigan populations sampled for this study, and prevalence was ≥ 70% in 4 of 6 infected populations (range -4 to 80%). Glugea sp. infection causes the formation of xenomas associated with the body wall, fat body, gonads, and kidneys. Infections range from mild to very heavy, with variable xenoma numbers and sizes. Female sculpin experience heavier infections and more frequent infection of the gonads relative to males. Glugea sp. is transmitted horizontally between hosts through ingestion of spores. Vertical transmission may also be possible, either by spores infecting eggs directly or by spores contaminating the surface of eggs in the ovary or in the nest. The frequency and route of vertical transmission requires further study, but if it occurs, it may partly explain the high prevalence of infection. Our study combined with previous research suggests that additional molecular data and cross-infection experiments should be conducted to clarify species designations in the genus Glugea.

Cucumispora ornata n. sp. (Fungi: Microsporidia) infecting invasive 'demon shrimp' (Dikerogammarus haemobaphes) in the United Kingdom

Dikerogammarus haemobaphes, the 'demon shrimp', is an amphipod native to the Ponto-Caspian region. This species invaded the UK in 2012 and has become widely established. Dikerogammarus haemobaphes has the potential to introduce non-native pathogens into the UK, creating a potential threat to native fauna. This study describes a novel species of microsporidian parasite infecting 72.8% of invasive D. haemobaphes located in the River Trent, UK. The microsporidium infection was systemic throughout the host; mainly targeting the sarcolemma of muscle tissues. Electron microscopy revealed this parasite to be diplokaryotic and have 7-9 turns of the polar filament. The microsporidium is placed into the 'Cucumispora' genus based on host histopathology, fine detail parasite ultrastructure, a highly similar life-cycle and SSU rDNA sequence phylogeny. Using this data this novel microsporidian species is named Cucumispora ornata, where 'ornata' refers to the external beading present on the mature spore stage of this organism. Alongside a taxonomic discussion, the presence of a novel Cucumispora sp. in the United Kingdom is discussed and related to the potential control of invasive Dikerogammarus spp. in the UK and the health of native species which may come into contact with this parasite.

Eaten alive: cannibalism is enhanced by parasites

Cannibalism is ubiquitous in nature and especially pervasive in consumers with stage-specific resource utilization in resource-limited environments. Cannibalism is thus influential in the structure and functioning of biological communities. Parasites are also pervasive in nature and, we hypothesize, might affect cannibalism since infection can alter host foraging behaviour. We investigated the effects of a common parasite, the microsporidian Pleistophora mulleri, on the cannibalism rate of its host, the freshwater amphipod Gammarus duebeni celticus. Parasitic infection increased the rate of cannibalism by adults towards uninfected juvenile conspecifics, as measured by adult functional responses, that is, the rate of resource uptake as a function of resource density. This may reflect the increased metabolic requirements of the host as driven by the parasite. Furthermore, when presented with a choice, uninfected adults preferred to cannibalize uninfected rather than infected juvenile conspecifics, probably reflecting selection pressure to avoid the risk of parasite acquisition. By contrast, infected adults were indiscriminate with respect to infection status of their victims, probably owing to metabolic costs of infection and the lack of risk as the cannibals were already infected. Thus parasitism, by enhancing cannibalism rates, may have previously unrecognized effects on stage structure and population dynamics for cannibalistic species and may also act as a selective pressure leading to changes in resource use.

Trait-Mediated Effects of Parasites on Invader-Native Interactions

Parasites have a variety of behavioural effects on their hosts, which can in turn affect species with which the host interacts. Here we review how these trait-mediated indirect effects of parasites can alter the outcomes of invader-native interactions, illustrating with examples from the literature and with particular regard to the invader-native crustacean systems studied in our laboratories. Parasites may potentially inhibit or exacerbate invasions via their effects on host behaviour, in addition to their direct virulence effects on hosts. In several crustacean systems, we have found that parasites influence both host predation rates on intra- and inter-guild prey and host vulnerability to being preyed upon. These trait effects can theoretically alter invasion impact and patterns of coexistence, as they indirectly affect interactions between predators and prey with the potential for further ramifications to other species in the food web. The fitness consequences of parasite-induced trait-mediated effects are rarely considered in traditional parasitological contexts, but demand attention in the context of ecological communities. We can regard these trait effects as a form of cryptic virulence that only becomes apparent when hosts are examined in the context of the other species with which they interact.

