Small Subunit Ribosomal RNA Sequences Unite Alternate Actinosporean and Myxosporean Stages of Myxobolus cerebralis the Causative Agent of Whirling Disease in Salmonid Fish

ABSTRACT. The alternating myxosporean and actinosporean stages of the myxozoan parasitc Myxobolus cerebralis (Hofer 1903) from its salmonid fish and aquatic oligochaete hosts, respectively, were compared for sequence homology of the small subunit (18S) ribosomal RNA genes. A 99.8% similarity between the sequences of these two stages was substantially greater than that of M. cerebralis compared to two other Myxobolus sp. from salmonid fish. Our results are the first molecular evidence confirming the alternating stages initially described by Wolf and Markiw [25] for the life cycle of M. cerebralis but found in two different taxonomic classes (Myxosporea and Actinosporea) are indeed forms of the same organism. Sequencing of rRNA genes of the actinosporean stage followed by development of specific primers for DNA amplification of the myxosporean stage, as in our study, should be applied to solve other myxozoan life cycles. Additionally, these approaches will in the future provide useful diagnostic reagents for the detection and study of this important group of fish pathogens.     

Myxobolus cerebralis internal transcribed spacer 1 (ITS-1) sequences support recent spread of the parasite to North America and within Europe

Molecular approaches for resolving relationships among the Myxozoa have relied mainly on small subunit (SSU) ribosomal DNA (rDNA) sequence analysis. This region of the gene is generally used for higher phylogenetic studies, and the conservative nature of this gene may make it inadequate for intraspecific comparisons. Previous intraspecific studies of Myxobolus cerebralis based on molecular analyses reported that the sequence of SSU rDNA and the internal transcribed spacer (ITS) were highly conserved in representatives of the parasite from North America and Europe. Considering that the ITS is usually a more variable region than the SSU, we reanalyzed available sequences on GenBank and obtained sequences from other M. cerebralis representatives from the states of California and West Virginia in the USA and from Germany and Russia. With the exception of 7 base pairs, most of the sequence designated as ITS-1 in GenBank was a highly conserved portion of the rDNA near the 3-prime end of the SSU region. Nonetheless, the additional ITS-1 sequences obtained from the available geographic representatives were well conserved. It is unlikely that we would have observed virtually identical ITS-1 sequences between European and American M. cerebralis samples had it spread naturally over time, particularly when compared to the variation seen between isolates of another myxozoan (Kudoa thyrsites) that has most likely spread naturally. These data further support the hypothesis that the current distribution of M. cerebralis in North America is a result of recent introductions followed by dispersal via anthropogenic means, largely through the stocking of infected trout for sport fishing.     

Comparison of 18S and ITS-1 rDNA sequences of selected geographic isolates of Myxobolus cerebralis.

Myxobolus cerebralis, the myxosporean parasite-causing salmonid whirling disease, was first reported among rainbow trout (Oncorhynchus mykiss) in Germany in 1903. The parasite was reported for the first time in North America in 1958 among hatchery-reared trout in the eastern USA, presumably arriving with frozen trout shipments from Europe. A comparison of 18S and ITS-1 ribosomal DNA sequences was conducted to identify potential strain differences between selected geographic isolates of this parasite from Europe and North America. Only fourteen of 1700 base pairs were different in the 18S rRNA gene from isolates obtained from California and West Virginia in the USA, and the Federal German Republic. No evidence for strain differences was obtained from ITS-1 sequences that were found to be identical among all parasite isolates. This finding is consistent with the hypothesis that the parasite was recently introduced to the USA from Europe.     

Recent Advances in Our Knowledge of the Myxozoa

In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis. and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete. Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that: 1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.     

