Three actinosporean types (Myxozoa) from the oligochaete Branchiura sowerbyi in China

First record and description of actinospore stages (raabeia, triactinomyxon, and aurantiactinomyxon types) of fish parasitic myxozoans from Malaysia

During a 2-month survey in 2023 at Tasik Telabak, Terengganu, Malaysia three distinct actinospore types, namely raabeia, triactinomyxon and aurantiactinomyxon were identified in three invertebrate host species: Aulodrilus acutus, Branchiodrilus sp., and Bothrioneurum sp. utilizing morphometric and molecular analyses. Maximum likelihood of 18S rDNA positioned the raabeia type within the Myxobolus clade from fish of the Order Cypriniformes, suggesting a detected actinospore has a potential life cycle development in Cypriniformes and the genus Myxobolus. Both triactinomyxon and aurantiactinomyxon types were described solely based on morphology and morphometrics due to preservation error preventing the acquisition of 18S rDNA sequences. The triactinomyxon type in this study exhibited distinct morphology in spore shape and dimensions, characterized by a short style and caudal processes. Conversely, the aurantiactinomyxon type described herein possesses prominent elongated pyriform polar capsules not resembling any previously known aurantiactinomyxon types. These distinctive features, along with host species and geographical location justify their classification as novel types. Histological and microscopic analyses revealed the development of pansporocysts in the intestinal epithelium of the oligochaete host. This study marks the first descriptions of actinospore stages of myxozoans in Malaysia and the initial report of actinospores infecting host species of Aulodrilus acutus, Branchiodrilus sp. and Bothrioneurum sp.

Characterization of novel aurantiactinomyxon types (Cnidaria, Myxosporea) from the oligochaete Ilyodrilus templetoni (Southern, 1909), with a comprehensive phylogeny of the collective group

Three new aurantiactinomyxon types are described from the oligochaete Ilyodrilus templetoni (Southern, 1909) (Naididae) collected from a northern Portuguese estuary, based on light microscopy and sequencing of the 18S rDNA. The addition of I. templetoni to the group of freshwater annelids known to be permissive for aurantiactinomyxon development reinforces the crucial role of naidids in the evolution and settlement of myxozoans in estuarine environments. Maximum likelihood and Bayesian inference analyses of a comprehensive 18S rDNA dataset placed the novel types within the Paramyxidium clade. This positioning suggests them as probable life cycle counterparts to Paramyxidium spp. that most likely infect the European eel Anguilla anguilla, as the sole representative of Elopomorpha in Portuguese rivers. Although distance estimation revealed a genetic difference of only 0.4 % between Aurantiactinomyxon types 1 and 3, this value was determined to be representative of interspecific variability based on the consistent matching of both genotypes with distinct actinospore morphologies, and potential richness of closely related species of Paramyxidium infecting the European eel in Portuguese waters. The clustering of aurantiactinomyxon types within distinct myxosporean lineages, representative of the suborders Variisporina and Platysporina, demonstrates that the aurantiactinomyxon morphotype is highly functional in promoting myxozoan infections in estuarine environments.

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.

Two new species of Bacillidium (Microsporidia) from coelomocytes of Branchiura sowerbyi (Oligochaeta: Naididae) in China

Bacillidium spp. exclusively infect oligochaetes and these microsporidian pathogens are typically characterized by their rod-shaped spores. Seven Bacillidium spp. are presently reported from different organs of oligochaetes. Here, we describe two new Bacillidium species, Bacillidium sinensis n. sp. and Bacillidium branchilis n. sp., from coelomocytes of Branchiura sowerbyi. This is the first report of Bacillidium spp. in oligochaetes from China. Both species of Bacillidium elicit the formations of opaque xenoma-like lesions in coelomocytes of the host. A diplokaryotic nucleus occurs in all life stages of these two new Bacillidium species. Mature spores of B. sinensis are 15.9 ±0.6 (14.7-17.1) μm long (average ± standard error, range, n = 50) and 2.5 ±0.1 (2.3-2.7) μm wide in fresh preparations. A new type of exospore (sixteen-layered exospore) is discovered from B. sinensis n. sp. which is distinctly different from B. branchilis n. sp., and other Bacillidium spp. (double-layered exospore) reported previously. These two Bacillidium species are morphologically distinguished from each other and all Bacillidium spp. described previously in terms of hosts, infection sites, spore size, spore wall or polar filament thickness. BLASTn searches indicated that these two new microsporidian parasites are surprisingly most similar to Janacekia tainanus (94.76% for B. sinensis and 90. 2% for B. branchilis) isolated from the fat body of midge larva (Kiefferulus tainanus). Phylogenetic analysis demonstrates that the two novel taxons cluster with J. debaisieuxi, J. tainanus, and Bacillidium sp. within the Jirovecia-Bacillidium-Janacekia clade. Other available 18S rRNA gene sequences for microsporidia that infect oligochaetes include J. sinensis, B. vesiculoformis, Neoflabelliforma aurantiae, and Bacillidium sp., but these do not form a single cluster with B. sinensis and B. branchilis, but are instead dispersed through the clade. Based on the ultrastructural features and molecular characteristics, two new species within the genus Bacillidium, B. sinensis n. sp. and B. branchilis n. sp., are designated.

