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

Widespread infection, diversification and old host associations of Nosema Microsporidia in European freshwater gammarids (Amphipoda)

The microsporidian genus Nosema is primarily known to infect insects of economic importance stimulating high research interest, while other hosts remain understudied. Nosema granulosis is one of the formally described Nosema species infecting amphipod crustaceans, being known to infect only two host species. Our first aim was to characterize Nosema spp. infections in different amphipod species from various European localities using the small subunit ribosomal DNA (SSU) marker. Second, we aimed to assess the phylogenetic diversity, host specificity and to explore the evolutionary history that may explain the diversity of gammarid-infecting Nosema lineages by performing a phylogenetic reconstruction based on RNA polymerase II subunit B1 (RPB1) gene sequences. For the host species Gammarus balcanicus, we also analyzed whether parasites were in excess in females to test for sex ratio distortion in relation with Nosema infection. We identified Nosema spp. in 316 individuals from nine amphipod species being widespread in Europe. The RPB1-based phylogenetic reconstruction using newly reported sequences and available data from other invertebrates identified 39 haplogroups being associated with amphipods. These haplogroups clustered into five clades (A-E) that did not form a single amphipod-infecting monophyletic group. Closely related sister clades C and D correspond to Nosema granulosis. Clades A, B and E might represent unknown Nosema species infecting amphipods. Host specificity seemed to be variable with some clades being restricted to single hosts, and some that could be found in several host species. We show that Nosema parasite richness in gammarid hosts is much higher than expected, illustrating the advantage of the use of RPB1 marker over SSU. Finally, we found no hint of sex ratio distortion in Nosema clade A infecting G. balcanicus. This study shows that Nosema spp. are abundant, widespread and diverse in European gammarids. Thus, Nosema is as diverse in aquatic as in terrestrial hosts.

Possible seasonal and diurnal modulation of Gammarus pulex (Crustacea, Amphipoda) drift by microsporidian parasites

In lotic freshwater ecosystems, the drift or downstream movement of animals (e.g., macroinvertebrates) constitutes a key dispersal pathway, thus shaping ecological and evolutionary patterns. There is evidence that macroinvertebrate drift may be modulated by parasites. However, most studies on parasite modulation of host drifting behavior have focused on acanthocephalans, whereas other parasites, such as microsporidians, have been largely neglected. This study provides new insight into possible seasonal and diurnal modulation of amphipod (Crustacea: Gammaridae) drift by microsporidian parasites. Three 72 h drift experiments were deployed in a German lowland stream in October 2021, April, and July 2022. The prevalence and composition of ten microsporidian parasites in Gammarus pulex clade E varied seasonally, diurnally, and between drifting and stationary specimens of G. pulex. Prevalence was generally higher in drifting amphipods than in stationary ones, mainly due to differences in host size. However, for two parasites, the prevalence in drift samples was highest during daytime suggesting changes in host phototaxis likely related to the parasite's mode of transmission and site of infection. Alterations in drifting behavior may have important implications for G. pulex population dynamics and microsporidians' dispersal. The underlying mechanisms are more complex than previously thought.

Cambaraspora faxoni n. sp. (Microsporidia: Glugeida) from native and invasive crayfish in the USA and a novel host of Cambaraspora floridanus

Crayfishes are among the most widely introduced freshwater taxa and can have extensive ecological impacts. Knowledge of the parasites crayfish harbor is limited, yet co-invasion of parasites is a significant risk associated with invasions. In this study, we describe a novel microsporidium, Cambaraspora faxoni n. sp. (Glugeida: Tuzetiidae), from two crayfish hosts in the Midwest USA, Faxonius virilis and Faxonius rusticus. We also expand the known host range of Cambaraspora floridanus to include Procambarus spiculifer. Cambaraspora faxoni infects muscle and heart tissue of F. rusticus and develops within a sporophorous vesicle. The mature spore measures 3.22 ± 0.14 μm in length and 1.45 ± 0.13 μm in width, with 8-9 turns of the polar filament. SSU sequencing indicates the isolates from F. virilis and F. rusticus were identical (100%) and 93.49% similar to C. floridanus, supporting the erection of a new species within the Cambaraspora genus. The novel parasite was discovered within the native range of F. rusticus (Ohio, USA) and within a native congeneric (F. virilis) in the invasive range of F. rusticus (Wisconsin, USA). Faxonius virilis is invasive in other regions. This new parasite could have been introduced to Wisconsin with F. rusticus or it may be a generalist species with a broad distribution. In either case, this parasite infects two crayfish species that have been widely introduced to new drainages throughout North America and could have future effects on invasion dynamics or impacts.