黄颡单尾虫(粘体门,双壳目)的重描述及基于28S rDNA和ITS-5.8S序列的系统地位分析

采用形态分类学方法与以28S rDNA和ITS-5.8S序列为基础的分子系统学研究方法,对采自嘉陵江重庆市磁器口江段的黄颡单尾虫Unicauda pelteobagrusMa,1998进行了形态学和分子生物学的研究。基于28S rDNA数据探讨了黄颡单尾虫以及单尾虫属与相邻种属粘孢子虫间的系统地位;基于5.8S rDNA数据比较分析了粘孢子虫的系统地位。补充了黄颡单尾虫重庆种群形态学信息和28S rDNA、ITS-5.8S rDNA序列的分子信息。     

Relationships among members of the genus Myxobolus (Myxozoa: Bilvalvidae) based on small subunit ribosomal DNA sequences

Sequences representing approximately 1,700 base pairs of the 18S rRNA gene from 10 different species in the genus Myxobolus were found to group them into 3 clusters that showed little correlation with spore morphology and size or host specificity, criteria currently used for both higher and lower taxonomic placements in the Myxozoa. Of the phenotypic criteria examined, tissue tropism was most correlated with the rRNA groupings observed. Spores of similar size and shape (Myxobolus cerebralis vs. Myxobolus squamalis) were distantly related in some instances, whereas spores with divergent morphology and size were sometimes found to be closely related (M. cerebralis and Myxobolus insidiosus). These initial investigations into the phylogenetic relationships of putative members of the genus Myxobolus clearly indicate the potential limitations of groupings based on size and morphological properties of the spores and host species infected. We propose that 18S rRNA gene sequences, combined with information on tissue tropism, host species infected, and developmental cycles in the fish and alternate host (when and if known) be given greater consideration in taxonomic placements of myxosporeans.     

Geographical variation in spore morphology, gene sequences, and host specificity of Myxobolus arcticus (Myxozoa) infecting salmonid nerve tissues

Myxobolus arcticus Pugachev and Khokhlov, 1979 is a freshwater myxosporean parasite infecting the nerve tissues of salmonid fishes throughout the Pacific region of Far East Asia and North America. The principal fish host is sockeye salmon Oncorhynchus nerka in North America and masu salmon O. masou in Japan. Actinospores of M. arcticus were isolated from the lumbriculid oligochaetes Lumbriculus variegatus and Stylodrilus heringianus in Japan and Canada, respectively. Morphological comparisons indicated that Japanese actinospores from L. variegatus have significantly shorter caudal projections than Canadian isolates from S. heringianus, whereas the corresponding myxospores are indistinguishable. Transmission experiments showed that sockeye salmon were rarely susceptible to the Japanese actinospores, while masu salmon are highly susceptible to this parasite. Sequences of 4560 base pairs of the ribosomal RNA (rRNA) gene, including small subunit (SSU) and internal transcribed spacer (ITS) regions, from Japanese and Canadian isolates had a high similarity over 99.9%, suggesting that they may be conspecific. However, the biological data indicate that they are at least distinct strains. M. arcticus may be geographically isolated due to the specific homing migration of the anadromous fish hosts and has specialized its morphology and host selection for its local environment in the ongoing process of differentiation, potentially leading to speciation.     

Molecular analysis of a haplosporidian parasite from cultured New Zealand abalone Haliotis iris

In the Austral summer and autumn of 2000 and 2001, mortalities of black-footed abalone Haliotis iris (Martyn, 1784) occurred in a commercial facility in New Zealand. Histological analyses suggested that infection by a haplosporidian parasite was responsible. To confirm identification as a haplosporidian and to help determine if this parasite represented a new, undescribed species, DNA was extracted from infected host tissues scored as positive for infection by histological examination. Small-subunit rRNA (SSU rRNA) gene sequences from both the host abalone and a parasitic organism were amplified by PCR and characterized. Although the sequence for this parasite was novel, not matching any known SSU rRNA gene sequences, phylogenetic analyses strongly supported grouping this parasite with the haplosporidians. Parsimony analyses placed the parasite at the base of the phylum Haplosporidia, ancestral to Urosporidium crescens and the Haplosporidium, Bonamia, and Minchinia species. Sequencing of multiple parasite DNA clones revealed a single polymorphic site in the haplosporidian SSU rRNA gene sequence.     

