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Myxobolus miyairii Kudo, 1919 was first reported from the intestines of the Amur catfish (Silurus asotus) in Japan and then in China and Russia, but with incomplete description. During the investigation of fish myxosporean diversity in Poyang Lake, the biggest lake along the Yangtze River, China, two Amur catfish highly infected with M. miyairii in the intestine wall were sampled. So, the complete description of this species with morphological and molecular data was presented here. A large number of whitish, round or ellipsoidal pseudocysts 0.32-0.78 mm in diameter could be found in the external intestinal wall after dissecting the infected fish. Mature spores of M. miyairii were elongated and ellipsoidal in the frontal view and narrow fusiform in the lateral view, with a slightly pointed anterior end and a bluntly rounded posterior end and measured 13.3 ± 0.49 (12.5-14.7) μm × 6.6 ± 0.27 (6.2-7.4) μm × 5.0 ± 0.26 (4.4-5.7) μm in size. Spore surface was smooth and two spore valves symmetrical, with a thin and straight sutural ridge. Interestingly, two types of caudal appendage (single or bifurcated) were occasionally present on the posterior end of some spores which has not previously been reported. The two equal pyriform polar capsules measured 6.5 ± 0.30 (6.2-7.5) μm long and 1.9 ± 0.14 (1.5-2.3) μm wide and situated at the anterior end of the spore. Polar filaments coiled with eight to nine turns, perpendicularly to the longitudinal axis of the polar capsules. Histopathological analysis showed that the plasmodium developed in the circular muscle layer of intestinal wall of Amur catfish, but no obvious inflammatory responses were observed. Phylogenetic analysis based on the partial 18S small subunit ribosomal DNA sequences indicates that M. miyairii cluster within a clade of Siluriforme-infecting Henneguya species with the support of a high bootstrap value, but also evolutionarily independent from the Henneguya clade infecting the epithelium of fish of the Ictaluridae family. Additionally, Myxobolus species reported with caudal processes dispersed within the Henneguya-Myxobolus clade.

BackgroundMyxozoa is a well-known economically and ecologically important group of metazoan parasites, phylogenetically related to Cnidaria. High diversity of myxosporeans has been recorded in Russia and China; however, most of the species were solely morphologically characterised. Here, we identified a new gibel carp-infecting Myxobolus species and morphologically and molecularly compared the Russian and Chinese isolates of this new myxosporean.Results Myxobolus pronini n. sp. was found free in the abdominal cavity of Carassius auratus gibelio (Bloch, 1782) in Lake Baikal watershed, Russia, and embedded in the visceral serous membranes of the same fish species in Lake Taibai, Hubei province, China. The morphometric data of the plasmodia and mature spores exhibited some differences between the Russian and Chinese isolates, but SSU rDNA sequences indicated that these two geographical isolates are conspecific. The mature spores from the two locations are obovate in frontal view, with wider anterior than posterior end and lemon-shaped in sutural view. Spores of the Russian isolate were 14.3–16.2 (mean 15.1 ± 0.2) μm long, 9.6–10.8 (10.1 ± 0.1) μm wide and 6.4–7.4 (6.7 ± 0.15) μm thick; those of the Chinese isolate were 13.8–15.6 (14.7 ± 0.24) μm long, 9.6–13.3 (9.6 ± 0.65) μm wide and 6.2–7.2 (6.6 ± 0.16) μm thick. The newly-generated rDNA sequences (including SSU rDNA, ITS and LSU rDNA) from the two isolates represented some variations within the intraspecific range. Homology search by BLAST showed that the newly obtained rDNA sequences do not match any sequences available on GenBank. Phylogenetic analysis based on the aligned partial SSU rDNA sequences indicated that this novel species clustered with several gibel carp-infecting Myxobolus spp. with round anterior end of spores. Additionally, phylogenetic analysis based on all obtained ITS sequences showed that distinct genetic geographical differentiation occurred for this new parasite.Conclusions Myxobolus pronini n. sp. is described by integrating morphological, ecological and molecular evidence. Two geographical isolates of this species showed some morphological and genetic differences but within the intraspecific range of variation.

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A new myxobolid species, Myxobolus zaikae n. sp., was found in the connective tissue near the kidney and liver blood vessels of the common roach Rutilus rutilus, while fish myxosporean fauna were being investigated in Lake Baikal, Russia. The parasites were studied on the basis of spore morphology as well as with histological and molecular methods. Mature spores of M. zaikae n. sp. are round or ellipsoidal in the frontal view and lemon-shaped in the lateral view, measuring 11.37 ± 0.11 µm (10.2-14.0 µm) in length, 10.29 ± 0.10 µm (9.6-11.0 µm) in width, and 6.3 ± 0.08 µm (5.8-7.1 µm) in thickness (mean ± SD; n = 50). Polar capsules are equal and pyriform, measuring 4.5 ± 0.07 µm (3.4-5.2 µm) in length and 2.9 ± 0.03 µm (2.6-3.3 µm) in width. Polar capsules contained polar filaments coiled with 5 to 6 turns. Phylogenetic analysis showed that this newly described species clusters with other myxobolid species infecting the connective tissue of different organs from Palearctic cyprinid fish.

Background The phylum Cnidaria consists of several morphologically diverse classes including Anthozoa, Cubozoa, Hydrozoa, Polypodiozoa, Scyphozoa, Staurozoa, and Myxozoa. Myxozoa comprises two subclasses of obligate parasites – Myxosporea and Malacosporea, which demonstrate various degrees of simplification. The subclass Myxosporea is especially simplified, lacking any normal multicellular body plan and embryonic development. Myxosporea lack the majority of core protein domains of apoptotic proteins including caspases, Bcl-2 and APAF-1 homologs. Other sequenced Cnidaria, including the parasite Polypodium hydriforme from Polypodiozoa do not share this genetic feature. Whether this loss of core apoptotic proteins is unique to Myxosporea or also present in its sister subclass Malacosporea was not previously investigated. It is also unclear whether Myxosporea lost their apoptotic proteins abruptly or gradually (starting with their common ancestors with Malacosporea) during their evolutionary history. Results Using comparative genomics and transcriptomics approaches we describe a detailed picture of apoptotic protein gene loss in parasitic cnidaria in relation to each other and free-living members of the phylum. We show that Polypodium hydriforme lost the main components of the extrinsic apoptotic pathway such as death receptor and adaptor proteins. For the intrinsic pathway it has one predicted initiator and one effector caspase, compared to 2 and 11 in Hydrarespectively. Malacosporea retain one predicted effector caspase while Myxosporea universally lack all main actors of apoptosis including caspases, Bcl-2 family proteins, calpains, inhibitors of apoptosis proteins (IAPs), APAF-1 and p53 homologs. As an exception some Myxosporea species retained potentially functional cytochrome C, whose gene is however absent in Myxobolus squamalis, Henneguya salminicola and is a pseudogene with multiple inner stop-codons in Kudoa iwatai, Sphaeromyxa zaharoni, and Enteromyxum leei. Conclusions We show that the presence of core apoptotic proteins (such as the number of different caspases) gradually diminishes from free-living Cnidaria to Polypodium to Malacosporea to Myxosporea. This observation does not favor the hypothesis of catastrophic simplification of Myxosporea at the genetic level, but rather supports a stepwise adaptation to parasitism that likely started from early parasitic ancestors that gave rise to Myxozoa.