G lugea sp. infecting Sardinella aurita in Algeria

Parasitological examination of the commercially important pelagic fish Sardinella aurita Valenciennes, 1847 (Clupeidae) from the Eastern coast of Algeria revealed xenomas in the peritoneal cavity, suggesting a microsporidian infection. The prevalence of the disease was approximately 30% on average, higher in smaller individuals and showing significant seasonal variation. The xenomas contained numerous ellipsoidal spores, surrounded by a dense layer of connective tissue. Spore sizes were 6.10 ± 0.38 µm length and 3.54 ± 0.43 µm width. Ultrastructural examination by transmission electron microscopy showed various development stages of the parasite, including meronts, sporonts, sporoblasts and mature spores. The internal organization of the mature spores, with a single nucleus, prominent posterior vacuole, a lamellar polaroplast and an isofilar polar tube arranged in a single row, was typical of the genus Glugea. The DNA sequence of the small subunit ribosomal RNA gene confirmed that this parasite belongs to the genus Glugea. Genetic and morphologic comparison with G. sardinellensis, a species previously described in the same host from Tunisia shows many similarities, although some molecular and morphometric inconsistencies precluded the unambiguous assignment of our samples to G. sardinellensis. At the same time, we do not find sufficient grounds to erect a new taxon for our parasite. We discuss the implications of our findings for the current state of the systematics of Glugea.

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.

What doesn’t kill you doesn’t make you stronger: Parasites modify interference competition between two invasive amphipods

We used a freshwater amphipod-microsporidian model (Ponto-Caspian hosts: Dikerogammarus villosus and D. haemobaphes, parasite: Cucumispora dikerogammari) to check whether parasites affect biological invasions by modulating behaviour and intra- and interspecific interactions between the invaders. We tested competition for shelter in conspecific and heterospecific male pairs (one or both individuals infected or non-infected). In general, amphipods of both species increased their shelter occupancy time when accompanied by infected rather than non-infected conspecifics and heterospecifics. Infected amphipods faced lower aggression from non-infected conspecifics. Moreover, D. villosus was more aggressive than D. haemobaphes and more aggressive towards conspecifics vs. heterospecifics. In summary, infection reduced the intra- and interspecific competitivity of amphipods, which became less capable of defending their shelters, despite their unchanged need for shelter occupancy. Dikerogammarus haemobaphes, commonly considered as a weaker competitor, displaced by D. villosus from co-occupied locations, was able to compete efficiently for the shelter with D. villosus when microsporidian infections appeared on the scene. This suggests that parasites may be important mediators of biological invasions, facilitating the existence of large intra- and interspecific assemblages of invasive alien amphipods.

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.

Dictyocoela microsporidia diversity and co-diversification with their host, a gammarid species complex (Crustacea, Amphipoda) with an old history of divergence and high endemic diversity

BACKGROUND

Although the processes of co-evolution between parasites and their hosts are well known, evidence of co-speciation remains scarce. Microsporidian intracellular parasites, due to intimate relationships with their hosts and mixed mode of transmission (horizontal but also vertical, from mother to offspring), may represent an interesting biological model for investigating co-speciation. Amphipod crustaceans, especially gammarids, are regular hosts of microsporidian parasites, in particular the Dictyocoela spp., which have so far been found limited to these amphipods and are known to use a vertical mode of transmission. The amphipod genus Gammarus has a diversification history spanning the last 50-60 Mya and an extensive cryptic diversity in most of the nominal species. Here, we investigated the degree of co-diversification between Dictyocoela and Gammarus balcanicus, an amphipod with high degrees of ancient cryptic diversification and lineage endemism, by examining the genetic diversity of these parasites over the entire geographic range of the host. We hypothesised that the strong host diversification and vertical transmission of Dictyocoela would promote co-diversification.

