Plasmodium structure of Intoshia linei (Orthonectida)

Orthonectids are enigmatic parasitic bilaterians whose exact position on the phylogenetic tree is still uncertain. Despite ongoing debate about their phylogenetic position, the parasitic stage of orthonectids known as "plasmodium" remains underexplored. There is still no consensus on the origin of the plasmodium: whether it is an altered host cell or a parasitic organism that develops in the host extracellular environment. To determine the origin of the orthonectid parasitic stage, we studied in detail the fine structure of the Intoshia linei orthonectid plasmodium using a variety of morphological methods. The orthonectid plasmodium is a shapeless multinucleated organism separated from host tissues by a double membrane envelope. Besides numerous nuclei, its cytoplasm contains organelles typical for other bilaterians, reproductive cells, and maturing sexual specimens. Reproductive cells, as well as developing orthonectid males and females, are covered by an additional membrane. The plasmodium forms protrusions directed to the surface of the host body and used by mature individuals for egress from the host. The obtained results indicate that the orthonectid plasmodium is an extracellular parasite. A possible mechanism for its formation might involve spreading parasitic larva cells across the host tissues with subsequent generation of a cell-within-cell complex. The cytoplasm of the plasmodium originates from the outer cell, which undergoes multiple nuclear divisions without cytokinesis, while the inner cell divides, giving rise to reproductive cells and embryos. The term "plasmodium" should be avoided and the term "orthonectid plasmodium" could be temporarily used instead.

Kudoa rousseauxii n. sp. (Cnidaria: Multivalvulida) Infects the Skeletal Muscles of the Freshwater Fish Brachyplatystoma rousseauxii in the Amazon River

PURPOSE

Members of the genus Kudoa Meglitsch, 1947 are known to infect the muscles of commercially important fishes worldwide, including those in the order Siluriformes. This paper describes the occurrence of a new species of Kudoa in the catfish Brachyplatystoma rousseauxii based on morphological study and molecular analysis of the ribosomal RNA gene (SSU rDNA).

METHODS

Fifteen specimens of Brachyplatystoma rousseauxii were purchased from fishing zones near Mosqueiro Island, Belém, Pará, Brazil. After necropsy, tissue samples and cysts were analyzed using a stereomicroscope, and fresh slides were viewed under a light microscope to confirm parasitic infection. The tissue fragments were removed and processed for molecular and histological analyses.

RESULTS

Microscopic pseudocysts were found in the epaxial region of skeletal muscle fibers in 80% of the analyzed specimens. The myxospores were quadrangular with four shell valves (SV), pyriform polar capsules (PC), and internal symmetry. Phylogenetic analyses indicated that the new species formed a cluster with the species previously described in the Amazon, being close to two freshwater species.

CONCLUSIONS

Morphological differences and molecular data of SSU rDNA support that Kudoa rousseauxii n. sp. is a new species that infects B. rousseauxii, a freshwater fish with intense migratory cycles that is widely captured and consumed in the Amazon.

Synopsis of the species of Myxobolus (Cnidaria, Myxozoa, Myxosporea) described between 2014 and 2020

A synopsis of the species of Myxobolus Bütschli, 1882 (Cnidaria, Myxosporea, Myxobolidae) described from 2014 up till now is presented. It includes 122 nominal species described all over the world. For each of the species, the most relevant morphological and morphometric data, as well as data are provided related to the location in the host, type host and type locality. The GenBank accession numbers are provided whenever possible, and the spores were redrawn based on the original descriptions. The bibliography includes all the papers containing the species descriptions.

Increasing the known biodiversity of cnidarian parasites of bryconid fishes from South America: two novel Myxobolus species with ultrastructure and ssrDNA-based phylogeny

This study increases the known biodiversity of cnidarian parasites in neotropical bryconid fishes. Two novel Myxobolus species are described based on morphology, ultrastructure and small subunit ribosomal DNA (ssrDNA) sequencing: Myxobolus vetuschicanus n. sp. infecting fins of Salminus franciscanus and Myxobolus mineirus n. sp. infecting the mesentery of Brycon orthotaenia from the São Francisco River basin, Minas Gerais State, Brazil. Ultrastructural analysis of the two species revealed an asynchronous sporogenesis process, with germinative cells and young developmental stages of myxospores in the periphery of the plasmodia. In M. vetuschicanus n. sp., the plasmodia were surrounded by a layer of fibroblasts and in M. mineirus n. sp., the plasmodial membrane had direct contact with the host tissue. The phylogenetic analysis based on the ssrDNA of Henneguya/Myxobolus species showed that the two novel Myxobolus species grouped in subclades together with other parasite species of bryconid fishes.