The parasite that causes whirling disease, Myxobolus cerebralis, is genetically variable within and across spatial scales

Understanding the genetic structure of parasite populations on the natural landscape can reveal important aspects of disease ecology and epidemiology and can indicate parasite dispersal across the landscape. Myxobolus cerebralis (Myxozoa: Myxosporea), the causative agent of whirling disease in the definitive host Tubifex tubifex, is native to Eurasia and has spread to more than 25 states in the USA. The small amounts of data available to date suggest that M. cerebralis has little genetic variability. We examined the genetic variability of parasites infecting the definitive host T. tubifex in the Madison River, MT, and also from other parts of North America and Europe. We cloned and sequenced 18S ribosomal DNA and the internal transcribed spacer-1 (ITS-1) gene. Five oligochaetes were examined for 18S and five for ITS-1, only one individual was examined for both genes. We found two different 18S rRNA haplotypes of M. cerebralis from five worms and both intra- and interworm genetic variation for ITS-1, which showed 16 different haplotypes from among 20 clones. Comparison of our sequences with those from other studies revealed M. cerebralis from MT was similar to the parasite collected from Alaska, Oregon, California, and Virginia in the USA and from Munich, Germany, based on 18S, whereas parasite sequences from West Virginia were very different. Combined with the high haplotype diversity of ITS-1 and uniqueness of ITS-1 haplotypes, our results show that M. cerebralis is more variable than previously thought and raises the possibility of multiple introductions of the parasite into North America.     

Henneguya mbakaouensis sp. nov., Myxobolus nounensis sp. nov. and M. hydrocyni Kostoingue & Toguebaye, 1994, Myxosporea (Myxozoa) parasites of Centropomidae, Cichlidae and Characidae (Teleosts) of the Sanaga basin in Cameroon (Central Africa)

The study of 102 teleost freshwater fishes of Sanaga basin in Cameroon revealed the presence of three myxosporean species, among which two were new. Host fishes were of three families: Centropomidae, Cichlidae and Characidae. New species were identified as Henneguya mbakaouensis sp. nov., a gill parasite of Lates niloticus and Myxobolus nounensis sp. nov. found in the kidney and spleen of Sarotherodon galilaeus and Tilapia mariae. Myxobolus hydrocyni Kosto?ngue & Toguebaye, 1994, previously described in Chad, was also found in Cameroon; complementary informations were given on that parasite which seemed to be specific to its host.     

A new parvicapsulid (Myxosporea) species in adult pink salmon, Oncorhynchus gorbuscha, from the Quinsam River, British Columbia, Canada

Myxospores consistent with species of Parvicapsula were observed in kidney of 15 of 95 (15.8%) adult pink salmon, Oncorhynchus gorbuscha, collected from the Quinsam River, British Columbia, Canada. The spores were elongate and curved with unequal valves, and 2 spherical-to-subspherical polar capsules within a highly refractile capsular region. The spores were unlike those of P. minibicornis found in nearby populations of Pacific salmon, Oncorhynchus spp. The spore dimensions were similar to those of Parvicapsula pseudobranchicola from Atlantic salmon, Salmo salar, in Norway, and the spores seemed similar to an undescribed Parvicapsula sp. from several Oncorhynchus spp. in Puget Sound, Washington. A sequence of 1,480 base pairs (bp) of the small subunit ribosomal RNA gene (SSU rDNA) of the parasite from pink salmon was most similar to, but distinct from, that of other Parvicapsula spp. The parasite is described as a new species, Parvicapsula kabatai n. sp. Polymerase chain reactions amplified a 158-bp sequence, unique to P. kabatai n. sp., from 22 of 93 (23.7%) adult pink salmon kidney samples, from 3 of 3 juvenile pink salmon collected in the ocean 125 km north of the Quinsam River, and from 2 of 5 archival coho salmon, Oncorhynchus kisutch, samples from Puget Sound. The parasite occurs within the lumen and epithelium of renal tubules and ducts, and within the renal interstitium. Concurrent infections with extrasporogonic stages of the myxosporean CKX, the microsporidian Loma salmonae, and a Myxidium sp. also were observed in the adult pink salmon.     

