Mass mortality of pond-reared Carassius gibelio caused by Myxobolus ampullicapsulatus in China

Immunosuppression and apoptosis activation mediated by p53-Bcl2/Bax signaling pathway -The potential mechanism of goldfish (Carassius auratus Linnaeus) gill disease caused by Myxobolus ampullicapsulatus

Myxobolus, a major harmful type of myxospora, is one of the main parasitic pathogens of freshwater fish. Once myxoboliosis occurs, treatment can be extremely difficult. Therefore, clear understandings of the etiology of myxoboliosis and its pathological mechanism are keys for prevention and control. Here, histology, transmission electron microscopy, transcriptome study, tunel assay, and immunohistochemistry were carried out, revealing the morphology, pathological effects as well as host response mechanism of goldfish gill to Myxobolus ampullicapsulatus. Histological studies showed that the mature spores of Myxobolus ampullicapsulatus were composed of three parts, the spore shell, sporoplasm and bottle shaped polar capsule containing double S-shaped polar filaments. Transcriptome analysis revealed that Myxobolus ampullicapsulatus -infected (Myx) goldfish gills were characterized by apoptosis activation mediated by "p53 signaling pathway" with significantly up-regulated apoptosis-related differential genes dominated by p53-Bcl2/Bax signaling pathway. In addition, tunel assay revealed severe gill apoptosis in the Myx group. Transcriptome analysis also revealed that Myx group showed changes in immune response and significantly down-regulated immune-related differential genes. Beyond that, immunohistochemistry showed that there was no significant increase in the number of gill lymphocyte after parasite infection. These results suggest that the pathological mechanism of Myxobolus ampullicapsulatus infection on gills of goldfish may be related to apoptosis and immunosuppression. Subsequent qRT-PCR showed that apoptosis-related genes (Caspase3,Bad, Bax) and anti-inflammatory gene IL-10 were significantly increased, while immune-related pro-inflammatory genes (IL-1β, IL-8) were markedly down-regulated, further verifying the transcriptome results. Based on the above results, we concluded that p53-Bcl2/Bax related networks that dominant the expression of apoptosis genes were activated while immunity was suppressed in the gills of Myxobolus ampullicapsulatus infected goldfish. Our study is not only of benefit to enrich the taxonomy of Myxobolus but also clarifies its pathogenic mechanism, thus providing targets for prevention and control of myxoboliosis.

Molecular and light microscopy evidence for the transfer of Myxobolus honghuensis from Carassius auratus gibelio broodfish to progeny

Myxozoans usually have a complex life cycle involving indirect transmission between vertebrate and invertebrate hosts. The vertical transmission of these parasites in vertebrate hosts has not been documented so far. Here, we assessed whether the myxozoan parasite Myxobolus honghuensis is vertically transmitted in naturally infected allogynogenetic gibel carp Carassius auratus gibelio (Bloch). M. honghuensis infection of broodfish, fertilized eggs and laboratory-cultured progeny was monitored in 2018 and 2019. The presporogonic stage was microscopically observed in the pharynx of broodfish and their progeny. In situ hybridization confirmed the presence of M. honghuensis presporogonic stage in the pharynx of broodfish and progeny. Nested PCR results showed that M. honghuensis was present in tissues and eggs of broodfish, fertilized eggs and their corresponding progeny. The sequences obtained from broodfish and progeny showed 98.0-99.8% similarity with ITS-5.8S rDNA of M. honghuensis. This study provides molecular and light microscopy evidence for the transfer of M. honghuensis from broodfish to progeny via the eggs, but it is insufficient to assert that M. honghuensis can transmit vertically in naturally infected allogynogenetic gibel carp. This is the first record about vertical transfer of myxozoan in the vertebrate host.

RNA-seq analysis of local tissue of Carassius auratus gibelio with pharyngeal myxobolosis: Insights into the pharyngeal mucosal immune response in a fish-parasite dialogue