Taxonomy, phylogeny and host-parasite interaction of two novel Myxobolus species infecting Brycon orthotaenia from the São Francisco River, Brazil

Two new Myxobolus species were described infecting Brycon orthotaenia from the São Francisco River, in the state of Minas Gerais, Brazil. From a total of 39 B. orthotaenia collected, two specimens (5.1%) exhibited infection of the ovary and 12 specimens (30.8%) displayed infection of the liver. The plasmodia of both Myxobolus species were white and spherical measuring around 1 mm in length. The plasmodium found in the ovary showed mature myxospores, which were oval shaped from the frontal view and measured 9.2-11.0 (9.8 ± 0.4) μm in length, 5.9-6.9 (6.5 ± 0.3) μm in width and 4.6-5 (4.9 ± 0.1) μm in diameter. The two polar capsules were the same size and measured 3.9-6.2 (4.7 ± 0.5) μm in length and 1.8-2.4 (2.1 ± 0.2) μm in width. The polar tubules had 9 coils. The plasmodium found in the liver showed mature myxospores which were ellipsoidal in shape from the frontal view and measured 10.0-11.4 (10.7 ± 0.5) μm in length, 7.3-8.6 (8.1 ± 0.4) μm in width and 5.3-7.0 (6.8 ± 0.4) μm in diameter. The two polar capsules were the same size and measured 4.2-5.4 (4.9 ± 0.3) μm in length and 1.9-2.9 (2.7 ± 0.3) μm in width. The polar tubules had 8 coils. Ultrastructural analysis revealed an asynchronous sporogenesis process, with young developmental myxospore stages more often found in the periphery of the plasmodium and mature myxospores in the centre of the plasmodium. The plasmodial wall was formed by a single membrane which was not surrounded by a layer of host tissue. A thick layer of fibrous material was found in the peripheral ectoplasm close to the plasmodial wall of the plasmodium found in the ovary. Phylogenetic analysis based on the small-subunit ribosomal DNA - ssrDNA sequences and using the closest myxozoan sequences to each one of the species studied here based on previous GenBank data and Henneguya/Myxobolus/Thelohanellus species parasitizing fish from South American, revealed that the new species are grouped in a subclade together with other Myxobolus species parasitizing bryconid hosts.

Morphological, ultrastructural, and phylogenetic analysis of two novel Myxobolus species (Cnidaria: Myxosporea) parasitizing bryconid fish from São Francisco River, Brazil

Twelve Myxobolus species have been previously described to parasitize Bryconidae fish in South America. Here, we describe two novel myxosporean species that parasitize economically important Bryconidae from the São Francisco River basin in Brazil. Myxospores morphometry, morphology, small-subunit ribosomal DNA - ssrDNA sequences, and other biological traits were used in the taxonomic analysis. Phylogenetic analysis was performed to assess the position of the new Myxobolus species among the closest Myxobolus/Henneguya. Myxobolus iecoris n. sp. was found infecting the liver of Salminus franciscanus (dourado). Myxospores were oval with the anterior region aculiform in frontal view and biconvex in lateral view and measured 11.4-14.2 (12.8 ± 0.8) μm long, 7.7-9.9 (8.7 ± 0.6) μm wide, 6.5-7.5 (6.9 ± 0.4) μm thick. Two pyriform and equal-sized polar capsules measuring 4.9-7.4 (5.9 ± 0.5) μm long and 2.3-3.5 (3.0 ± 0.2) μm wide contained polar tubules with 8-9 turns. Myxobolus lienis n. sp. was found infecting the spleen of Brycon orthotaenia (matrinxã). Myxospores were round to oval in frontal view and biconvex in lateral view and measured 10.3-13.8 (12 ± 0.6) μm long, 6.8-9.3 (8.3 ± 0.5) μm wide, and 6.9-7.0 (7.0 ± 0.6) μm thick. Two oval and equal-sized polar capsules measured 3.9-5.8 (4.6 ± 0.5) μm long and 2.0-3.5 (2.8 ± 0.3) μm wide contained polar tubules with 5-6 turns. Ultrastructural analysis revealed asynchronous sporogenesis with germinative cells and young sporogonic stages in the periphery of the plasmodia. A connective tissue capsule was observed surrounding Myxobolus lienis n. sp., but it was absent for Myxobolus iecoris n. sp. Maximum likelihood and Bayesian inferences showed the two novel species clustering in a well-supported subclade composed by Myxobolus spp. of bryconids. Myxobolus iecoris n. sp. appeared as a sister species of M. aureus and Myxobolus lienis n. sp. as sister to M. umidus.

