Intraspecific morphometric variation in myxosporeans

Morphological and molecular studies on two myxosporean infections of cyprinid fishes: Thelohanellus pyriformis from tench and Thelohanellus cf. fuhrmanni from nase

During surveys on myxosporean parasites of Lake Balaton and River Danube fishes, two Thelohanellus spp. were found on tench (Tinca tinca) and on common nase (Chondrostoma nasus). They were identified as Thelohanellus pyriformis and Thelohanellus cf. fuhrmanni, respectively. Myxospores of T. pyriformis from tench were collected from arteria branchialis afferens of gill filaments. The mature myxospores of this species were pyriform in shape and 19 ± 0.6l (18–19.5) long, 8.2 ± 0.54 (7.5–9) wide, 7.3 ± 0.25 (7–7.5) thick containing polar tubules with 9–10 turns. The plasmodia of Thelohanellus cf. fuhrmanni were collected from under the skin of snout of the common nase. The myxospores were pyriform, 16.3 ± 0.39l (15.5–16.5) long, 6.5 ± 0.55 (6.3–7) wide, 6.3 ± 0.53 (5.8–7) thick containing polar tubules with 6 turns. Small subunit ribosomal DNA sequences of both Thelohanellus species differed from other known myxozoans. The myxospores morphology, histopathology and ssrDNA sequences supported a diagnosis of T. pyriformis from tench and T. cf. fuhrmanni from common nase.

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Two Thelohanellus spp. were redescribed from cyprinid fishes.

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Thelohanellus pyriformis was recorded and described from the type host tench (Tinca tinca).

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Thelohanellus cf. fuhrmanni was found in an atypical host, common nase (Chondrostoma nasus).

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The morphology of both Thelohanellus species was supported by histology and ssrDNA sequence data.

GENETIC DIVERSITY OF CYSTODISCUS SPECIES IN AMPHIBIANS IN THE SOUTHERN UNITED STATES

Myxosporean species in the genus Cystodiscus are parasites of amphibians and have been reported from several continents. Typically used for the identification of myxozoans, the spores produced by these species are similar to one another, possessing 2 polar capsules and being ovoid. The number of transverse depressions on the spore can be useful for delineating species, but these can sometimes be difficult to distinguish. In North America, Cystodiscus serotinus and Cystodiscus melleni have been described, and for C. serotinus in particular, numerous reports and a wide range of hosts have been associated with this species. Given the challenges of identifying some of these species, we questioned whether all encounters of Cystodiscus species can be attributed to these 2 described species, or if there may be additional undescribed species or cryptic species. Over 7 yr, 383 amphibians representing 13 species of toads, frogs, and salamanders were collected from sites in Oklahoma and Arkansas. Cystodiscus infections were found in 56 individuals (14.6%). Tissues from these infected individuals were preserved in alcohol for genetic analysis. The small subunit (SSU) and large subunit (LSU) ribosomal RNA genes were partially sequenced and analyzed phylogenetically. Nine distinct SSU sequence types and 7 distinct LSU sequence types were identified. Phylogenetically, sequence types were attributable to C. serotinus, C. melleni, Cystodiscus axonis, and an undescribed species. For the previously described species, there were multiple SSU sequence types: 4 for C. serotinus and 2 for both C. melleni and C. axonis. Phylogenetic patterns were similar for the LSU sequence analysis using a shorter sequence than the SSU, and we propose that the LSU is useful for initial barcoding of Cystodiscus species in any future surveys. In our qualitative assessment of sequence types compared to geography and host species, SSU types C1 and C2 (C. axonis) were only found in Union County, Arkansas, and McCurtain County, Oklahoma, respectively. Also, salamanders were only infected with SSU types B or D (C. melleni), and type B was only found in salamanders. Our finding of C. axonis in North America is notable because this species was described in Australia and is associated with host pathology. Our work reveals that there are cryptic species of Cystodiscus in the United States, one of which may be a pathogen, highlighting the importance of genetic analysis for future surveys of these species.

Molecular diagnostic based on 18S rDNA and supplemental taxonomic data of the cnidarian coelozoic Ceratomyxa (Cnidaria, Myxosporea) and comments on the intraspecific morphological variation

Ceratomyxa amazonensis is a cnidarian myxosporean originally described with strongly arcuate crescent-shaped myxospores, absence of vegetative stages and infecting Symphysodon discus, an important Amazonian ornamental fish in the aquarium industry. As part of a long-term investigation concerning myxosporeans that infect discus fish Symphysodon spp. from different rivers of the Amazon Basin, thirty specimens of S. discus collected from Unini River were examined. Plasmodial vegetative stages therefrom were found freely floating in the bile of gall bladders from eighteen fish. Mature myxospores were slightly crescent-shaped, measuring 4.72 ± 0.1 (4.52–4.81) μm in length, 24.2 ± 0.4 (23.9–25.3) μm in thickness with polar capsules 2.31 ± 0.1 (2.29–2.33) μm in length and 2.15 ± 0.1 (2.13–2.17) μm in width. Strong morphological differences were observed between the newly isolated myxospores obtained and the previously described C. amazonensis; however, molecular assessment, based on 18S rDNA, revealed a high similarity (99.91%), with only a single nucleotide base change. This study provides new data, expanding the original description of the species with a discussion on differences in myxospore-morphology in the context of intraspecific morphological plasticity.