The lack of practical control measures for pharyngeal myxobolosis is becoming an important limiting factor for the sustainable development of the gibel carp (Carassius auratus gibelio) culture industry in China. Myxobolus honghuensis has been identified as the causative agent of this pandemic disease, which exclusively infects the pharynx of gibel carp, a potential important mucosal lymphoid-associated tissue (MLAT). Myxozoa generally initiate invasion through the mucosal tissues of fish, where some of them also complete their sporogonial stages. However, the pharynx-associated immune responses of teleost against myxosporeans infection remain unknown. Here, a de novo transcriptome assembly of the pharynx of gibel carp naturally infected with M. honghuensis was performed for the first time, using RNA-seq. Comparative analysis of severely infected and mildly infected pharyngeal tissues (SI group and MI group) from the same fish individuals and control pharyngeal tissues (C group) from the uninfected fish was carried out to investigate the potential mucosal immune function of the fish pharynx, and characterize the panoramic picture of pharynx local mucosal immune responses of gibel carp against the M. honghuensis infection. A total of 242,341 unigenes were obtained and pairwise comparison resulted in 13,009 differentially-expressed genes (DEGs) in the SI/C group comparison, 6014 DEGs in the MI/C group comparison, and 9031 DEGs in the SI/MI group comparison. Comprehensive analysis showed that M. honghuensis infection elicited a significant parasite load-dependent alteration of the expression of numerous innate and adaptive immune-related genes in the local lesion tissue. Innate immune molecules, including mucins, toll-like receptors, C-type lectin, serum amyloid A, cathepsins and complement components were significantly up-regulated in the SI group compared with the C group. Up-regulation of genes involved in apoptosis signaling pathway and the IFN-mediated immune system were found in the SI group, suggesting these two pathways played a crucial role in innate immune response to M. honghuensis infection. Up-regulation of chemokines and chemokine receptors and the induction of the leukocyte trans-endothelial migration pathways in the severely and mildly infected pharynx suggested that many leucocytes were recruited to the local infected sites to mount a strong mucosal immune responses against the myxosporean infection. Up-regulation of CD3D, CD22, CD276, IL4/13A, GATA3, arginase 2, IgM, IgT and pIgR transcripts provided strong evidences for the presence of T/B cells and specific mucosal immune responses at local sites with M. honghuensis infection. Our results firstly demonstrated the mucosal function of the teleost pharynx and provided evidences of intensive local immune defense responses against this mucosa-infecting myxosporean in the gibel carp pharynx. Pharyngeal myxobolosis was shaped by a prevailing anti-inflammatory response pattern during the advanced infection stages. Further understanding of the functional roles of fish immune molecules involved in the initial invasion and/or final sporogony site may facilitate future development of control strategies for this myxobolosis.

Myxobolus pelecicola Voronin et Dudin 2015 is a junior synonym of Myxobolus ladogensis Rumyantsev et Schulman 1997 (Myxosporea: Myxobolidae) infecting the skeletal muscle of sichel Pelecus cultratus (Actinopterygii: Cyprinidae) in Russia

Myxobolus pelecicola Voronin et Dudin, 2015 was recently described from the skeletal musculature of sichel Pelecus cultratus. However, another species, Myxobolus ladogensis Rumyantsev et Schulman, 1997, was described previously from the same host, displaying identical tissue localization and spore morphology as in M. pelecicola. Unfortunately, M. ladogensis was overlooked when M. pelecicola was described, resulting in the superfluous description of the latter species, which, according to the International Code of Zoological Nomenclature, is a junior synonym of M. ladogensis. The description of M. ladogensis is supplemented with SSU rDNA sequence analysis supporting the conspecificity with M. pelecicola. The closest relatives of Myxobolus ladogensis (syn. M. pelecicola) include several muscle-infecting Myxobolus spp. with sequence similarity below 97%.

Molecular characterization of Myxoboluscatmrigalae (Myxosporea: Myxobolidae) infecting the gill lamellae of carp Cirrhinusmrigala (Hamilton)

The present study attempted sequencing the 18S rRNA gene of Myxoboluscatmrigalae infecting the gill lamellae of carp, Cirrhinusmrigala and compared its genetic homology and phylogenetic characteristics with 18S rRNA genes of other Myxobolus spp. The infected fish had up to 3 small, creamy white plasmodia per gill filament with 30-50 spores each. The spore size was 17.90 ± 0.70 × 7.40 ± 0.40 μm. The sporoplasm contained two large nuclei of size 0.57 ± 0.09 μm and no iodinophilous vacuole. The DNA sequence of M.catmrigalae was clustered phylogenetically with other Myxobolus spp. infecting the gills of cyprinids available in GenBank, which showed 77-87 % homogeneity. On the phylogenetic tree, M.catmrigalae (KC933944) was clustered with M.pavlovskii (HM991164) infecting the gill lamellae of silver carp, Hypophthalmichthysmolitrix. The species most closely related to M.catmrigalae in GenBank was M.pavlovskii (AF507973) infecting the gill lamellae of big head carp, Aristichthysnobilis with 87 % homogeneity. This is the first report on molecular characterization of gill lamellae infecting M. catmrigalae.

