Small subunit ribosomal RNA sequence of Henneguya exilis (class Myxosporea) identifies the actinosporean stage from an oligochaete host

Parvicapsula pseudobranchicola in the northeast Pacific Ocean is rare in farmed Atlantic salmon Salmo salar despite widespread occurrence and pathology in wild Pacific salmon Oncorhynchus spp

BACKGROUND

Infection with the myxozoan parasite Parvicapsula pseudobranchicola causes disease in wild and farmed salmonids in Norway. In the northeast Pacific Ocean, the parasite has been reported in Pacific salmon Oncorhynchus spp. without evidence of disease. The objectives of the present study were to confirm the identity of P. pseudobranchicola in the Pacific, document its host and geographic ranges, and describe associated pathological changes.

METHODS

Ocean-entry year wild pink salmon Oncorhynchus gorbuscha, chum salmon O. keta, Chinook salmon O. tshawytscha, coho salmon O. kisutch and sockeye salmon O. nerka were collected in summer and autumn surveys near Vancouver Island (VI) and from a winter survey in the Gulf of Alaska. Samples were also obtained from farmed Atlantic salmon Salmo salar and Chinook salmon near VI. Samples were analysed by qPCR and histology using conventional staining or in situ hybridisation. Parasite sequence was obtained from small subunit ribosomal RNA gene (SSU rDNA).

RESULTS

Identical 1525 base-pair SSU rDNA sequences from infected pink salmon, chum salmon and Chinook salmon shared 99.93% identity with a P. pseudobranchicola sequence from Norwegian Atlantic salmon. In autumn surveys, the prevalence was greatest in chum salmon (91.8%) and pink salmon (85.9%) and less so in Chinook salmon (68.8%) and sockeye salmon (8.3%). In farmed salmon, the prevalence was zero in Atlantic salmon (n = 967) and 41% in Chinook salmon (n = 118). Infections were preferentially sited in pseudobranch and visualised by in situ hybridisation. Heavy parasite burdens in all species of Pacific salmon were inconsistently associated with focal granulomatous pseudobranchitis.

CONCLUSIONS

In the northeast Pacific, widespread occurrence of P. pseudobranchicola in Pacific salmon together with its absence or sporadic occurrence in farmed Atlantic salmon differs from its epidemiology in Norway, despite similar pathological development in the pseudobranch. Consequences of the infections to the health of wild Pacific salmon, identity of the invertebrate host and the distribution and abundance of infective actinospores are unknown and remain high priorities for research.

Synopsis of the aurantiactinomyxon collective group (Cnidaria, Myxozoa), with a discussion on the validity of morphotype definition and demise of guyenotia

Aurantiactinomyxon is one of the most diverse myxozoan collective groups, comprising types that mostly infect freshwater and marine oligochaetes belonging to the family Naididae Ehrenberg, 1828, but also Lumbriculidae Claus, 1872. In this study, a comprehensive revision of all known aurantiactinomyxon types is performed and highlights the fallibility of using the form and length of the valvular processes as main criterion for differentiating among style-less actinospore morphotypes. The demise of the guyenotia collective group is proposed based on the ambiguous features of several types that allow conformity with both the aurantiactinomyxon and guyenotia definitions. Nonetheless, the information presently available clearly shows that a general shift is needed in our approach to actinospore grouping, which should probably be based on actinospore functionality relative to environment and host ecology, rather than on morphology. Life cycle studies based on experimental transmission and molecular inferences of the 18S rDNA have linked aurantiactinomyxon (including former guyenotia) to myxozoans belonging to a diverse array of genera, including Chloromyxum, Henneguya, Hoferellus, Myxobolus, Paramyxidium, Thelohanellus and Zschokkella. This undoubtedly shows a high capacity of the aurantiactinomyxon morphotype to promote infection in intrinsically distinct vertebrate hosts and environmental habitats, consequently increasing interest in its study for attaining a better understanding of myxozoan-host interactions. The identification of novel and known types, however, is impeded by the lack of concise information allowing a comprehensive analysis of biological, morphological, and molecular criteria. In this sense, the compilation of data presented in this study will ultimately help researchers seeking to perform reliable identifications.

Synopsis of the species of Henneguya Thélohan, 1892 (Cnidaria: Myxosporea: Myxobolidae) described since 2012

The genus Henneguya Thélohan, 1892 (Cnidaria: Myxosporea: Myxobolidae) encompasses a large number of species that mostly infect freshwater fish belonging to 71 families of Actinopterygii. A synopsis of Henneguya species described between 2012 and 2022 is herein presented. It includes 57 species described during the last decade, and one species missing from the previous synopses, adding to a total of 254 species that have been formally described within this genus. Biological characters and myxospore morphometry are presented for each species record.

New data on Henneguya postexilis Minchew, 1977, a parasite of channel catfish Ictalurus punctatus, with notes on resolution of molecular markers for myxozoan phylogeny

Previous morphological and histological data are supplemented with molecular and ultrastructural data for a Henneguya sp. isolated from farm-raised channel catfish Ictalurus punctatus in Mississippi, USA. Myxospores were cryptic, encapsulated within a thin layer of epithelium in the gill lamellae with spore measurements consistent with the original description of Henneguya postexilis Minchew, 1977. Myxospores were 42.7-49.1 µm in total length with spore bodies 12.1-17.2 × 3.6-4.8 × 2.9-3 µm. Polar capsules were of unequal length, with the longer capsule being 4.4-6.7 × 1.1-1.6 µm and the shorter capsule being 4.4-6.4 × 1.1-1.6 µm. Polar tubules had 6-8 turns. Caudal processes were 25.7-38.1 µm in length. Spores were encapsulated in a thin layer of epithelium in the gill lamellae. Molecular data from the most commonly used markers for myxozoan identification and phylogeny, partial 18S small subunit ribosomal gene (SSU), partial 28S large subunit ribosomal gene (LSU), and elongation factor 2 (EF2) were generated for H. postexilis. Additionally, novel data for LSU and EF2 were generated for archived myxozoan specimens from farm-raised catfish (H. mississippiensis, H. ictaluri, H. exilis, H. adiposa, H. sutherlandi, H. bulbosus, Unicauda fimbrethilae), as well as archived specimens from wild fish (H. laseeae [from Pylodictis olivaris], Hennegoides flockae [from Aphredoderus sayanus], Myxobolus cloutmani [from Cycleptus elongatus]. These include the first EF2 sequence data for the genera Hennegoides and Unicauda. Phylogenetic analyses using these data placed H. postexilis in well supported clades with other ictalurid-infecting Henneguya species. Phylogenetic signal assessments on these analyses suggest that while SSU provided the greatest phylogenetic signal, LSU yielded comparable signal, supporting previous work implying this region may be underutilised in reconstructing myxobolid phylogenies.