Supplemental diagnosis and phylogeny of Myxobolus absonus (Cnidaria, Myxozoa) from the eye of the freshwater fish Pimelodus maculatus (Siluriformes, Pimelodidae)

Myxobolus absonusCellere et al., 2002 was originally described as having free cysts in the opercular cavity of the freshwater fish Pimelodus maculatus in Brazil. The present study provides a supplemental description of this parasite from the eye of its type host, with basis on morphological, ultrastructural, and molecular data of the SSU rRNA gene. The parasite formed spherical whitish cysts, which wall presented numerous microvilli that attached to the collagen layers of the corneal stroma. Myxospores were oval in valvular and sutural view, measuring 13.2 ± 0.4 μm in length, 8.5 ± 0.4 μm in width, and 6.6 ± 0.3 μm in thickness. Two asymmetric pyriform polar capsules were located at the anterior pole: the larger 6.2 ± 0.4 μm long and 3.6 ± 0.3 μm wide, containing a polar filament coiled in 6 turns; and the smaller 3.5 ± 0.3 μm long and 1.9 ± 0.1 μm wide, containing a polar filament coiled in 4 turns. At the posterior pole, the sporoplasm displayed two nuclei and numerous spherical sporoplasmosomes. Phylogenetic analysis using maximum likelihood, Bayesian inference and maximum parsimony revealed M. absonus clustering within a well-supported clade with poorly-resolved internal nodes, amongst the SSU rRNA sequences of other myxobolids that infect siluriform and characiform fish hosts, as well as the perciform-infecting species Myxobolus acanthogobii, Triangula percae and Cardimyxobolus japonensis. This clade appeared separated from the other clades comprising most of the species that infect siluriform and characiform hosts, showing that more than one myxobolid lineage evolved while parasitizing these taxonomic groups of fish.

Phylogeny and comprehensive revision of mugiliform-infecting myxobolids (Myxozoa, Myxobolidae), with the morphological and molecular redescription of the cryptic speciesMyxobolus exiguus

Mullets inhabit a wide range of habitats from tropical to temperate regions and play a critical role in their ecosystems. This commercially important fish group constitutes a significant source of food in several geographic regions, and the production of some species for consumption is an increasing trend. About 64 myxosporean species have been reported in mullets, some of which are cryptic, as is the case ofMyxobolus exiguus, andM. muelleri. This paper provides, for the first time, a detailed and critical revision of the data available for myxobolids reported in mullets, determining the species that havebona fidemugiliform fish hosts, in accordance with the original species descriptions, the available molecular data and the currently accepted taxonomic and phylogenetic criteria. Phylogenetic analyses using Bayesian inference and maximum-likelihood methodologies suggest that the evolutionary history of myxobolids withbona fidemugiliform fish hosts reflects that of its vertebrate hosts, while reinforcing known evolutionary factors and old systematic issues of the clade of myxobolids. A comprehensive morphological, ultrastructural and molecular redescription is also provided for the cryptic speciesM. exiguus, from infections in the visceral peritoneum of the thinlip-grey mulletChelon ramadain the River Minho, Portugal.

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.