Description of two new gill myxozoans from smallmouth (Micropterus dolomieu) and largemouth (Micropterus salmoides) bass

Two previously undescribed species of myxozoan parasites were observed in the gills of bass inhabiting the Potomac and James River basins. They are described using morphological characteristics and small-subunit (SSU) rDNA gene sequences. Both were taxonomically identified as new species of Myxobolus; Myxobolus branchiarum n. sp. was found exclusively in smallmouth bass, and Myxobolus micropterii n. sp. was found in largemouth and smallmouth bass. Small, spherical, white plasmodia of M. branchiarum from smallmouth bass were observed grossly in the gills; these plasmodia had an average length of 320.3 μm and width of 246.1 μm. The development of the plasmodia is intralamellar in the secondary lamellae of the gills. Mature spores were pyriform in shape with a length of 12.8 ± 1.4 (8.1-15.1) μm and width of 6.9 ± 1.1 (4.0-9.0) μm. Analysis of SSU rDNA identified M. branchiarum in a sister-group to 3 species of Henneguya , although morphologically caudal appendages were absent. Myxobolus micropterii observed in the gills of largemouth and smallmouth bass had larger, ovoid, cream-colored plasmodia with an average length of 568.1 μm and width of 148.1 μm. The cysts developed at the distal end of the gill filament within the primary lamellae. The mature spores were ovoid in shape with a length of 10.8 ± 0.7 (9.2-12.2) μm and width of 10.6 ± 0.6 (9.0-11.8) μm. SSU rDNA analysis placed M. micropterii in a sister group with Henneguya lobosa and Myxobolus oliveirai . The highest prevalence of M. branchiarum was observed in the gills of bass collected from the Cowpasture River (50.9%). Prevalence was 44.6% in bass from the Potomac River and only 4.3% in bass collected from the Shenandoah River. A seasonal study of M. branchiarum , which included both infected and uninfected smallmouth bass, determined that a significantly higher intensity was observed in the spring than in the summer (P < 0.001) or fall (P = 0.004). In an analysis excluding uninfected bass, a higher intensity was observed in the spring than in the summer (P = 0.001) or fall (P = 0.008). Prevalence and seasonal differences were not determined for M. micropterii .     

Sequence analysis of OmNramp alpha and quantitative expression of Nramp homologues in different trout strains after infection with Myxobolus cerebralis

Salmonid whirling disease caused by the metazoan parasite Myxobolus cerebralis is an ongoing problem in wild and farmed rainbow trout Oncorhynchus mykiss populations. Rainbow trout from different strains vary in susceptibility to the parasite. Identification of underlying mechanisms could be a starting point for improved control of the disease. We conducted infection trials using 2 rainbow trout strains and brown trout Salmo trutta fario, a species not susceptible to the parasite, to investigate host immune response and resistance mechanisms. We compared expression levels of 2 natural resistance-associated macrophage proteins (Nramp alpha and beta) after infection with M. cerebralis. Total RNA was extracted from skin, muscle, kidney, head and spinal column, and gene expression was quantified by real-time PCR. Significant decreases in expression of both genes were observed at different time points in the infected susceptible rainbow trout compared to the non-infected group. Furthermore, the OmNramp alpha (O. mykiss natural resistance-associated macrophage protein alpha) sequences in 2 resistant and 1 non-resistant rainbow trout strain were analysed and compared for sequence aberrations.     

Myxobolus albi n. sp. (Myxozoa) from the Gills of the Common Goby Pomatoschistus microps Krøyer (Teleostei: Gobiidae)

ABSTRACT. A recent investigation into the myxozoan fauna of common gobies, Pomatoschistus microps , from the Forth Estuary in Scotland, revealed numerous myxosporean cysts within the gill cartilage. They were composed of polysporous plasmodia containing myxobolid spores that were morphologically different from the other known species of Myxobolus and from the myxosporeans previously recorded from this host (i.e. the ceratomyxid Ellipsomyxa gobii , infecting the gall bladder, and the kudoid Kudoa camarguensis , infecting the muscle tissues). Spores were ovoid, 9.4 × 9.1 μm with a thickness of 6.6 μm, with two pyriform polar capsules, the polar filaments of which had four to five turns. Molecular analysis of the parasite's small subunit rDNA region, based upon a contiguous sequence of 1,558 base pairs, discriminated it from other myxosporean species that have been characterized so far. A comparison of the spore morphology and the molecular sequences determined for this new isolate with other myxozoans described to date, confirmed its identity as a previously unknown myxobolid supporting the proposal that this isolate be elevated to the species level as a new species within the genus Myxobolus . A phylogenetic analysis places this new myxobolid, Myxobolus albi n. sp., in a basal position of a clade containing the majority of Henneguya spp. sequenced to date and various Myxobolus spp.     