A co-invasive microsporidian parasite that reduces the predatory behaviour of its host Dikerogammarus villosus (Crustacea, Amphipoda)

Parasites are known to affect the predatory behaviour or diet of their hosts. In relation to biological invasions, parasites may significantly influence the invasiveness of the host population and/or mediate the relationships between the invader and the invaded community. Dikerogammarus villosus, a recently introduced species, has had a major impact in European rivers. Notably, its high position in trophic web and high predatory behaviour, have both facilitated its invasive success, and affected other macroinvertebrate taxa in colonized habitats. The intracellular parasite Cucumispora dikerogammari, specific to D. villosus, has successfully dispersed together with this amphipod. Data presented here have shown that D. villosus infected by this parasite have a reduced predatory behaviour compared with healthy individuals, and are much more active suggesting that the co-invasive parasite may diminish the predatory pressure of D. villosus on newly colonized communities.

A new microsporidium, Triwangia caridinae gen. nov., sp. nov. parasitizing fresh water shrimp, Caridina formosae (Decapoda: Atyidae) in Taiwan

A new microsporidium was isolated from the endemic, Taiwanese shrimp, Caridina formosae (Decapoda, Atyidae) from northern Taiwan. A conspicuous symptom of infection was presence of opaque white xenomas located in the proximity of the alimentary tract, the surface of the hepatopancreas, and the gills. A fully developed xenoma consisted of a hard, thick capsule filled with sporophorous vesicles containing multiple spores. Microsporidia developed synchronously within the same sporophorous vesicle, although the stage of parasite development differed among the vesicles. Fresh spores were pyriform, mononucleated and measured 6.53 × 4.38 μm. The polar filament was anisofilar with 9-11 coils. Phylogenetic analysis based on the small subunit ribosomal DNA sequence showed that the isolate is most similar to the fish microsporidian clade containing the genera Kabatana, Microgemma, Potaspora, Spraguea, and Teramicra. The highest sequence identity, 80%, was with Spraguea spp. Based on pathogenesis, life cycle and phylogenetic analysis, we erect a new genus and species, Triwangia caridinae for the novel microsporidium.

Should sex-ratio distorting parasites abandon horizontal transmission?

BACKGROUND

Sex-ratio distorting parasites are of interest due to their effects upon host population dynamics and their potential to influence the evolution of host sex determination systems. In theory, the ability to distort host sex-ratios allows a parasite with efficient vertical (hereditary) transmission to dispense completely with horizontal (infectious) transmission. However, recent empirical studies indicate that some sex-ratio distorting parasites have retained the capability for horizontal transmission.

RESULTS

Numerical simulations using biologically realistic parameters suggest that a feminising parasite is only likely to lose the capability for horizontal transmission if its host occurs at low density and/or has a male-biased primary sex ratio. It is also demonstrated that even a small amount of horizontal transmission can allow multiple feminising parasites to coexist within a single host population. Finally it is shown that, by boosting its host's rate of population growth, a feminising parasite can increase its own horizontal transmission and allow the invasion of other, more virulent parasites.

CONCLUSIONS

The prediction that sex-ratio distorting parasites are likely to retain a degree of horizontal transmission has important implications for the epidemiology and host-parasite interactions of these organisms. It may also explain the frequent co-occurrence of several sex-ratio distorting parasite species in nature.

Cucumispora dikerogammari n. gen. (Fungi: Microsporidia) infecting the invasive amphipod Dikerogammarus villosus: a potential emerging disease in European rivers

Dikerogammarus villosus is an invasive amphipod that recently colonized the main rivers of Central and Western Europe. Two frequent microsporidian parasites were previously detected in this species, but their taxonomic status was unclear. Here we present ultrastructural and molecular data indicating that these two parasites are in fact a single microsporidian species. This parasite shares numerous characteristics of Nosema spp. It forms elongate spores (cucumiform), developing in direct contact with host cell cytoplasm; all developmental stages are diplokaryotic and the life cycle is monomorphic with disporoblastic sporogony. Initially this parasite was described as Nosema dikerogammari Ovcharenko and Kurandina 1987. However, phylogenetic analysis based on the complete sequence of SSU rDNA places the parasite outside the genus Nosema and it is therefore ascribed to a new genus Cucumispora. The key features characteristic to this genus are: presence of a very well-developed, umbrella-shape anchoring disk covering the anterior part of polaroplast; arrangement of isofilar polar filament into 6-8 coils convoluted with different angles, voluminous diplokaryon, thin spore wall and relatively small posterior vacuole containing posterosome. The parasite infects most host tissues but mainly muscles. It showed high rates of horizontal trophic transmission and lower rates of vertical transmission.