Occurrence of two novel actinospore types (Cnidaria: Myxozoa) in fish farms in Mato Grosso do Sul state, Brazil

We investigated the involvement of oligochaetes in the life cycles of fresh water myxozoan parasites in Brazil. In a fish farm in the State of Mato Grosso do Sul, we examined 192 oligochaetes and found that two (1%) released Aurantiactinomyxon type actinospores. We identified infected oligochaetes by morphology: both were Pristina synclites, from family Naididae. This is the first report of the involvement of this species in the life cycle of myxozoans. Small-subunit ribosomal DNA sequences of Aurantiactinomyxon type 1 (1882 nt) and Aurantiactinomyxon type 2 (1900 nt) did not match any previously sequenced myxozoan in the NCBI database, with the highest BLAST search similarities of 83% with Myxobolus batalhensis MF361090 and 93% with Henneguya maculosus KF296344, respectively, and the two aurantiactinomyxons were only 75% similar to each other (over ~ 1900 bases). Phylogenetic analyses showed that Aurantiactinomyxon type 1 had closest affinities with myxozoans from fish hosts in Order Characiformes, and Aurantiactinomyxon type 2 had affinities with myxozoans from fish of Order Siluriformes.

New myxosporeans parasitizing Phractocephalus hemioliopterus from Brazil: morphology, ultrastructure and SSU-rDNA sequencing

Myxozoans are a diverse group of parasitic cnidarians, with some species recognized as serious pathogens to their hosts. The present study describes 2 new myxobolid species (Myxobolus figueirae sp. nov. and Henneguya santarenensis sp. nov.) infecting skin and gill filaments of the Amazonian pimelodid fish Phractocephalus hemioliopterus, based on ultrastructural, histology and phylogenetic analysis. The fish were caught in the Amazon River, Pará, Brazil. The plasmodial development of M. figueirae sp. nov. was in the dermis and those of H. santarenensis sp. nov. were of the intralamellar type. For both species, the plasmodia were surrounded by a connective tissue layer, but there was no inflammatory infiltrate. For M. figueirae sp. nov., mature spores were ovoid measuring 9.1 to 10 (9.5 ± 0.3) µm in length, 5.8 to 6.9 (6.4 ± 0.3) µm in width and 4.4 to 4.5 (4.5 ± 0.1) µm in thickness. Two polar capsules were elongated and of unequal size. For H. santarenensis sp. nov., mature spores were ellipsoidal in the frontal view, measuring 26.3 to 36.1 (31.9 ± 3) µm in total length, 9.6 to 11.9 (10.8 ± 0.5) µm in body length, 3.7 to 4.9 (4.3 ± 0.3) µm in width and 16.6 to 25.6 (21 ± 3.1) µm in caudal process. The polar capsules were elongated and of equal size. Phylogenetic analysis, based on partial small subunit ribosomal DNA (SSU rDNA) sequences and using the closest myxozoan sequences to each one of the species studied here based on previous GenBank data, showed M. figueirae sp. nov. and H. santarenensis sp. nov. clustering in distinct lineages. While H. santarenensis sp. nov. clustered in a well-supported subclade composed of Henneguya species that infect gills of South American pimelodid hosts, M. figueirae sp. nov. clustered in a weakly supported subclade containing parasite species of bryconid hosts.

Correlation between host specificity and genetic diversity for the muscle-dwelling fish parasite Myxobolus pseudodispar: examples of myxozoan host-shift?

Myxobolus pseudodispar Gorbunova, 1936 (Myxozoa) is capable of infecting and developing mature myxospores in several cyprinid species. However, M. pseudodispar isolates from different fish show up to 5% differences in the SSU rDNA sequences. This is an unusually large intraspecific difference for myxozoans and only some of the muscle-dwelling myxozoan species possess such a high genetic variability. We intended to study the correlation between the host specificity and the phylogenetic relationship of the parasite isolates, and to find experimental proof for the putatively wide host range of M. pseudodispar with cross-infection experiments and phylogenetic analyses based on SSU rDNA. The experimental findings distinguished 'primary' and less-susceptible 'secondary' hosts. With some exceptions, M. pseudodispar isolates showed a tendency to cluster according to the fish host on the phylogenetic tree. Experimental and phylogenetic findings suggest the cryptic nature of the species. It is likely that host-shift occurred for M. pseudodispar and the parasite speciation in progress might explain the high genetic diversity among isolates which are morphologically indistinguishable.