Involvement of sphaeractinomyxon in the life cycle of mugiliform-infecting Myxobolus (Cnidaria, Myxosporea) reveals high functionality of actinospore morphotype in promoting transmission

Four new actinospore types belonging to the sphaeractinomyxon collective group (Cnidaria, Myxosporea) are described from the coelomic cavity of a marine Baltidrilus sp. (Oligochaeta, Naididae) inhabiting a northern Portuguese estuary. Host identification supports the usage of marine oligochaetes, namely of the family Naididae Ehrenberg, 1828, as definitive hosts for myxosporeans inhabiting estuarine/marine environments. The absence of mixed infections in the host specimens analysed is suggested to reflect the influence of host-, parasite- and environmental-related factors regulating myxosporean-annelid interactions. Molecular analyses matched the SSU rDNA sequences of three of the four new types with those of mugiliform-infecting Myxobolus spp., namely Myxobolus mugiliensis and a Myxobolus sp. from flathead grey mullet Mugil cephalus, and Myxobolus labrosus from thicklip grey mullet Chelon labrosus. These results directly link, for the first time, the sphaeractinomyxon collective group to a myxospore counterpart, further confirming their previously hypothesized specific involvement in the life cycle of myxobolids that infect mullets. Acknowledging this life cycle relationship, the functionality of the sphaeractinomyxon morphotype is suggested to have been decisive for the evolutionary hyperdiversification of the genus Myxobolus in mullets. Unlike other actinospore morphotypes, sphaeractinomyxon lack valvular processes, which implies a limited capability for buoyancy. Considering the benthic-feeding nature of mullets, this feature is most likely crucial in promoting successful transmission to the vertebrate host.

Characterisation of sphaeractinomyxon types (Cnidaria: Myxozoa) from marine and freshwater oligochaetes in a Portuguese estuary, with the demise of the endocapsa collective group

Six types of sphaeractinomyxon are reported from the coelomic cavity of oligochaetes collected from the Minho River estuary in northern Portugal. Four new types are morphologically and molecularly described from freshwater species belonging to the genera Psammoryctides Hrabě and Potamothrix Vejdovský et Mrázek in the upper estuary, thus significantly increasing the number of known freshwater sphaeractinomyxon. In the lower estuary, sphaeractinomyxon types 8 and 10 of Rangel et al. (2016) are recorded infecting the marine oligochaete Tubificoides pseudogaster (Dahl). A single specimen of T. pseudogaster further displayed infection by one of the four new types found in the upper estuary, suggesting the involvement of sphaeractinomyxon in the life cycles of myxosporean species that infect migratory fish hosts. The acquisition of these second hosts is proposed to have allowed the myxosporean counterparts of sphaeractinomyxon to cross environmental barriers and conquer new habitats. Phylogenetic analyses of the SSU rRNA gene reveal the four new types clustering within the monophyletic clade of mugiliform-infecting myxobolids, strengthening the previously proposed involvement of the sphaeractinomyxon collective group in the life cycles of this specific group of myxosporeans. Endocapsa types also cluster within the latter clade, having actinospores that differ from those of sphaeractinomyxon only in the presence of valvular swellings that do not change when in contact with water. In this study, however, one type was found displaying actinospores with and without valvular swellings in the same oligochaete specimen. This overlap in actinospore morphology is given as grounds for the demise of the endocapsa collective group.

Transcriptomic analysis of oligochaete immune responses to myxosporeans infection: Branchiura sowerbyi infected with Myxobolus cultus

The Myxozoa are endoparasites characterized by a two-host life cycle that typically involves invertebrates and vertebrates as definitive and intermediate hosts, respectively. However, little is known about invertebrate-myxosporean interactions, particularly about patterns of host immune defense. We used RNA-sequencing to identify genes that are possibly involved in the immune responses of the oligochaete Branchiura sowerbyi naturally infected with Myxobolus cultus. De novo assembly of the B. sowerbyi transcriptome yielded 119,031 unigenes, with an average length of 896 bp and an N50 length of 1754 bp. Comparative transcriptome analysis revealed 4059 differentially expressed genes (DEGs) between M. cultus-infected and uninfected B. sowerbyi groups, including 3802 upregulated genes and 257 downregulated genes. Among the B. sowerbyi immune factors implicated in the responses to M. cultus infection, DEGs related to lectins, ubiquitin-mediated proteolysis, phagocytosis, oxidative-antioxidative responses, proteases, and protease inhibitors were upregulated. The expression of some immune-related molecules such as calmodulin, heat shock proteins, antimicrobial peptides, lysenin, and serum amyoid A protein were also significantly upregulated. The expression patterns of 14 immune-related DEGs identified by RNA-seq were validated by quantitative real-time polymerase chain reaction. This study is the first attempt to characterize the B. sowerbyi transcriptome and identify immune-related molecules possibly associated with M. cultus infection. It is also the first report of invertebrate host-myxosporean interactions at the transcriptomic level. Our results will facilitate the elucidation of adaptive evolution mechanisms of myxosporean parasites in the definitive host and the genetic basis for differences in resistance of invertebrate hosts of different genotypes to a myxosporean species.