RESULTS

Using the parasite SSU rDNA as a molecular marker, analyzing 2225 host specimens from 88 sites covering whole host range, we found 31 haplogroups of Dictyocoela, 30 of which were novel, belonging to four Dictyocoela species already known to infect other Gammarus spp. The relationships between Dictyocoela and gammarids is therefore ancient, with the speciation in parasites preceding those of the hosts. Each novel haplogroup was nevertheless specific to G. balcanicus, leaving the possibility for subsequent co-diversification process during host diversification. A Procrustean Approach to Co-phylogeny (PACo) analysis revealed that diversification of Dictyocoela was not random with respect to that of the host. We found high degrees of congruence between the diversification of G. balcanicus and that of Dictyocoela roeselum and D. muelleri. However, we also found some incongruences between host and Dictyocoela phylogenies, e.g. in D. duebenum, probably due to host shifts between different G. balcanicus cryptic lineages.

CONCLUSION

The evolutionary history of Dictyocoela and Gammarus balcanicus represents an example of an overall host-parasite co-diversification, including cases of host shifts.

Shared geographic histories and dispersal contribute to congruent phylogenies between amphipods and their microsporidian parasites at regional and global scales

In parasites that strongly rely on a host for dispersal, geographic barriers that act on the host will simultaneously influence parasite distribution as well. If their association persists over macroevolutionary time it may result in congruent phylogenetic and phylogeographic patterns due to shared geographic histories. Here, we investigated the level of congruent evolutionary history at a regional and global scale in a highly specialised parasite taxon infecting hosts with limited dispersal abilities: the microsporidians Dictyocoela spp. and their amphipod hosts. Dictyocoela can be transmitted both vertically and horizontally and is the most common microsporidian genus occurring in amphipods in Eurasia. However, little is known about its distribution elsewhere. We started by conducting molecular screening to detect microsporidian parasites in endemic amphipod species in New Zealand; based on phylogenetic analyses, we identified nine species-level microsporidian taxa including six belonging to Dictyocoela. With a distance-based cophylogenetic analysis at the regional scale, we identified overall congruent phylogenies between Paracalliope, the most common New Zealand freshwater amphipod taxon, and their Dictyocoela parasites. Also, hosts and parasites showed similar phylogeographic patterns suggesting shared biogeographic histories. Similarly, at a global scale, phylogenies of amphipod hosts and their Dictyocoela parasites showed broadly congruent phylogenies. The observed patterns may have resulted from covicariance and/or codispersal, suggesting that the intimate association between amphipods and Dictyocoela may have persisted over macroevolutionary time. We highlight that shared biogeographic histories could play a role in the codiversification of hosts and parasites at a macroevolutionary scale.

Pathogens of Dikerogammarus haemobaphes regulate host activity and survival, but also threaten native amphipod populations in the UK