Characterisation of myxozoan fauna of western mosquitofish, Gambusia affinis (Baird and Gerard) (Cyprinodontiformes: Poeciliidae), inhabiting experimental catfish ponds in Mississippi, USA

Characterising myxozoan taxa parasitising fish hosts in catfish aquaculture ponds is crucial to understanding myxozoan community dynamics in these diverse and complex ecological systems. This work investigated the myxozoan fauna of the western mosquitofish, Gambusia affinis, a common, incidental species found in catfish aquaculture ponds in the southeastern United States. 598 fish were sampled in May of 2018 and 2019 from the pond facility of the Thad Cochran National Warmwater Aquaculture Center in Stoneville, Mississippi, USA. Fish were examined microscopically using wet mount preparations of fresh tissue and histology for myxozoans. 18S rRNA gene sequences were amplified from myxospores obtained at necropsy. Updated morphologic, histologic, and 18S rRNA gene sequence features are provided for Henneguya gambusi, Myxobolus pharyngeus, and Myxidium phyllium. Two potentially novel myxozoans were observed during this survey, an undocumented Myxobolus sp. associated with chondrolysis of bones throughout the body and a putative Myxobilatus sp. observed histologically in the renal tubules, ureters, and urinary bladder. However, inadequate samples were obtained for proper species descriptions. Lastly, the life cycle of M. pharyngeus, which is thought to utilize the oligochaete worm Dero digitata as their definitive host, was putatively confirmed by 18S rRNA sequence matching to actinospore stages from oligochaetes in catfish ponds in Mississippi. This work provides novel and expanded morphologic, histologic, molecular and biologic data of five myxozoan parasites of G. affinis, expanding our knowledge of myxozoan diversity in catfish aquaculture ponds.

Molecular data infers the involvement of a marine aurantiactinomyxon in the life cycle of the myxosporean parasite Paramyxidium giardi (Cnidaria, Myxozoa)

An aurantiactinomyxon type is described from the marine naidid Tubificoides pseudogaster (Dahl, 1960), collected from the lower estuary of a Northern Portuguese River. This type constitutes the first of its collective group to be reported from Portugal, and only the fourth described from a marine oligochaete worldwide. Extensive morphological comparisons of new aurantiactinomyxon isolates to all known types without available molecular data are proposed to be unnecessary, given the artificiality of the usage of morphological criteria for actinosporean differentiation and the apparent strict host specificity of the group. Recognition of naidid oligochaetes as the hosts of choice for marine types of aurantiactinomyxon and other collective groups, suggests that the family Naididae played a preponderant role in the myxosporean colonization of estuarine communities. Molecular analyses of the type in study further infer its involvement in the life cycle of Paramyxidium giardi (Cépède, 1906) Freeman and Kristmundsson, 2018, a species that infects the kidney of European eel Anguilla anguilla (Linnaeus, 1758) and that has been reported globally, including from Portuguese waters. The low intraspecific difference registered in relation to Icelandic isolates of P. giardi (0.6%) is hypothesized to result from the emergence of genotypically different subspecies due to geographic isolation.

Molecular confirmation of Henneguya adiposa (Cnidaria: Myxozoa) and associated histologic changes in adipose fins of channel catfish, Ictalurus punctatus (Teleost)

Henneguya adiposa is one of ten known, closely related myxozoan species that parasitize a variety of tissue sites in the channel catfish, Ictalurus punctatus. Reported to specifically target the adipose fin, H. adiposa is not associated with morbidity or mortality, although detailed descriptions of its associated histologic pathology are lacking. The objective of this work was to confirm the presence of H. adiposa within fin lesions of affected channel catfish using DNA sequenced from histologic sections obtained by laser capture microdissection, as well as to describe pathologic changes induced by infection. The parasite formed large, white, elongate, nodular plasmodia that caused localized tissue damage and incited a granulomatous inflammatory response within a deep connective tissue layer at the base of the adipose fin. Myxospores released from ruptured plasmodia into adjacent tissue were observed to migrate superficially in tracts through the skin, indicating a portal of exit for environmental dispersal. Defects in the connective tissue layer created by ruptured plasmodia were infiltrated by granulomatous inflammation and fibroplasia, suggesting lesion resolution by scar formation over time. Sequencing of the 18S rRNA gene amplified from excised myxospores confirmed the myxozoan's identity as H. adiposa, with 100% similarity to the reference sequence from previous published work.

A new species of Myxosporea, Henneguya quelen, from silver catfish Rhamdia quelen (Siluriforme: Pimelodidae) in the Amazonian region

Henneguya quelen n. sp. (Myxosporea) is described from the kidney of the silver catfish, Rhamdia quelen (Quoy & Gaimard, 1824), from Marajó Island. In all, 40% of the 57 silver catfish collected from the Paracauari River in the municipality of Salvaterra presented cysts in the kidney. The cysts were whitish, spherical, and 0.45 mm in diameter. The histological analyses revealed tubular renal compression and cellular degeneration in the area adjacent to the xenomas. The spores are ellipsoid, with well-marked valves, caudal projection, and two elongated polar capsules of equal size. The mature spores measure approximately 15.6 μm in length, with the caudal appendage split posteriorly; 24.3 μm in length, for a total spore length of 40.0 μm; and a width of 4.1 μm. The polar capsules are pyriform, 5.5 μm long and 1.7 μm wide. A phylogenetic analysis based on Bayesian inference confirmed that the specimens represented a new species, which was denominated Henneguya quelen n. sp.