Description of a new Neoactinomyxum type actinosporean from the oligochaete Branchiura sowerbyi Beddard

Actinosporean infection of oligochaetes living in the mud of a commercial gibel carp pond with myxosporean disease was studied. Six actinospore types were detected exclusively from the oligochaete Branchiura sowerbyi Beddard with very high prevalence (18%). Five out of the six types were identified as the same actinosporeans described in previous reports, the sixth actinosporean was identified as a new Neoactinomyxum type and described here based on morphological and molecular characterisation. Spore body of the actinospore was globular, much smaller than caudal processes. Three caudal processes were disc-like in apical view, hemispherical in side view, closer together and encircling the spore body. The number of sporoplasm cells was detected as eight in one specimen. The new actinosporean markedly differed from other Neoactinomyxum types in literature having much bigger caudal processes. DNA sequence analyses further confirmed the morphological identification, and revealed the actinosporean described here (KU641392) possessed less than 94% sequence similarity with myxozoans available in the GenBank database.

Histopathological and ultrastructural studies of Myxobolus turpisrotundus from allogynogenetic gibel carp Carassius auratus gibelio in China

During an ongoing systematic survey on species diversity of myxozoans parasitising allogynogenetic gibel carp Carassius auratus gibelio (Bloch) in China, plasmodia were detected in the fins, lip, jaw, gill chamber, gill arches, operculum and oral cavity of infected fish. Combining the morphological and molecular data, the present species was identified as Myxobolus turpisrotundus Zhang, Wang, Li et Gong, 2010. Histopathological examination revealed that despite infecting different organs, M. turpisrotundus always occurred in dermis, demonstrating its affinity to this tissue. Histopathological effect of M. turpisrotundus on the host is relatively mild except parasites in the gill arches producing compression of the adipose tissue and heavy adductor muscles deformation with lymphohistiocytic infiltrates. In addition, the plasmodia in different sites were with the same complex structure arrangement: cup-like cells with unknown derivation, a thin collagenous fibril layer, areolar connective tissue, basement membrane and host epithelial cell. Ultrastructural analysis showed that the parasite has monosporic pansporoblast and sporogenesis followed the usual pattern of most of the myxosporeans.

Morphological and molecular characterization of actinosporeans infecting oligochaete Branchiura sowerbyi from Chinese carp ponds

We surveyed the actinosporean stages of fish myxosporeans at fish farms in Jiangsu Province, China, from 2011 to 2014. During the surveys, we identified 7 actinosporean types from 4 collective groups: echinactinomyxon (1 type), triactinomyxon (1 type), aurantiactinomyxon (1 type), and neoactinomyxum (4 types), released by the oligochaete Branchiura sowerbyi. The morphological characteristics and DNA sequences of these types are described here. Based on 18S rDNA sequence analysis, the actinosporean of echinactinomyxon type CZ with 4 branches at the end of the caudal processes was identified as Myxobolus wulii, and the neoactinomyxum type JD was identified as Thelohanellus wangi Yuan, Xi, Wang, Xie, Zhang, 2015 (JX458816), a recently nominated species from the gills of allogynogenetic gibel carp Carassius auratus gibelio. In addition, actinosporeans of aurantiactinomyxon type JD, neoactinomyxum type CZ-1, neoactinomyxum type CZ-2, and neoactinomyxum type CZ-3 showed high genetic similarity to T. wuhanensis (96.3-96.5%), T. nikolskii (98.0-99.1%), T. wuhanensis (97.8-98.9%), and T. hovorkai (98.7-98.9%), respectively. Phylogenetic analyses showed that these actinosporeans were robustly clustered in the Thelohanellus spp. clade.

Henneguya doneci (Myxosporea: Bivalvulida) in the gill filaments of Prussian carp Carassius gibelio (Bloch) from the upper Yellow River running through Inner Mongolia, China

We examined 11 Prussian carp, Carassius gibelio (Bloch), from the upper Yellow River running through Inner Mongolia (Wuhai City) to record myxosporean species. Between 6 and 15 elongated cysts of Henneguya doneci were located at the basal part of the gill filaments of 3 carp (27.3%); no more myxosporean plasmodia were found in other organs. Although the morphology and morphometric values of the spores (average measurements of 14 spores in µm: 11.4 long by 9.2 wide with 7.5 in thickness; 2 polar capsules, equal, 5.5 long by 3.2 wide; and a bifurcated caudal process, 51.6 long) with an evident intercapsular appendix were basically coincident with the species, the dimensions of the spore bodies were marginally larger, and the length of the caudal processes was distinctly longer than previously reported values for H. doneci (44.2-59.2 µm vs. 26.8-42.6 µm, respectively). Genetic analysis of the 18S ribosomal RNA gene (rDNA) found few nucleotide substitutions when compared with 3 deposited sequences of H. doneci collected around the Yangtze River (Sichuan and Hubei Provinces), China, indicating that the uniqueness of some of the morphological features exhibited by the present Wuhai isolate should be ascribed to intraspecific variation.