Dikerogammarus haemobaphes is a non-native amphipod in UK freshwaters. Studies have identified this species as a low-impact invader in the UK, relative to its cousin Dikerogammarus villosus. It has been suggested that regulation by symbionts (such as Microsporidia) could explain this difference in impact. The effect of parasitism on D. haemobaphes is largely unknown. This was explored herein using 2 behavioural assays measuring activity and aggregation. First, D. haemobaphes were screened histologically post-assay, identifying 2 novel viruses (D. haemobaphes bi-facies-like virus [DhbflV], D. haemobaphes bacilliform virus [DhBV]), Cucumispora ornata (Microsporidia), Apicomplexa, and Digenea, which could alter host behaviour. DhBV infection burden increased host activity, and C. ornata infection reduced host activity. Second, native invertebrates were collected from the invasion site at Carlton Brook, UK, and tested for the presence of C. ornata. PCR screening identified that Gammarus pulex and other native invertebrates were positive for C. ornata. The host range of this parasite, and its impact on host survival, was additionally explored using D. haemobaphes, D. villosus, and G. pulex in a laboratory trial. D. haemobaphes and G. pulex became infected by C. ornata, which also lowered survival rate. D. villosus did not become infected. A PCR protocol for DhbflV was also applied to D. haemobaphes after the survival trial, associating this virus with decreased host survival. In conclusion, D. haemobaphes has a complex relationship with parasites in the UK environment. C. ornata likely regulates populations by decreasing host survival and activity, but despite this benefit, the parasite threatens susceptible native wildlife.

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.

Core and Dynamic Microbial Communities of Two Invasive Ascidians: Can Host-Symbiont Dynamics Plasticity Affect Invasion Capacity?

Ascidians (Chordata, Ascidiacea) are considered to be prominent marine invaders, able to tolerate highly polluted environments and fluctuations in salinity and temperature. Here, we examined the seasonal and spatial dynamics of the microbial communities in the inner-tunic of two invasive ascidians, Styela plicata (Lesueur 1823) and Herdmania momus (Savigny 1816), in order to investigate the changes that occur in the microbiome of non-indigenous ascidians in different environments. Microbial communities were characterized using next-generation sequencing of partial (V4) 16S rRNA gene sequences. A clear differentiation between the ascidian-associated microbiome and bacterioplankton was observed, and two distinct sets of operational taxonomic units (OTUs), one core and the other dynamic, were recovered from both species. The relative abundance of the dynamic OTUs in H. momus was higher than in S. plicata, for which core OTU structure was maintained independently of location. Ten and seventeen core OTUs were identified in S. plicata and H. momus, respectively, including taxa with reported capabilities of carbon fixing, ammonia oxidization, denitrification, and heavy-metal processing. The ascidian-sourced dynamic OTUs clustered in response to site and season but significantly differed from the bacterioplankton community structure. These findings suggest that the associations between invasive ascidians and their symbionts may enhance host functionality while maintaining host adaptability to changing environmental conditions.

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.

Reintroduction of freshwater macroinvertebrates: challenges and opportunities

Species reintroductions - the translocation of individuals to areas in which a species has been extirpated with the aim of re-establishing a self-sustaining population - have become a widespread practice in conservation biology. Reintroduction projects have tended to focus on terrestrial vertebrates and, to a lesser extent, fishes. Much less effort has been devoted to the reintroduction of invertebrates into restored freshwater habitats. Yet, reintroductions may improve restoration outcomes in regions where impoverished regional species pools limit the self-recolonisation of restored freshwaters. We review the available literature on macroinvertebrate reintroductions, focusing on identifying the intrinsic and extrinsic factors that determine their success or failure. Our study reveals that freshwater macroinvertebrate reintroductions remain rare, are often published in the grey literature and, of the attempts made, approximately one-third fail. We identify life-cycle complexity and remaining stressors as the two factors most likely to affect reintroduction success, illustrating the unique challenges of freshwater macroinvertebrate reintroductions. Consideration of these factors by managers during the planning process and proper documentation - even if a project fails - may increase the likelihood of successful outcomes in future reintroduction attempts of freshwater macroinvertebrates.