New host record and molecular characterization of Dicauda atherinoidi Hoffman & Walker (Bivalvulida: Myxobolidae): a parasite of the emerald shiner Notropis atherinoides Rafinesque, 1818 and mimic shiner Notropis vollucellus Cope, 1865

Updated morphological and histopathological descriptions for Dicauda atherinoidi (Bivalvulida:Myxobolidae) and an expanded host range are supplemented with the first molecular data and phylogenetic analyses of the genus. Plasmodia were located on the head, ventrum/body and fins of infected emerald shiner Notropis atherinoides Rafinesque, 1818 and mimic shiner Notropis vollucellus Cope, 1865, a new host species. Myxospores were spherical, ranging 9.3-11.4 μm (10.5 ± 0.4) in length, 9.0-11.0 μm (9.7 ± 0.4) in width and 6.6-7.0 μm (6.8 ± 0.2) thick in sutural view, and possessed 2-3 caudal processes (5.3-68.3 μm, 31.1 ± 13.6) connected to the spore body at the sutural groove, all of which are consistent with the genus Dicauda. In the absence of available Dicauda sequence data, the 18S rDNA sequences from Michigan isolates were most similar to Myxobolus spp. Phylogenetic analyses clustered these isolates with myxobolid species from cyprinid fish, suggesting these parasites may represent an underpopulated group of cyprinid-infecting myxozoans. Histopathology revealed thin-walled plasmodial pseudocysts in the dermis and associated connective tissue, where granulomatous inflammation and focal scale atrophy were also present. Further sampling/sequencing of myxobolids from Notropis spp. should expand these underrepresented myxozoans and offer further insight into Myxobolidae host family tropisms.

New morphological data and molecular diagnostic of Henneguya friderici (Myxozoa: Myxobolidae), a parasite of Leporinus friderici (Osteichthyes: Anostomidae) from southeastern Brazil

The myxozoan Henneguya friderici is a parasite of the gills, intestine, kidney and liver of Leporinus friderici, a characiform fish belonging to the family Anostomidae. Forty-two specimens of L. friderici that had been caught in the Mogi Guaçú River, state of São Paulo, were studied. Elongated white plasmodia were found in the gill filaments of 10 host specimens (24%). The mature spores had an ellipsoidal body with polar capsules of equal size and caudal length greater than body length. This study also described 18S rDNA sequencing of H. friderici infecting the gill filaments. This produced a sequence of 1050 bp that demonstrated significant genetic differences with previously described species of Henneguya. Similarity analysis using sequences from species that clustered closest to those produced by this study showed that the species with greatest genetic similarity to H. friderici was H. leporinicola, with 94% similarity.

Henneguya laseeae n. sp. from flathead catfish (Pylodictis olivaris) in the upper Mississippi River

A novel species of Henneguya was isolated from flathead catfish (Pylodictis olivaris) captured in the upper Mississippi River near Lansing (Allamakee County), IA, and La Crosse (La Crosse County), WI. Designated Henneguya laseeae n. sp., this novel species is described using critical morphological features, histology, and 18S ribosomal RNA gene sequence. Ovoid cysts, ranging from 1200 to 1800 μm in width, tended to be at filament tips or in the distal third, often directly on the filament midline, but occasionally paramedian. Lanceolate-shaped myxospores were consistent with those of the genus Henneguya. The spore body was 16.2 ± 0.5 μm (mean ± standard deviation; range = 15.1-17.0 μm) in length, 6.0 ± 0.4 μm (5.1-6.6 μm) in width, and 4.7 ± 0.2 μm (4.4-4.9 μm) thick. The two polar capsules at the anterior of the spore body were 5.9 ± 0.3 μm (5.3-6.3 μm) in length and 1.8 ± 0.1 μm (1.6-2.1 μm) in width and contained six to seven turns in the polar filament. The caudal processes tapered to fine points and were 54.3 ± 2.9 μm (49.1-61.7 μm) in length. Total spore length was 70.4 ± 3.3 μm (64.5-79.4 μm). The spores and plasmodium of this species are of similar size and morphology to other species of Henneguya from ictalurid fishes. Additionally, the 18S rRNA gene sequences placed this isolate within a clade populated by Henneguya spp. from North American ictalurids. This is the first reported species of Henneguya from flathead catfish.

Specific PCR for Myxobolus arcticus SSU rDNA in juvenile sockeye salmon Oncorhynchus nerka from British Columbia, Canada

A PCR for the specific detection of the salmon brain parasite Myxobolus arcticus (Pugachev and Khokhlov, 1979) was developed using primers designed to amplify a 1363 base pair fragment of the small subunit rDNA. The assay did not amplify DNA from 5 other Myxobolus species or from 7 other myxozoan species belonging to 5 other genera. For juvenile sockeye salmon Oncorhynchus nerka (Walbaum) collected from Chilko Lake, British Columbia (BC), Canada, in 2011, the prevalence by PCR was 96%, in contrast to 71% by histological examination of brain tissue. In 2010, the histological prevalence was 52.5%. Sequence identity between M. arcticus from Chilko Lake and other sites in BC ranged from 99.7 to 99.8% and was 99.6% for a Japanese sequence. In contrast, an M. arcticus sequence from Norway shared 95.3% identity with the Chilko Lake sequence, suggesting misidentification of the parasite. Chilko Lake sockeye salmon were previously reported free of infection with M. arcticus, and more research is required to understand the processes involved in the local and global dispersion of this parasite.

Molecular and morphological characterization of myxozoan actinospore types from a commercial catfish pond in the Mississippi delta

The actinospore diversity of infected Dero digitata was surveyed (May 2011) from a channel catfish (Ictalurus punctatus) production pond in the Mississippi Delta region for the elucidation of unknown myxozoan life cycles. At present, only 2 myxozoan life cycles have been molecularly confirmed in channel catfish, linking the actinospore stage from an aquatic oligochaete (D. digitata ) and the myxospore stage from the catfish. In this study D. digitata (n = 2,592) were isolated from oligochaetes collected from the bottom sediment of a channel catfish production pond. After 1 wk of daily observation, a total of 6 genetically different actinospore types were observed. The collective groups were classified as 2 aurantiactinomyxons, 2 helioactinomyxons, 1 raabeia, and 1 triactinomyxon. Overall prevalence of myxozoan infections in the isolated oligochaetes was 4.4%. Actinospores were photographed and measured for morphological characterization. Four previously undescribed actinospore types were identified and characterized molecularly and morphologically. Phylogenetic analysis revealed the raabeia and one of the helioactinomyxon (type 1) actinospores were closely related to the group of myxozoans known to parasitize ictalurids in North America. To date, no myxospores have been linked to the newly sequenced actinospores reported in this survey. The morphological and molecular data generated from this study will assist in the identification of myxospore counterparts for these actinospore stages and aid in the elucidation of unknown myxozoan life cycles in closed production systems.