Description of Myxidium pseudocuneiforme n. sp. (Myxosporea: Myxidiidae) from Cyprinus carpio in China, with the Resolution on a Taxonomic Dilemma of Myxidium cuneiforme

Investigations on myxozoan parasites of fish from Chongqing in China, revealed two Myxidium cuneiforme-like myxosporeans infecting the gallbladder of Cyprinus carpio carpio and Carassius auratus. We researched their myxospore morphology, and analyzed their genetic similarity and phylogenic relationships to other myxozoans based on small subunit ribosomal DNA (18S rDNA) sequences. Although both parasites recovered were morphologically similar, the myxosporean isolated from C. auratus was consistent in morphology to Myxidium cuneiforme, which was described from this host species. The parasite isolated from C. c. carpio had overlapping myxospore dimensions to M. cuneiforme, but on average, the polar capsules were not as long. More importantly, this parasite was genetically distinct from M. cuneiforme with 96.3% and 96.5% similarity in two sequences of 18S rDNA, and we propose the name Myxidium pseudocuneiforme n. sp. for this myxozoan from common carp. Its mature myxospores are ellipsoidal and asymmetric with pointed ends in valvular view, arc-shaped or fusiform in sutural view. The pyriform polar capsules are equal in size, and polar filament with 5-6 coils. This study highlights that molecular characteristics and host specificity are indispensable for myxozoan species identification when presented with the taxonomic dilemma of whether we are observing one species that exhibits slight morphological differences or multiple, but similar, species in different hosts.

Morphological, histological and molecular characterization of Myxidium cf. rhodei infecting the kidney of Rutilus rutilus

In the present study, we provide supplementary data for Myxidium cf. rhodei Léger, 1905 based on morphological, histological and molecular characterization. M. cf. rhodei was observed in the kidneys of 918 out of 942 (97%) roach Rutilus rutilus (Linnaeus, 1758). Myxospores of M. cf. rhodei were fusiform with pointed ends, measuring 12.7 ± 0.1 SD (11.8-13.4) µm in length and 4.6 ± 0.1 (3.8-5.4) µm in width. Two similar pear-shaped polar capsules were positioned at either ends of the longitudinal axis of the myxospore: each of these capsules measured 4.0 ± 0.1 (3.1-4.7) µm in length and 2.8 ± 0.1 (2.0-4.0) µm in width. Polar filaments were coiled into 4 to 5 turns. Approximately 18-20 longitudinal straight ridges were observed on the myxospore surface. The suture line was straight and distinctive, running near the middle of the valves. Histologically, the plasmodia of the present species were found in the Bowman's capsules, and rarely in the interstitium of the host. Phylogenetic analysis revealed that M. cf. rhodei was sister to M. anatidum in the Myxidium clade including most Myxidium species from freshwater hosts.

Morphological and molecular characterization of the muscle-infecting myxosporean Myxobolus xinyangensis sp. nov. from Abbottina rivularis in China

During an environmental assessment on the Huang River in Xinyang City (Henan Province, China), a novel Myxobolus species (Myxozoa: Myxobolidae) was found infecting the trunk muscle of Chinese false gudgeon Abbottina rivularis Basilewsky, 1855 (Gobioninae, Cyprinidae). Plasmodia of the new myxozoan, nominated herein as Myxobolus xinyangensis sp. nov., are round and yellowish, symmetrically and bilaterally located dorsal to the openings of the 2 opercula, and measure about 4.5 mm in diameter. The mature myxospores are orbicular in frontal view and fusiform in sutural view, with slightly tapered anterior end and rounded posterior end, and measure 9.4 ± 0.5 (8.7-10.6) µm long, 8.6 ± 0.6 (7.3-9.5) µm wide and 6.4 ± 0.3 (5.8-7.1) µm thick (mean ± SD, range). The ratio of spore length to spore width is close to 1. Two slightly unequal pyriform polar capsules, with tapering anterior ends and rounded posterior ends, measure 5.6 ± 0.67 (4.3-6.8) µm long and 3.0 ± 0.3 (2.4-3.6) µm wide, present as a figure 8 in the anterior part of spores and tightly converge at the top end of spores. Polar filament coils show 4 to 5 turns and are situated perpendicularly to the longitudinal axis of the polar capsules. No intercapsular appendix or sutural folds at the posterior end of spores were observed. The obtained partial small subunit ribosomal DNA sequence did not match any available data in GenBank and showed the highest sequence identity (93%) with 2 cyprinid trunk muscle-infecting Myxobolus species, M. pseudodispar and M. klamathellus. Phylogenetic analysis clearly showed that M. xinyangensis sp. nov. clustered within a cyprinid trunk muscle-infecting Myxobolus subclade at the basal position, but as an independent branch which was a possible reflection of its distinct myxospore morphology. This is the first record of infection of Myxobolus species in the trunk muscle of Abbotina fish.