A new microsporidium Fibrillaspora daphniae g. n. sp. n. infecting Daphnia magna (Crustacea: Cladocera) in Siberia and its taxonomic placing within a new family Fibrillasporidae and new superfamily Tubulinosematoidea (Opisthosporidia: Microsporidia)

Infection with a new microsporidium, Fibrillaspora daphniae g. n. sp. n., was found in a local Daphnia magna population in Tomsk region (Western Siberia, Russia) at the prevalence rate of 52%. Histological sections showed parasite cells entirely encompassing the host haemocoel. Methanol-fixed spores were elongate, oval, 4.8 ± 0.3 μm × 2.3 ± 0.2 μm in size. All developmental stages were in direct contact with the host cell cytoplasm, with single nuclei, and division by binary fission. The sporont surface was covered with an additional outer layer composed of fine tubules. The spores possessed a thick endospore, large posterior vacuole filled with electron-dense granules, and a bipartite polaroplast composed of anterior lamellar and posterior globular elements. The polar tube was slightly anisofilar, with 13-19 coils arranged in one row; the two posterior coils were of lesser diameter. The small subunit ribosomal RNA gene sequence was deposited in Genbank under accession # MF278272. Considering the sister relationship between Fibrillanosema crangonycis and our new isolate described here as Fibrillaspora daphniae, we propose a new family Fibrillasporidae fam. n. to contain these two genera and the descendants of their common ancestor. A new superfamily Tubulinosematoidea superfam. n. is proposed as a monophyletic assemblage of Fibrillasporidae fam. n. and Tubulinosematidae.

Pathological analysis of silkworm infected by two microsporidia Nosema bombycis CQ1 and Vairimorpha necatrix BM

Microsporidia Nosema bombycis CQ1 can be vertically transmitted in silkworm Bombyx mori but Vairimorpha necatrix BM cannot. Therefore, the pathological differences in silkworm infected with these two microsporidia required clarification. Here, we compared the virulence of N. bombycis CQ1 and V. necatrix BM against silkworm. The pathological characteristics in intestine, testis and ovary were surveyed using paraffin sections, scanning electron microscopy and transmission electron microscopy. Our data firstly showed that the virulence of V. necatrix BM was weaker than that of N. bombycis CQ1. Secondly, the typical symptom of V. necatrix BM infection is making xenomas, which are full of pathogens in different stages, at the posterior of intestine. However, no xenomas were formed surrounding intestines infected with N. bombycis CQ1. Thirdly, N. bombycis CQ1 can cluster spores near the trachea while infecting ovaries. It is worth noting that N. bombycis CQ1 infected epithelial cells and connective tissues of ovaries, while V. necatrix BM did not. Although silkworm ovaries can not be infected by V. necatrix BM in vivo, it can infect embryonic and ovarian cell lines in vitro. This study is the first report about comparing infection features of N. bombycis CQ1 and V. necatrix BM in silkworm tissues and it provided elaborate and visual information of pathological characteristics which can help to explain the different transmission strategies of these two microsporidia.

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.

Intragenomic conflict produces sex ratio dynamics that favor maternal sex ratio distorters

Maternal sex ratio distorters (MSDs) are selfish elements that enhance their transmission by biasing their host's sex allocation in favor of females. While previous models have predicted that the female-biased populations resulting from sex ratio distortion can benefit from enhanced productivity, these models neglect Fisherian selection for nuclear suppressors, an unrealistic assumption in most systems. We used individual-based computer simulation modeling to explore the intragenomic conflict between sex ratio distorters and their suppressors and explored the impacts of these dynamics on population-level competition between species characterized by MSDs and those lacking them. The conflict between distorters and suppressors was capable of producing large cyclical fluctuations in the population sex ratio and reproductive rate. Despite fitness costs associated with the distorters and suppressors, MSD populations often exhibited enhanced productivity and outcompeted non-MSD populations in single and multiple-population competition simulations. Notably, the conflict itself is beneficial to the success of populations, as sex ratio oscillations limit the competitive deficits associated with prolonged periods of male rarity. Although intragenomic conflict has been historically viewed as deleterious to populations, our results suggest that distorter-suppressor conflict can provide population-level advantages, potentially helping to explain the persistence of sex ratio distorters in a range of taxa.