Small subunit ribosomal RNA sequence links the myxospore stage of Henneguya mississippiensis n. sp. from channel catfish Ictalurus punctatus to an actinospore released by the benthic oligochaete Dero digitata

There are more than 200 species of Henneguya described from fish. Of these, only three life cycles have been determined, identifying the actinospore and myxospore stages from their respective hosts. Two of these life cycles involve the channel catfish (Ictalurus punctatus) and the freshwater oligochaete Dero digitata. Herein, we molecularly confirm the life cycle of a previously undescribed Henneguya sp. by matching 18S ribosomal RNA (rRNA) gene sequence of the myxospore stage from channel catfish with the previously described actinospore stage (Aurantiactinomyxon mississippiensis) from D. digitata. Gill tissue from naturally infected channel catfish contained pseudocysts restricted to the apical end of the primary lamellae. Myxospores were morphologically consistent with Henneguya spp. from ictalurid fishes in North America. The spores measured 48.8 ± 4.8 μm (range = 40.7-61.6 μm) in total spore length. The lanceolate spore body was 17.1 ± 1.0 μm (14.4-19.3 μm) in length and 5.0 ± 0.3 μm (4.5-5.5 μm) in width. The two polar capsules were 6.2 ± 0.4 μm (5.8-7.0 μm) long and 5.0 ± 0.3 μm (4.5-5.5 μm) wide. The polar capsule contained eight to nine coils in the polar filament. The two caudal processes were of equal length, measuring 31.0 ± 4.1 μm (22.9-40.6 μm). The 1980-bp 18S rRNA gene sequence obtained from two excised cysts shared 99.4% similarity (100% coverage) to the published sequence of A. mississippiensis, an actinospore previously described from D. digitata. The sequence similarity between the myxospore from channel catfish and actinospore from D. digitata suggests that they are conspecific, representing alternate life stages of Henneguya mississippiensis n. sp.

18S rRNA gene sequencing identifies a novel species of Henneguya parasitizing the gills of the channel catfish (Ictaluridae)

In the southeastern USA, the channel catfish Ictalurus punctatus is a host to at least eight different species of myxozoan parasites belonging to the genus Henneguya, four of which have been characterized molecularly using sequencing of the small subunit ribosomal RNA (SSU rRNA) gene. However, only two of these have confirmed life cycles that involve the oligochaete Dero digitata as the definitive host. During a health screening of farm-raised channel catfish, several fish presented with deformed primary lamellae. Lamellae harbored large, nodular, white pseudocysts 1.25 mm in diameter, and upon rupturing, these pseudocysts released Henneguya myxospores, with a typical lanceolate-shaped spore body, measuring 17.1 ± 1.0 μm (mean ± SD; range = 15.0-19.3 μm) in length and 4.8 ± 0.4 μm (3.7-5.6 μm) in width. Pyriform-shaped polar capsules were 5.8 ± 0.3 μm in length (5.1-6.4 μm) and 1.7 ± 0.1 μm (1.4-1.9 μm) in width. The two caudal processes were 40.0 ± 5.1 μm in length (29.5-50.0 μm) with a spore length of 57.2 ± 4.7 (46.8-66.8 μm). The contiguous SSU rRNA gene sequence obtained from myxospores of five excised cysts did not match any Henneguya sp. in GenBank. The greatest sequence homology (91% over 1,900 bp) was with Henneguya pellis, associated with blister-like lesions on the skin of blue catfish Ictalurus furcatus. Based on the unique combination of pseudocyst and myxospore morphology, tissue location, host, and SSU rRNA gene sequence data, we report this isolate to be a previously unreported species, Henneguya bulbosus sp. nov.

A revision of the genus Neogrubea Dillon & Hargis, 1968 (Monogenea: Mazocraeidae): new morphological and molecular data from off the Patagonian coast of Argentina

The genus Neogrubea Dillon & Hargis, 1968 (syn. Asymmetria Suriano, 1975) is revised based on examination of type- and voucher material, and from new specimens collected from the gills of Seriolella porosa Guichenot and Stromateus brasiliensis Fowler from off Patagonia, Argentina. Morphological comparisons based on light and scanning electron microscopy and molecular data (partial SSU and LSU rDNA sequences) of the monogeneans from off Patagonia suggest that N. seriolellae Dillon & Hargis, 1968 (syns N. stromateae Gibson, 1976, A. asymmetria Suriano, 1975 and A. platensis Rey & Meneses, 1985) is currently the only species of the genus. Neogrubea soni Evdokimova, 1969 is considered a species inquirenda. An emended diagnosis of Neogrubea is presented, and new host and locality records for N. seriolellae are given in detail. Morphological characters of the members of the mazocraeid subfamily Grubeinae Price, 1961 are also discussed.

Aporocotyle mariachristinae n. sp., and A. ymakara Villalba & Fernández, 1986 (Digenea: Aporocotylidae) of the pink cusk-eel, Genypterus blacodes (Ophidiiformes: Ophidiidae) from Patagonia, Argentina

Aporocotyle mariachristinae n. sp. and A. ymakara Villalba & Fernández, 1986 were collected from the bulbus arteriosus and ventral aorta of pink cusk-eels, Genypterus blacodes (Forster, 1801) from Patagonia, Argentina. A. mariachristinae n. sp. can be distinguished from all the species of Aporocotyle by the asymmetrical extension of posterior caeca (right posterior caecum longer, terminating at the area between mid-level of ovary and posterior body end; left posterior caecum shorter, terminating at the area between mid-level of cirrus sac and posterior to reproductive organs), the distribution of spines along the ventro-lateral body margins and the number of testes. The new species clearly differs from A. ymakara, from the same host species, in the esophagus / body length ratio, the absence of distal loops at caeca, the anterior caeca / posterior caeca length ratio, and the number of testes. Additionally, in A. ymakara the left posterior caecum may be longer than right posterior caecum, while in the new species left posterior caecum is always shorter. The specimen of A. ymakara collected from Argentina is also described. We also provide observations of the distribution of spines in different species of Aporocotyle, including new specimens of A. argentinensis Smith, 1969 from Merluccius hubbsi Marini, 1933. Molecular sequence data obtained from partial 18S and 28S rDNA regions were compared between the new species and other two species of Aporocotyle (A. argentinensis and A. spinosicanalis Williams, 1958). This is a new locality record for A. ymakara, extending the known geographical distribution for this species from Chile to Argentina, and the first report of two species of Aporocotyle in the same host species and locality.