Myxosporean infections in cultured tilapias in Israel

Five new species of myxosporean parasite are described from cultured tilapias in Israel. These are: Myxosoma sarigi, Myxosoma equatorialis, Myxobolus israelensis, Myxobolus agolus, and Myxobolus galilaeus. The first four were found in hybrids of Oreochromis aureus X Oreochromis niloticus while Myxobolus galilaeus was found in Sarotherodon galilaeus. In addition, M. sarigi, M. israelensis, and Myxobolus sp. were also found in S. galilaeus. In the light of the present study, the taxonomy of myxosporean infections in tilapias is modified. Mature spores may localize in the melano-macrophage centers of the spleen and kidney where they may eventually be destroyed. No cases of mortality have so far been associated with these parasites.     

Development of a polymerase chain reaction diagnostic assay for Ceratomyxa shasta, a myxosporean parasite of salmonid fish.

A diagnostic procedure based on the polymerase chain reaction (PCR) was developed for the myxosporean parasite Ceratomyxa shasta. Three sets of oligonucleotide primers were designed to specifically amplify C. shasta ribosomal RNA genes and several parameters of the assay were tested and optimised. A simple protocol for the processing of fish tissue samples was also developed. In a single round, 20 microliters volume reaction the optimised procedure allows the detection of 50 fg of purified C. shasta genomic DNA, or 0.01 spore from a seeded fish intestine sample. This protocol is considerably faster, cheaper and more reliable than any previous diagnostic procedure for a myxosporean parasite, and can be an invaluable tool for the monitoring of early and/or subclinical C. shasta infections in wild and cultured salmon populations.     

池塘养殖异育银鲫寄生黏孢子虫的种群动态

研究对湖北洪湖地区一养殖池塘的异育银鲫开展了黏孢子虫的流行病学调查, 以利于黏孢子虫病的综合防治。发现异育银鲫中寄生了4种黏孢子虫, 分别为多涅茨尾孢虫(Henneguya doneci Schulman 1962)、住心碘泡虫(Myxobolus hearti Chen, 1998)、瓶囊碘泡虫(Myxobolus ampullicapsulatus Zhao 2008)和尾孢虫未定种(Henneguya sp.)。鳃寄生的多涅茨尾孢虫在不同月份中感染率有显著差异(P 0.05), 在46月未见包囊, 79月感染率突增, 明显高于其他月份; 多涅茨尾孢虫包囊的平均丰度与其感染率的变化趋势相同, 不同月份间, 多涅茨尾孢虫平均丰度在统计学上差异显著(P 0.05); 多涅茨尾孢虫平均包囊直径从7月至12月逐渐增大, 随后月份逐渐减小, 不同月份间, 多涅茨尾孢虫平均包囊直径在统计学上差异显著(P 0.05); 在不同的鳃片之间, 多涅茨尾孢虫平均感染强度差异不显著(P 0.05), 但第四片鳃的平均感染强度明显要高于其他鳃片。鳃寄生的瓶囊碘泡虫只在36月发现感染, 感染率6月最高; 心脏寄生的住心碘泡虫全年都发现感染, 在不同月份中感染率差异不显著(P 0.05); 膀胱寄生的尾孢虫未定种除了8月, 其他月份均有感染, 14月的感染率显著高于其他月份(P 0.05)。       

Comparative susceptibility of Atlantic salmon, lake trout and rainbow trout to Myxobolus cerebralis in controlled laboratory exposures