The gill microbiota of invasive and indigenous Spondylus oysters from the Mediterranean Sea and northern Red Sea

The gill tissue of bivalve mollusks hosts rich symbiotic microbial communities that may contribute to the animal's metabolism. Spondylus spinosus is an invasive oyster that has become highly abundant along the eastern Mediterranean Sea (EMS) coastline, but is scarce in the northern Red Sea (NRS), its indigenous region. The composition and seasonal dynamics of the gill microbial communities of S. spinosus were examined in both regions, using 16S rRNA gene amplicon sequencing. Additionally, two Red Sea Spondylus species, S. avramsingeri and S. pickeringae, were investigated using the same approach. Significant differences were found between microbial communities of the EMS S. spinosus and the three NRS species. Bacteria from the family Hahellaceae dominated the communities of the EMS S. spinosus and the NRS S. avramsingeri, oysters that are dominant in their habitat, yet were rare in the NRS S. spinosus and S. pickeringae, which are only seldom encountered. Bacterial communities of EMS S. spinosus were more similar to those of NRS S. spinosus than to those of other NRS Spondylus species, indicating that either part of the microbiota had co-invaded with their host into the Mediterranean Sea, or that there are species-specific selective constraints on microbial composition.

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.

Biology of a new xenoma-forming gonadotropic microsporidium in the invasive blotchfin dragonet Callionymus filamentosus

A gonadotropic microsporidian parasite, Obruspora papernae gen. et sp. nov. (Microsporidia: Enterocytozoonidae), is described from Callionymus filamentosus (Teleostei: Callionymidae) in the Mediterranean Sea. The host, a Red Sea invasive species which entered the Mediterranean through the Suez Canal, was first collected in the Levant Basin in 1953, whereas its parasite went unobserved until 2008. Analysis of partial small subunit ribosomal gene sequences (SSU rDNA) placed the new species within the Nucleospora, Desmozoon, and Paranucleospora clade, and as it differs from each of them, it is assigned to a new genus. The development of the parasite is described, and the biological mechanisms underlying this parasite-host system are analyzed. Prevalence of infection approached 80% in female samples throughout most of the year. Males showed no signs of infection, but parasite rDNA was detected in male internal organs. The parasite-induced xenomas progressively occupied and eventually replaced much of the ovary, in some cases producing effective castration. Despite high levels of parasite infection, current trawl fishery statistics indicate that the abundance of Mediterranean populations of the host remains high. The parasite impact on the host population dynamics is unclear. Possible effects of the new microsporidian parasite on the reproductive effort of C. filamentosus and the potential role of another parasite, the ectoparasitic copepod Lernanthropus callionymicola, as an additional host in the life cycle of O. papernae, require further investigation.

Bacterial symbionts in insects or the story of communities affecting communities

Bacterial symbionts are widespread in insects and other animals. Most of them are predominantly vertically transmitted, along with their hosts' genes, and thus extend the heritable genetic variation present in one species. These passengers have a variety of repercussions on the host's phenotypes: besides the cost imposed on the host for maintaining the symbiont population, they can provide fitness advantages to the host or manipulate the host's reproduction. We argue that insect symbioses are ideal model systems for community genetics. First, bacterial symbionts directly or indirectly affect the interactions with other species within a community. Examples include their involvement in modifying the use of host plants by phytophagous insects, in providing resistance to natural enemies, but also in reducing the global genetic diversity or gene flow between populations within some species. Second, one emerging picture in insect symbioses is that many species are simultaneously infected with more than one symbiont, which permits studying the factors that shape bacterial communities; for example, horizontal transmission, interactions between host genotype, symbiont genotype and the environment and interactions among symbionts. One conclusion is that insects' symbiotic complements are dynamic communities that affect and are affected by the communities in which they are embedded.