New actinosporean description prompts union of the raabeia and echinactinomyxon collective groups (Cnidaria, Myxozoa)

We describe, morphologically and molecularly, a new actinosporean from the intestinal epithelium of the freshwater oligochaete Ilyodrilus templetoni in the upper estuary of the River Minho, Northern Portugal. Mature actinospores resembled both echinactinomyxon and raabeia types, emphasizing the previously known lack of a clear boundary between these 2 collective groups. Historically, raabeia and echinactinomyxon types have been differentiated solely based on the shape of the valvular processes being curved or straight, respectively. Our observations, however, show that this morphological character is too variable for distinguishing between these 2 collective groups, since the actinospores of the raabeia described here displayed valvular processes that could either be straight, downward or upward curved. Several similar cases can be found in the available literature. Considering this overlap in actinospore morphology, we propose that echinactinomyxon be deemed invalid and its types be included in raabeia, as the latter constitutes the older of the 2 groups. Known echinactinomyxon types, however, should not be renamed as raabeia, as this would create unnecessary confusion. Accordingly, a more comprehensive definition of the raabeia collective group is provided. Phylogenetic analyses revealed polyphyletic clustering of raabeia/echinactinomyxon types among members of the myxosporean suborders Variisporina and Platysporina, reiterating the lack of agreement between actinosporean morphotypes and myxosporean genera. The new type described here specifically clusters within the Paramyxidium clade, alongside other SSU rDNA sequences of raabeia, echinactinomyxon, aurantiactinomyxon and synactinomyxon. Considering that most Paramyxidium spp. parasitize Anguilla anguilla (Linnaeus, 1758), future myxozoan surveys in the River Minho should include this species.

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.

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.

Morphological, histopathological and molecular characterization of Thelohanellus theinensis n. sp. (Cnidaria: Myxosporea) infecting an Indian major carp, Labeo bata in a cold water wetland in Punjab (India)

A myxozoan parasite belonging to the genus Thelohanellus Kudo, 1933 (Myxosporea, Bivalvulida) was isolated from the gill lamellae of Labeo bata (Hamilton, 1822) inhabiting Ranjit Sagar Wetland in Northern parts of Punjab, India. Histopathology of the infected gills indicated lamellar distortion due to the presence of plasmodia. The intensity of infection was recorded to be moderate as indicated by gill plasmodial index (GPI = 2). The plasmodia were microscopic, ovoid, creamish white, 2.0-4.0 mm in diameter containing 300-400 number of myxospores. The myxospores were broadly pyriform in shape, measuring 8.27 × 4.41 µm, having a prominent pore at the anterior end. The polar capsule was pyriform in shape, measuring 4.15 × 4.10 µm, containing a polar filament coiled perpendicular to the longitudinal axis of myxospore body making 6-7 turns. At molecular level, the analysis of 18S rDNA sequence of T. theinensis n. sp. demonstrated maximum homogeneity of 99 % with T. filli (KR340464) as only 2 gaps were found due to the difference of the last 20 base pairs at the 3' end of T. theinensis n. sp. The current study emphasizes the need for caution when dealing with members of species complex.

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.

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.

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.

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.

Description of raabeia, synactinomyxon and neoactinomyxum developing stages of myxosporeans (Myxozoa) infecting Isochaetides michaelseni Lastočkin (Tubificidae) in Lake Balaton and Kis-Balaton Water Reservoir, Hungary

Molecular and morphometric investigations were conducted on the actinosporean morphotypes of myxosporeans surveyed in oligochaetes of Lake Balaton and Kis-Balaton Water reservoir. Oligochaetes belonging to the species Isochaetides michaelseni Lastočkin and Branchiura sowerbyi Beddard as well as to the genera Nais Dujardin, Dero Müller and Aeolosoma Ehrenberg were studied during an 18-month period. Actinosporeans were obtained exclusively from I. michaelseni (7,818 specimens) with very low prevalence (0.01-0.06%). Four new actinosporean morphotypes of the collective groups raabeia (2 types), synactinomyxon (1 type) and neoactinomyxum (1 type) were found and described, including the first synactinomyxon collective group from Hungarian biotopes and a new raabeia morphotype. Except for Synactinomyxon type 1, the 18S rDNA analysis revealed that the spores did not match any myxospore entity found in the GenBank.