The susceptibility of lake trout Salvelinus namaycush, rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar to Myxobolus cerebralis, the causative agent of whirling disease, was compared in controlled laboratory exposures. A total of 450 (225 for each dose) fry for each species were exposed to a low (200 spores per fish) or high (2000 spores per fish) dose of the infective triactinomyxon. At 22 wk post-exposure, 60 fish from each group, as well as controls for each species, were examined for clinical signs (whirling behavior, blacktail, deformed heads and skeletal deformities), microscopic lesions, and presence of spores. Rainbow trout were highly susceptible to infection, with 100% being positive for spores and with microscopic pathological changes in both exposure groups. Rainbow trout were the only species to show whirling behavior and blacktail. Atlantic salmon were less susceptible, with only 44 and 61% being positive for spores, respectively, in the low and high dose groups, while 68 and 75%, respectively, had microscopic pathology associated with cartilage damage. Rainbow trout heads contained mean spore concentrations of 2.2 (low dose) or 4.0 (high dose) x 10(6) spores g tissue(-1). The means for positive Atlantic salmon (not including zero values) were 1.7 (low) and 7.4 (high) x 10(4) spores g tissue(-1). Lake trout showed no clinical signs of infection, were negative for spores in both groups and showed no histopathological signs of M. cerebralis infection.     

Novel and known myxobolids (Cnidaria,Myxozoa) infecting Chondrostoma angorense (Cypriniformes: Leuciscidae) in Turkey

Turkey has more than 200 endemic freshwater fish species, one of which is the Ankara nase, Chondrostoma angorense Elvira, 1987 (Cypriniformes: Leuciscidae), a food fish in northern Turkey. Like most endemic fish species in Turkey, its myxosporean parasite fauna (Cnidaria: Myxosporea) are not yet described. We surveyed twenty C. angorense from Lâdik Lake in northern Turkey, and identified two myxosporean parasites from gills of these fish: Myxobolus arrabonensis Cech, Borzák, Molnár, Székely, 2015, and a co-infection of a novel species, Myxobolus polati sp. nov. We characterized both infections based on myxospore morphology, morphometry, tissue tropism, small subunit ribosomal DNA sequence and phylogenetic analysis. Plasmodia of both species were observed in gills, but had distinct tropism: M. arrabonensis is an intrafilamental vascular type, and M. polati sp. nov. is an intralamellar vascular type. We identified M. arrabonensis on the basis of myxospore characters and 100% similarity to the type DNA sequence from the closely-related host C. nasus. The small subunit ribosomal DNA sequence of M. polati sp. nov. (1946 base pairs; GenBank Accession number MH392318) had a maximum similarity of 98% with any Myxobolus sp. from other Eurasian cypriniforms. Phylogenetic analysis revealed that M. polati sp. nov. is most closely related to gill-infecting Myxobolus diversicapsularis from Rutilus rutilus (L.). The present study is the first record of myxosporean species infecting C. angorense comprising a novel species, M. polati sp. nov. and a known species M. arrabonensis.     

Variability in triactinomyxon production from Tubifex tubifex populations from the same mitochondrial DNA lineage infected with Myxobolus cerebralis, the causative agent of whirling disease in salmonids

Myxobolus cerebralis, the causative agent of whirling disease, infects both salmonid fish and an aquatic oligochaete, Tubifex tubifex. Although M. cerebralis has been detected in river drainages throughout the United States, disease severity among wild fish populations has been highly variable. Tubifex tubifex populations have been genetically characterized using sequences from the 16S mitochondrial DNA (mtDNA) gene, the 18S ribosomal RNA gene, the internal transcribed spacer region 1 (ITS1), and randomly amplified polymorphic DNA (RAPD). Our earlier work indicated that large differences in compatibility between the parasite and populations of T. tubifex may play a substantial role in the distribution of whirling disease and resulting mortality in different watersheds. In the present study, we examined 4 laboratory populations of T. tubifex belonging to 16S mtDNA lineage III and 1 population belonging to 16S mtDNA lineage I for triactinomyxon (TAM) production after infection with M. cerebralis myxospores. All 4 16S mtDNA lineage III populations produced TAMs, but statistically significant differences in TAM production were observed. Most individuals in the 16S mtDNA lineage III-infected populations produced TAMs. The 16S mtDNA lineage I population produced few TAMs. Further genetic characterization of the 16S mtDNA lineage III populations with RAPD markers indicated that populations producing similar levels of TAMs had more genetic similarity.