Diversity and ancestry of flatworms infecting blood of nontetrapod craniates "fishes"

We herein review all published molecular studies (life history, taxonomy, and phylogeny) and summarize all GenBank sequences and primer sets for the "fish blood flukes". Further, by analysing new and all available sequence data for the partial D1-D2 domains of 28S from 83 blood fluke taxa, we explore the evolutionary expansion of flatworm parasitism in the blood of craniates. Based on this analysis, the blood flukes infecting marine bony fishes (Euteleostei) are monophyletic. The clade comprising the chondrichthyan blood fluke plus the marine euteleost blood flukes is the sister group to tetrapod blood flukes (spirorchiids and schistosomes). The innominate blood fluke cercariae from freshwater gastropods were monophyletic and sister to the clade comprising spirorchiids and schistosomes, but low nodal support indicated that they may represent a distinct blood fluke lineage with phylogenetic affinities also to fish blood flukes. Blood flukes that utilize gastropod intermediate hosts were monophyletic (unidentified gastropod cercariae+tetrapod blood flukes) and those utilizing bivalves and polychaetes were monophyletic (marine fish blood flukes). Low or no taxon sampling among blood flukes of basal fish lineages and primary division freshwater fish lineages are significant data gaps needing closure. We also note that no record of an infection exists in a hagfish (Myxiniformes), lamprey (Petromyzontiformes), or nontetrapod sarcopterygiian, i.e., coelacanth (Coelacanthimorpha) or lungfish (Dipnoi). The present phylogenetic analysis reiterated support for monophyly of Schistosomatidae and paraphyly of spirorchiids, with the blood flukes of freshwater turtles basal to those of marine turtles and schistosomes.

Host-parasite relationship of Ceratomyxa puntazzi n. sp. (Myxozoa: Myxosporea) and sharpsnout seabream Diplodus puntazzo (Walbaum, 1792) from the Mediterranean with first data on ceratomyxid host specificity in sparids

Sparidae are economically important fishes to both, fisheries and aquaculture in the Mediterranean. Species diversification is an important strategy for the development of Mediterranean aquaculture. One of the species recently introduced is the sharpsnout seabream Diplodus puntazzo (Walbaum, 1792). During a parasitological study of fish from the Gulf of Valencia and the Mar Menor (Spain), myxozoan spores belonging to the genus Ceratomyxa were found in the gall bladder of D. puntazzo. A morphological description of the spores, which includes histology and transmission electron microscopy (TEM) as well as molecular (SSU ribosomal DNA) data resulted in the erection of a new species, Ceratomyxa puntazzi n. sp. A histopathological study of C. puntazzi n. sp. infection in D. puntazzo showed that the parasite causes necrosis and loss of epithelial cells in the gall bladder, and provokes a pericholangitis in the liver tissue surrounding the bile ducts. Furthermore, molecular data obtained from C. puntazzi n. sp. and three other ceratomyxids from the closely related fish species Diplodus annularis L. and Sparus aurata L. which share the same habitat suggest that the genus Ceratomyxa is host-specific in sparids, which agrees with data previously obtained from Serranidae, Labridae and Pomacentridae, and that ceratomyxid species from sparids in the Mediterranean originated from a common ancestor.

Real-time PCR detection of Parvicapsula pseudobranchicola (Myxozoa: Myxosporea) in wild salmonids in Norway

The myxozoan genus Parvicapsula contains 14 species infecting fish, some of which are known to cause severe disease in farmed and wild salmonids. Parvicapsula pseudobranchicola infections were first reported from seawater-reared Atlantic salmon, Salmo salar, in Norway in 2002 and have since then been an increasing problem. The present study describes a Taqman real-time PCR assay for specific detection of P. pseudobranchicola. The Taqman assay targets the 18S rRNA gene of P. pseudobranchicola and is able to detect as few as ten copies of the target sequence. Using the described assay, P. pseudobranchicola was detected in both farmed and wild salmonids, indicating that wild Atlantic salmon, sea trout, Salmo trutta, and Arctic char, Salvelinus alpinus, may be natural hosts of the parasite. Parvicapsula pseudobranchicola was found in samples from wild salmonids in the far south and the far north of Norway, displaying a wide geographic range of the parasite. Farmed salmonids showed P. pseudobranchicola infection levels many folds higher than that observed for wild sea trout, indicating that farmed Atlantic salmon are subjected to an elevated infection pressure compared with wild salmonids.

A novel Henneguya species from channel catfish described by morphological, histological, and molecular characterization

Channel catfish Ictalurus punctatus from a commercial farming operation in the Mississippi Delta were submitted for examination for the presence of infection by the trematode Bolbophorus damnificus. The fish were instead found to possess skin nodules suggestive of Henneguya pellis, a species previously described in the blue catfish I. furcatus. Despite the dermal location and distribution of lesions, morphological characteristics of the myxospores were inconsistent with H. pellis. Spores possessed a lanceolate spore body 15.4 +/- 1.5 microm (mean +/- SD; range = 12.2-19.3 microm) in length and 5.5 +/- 0.6 microm (range = 4.5-6.8 microm) in width in valvular view, and 4.7 +/- 0.2 microm (range = 4.2-5.0 microm) in width in sutural view. Polar capsules were pyriform and unequal in both length and width and contained polar filaments with six coils. Polar capsules measured 6.1 +/- 0.8 microm (range = 4.0-7.9 microm) long and 1.7 +/- 0.3 microm (range = 1.0-2.2 microm) wide. The caudal appendages were 50.5 +/- 8.3 microm (range = 34.8-71.4 micorm) long and the total length of the spore was 65.9 +/- 8.6 microm (range = 48.2-90.0 microm). The "blister like" plasmodia were round or ovoid, up to 2 mm in diameter, and randomly distributed throughout the epidermis of the fish. Histologically, plasmodia were confined to the dermis and elicited no inflammatory reaction from the fish. A blast search of the 18S small subunit rDNA sequence obtained by polymerase chain reaction amplification resulted in no identical sequence matches but indicated a close relationship to H. gurlei, H. ictaluri, and H. exilis. The unique host record, spore morphology, and novel genetic sequence derived from this isolate lead us to propose this isolate as a novel species, H. sutherlandi.

Morphology and 18S rDNA of Henneguya gurlei (Myxosporea) from Ameiurus nebulosus (Siluriformes) in North Carolina

Henneguya gurlei was isolated from Ameiurus nebulosus captured in North Carolina and redescribed using critical morphological features and 18S small-subunit ribosomal RNA (SSU rDNA) gene sequence. Plasmodia are white, spherical, or subspherical, occur in clusters, measure up to 1.8 mm in length, and are located on the dorsal, pectoral, and anal fins. Histologically, plasmodia are located in the dermis and subdermally, and the larger cysts disrupt the melanocyte pigment layer. The spore body is lanceolate, 18.2 +/- 0.3 microm (range 15.7-20.3) in length, and 5.4 +/- 0.1 microm (range 3.8-6.1) in width in valvular view. The caudal appendages are 41.1 +/- 1.1 microm (range 34.0-49.7) in length. Polar capsules are pyriform and of unequal size. The longer polar capsule measures 6.2 +/- 0.1 microm (range 5.48-7.06), while the shorter is 5.7 +/- 0.1 microm (range 4.8-6.4) in length. Polar capsule width is 1.2 +/- 0.03 microm (range 1.0-1.54). The total length of the spore is 60.9 +/- 1.2 microm (range 48.7-68.5). Morphologically, this species is similar to other species of Henneguya that are known to infect ictalurids. Based on SSU rDNA sequences, this species is most closely related to H. exilis and H. ictaluri, which infect Ictalurus punctatus.