Genetic diversity of the feminising microsporidian parasite Dictyocoela: new insights into host-specificity, sex and phylogeography

Microsporidia of the genus Dictyocoela are parasites of gammarid amphipod Crustacea. They typically exhibit low virulence and efficient vertical transmission and at least some strains are capable of feminising their hosts. Sequencing of a region of the 16S rDNA of Dictyocoela spp. from various gammarid host species and localities in Europe and northern Asia indicates that Dictyocoela is genetically diverse and that different strains predominate in different host species. However, the presence of intermediate sequences casts doubt upon previous attempts to describe Dictyocoela spp. on the basis of rDNA divergence alone. Phylogenetic analysis provides little support for coevolution between gammarids and Dictyocoela. Furthermore, observations of heavily infected individuals, together with genetic evidence of recombination, suggest that some strains of Dictyocoela may be horizontally transmitted and are sexually reproducing. These findings suggest that Dictyocoela may be phenotypically, as well as genotypically, diverse, with the potential to exhibit a range of different interactions with its host populations.

Composition and dynamics of the gill microbiota of an invasive Indo-Pacific oyster in the eastern Mediterranean Sea

Gill bacterial communities of Chama pacifica, an Indo-Pacific invasive oyster to the eastern Mediterranean Sea, were compared with those of Chama savignyi, its northern Red Sea congeneric species. Summer and winter bacterial populations were characterized and compared using 16S rDNA clone libraries, and seasonal population dynamics were monitored by automated ribosomal intergenic spacer analysis (ARISA). Clone libraries revealed a specific clade of bacteria, closely related to marine endosymbionts from the Indo-Pacific, found in both ecosystems, of which one taxon was conserved in oysters from both sites. This taxon was dominant in summer libraries and was weakly present in winter ones, where other members of this group were dominant. ARISA results revealed significant seasonal variation in bacterial populations of Mediterranean Sea oysters, as opposed to Red Sea ones that were stable throughout the year. We suggest that this conserved association between bacteria and oyster reflects either a symbiosis between the oyster host and some of its bacteria, a co-invasion of both parties, or both.

The ecology and evolution of microsporidian parasites

The phylum Microspora is ancient and diverse and affects a wide range of hosts. There is unusually high use of vertical transmission and this has significant consequences for transmission and pathogenicity. Vertical transmission is associated with low pathogenesis but nevertheless can have significant impact through associated traits such as sex ratio distortion. The majority of microsporidia have mixed transmission cycles and it is not clear whether they are able to modify their phenotype according to environmental circumstances. There is a great need to understand the mechanisms controlling transmission and one of the first challenges for the genomics era is to find genes associated with life cycle stages. Similarly we cannot currently predict the ease with which these parasites might switch between host groups. Phylogenetic analysis suggests that there are strong relationships between Microsporidia and their hosts. However closer typing of parasite isolates, in relation to host range and disease phenotype, is required to assess future environmental risk from these pathogens.

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.

Infection by a vertically-transmitted microsporidian parasite is associated with a female-biased sex ratio and survival advantage in the amphipod Gammarus roeseli

Vertically transmitted parasites may have positive, neutral or negative effects on host fitness, and are also predicted to exhibit sex-specific virulence to increase the proportion or fitness of the transmitting sex. We investigated these predictions in a study on the survival and sex ratio of offspring of the amphipod Gammarus roeseli from females infected by the vertically transmitted microsporidia Nosema granulosis. We found, to our knowledge, the first evidence for a positive relationship between N. granulosis infection and host survival. Infection was associated with sex ratio distortion, not by male-killing, but probably by parasite-induced feminization of putative G. roeseli males. This microsporidia also feminizes another amphipod host, Gammarus duebeni, which is phylogenetically and biogeographically distant from G. roeseli. Our study suggests that the reproductive system of gammarids is easily exploited by these vertically-transmitted parasites, although the effects of infections on host fitness may depend on specific host-parasite species interactions.

Multiple losses of sex within a single genus of Microsporidia

Background

Most asexual eukaryotic lineages have arisen recently from sexual ancestors and contain few ecologically distinct species, providing evidence for long-term advantages of sex. Ancient asexual lineages provide rare exceptions to this rule and so can yield valuable information relating to the evolutionary forces underlying the maintenance of sex. Microsporidia are parasitic, unicellular fungi. They include many asexual species which have traditionally been grouped together into large, presumably ancient taxonomic groups. However, these putative ancient asexual lineages have been identified on the basis of morphology, life cycles and small subunit ribosomal RNA (16S rRNA) gene sequences, all of which hold questionable value in accurately inferring phylogenetic relationships among microsporidia.