Thelohanellus wangi n. sp. (Myxozoa, Myxosporea), a new gill parasite of allogynogenetic gibel carp (Carassius auratus gibelio Bloch) in China, causing severe gill myxosporidiosis

We describe here a new myxozoan, Thelohanellus wangi n. sp., infecting the allogynogenetic gibel carp, Carassius auratus gibelio (Bloch), in a fry nursery farm in Jiangsu Province, China. Polysporous gray white round or ovoid plasmodia, 500-1,500 μm in size, were found exclusively in the gill filaments. The diagnostic characteristics of the myxospores are as follows: spore melon seed shaped in frontal view with smooth surface and asymmetrical valves; convex-shaped in sutural view with straight or slightly bent and thick sutural line, averaging 20.2 (16.5-22.3) × 9.9 (9.1-10.8) × 9.3 μm (8.2-10.4) in size; and one elliptical polar capsules with subulate anterior end and round posterior end, averaging 10.1 (8.4-11.2) × 6.5 μm (6.1-7.0) in size. Polar filaments coil six to seven turns and are slightly oblique to the longitudinal axis of the polar capsule. An elongate melon seed-shaped mucus envelope surrounds almost completely the spore, but with largest distance in posterior end between spore valve and mucus envelope. Granular sporoplasm contains two slightly oval nuclei, and no iodinophilous vacuole is observed. Histopathological analysis showed that the plasmodia embedded in the gill filaments and occasionally extended into the connective tissue of the gill arch. No significant inflammatory responses were provoked by the infection. The development of parasite is asynchronous, with mature spores centrally locating and trophozoites and pre-sporogonic stages peripherally locating within the plasmodia. Overall prevalence was 74% (37/50). Infection intensity was usually high, with about 10-30 cysts per fish. Phylogenetical analysis shows that T. wangi n. sp. is most closely related to several Thelohanellus species infecting cyprinid fish and formed a Thelohanellus clade based on the ribosomal DNA data. The cysts of this myxosporean were only found from fry seed with body length below 7 cm, generally from late April to early June when the fry fish of allogynogenetic gibel carp were nursed in the epizootic areas.

Remarks on the validity of Myxobolus ampullicapsulatus and Myxobolus honghuensis (Myxozoa: Myxosporea) based on SSU rDNA sequences

In the present study, we isolated three populations of Myxobolus ampullicapsulatus from the gills of crucian carp, Carassius auratus auratus, two from Yongchuan, Chongqing area and one from Poyang Lake, Jiangxi area, China, sequenced their complete small subunit ribosome RNA gene, analyzed their genetic distance and gene similarity, and explored their relationship based on Bayesian inference and maximum likelihood analyses of their small subunit ribosomal DNA. The results combined with their morphological characteristics suggest that M. ampullicapsulatus infecting the gills and pharynx of allogynogenetic gibel carp, Carassius auratus gibelio, should be Myxobolus honghuensis. This study highlights the importance of DNA sequence comparisons for distinguishing Myxobolus species and indicates that the intra-species identification for the two Myxobolus species mentioned in the present research should be less than ten variation sites. In morphology, M. honghuensis Liu et al. (2012) parasitic on the gills of C. auratus auratus (goldfish) was collected from Chongqing area, and its mature spore was 16.5–19.5 × 8.5–10.0 μm in size, polar capsule was 7.0–10.0 × 2.5–4.0 μm in size, and polar filament had 9–10 coils. M. honghuensis Liu et al. (2012) isolated from the pharynx of C. auratus gibelio was sampled in Hubei area, and its mature spore was 15.1–19.5 × 9.0–11.3 μm in size, polar capsule was 7.9–8.1 × 3.0–4.5 μm in size, and polar filament had 7–8 coils.

Myxozoan infection of the Malaysian mahseer, Tor tambroides, of Tasik Kenyir Reservoir, Malaysia: description of a new species Myxobolus tambroides sp.n

Tor tambroides, a common and appreciated cyprinid fish of the Tasik Kenyir water reservoir in Malaysia, is one of the species selected for propagation. This fish was first successfully propagated in Malaysia by the Department of Agriculture, Sarawak, Malaysia, and the breeding program continued throughout the country. The gills were frequently infected by a Myxobolus species to be described as Myxobolus tambroides sp. n. The small, 50 to 70 μm, round plasmodia of this species is located intralamellarly. Plasmodia were filled with pyriform myxospores, 9.9 and 7.4 μm wide. In sutural view, the caudal end of the myxospores had a distinctive valvular groove, parallel with the suture. Plasmodia caused deformations on the affected and the neighbouring gill lamellae. The 18S rDNA sequence of M. tambroides sp.n. did not show a close relationship with any other Myxobolus spp., represented in the GenBank. This might be an emerging parasite likely to impact the propagation of this fish.