Kudoa unicapsula n. sp. (Myxosporea: Kudoidae) a parasite of the Mediterranean mullets Liza ramada and L. aurata (Teleostei: Mugilidae)

A new multivalvulid myxozoan parasite, Kudoa unicapsula n. sp., is described from the intestinal mesentery, intestine and pyloric caeca of the thin-lipped grey mullet Liza ramada (Risso 1826) and the golden grey mullet L. aurata (Risso, 1810) from the Mediterranean coastal waters of Spain. It is characterized by the presence of elongated, rice corn-like white cysts of 0.47-0.56 x 0.18-0.38 mm, filled with tetracapsulate, slightly asymmetric spores, rectangular in apical view and tear-shaped in lateral view with four polar capsules of considerably different size and slightly unequal spore valves with rounded edges, overlapping each other on the apex of the spore. One large polar capsule includes a polar filament coiled in two to three turns, and the other three polar capsules, which are very small, posses only a rudimental filament. Both light and electron microscopy data showed that this species differs from all previously described Kudoa spp. with unequal polar capsules. The molecular analysis based on 18S and 28S ribosomal deoxyribonucleic acid DNA sequence data of K. unicapsula n. sp. indicates a close relationship and thus phylogenetic clustering together with K. trifolia, a myxozoan from the same host and the same geographical location.

The secondary structure of the unusually long 18S ribosomal RNA of the myxozoan Sphaerospora truttae and structural evolutionary trends in the Myxozoa

The nearly complete 18S rRNA sequence of the myxozoan parasite Sphaerospora truttae shows an extraordinary length (2,552bp) in comparison with other myxozoans and with metazoans in general (average 1,800-1,900bp). The sequence shows nucleotide insertions in most variable regions of the 18S rRNA (V2, V4, V5 and V7), with especially large expansion segments in V4 and V7. In the myxozoans, nucleotide insertions and specific secondary structures in these regions of the gene were found to be strongly related to large scale phylogenetic clustering and thus with the invertebrate host type. Whereas expansion segments were generally found to be absent in the malacasporeans and the clade of primary marine myxozoan species, they occur in all taxa of the clade containing freshwater species, where they showed a consistent secondary structure throughout. The longest expansion segments occur in S. truttae, Sphaerospora elegans and Leptotheca ranae, which represent a clade that has emerged after the malacosporeans and before the radiation of all other myxozoan genera. These three species demonstrate structural links to the malacosporeans as well as other unique features. A smaller number of nucleotide insertions in different subhelices and specific secondary structures appear to have evolved independently in two marine genera, i.e. Ceratomyxa and Parvicapsula. The secondary structural elements of V4 and V7 of the myxozoan 18S rRNAs were found to be highly informative and revealed evolutionary trends of various regions of the gene hitherto unknown, since previous analyses have been based on primary sequence data excluding these regions. Furthermore, the unique features of the V4 region in S. truttae allowed for the design of a highly specific PCR assay for this species.

The life cycle of Chloromyxum auratum (Myxozoa) from goldfish, Carassius auratus (L.), involves an antonactinomyxon actinospore

The myxozoan parasite Chloromyxum auratumHallett, Atkinson, Holt, Banner & Bartholomew, 2006, was shown experimentally to have a two-host life cycle which involved a previously undescribed antonactinomyxon actinospore stage. Myxospores obtained from gall bladders of naturally infected feral goldfish, Carassius auratus (L.), were used to infect samples of mixed species of oligochaete worms obtained from the same locality as the fish: Fern Ridge Dam, Oregon, USA. After some 110 days post-exposure, actinospores were detected from the water above the oligochaetes. The 18S rDNA sequence of these actinospores was identical to the original myxospores. Spore release was sporadic, of low intensity and short duration, which confounded efforts to identify the host oligochaete species and infect naïve fish. This is the first life cycle that incorporates an actinospore of the collective group Antonactinomyxon, and the first life cycle demonstrated in the laboratory for a species of Chloromyxum.

Kudoa trifolia sp. n. - molecular phylogeny suggests a new spore morphology and unusual tissue location for a well-known genus

A new species of myxozoan, Kudoa trifolia sp. n., was found in various organs of the golden grey mullet, Liza aurata (Risso), and the thinlip mullet, L. ramada (Risso), from the western Mediterranean. Spores developed in subspherical plasmodia of 0.28-1 mm diameter within connective tissue, predominantly in the spleen, the outer wall of the gall bladder and the gut, the mesenteries and occasionally also in the gills. The spores of K. trifolia differ from the commonly known shape of Kudoa by considerable enlargement of one of the four valve cells, thus forming a 'spore body', which contains the major part of the binucleate sporoplasm. Scanning electron microscopy of the spores revealed the presence of grape-like appendages, which occur in bundles terminally on the valve cells. Phylogenetic analysis based on the 18S rDNA sequence of K. trifolia showed that this species is deeply embedded in the genus Kudoa despite its aberrant morphology and host tissue location. This suggests important amendments to the morphological diagnosis of the genus Kudoa.

Transmission of freshwater myxozoans during the asexual propagation of invertebrate hosts

The phylum Myxozoa contains over 1350 species almost all of which are considered to be obligate parasites of aquatic animals. The phylum is composed of two classes, the Myxosporea and the Malacosporea, species of which are important pathogens responsible for severe economic losses in cultured fisheries. The life cycles of freshwater Myxozoa are believed to involve horizontal, indirect transmission, involving an invertebrate (oligochaetes or bryozoans) and a vertebrate host (fish or amphibians). Here, we describe myxozoan propagation through the fragmentation of invertebrate hosts to form new infected individuals. The two hosts examined are an oligochaete Lumbriculus variegatus infected with an unidentified myxosporean (Triactinomyxon sp.) and the bryozoan Fredericella sultana infected with the malacosporean Tetracapsuloides bryosalmonae which causes proliferative kidney disease, a major constraint of the European rainbow trout industry. Such intra-clonal propagation is a novel form of vertical transmission that is likely to be widespread within the Myxozoa and could form an important method by which some of these parasites maintain and proliferate within the aquatic environment.