Evidence for sex ratio distortion by a new microsporidian parasite of a Corophiid amphipod

In this paper, we describe the occurrence of a microsporidian parasite in female-biased populations of an intertidal amphipod,Corophium volutator(Pallas), at mudflat sites in the Bay of Fundy, Canada. Sequence data for the parasite's 16S rDNA indicate that it is a novel microsporidian species. This parasite was found principally in female host gonads, indicating that it might be a vertically transmitted, sex-distorting microparasite. At 4 sites each sampled in early and mid-summer, parasite prevalence varied from 0 to 21%. In the lab, infected mothers gave rise to more female-biased broods, than did uninfected mothers. Infection was not associated with size of females or with lowered survivorship of their young. Surprisingly, infected mothers actually had higher fertility controlling for body length than did uninfected mothers. Taken together, our results suggest that this novel microsporidian is likely a feminizing microparasite and is a contributing factor to local and widespread sex ratio distortion inC. volutator.

Phylogenetic Distance of Thelohania butleri (Microsporidia; Thelohaniidae), a Parasite of the Smooth Pink Shrimp Pandalus jordani, from its Congeners Suggests Need for Major Revision of the Genus Thelohania Henneguy, 1892

Thelohania butleri, a microsporidian that causes mortality and commercial losses in the smooth pink shrimp Pandalus jordani, is of taxonomic interest as a species resembling the poorly studied type species, Thelohania giardi, of the large, polyphyletic genus Thelohania. We examined the ultrastructure of T. butleri to confirm its identity and reconstructed phylogenies using ribosomal DNA to find the relationship of T. butleri with other Thelohania species in crayfish and ants. Light and transmission electron microscopy from specimens collected from the type locality, the Pacific coast of Canada, confirmed the identity and demonstrated a development similar to that of T. giardi, involving a series of binary fissions without formation of a plasmodium. Phylogenetic analyses consistently showed T. butleri to be distantly related to other Thelohania species, and closely related to species from marine decapods within a larger fish-parasitic clade. Together, features such as host group and habitat, developmental morphology, and phylogeny suggest T. butleri may be a closer relative to T. giardi than any other Thelohania species represented by DNA data so far, and thus imply species from crayfish and ants may not belong in this genus. Results also confirm that genus Thelohania and family Thelohanidae are in need of revision.

Phylogenetic Analysis of Nosema antheraeae (Microsporidia) Isolated from Chinese Oak Silkworm, Antheraea pernyi

The microsporidian Nosema antheraeae is a pathogen that infects the Chinese oak silkworm, Antheraea pernyi. We sequenced the complete small subunit (SSU) rRNA gene and the internal transcribed spacer (ITS) of N. antheraeae, and compared the SSU rRNA sequences in other microsporidia. The results indicated that Nosema species, including N. antheraeae, formed two distinct clades, consistent with previous observations. Furthermore, N. antheraeae is clustered with N. bombycis with high bootstrap support. The organization of the rRNA gene of N. antheraeae is LSU-ITS1-SSU-ITS2-5S, also following a pattern similar to the Nosema type species, N. bombycis. Thus, N. antheraeae is a Nosema species and has a close relationship to N. bombycis.

Ecological impacts of the microsporidian parasite Pleistophora mulleri on its freshwater amphipod host Gammarus duebeni celticus

The microsporidian parasite, Pleistophora mulleri, infects the abdominal muscle of the freshwater amphipod Gammarus duebeni celticus. We recently showed that P. mulleri infection was associated with G. d. celticus hosts being more vulnerable to predation by the invasive amphipod Gammarus pulex. Parasitized G. d. celticus also had a reduced ability to prey upon other co-occurring amphipods. We suggested the parasite may have pervasive influences on host ecology and behaviour. Here, we examine the association between P. mulleri parasitism and parameters influencing individual host fitness, behaviour and interspecific interactions. We also investigate the relationship between parasite prevalence and host population structure in the field. In our G. d. celticus study population, P. mulleri prevalence was strongly seasonal, ranging from 8.5% in summer to 44.9% in winter. The relative abundance of hosts with the heaviest parasite burden increased during summer, which coincided with high host mortality, suggesting that parasitism may regulate host abundance to some degree. Females were more likely to be parasitized than males and parasitized males were paired with smaller females than unparasitized males. Parasitism was associated with reduction in the host's activity level and reduced both its predation on the isopod Asellus aquaticus and aggression towards precopula pairs of the invasive G. pulex. We discuss the pervasive influence of this parasite on the ecology of its host.