Results

The hypothesis of a single, ancient loss of sex within the Nosema/Vairimorpha group of microsporidia was tested using phylogenetic analyses based on alignments of rRNA and RPB1 gene sequences from sexual and asexual species. Neither set of gene trees supported ancient asexuality, instead indicating at least two, recent losses of sex.

Conclusion

Sex has been lost on multiple, independent occasions within the Nosema/Vairimorpha group of microsporidia and there is no evidence for ancient asexual lineages. It appears therefore that sex confers important long-term advantages even upon highly simplified eukaryotes such as microsporidia. The rapid evolution of microsporidian life cycles indicated by this study also suggests that even closely related microsporidia cannot be assumed to have similar life cycles and the life cycle of each newly discovered species must therefore be completely described. These findings are relevant to the use of microsporidia as biological control agents, since several species under consideration as potential agents have life cycles that have been incompletely described.

Morphological and Molecular Investigations of Tubulinosema ratisbonensis gen. nov., sp. nov. (Microsporidia: Tubulinosematidae fam. nov.), a Parasite Infecting a Laboratory Colony of Drosophila melanogaster (Diptera: Drosophilidae)

A new species of microsporidia from Drosophila melanogaster was investigated by light and electron microscopy and by ribosomal RNA (rRNA) sequencing. This microsporidium and the previously described Nosema kingi and Nosema acridophagus have been transferred to the new genus Tubulinosema gen. nov. with the following characters: nuclei are in diplokaryotic arrangement during the life cycle. All stages are in direct contact with the host cell cytoplasm, slightly anisofilar polar tube with the last coils being smaller in diameter arranged in one or two rows on both sides of the diplokaryon and small tubuli on the surface of late meronts. Spores are oval or slightly pyriform. Thick endospore wall, thinner over anchoring disc. This new genus and the genus Brachiola have been placed in a new family Tubulinosematidae fam. nov. Phylogenetic analysis of small subunit rRNA sequences by different methods placed Tubulinosema spp. in one clade with the genus Brachiola forming its sister clade, which is distant from the clade containing the true Nosema spp. including Nosema bombycis.

Coexistence of three microsporidia parasites in populations of the freshwater amphipod Gammarus roeseli: evidence for vertical transmission and positive effect on reproduction

We investigated the prevalence, transmission mode and fitness effects of infections by obligatory intracellular, microsporidian parasites in the freshwater amphipod Gammarus roeseli. We found three different microsporidia species in this host, all using transovarial (vertical) transmission. All three coexist at different prevalences in two host populations, but bi-infected individuals were rarely found, suggesting no (or very little) horizontal transmission. It is predicted that vertically-transmitted parasites may exhibit sex-specific virulence in their hosts, or they may have either positive or neutral effects on host fitness. All three species differed in their transmission efficiency and infection intensity and our data suggest that these microsporidia exert sex-specific virulence by feminising male hosts. The patterns of infection we found exhibit convergent evolution with those of another amphipod host, Gammarus duebeni. Interestingly, we found that infected females breed earlier in the reproductive season than uninfected females. This is the first study, to our knowledge, to report a positive effect of microsporidian infection on female host reproduction.

Roles of parasites in animal invasions

Biological invasions are global threats to biodiversity and parasites might play a role in determining invasion outcomes. Transmission of parasites from invading to native species can occur, aiding the invasion process, whilst the 'release' of invaders from parasites can also facilitate invasions. Parasites might also have indirect effects on the outcomes of invasions by mediating a range of competitive and predatory interactions among native and invading species. Although pathogen outbreaks can cause catastrophic species loss with knock-on effects for community structure, it is less clear what impact persistent, sub-lethal parasitism has on native-invader interactions and community structure. Here, we show that the influence of parasitism on the outcomes of animal invasions is more subtle and wide ranging than has been previously realized.