Morphological and molecular comparison of Myxobolus procerus (Kudo, 1934) and M. intramusculi n. sp. (Myxozoa) parasitising muscles of the trout-perch Percopsis omiscomaycus

Inter- and intracellular cysts from a presumed single species, Myxobolus procerus (Kudo, 1934), parasitising the trout-perch Percopsis omiscomaycus were compared with respect to their spore morphology and small subunit 18S ribosomal DNA. The two cyst types have similar pyriform spores and comparable ranges in spore length and width, size of the polar capsules and number of filament coils. However, fixed spores of the intracellular cysts are significantly shorter (p<0.05) in mean length than those from intercellular cysts, giving them a broader appearance. Comparing 991 base pairs of 18S rDNA revealed a 20 bp difference between the two cyst types (97.9% similarity), 10 due to base substitutions and 10 due to the presence or absence of a base. The described morphological differences and consistent 2.1% difference in the 18S rDNA reveal that the intracellular form is a separate species, which is described herein as Myxobolus intramusculi n. sp. M. intramusculi n. sp. and M. procerus may be sister species and it is suggested that the two species arose through sympatric speciation involving a switch in site specificity within a single host species.

The life cycle of Henneguya nuesslini Schuberg & Schroder, 1905 (Myxozoa) involves a triactinomyxon-type actinospore

The life cycle of the histozoic myxozoan parasite Henneguya nuesslini was investigated in two salmonid host species. Naive brown trout, Salmo trutta, and brook trout, Salvelinus fontinalis, were experimentally infected in two trials by triactinomyxon type actinospores from naturally infected Tubifex tubifex. In exposed common carp, Cyprinus carpio, no myxospore production was detected. The parasite formed cysts with mature myxospores in the connective tissue of the fish 102 days post-exposure. The morphology of both actinosporean and myxosporean stages was described by light microscopy and a 1417-bp fragment of the 18S rDNA gene was sequenced. Sequence analysis confirmed the absolute congruence of the two developmental stages and assisted in determining species identity. Host range, tissue specificity and myxospore measurements provided sufficiently distinctive features to confirm species validity and were thus crucial for identification. The triactinomyxon spores had 16 secondary germ cells, unique dimensions, a very opaque sporoplasm matrix and three conspicuously protruding, pyriform polar capsules. This is the first record of a Henneguya sp. life cycle with a triactinomyxon-type actinospore, which suggests a close relationship with the Myxobolus group and a polyphyletic origin of the genus Henneguya.

Molecular relationships and phylogeny in a community of myxosporeans and actinosporeans based on their 18S rDNA sequences

The community of myxosporeans and actinosporeans inhabiting a typical Scottish highland stream and the outflow area of an adjacent salmon hatchery was analysed on the basis of their 18S rDNA sequences. Nine myxosporeans belonging to the genera Sphaerospora, Chloromyxum, Zschokkella, Myxidium, Hoferellus and Myxobilatus were identified from mature spores in different organs of the fish species present. Twelve actinosporean types belonging to the collective groups of neoactinomyxum, aurantiactinomyxon, raabeia, echinactinomyxon and synactinomyxon were found to be released from oligochaete worms collected from sediments. Twenty of the 21 sequences obtained from these myxozoans are new entries to the myxozoan database, and the genera Chloromyxum, Hoferellus and Myxobilatus were entered for the first time. Study of the molecular relationships between the different taxa and with other myxozoan sequences available showed that the myxosporeans inhabiting the urinary system clearly cluster together, independently of host species or spore morphology. However, the sequences of the two Sphaerospora species encountered show considerable differences from other members of this group and all other freshwater myxosporeans, and they were found to occupy an ancestral marine position. Three actinosporeans, i.e. Neoactinomyxum eiseniellae, Aurantiactinomyxon pavinsis and Raabeia 'type 3' were found to represent alternate life cycle stages of Chloromyxum sp., Chloromyxum truttae and Myxidium truttae, respectively (approximately 1400 identical base pairs each). Three other actinosporeans encountered (two echinactinomyxon and one raabeia type) showed over 92% sequence identity with myxosporeans from GenBank, whereas all other actinosporeans formed a closely related group devoid of any known myxosporeans.

Molecular methods clarify morphometric variation in triactinomyxon spores (Myxozoa) released from different oligochaete hosts

Thirty-nine freshwater tubificid oligochaetes were isolated, each of which harboured a triactinomyxon infection. Spore characteristics include the typical triactinomyxon anchor shape, eight germ cells within the sporoplasm and three unequal (two long and one shorter) caudal processes with square tips. Despite morphological similarities between the spores from the different hosts, their morphometrical data varied considerably; significantly, the ranges of dimensions of the smallest and largest exemplars were mutually exclusive. In order to ascertain the true number of spore types present, molecular methods were employed. Samples of waterborne spores, including the smallest and largest representatives, were selected from 11 host oligochaetes (all Tubifex tubifex Müller) and a nested PCR-RFLP 'riboprint' analysis performed. The small subunit ribosomal DNA gene (18S rDNA) was targetted and amplified through two rounds of PCR, then digested with the restriction enzymes Dde I and Hha I. The resultant major cleavage patterns produced by both enzymes indicated a single triactinomyxon form; however, the pattern of several less intense bands varied between the samples. From a subset of five samples drawn from across the full spectrum of spore sizes, a 327 bp region near the 5' was sequenced and was identical for all five samples. Comparison of this 327 bp region with that of 12 other triactinomyxons in GenBank showed 68.7-96.9% similarity (at least 9 base differences). A further 469 bp generated for each of the smallest, largest and mid-range (= reference) spore samples was identical also. The reference sample was sequenced further to yield 1,554 bp of 18S rDNA (GenBank accession number AY162270); comparison with other Myxozoa indicated this sequence was novel. The morphometrics of our triactinomyxon did not correlate with any published description. The morphometrical variation exhibited by spores of the triactinomyxon type in this study raises questions about the validity of using morphometrical data to distinguish spore types and suggests that there could be taxonomic redundancy in the diversity of actinosporeans recorded in the literature. The additional information provided by molecular data in this study was pivotal in the clarification of morphometrical variation exhibited by morphologically similar triactinomyxon spores released from different oligochaete hosts.