Development and Molecular Characterisation of the Microsporidian Schroedera airthreyi n. sp. in a Freshwater Bryozoan Plumatella sp. (Bryozoa: Phylactolaemata)

The development of a new species of microsporidian, infecting a freshwater Plumatellid bryozoan, is described. The small-subunit rDNA, internal transcribed spacer region (ITS), and partial large-subunit rDNA genes were sequenced. Phylogenetic analysis demonstrated that the parasite clustered with Schroedera plumatellae. However, while there were morphological affinities with this species, significant differences were also observed. The infection initially appeared as a roughening of the peritoneum lining the metacoelom of the bryozoan. This roughening resolved into meront-infected syncytia, composed of interconnected cells of the body wall that detached to float in the coleomic cavity. Spores were observed to develop within these syncytia. All stages of development were diplokaryotic in contrast to S. plumatellae, which has a distinct monokaryotic merogony preceding sporogony. The infection was pathogenic to the host. Direct bryozoan-bryozoan transmission was not observed. We propose to name the microsporidian Schroedera aithreyi n. sp.

Ultrastructural and molecular characterization of Bacillidium vesiculoformis n. sp. (Microspora: Mrazekiidae) in the freshwater oligochaete Nais simplex (Oligochaeta: Naididae)

The development of a new species, Bacillidium vesiculoformis n. sp. (Microspora, Mrazekiidae), is described from the freshwater oligochaete Nais simplex (Oligochaeta, Naididae). Initial stages of parasite development consist of a monokaryotic merogony within a haemocyte of the intestinal blood sinus. The resulting hypertrophied haemocyte is attached to the chloragocytes of the sinus by fine cytoplasmic extensions with the sinus around the cell becoming greatly enlarged. The meronts within the haemocyte form diplokaryotic sporonts that undergo sporogenesis directly within the cytoplasm of the host cell. The infected cell becomes packed with spores and developmental stages, causing it dramatically to increase in size, eventually rupturing the oligochaete and cell. Sporogony appears to be disporoblastic. Released spores were observed to have an adhesive quality. Transmission studies conducted with mature spores failed to transmit the parasite horizontally although vertical transmission was observed. Phylogenetic analysis of the parasite demonstrated that B. vesiculoformis clustered with microsporidian parasites of bryozoa and two other microsporidians, Janacekia debaiseuxi and an unidentified Bacillidium sp.

Widespread vertical transmission and associated host sex-ratio distortion within the eukaryotic phylum Microspora

Vertical transmission (VT) and associated manipulation of host reproduction are widely reported among prokaryotic endosymbionts. Here, we present evidence for widespread use of VT and associated sex-ratio distortion in a eukaryotic phylum. The Microspora are an unusual and diverse group of eukaryotic parasites that infect all animal phyla. Following our initial description of a microsporidian that feminizes its crustacean host, we survey the diversity and distribution of VT within the Microspora. We find that vertically transmitted microsporidia are ubiquitous in the amphipod hosts sampled and that they are also diverse, with 11 species of microsporidia detected within 16 host species. We found that infections were more common in females than males, suggesting that host sex-ratio distortion occurs in five out of eight parasite species tested. Phylogenetic reconstruction demonstrates that VT occurs in all major lineages of the phylum Microspora and that sex-ratio distorters are found on multiple branches of the phylogenetic tree. We propose that VT is either an ancestral trait or evolves with peculiar frequency in this phylum. If the association observed here between VT and host sex-ratio distortion holds true across other host taxa, these eukaryotic parasites may join the bacterial endosymbionts in their importance as sex-ratio distorters.

Invasion success of Fibrillanosema crangonycis, n.sp., n.g.: a novel vertically transmitted microsporidian parasite from the invasive amphipod host Crangonyx pseudogracilis

Parasitism is known to be an important factor in determining the success of biological invasions. Here we examine Crangonyx pseudogracilis, a North American amphipod invasive in the United Kingdom and describe a novel microsporidium, Fibrillanosema crangonycis n.sp., n.g. The primary site of infection is the female gonad and the parasite is transovarially transmitted to the eggs. PCR screening reveals a female bias in the distribution of parasites (96.6% of females, N=29; 22.2% of males, N=27), which is indicative of host sex ratio distortion. The morphological and molecular characterisations of this new microsporidium place it outside all currently established genera. On the basis of these differences, we erect the new genus Fibrillanosema n.g. While F. crangonycis is morphologically identical to uncharacterised microsporidia from populations of North American amphipods, it is distinct from microsporidia found in European populations of amphipods. These data support the hypothesis that vertically transmitted parasites may be selectively retained during invasion events. Furthermore where vertical transmission is combined with host sex ratio distortion these parasites may directly enhance host invasion success through increased rates of population growth.