Morphological plasticity in Myxobolus Bütschli, 1882: a taxonomic dilemma case and renaming of a parasite species of the common carp

BACKGROUND

Myxozoans are a group of cnidarian parasites, the present taxonomy of which favors a more comprehensive characterization strategy combining spore morphology, biological traits (host/organ specificity, tissue tropism), and DNA data over the classical morphology-based taxonomy. However, a systematist might again run into a taxonomic dilemma if more than two of the following exceptional cases were encountered at the same time: extensive intraspecific polymorphism, interspecific morphological similarity, identical interspecific biological traits and blurred small-subunit (SSU) rDNA-based species boundaries. In the present study, spores of a species of Myxobolus Bütschli, 1882 with two morphotypes (wide type and narrow type) were collected from the gills of common carp Cyprinus carpio Linnaeus. Confusingly, the wide type was found to be identical to Myxobolus paratoyamai Kato, Kasai, Tomochi, Li & Sato, 2017 in spore morphology and SSU rDNA sequence, which confidently suggested their conspecificity; while the narrow type, was highly similar to Myxobolus toyamai Kudo, 1917 based on spore morphology and SSU rDNA sequence and thus could not be easily classified. This discordance between wide type and narrow type has caused a taxonomic dilemma. To address this problem, a hypothesis about the conspecificity of the narrow type and M. toyamai was addressed.

RESULTS

It was found that if the narrow type is conspecific with M. toyamai, it would be paradoxical for the SSU rDNA sequence of the narrow type to be more similar to M. paratoyamai (99.3%), Myxobolus acinosus Nie & Li, 1973 (98.6%) and Myxobolus longisporus Nie & Li, 1992 (98.7%) than to M. toyamai (97.6%). According to the results of the above what-if analysis, the narrow type and M. toyamai were considered to be different species. All in all, the present dual-morphotype species is estimated to be conspecific with M. paratoyamai Kato, Kasai, Tomochi, Li & Sato, 2017. Considering that this species name was preoccupied by Myxobolus paratoyamai Nie & Li, 1992, the replacement name Myxobolus pseudoacinosus nom. nov. is proposed.

CONCLUSIONS

This work addresses the taxonomic dilemma in polymorphic myxozoans and demonstrates that M. pseudoacinosus is a distinct species with two morphotypes. The present study may serve as a baseline for future studies that encounter similar classification complexities.

Ultrastructural, ssrDNA sequencing of Myxobolus prochilodus and Myxobolus porofilus and details of the interaction with the host Prochilodus lineatus

Myxobolus prochilodus and Myxobolus porofilus are parasites of Prochilodus lineatus, an economically important South American fish found in La Plata and Paraiba do Sul river basins. This study focusing on parasite-host interaction provides an ultrastructural and phylogenetic analysis, the latter based on ssrDNA sequencing of these parasites respectively infecting the gill filaments and fins of P. lineatus taken from the Mogi Guaçu River, São Paulo, Brazil. A total of 13 adult specimens were examined in this study. The prevalence of infection was 7.69 % for M. prochilodus and 15.38 % for M. porofilus. Phylogenetic analysis showed M. prochilodus and M. porofilus clustered in a subclade composed of parasites of the Prochilodontidae family. In M. prochilodus infecting gill filaments, where cellular degeneration in the epithelium was observed, the plasmodia were surrounded by a capsule composed of layers of fibrocyte-like cells, with cellular projections joined to the projections of other fibrocyte-like cells by desmosomes, and more externally typical fibroblast layers. Some granular leukocytes were seen interspersed among these layers. In M. porofilus infecting the fins, the capsule of connective tissue was represented only by loosely arranged collagen fibers, and no granular leucocytes were observed. Finally, several unusual vacuoles with filamentous content and some characteristics usually described as degenerative alterations, as myelin figure, were noted in plasmodia and pansporoblasts of both myxosporean species. The possible influence of inflammatory response and xenobiotics was considered to be the explanation for the alterations observed in Myxobolus species and its host.