Tracing the route of Sphaerospora truttae from the entry locus to the target organ of the host, Salmo salar L., using an optimized and specific in situ hybridization technique

Sphaerospora truttae is an important pathogen of Atlantic salmon parr in Scottish aquaculture. To trace the early development of S. truttae and to overcome the common problem of detecting low numbers of cryptic, early myxosporean stages, a DNA-based approach was applied in this study. Specific primers were designed for S. truttae from Atlantic salmon, based on 18S rDNA sequences, obtained from isolated myxosporean spores. These were 5' biotin-labelled and used in an optimized and rapid in situ hybridization (ISH) protocol, which provided a strong and specific signal of the parasite within host tissue sections and, at the same time, minimized structural damage to tissues due to processing. This methodology provided a reliable tool enabling the detection of S. truttae stages down to single cell level. Using ISH the epithelium of the gills was identified as the predominant entry locus of the parasite. By 3 days after infection S. truttae had penetrated the vascular epithelia and thereafter proliferated in the blood for at least 10 days before exiting the vascular system through capillary walls. From day 12 post-infection onwards, the kidney, as well as the spleen and the liver, were invaded. Numbers of S. truttae invading the kidney (37.3%) differed little from numbers found in the spleen (35.3%) and the liver (27.4%). The latter organs represented a dead end in the development of S. truttae as all stages in these organs degenerated and sporogony was found to take place exclusively inside the renal tubules. Early sporogonic stages were found from day 25 post-infection but mature spores only developed after at least 15 days of proliferation within the tubules.

Experimental transmission of Enteromyxum scophthalmi (Myxozoa), an enteric parasite of turbot Scophthalmus maximus

Several experiments were designed to elucidate the modes of transmission of the myxozoan parasite Enteromyxum scophthalmi to turbot Scophthalmus maximus. Direct transmission of the infections was achieved by cohabitation of infected and test fish, through waterborne contamination from the effluent of a tank containing infected fish, and via the oral route using parasite-infected intestines. The transmission of the turbot enteromyxosis was successful in all the fish exposed to the parasite by the 3 routes; accumulated mortality reached 100% at the end of most experiments. The progress of the infections was monitored by study of the histopathology. Influence of the mode of exposure was observed, with the oral route the fastest to initiate the parasite infections. The temperature also affected the course of the infections, which were established earlier at higher water temperature. Direct fish-to-fish transmission of the disease explains the rapid spreading of the turbot enteromyxosis in farms.

Early developmental stages of two actinosporeans, Raabeia and Aurantiactinomyxon (Myxozoa), as detected by light and electron microscopy

The development of actinosporeans in their oligochaete host proceeding pansporocyst formation is relatively well documented, however, phases preceding it are not as well known. The initial stages in the development of two actinosporeans, Raabeia type 1 of Oumouna et al. [Parasitol. Res. 2002] and Aurantiactinomyxon pavinsis (Ormières, 1968) Marquès [Languedoc, Universite des Sciences et Techniques, Dissertation, 1984] from schizogony to gametogony and sporogony are described. Both actinosporeans begin their development as multinucleate stages near the basal lamina of the oligochaete intestine. Proximal to these stages and between the host epithelium cells are uninucleate cells whose nuclei divide to produce binucleate cells. These divide mitotically to produce cells with four nuclei which then undergo plasmotomy to yield a tetracellular stage and the first phase in pansporocyst formation. From the uninucleate stage to the tetranucleate stage, the cell membrane of the parasite is associated closely via finger-like projections with the intestinal epithelial and glandular cells of the host.

Molecular approaches for inferring evolutionary relationships among protistan parasites

Within the diverse group of parasites broadly recognized as protists, there are limited morphological characters that can be used to distinguish species and even fewer characters that can infer evolutionary relationships among species. For this reason, molecular data are commonly used to infer relationships among species and strains. These studies most commonly rely on sequences associated with the ribosomal RNA genes but increasingly other nuclear, mitochondrial or plastid genes are contributing data. Molecular systematics has been invaluable in expanding the range of characters that are available for inferring relationships among protistan taxa. As an adjunct to morphological characters, sequence data allow us to better understand the evolutionary history of this group of parasites and thereby erect a well-supported taxonomic framework that reflects these historical relationships. Importantly, the predictive nature of such a framework can aid the search for therapeutic compounds (e.g. via shared biochemical pathways) and highlight organisms that should be tested for cross-reactivity in immunological or molecular diagnostic methods (e.g. use of the closest relatives to assess test specificity). For emergent and poorly known parasites, molecular characterization and placement within the broader phylogenetic framework can help predict likely life history traits, including possible or likely definitive hosts.

The use of polymerase chain reaction assay to diagnose proliferative gill disease in channel catfish (Ictalurus punctatus)

To assess the potential of a polymerase chain reaction (PCR) assay as a diagnostic tool in the detection of proliferative gill disease (PGD) in channel catfish (Ictalurus punctatus), PCR assays were compared with the traditional diagnostic methods of gill wet mounts and histology. A PCR assay using primers for Aurantiactinomyxon ictaluri, the actinospore associated with PGD, was performed with tissues from fish from commercial ponds. Using histology as the "gold standard," the sensitivity, specificity, and accuracy of the PCR assay were all >90%. In comparison, the wet mount examinations had a lower sensitivity and specificity. Using the chi-square test and a test for strength of association, there was a significant, strong association between results obtained by PCR and those obtained by the other 2 methods. These results demonstrate that the PCR assay is a good diagnostic tool for the detection of PGD.

Recent advances in our knowledge of the Myxozoa

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis. and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete. Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that: 1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.

Molecular characterization of myxozoan parasites from Lake Sasajewun, Algonquin Park, Ontario, by riboprinting

The small subunit-rRNA genes of 18 myxozoans from Lake Sasajewun, Algonquin Park were amplified and digested with restriction endonucleases for riboprinting analysis. Identical riboprints were not found between the myxosporeans and the actinosporeans. The distinct riboprinting patterns observed among these myxozoans indicate considerable genetic diversity within this group. Identical riboprints were found between Myxobolus pendula and Myxobolus pellicides, and between triactinomyxon 'C' and Triactinomyxon ignotum. Parsimony analysis of the riboprints demonstrated that neither the myxosporeans nor the actinosporeans formed a monophyletic group. Some species of Myxobolus are more closely related to forms of triactinomyxon, echinactinomyxon or raabeia than to other Myxobolus species. These results are consistent with the two-host life cycle hypothesis of myxozoans that myxosporeans and actinosporeans are alternating stages of the same organisms.