Jiří Lom

Fish microsporidia: fine structural diversity and phylogeny.

Structural diversity of fish microsporidian life cycle stages and of the host-parasite interface is reviewed. In the infected cell of the fish host, microsporidia may either cause serious degradation of the cytoplasm and demise of the cell, or they may elicit host cell hypertrophy, producing a parasite-hypertrophic host cell complex, the xenoma. The structure of the xenoma and of its cell wall may differ according to the genus of the parasite, and seems to express properties of the parasite rather than those of the host. In merogony, the parasite cell surface interacts with the host cell in diverse ways, the most conspicuous being the production of thick envelopes of different types. Sporogony stages reveal different types of walls or membranes encasing the sporoblasts and later the spores and these envelopes may be of host or parasite origin. Nucleospora differs from all other fish microsporidia by its unique process of sporogony. Except for the formation of conspicuous xenomas, there are no essentially different structures in fish-infecting microsporidia compared with microsporidia from other hosts. Although the structures associated with the development of fish microsporidia cannot be attributed importance in tracing the phylogeny, they are relevant for practical determination and assessing the relation to the host. The possibility of the existence of an intermediate host is discussed. Higher-level classification of Microsporidia is briefly discussed and structure and evolutionary rates in microsporidian rDNA are reviewed. Discussion of rDNA molecular phylogeny of fish-infecting microsporidia is followed by classification of these parasites. Most form a rather cohesive clade. Outside this clade is the genus Nucleospora, separated at least at the level of Order. Within the main clade, however, there are six species infecting hosts other than fish. Based on data available for analysis, a tentative classification of fish-infecting microsporidia into five groups is proposed. Morphologically defined groups represent families, others are referred to as clades. Group 1, represented by family Pleistophoridae, includes Pleistophora, Ovipleistophora and Heterosporis; Vavraia and Trachipleistophora infect non-fish hosts. Group 2, represented by family Glugeidae, is restricted to genus Glugea and Tuzetia weidneri from crustaceans. Group 3 comprises three clades: Loma and a hyperparasitic microsporidian from a myxosporean; Ichthyosporidium and Pseudoloma clade and the Loma acerinae clade. For the latter species a new genus has to be established. Group 4 contains two families, Spragueidae with the genus Spraguea and Tetramicridae with genera Microgemma and Tetramicra, and the Kabatana and Microsporidium seriolae clade. Group 5 is represented by the family Enterocytozoonidae with the genus Nucleospora and mammal-infecting genus Enterocytozoon.

Taxonomy of the genera of the Myxobolus/Myxosoma group (Myxobolidae: Myxosporea), current listing of species and revision of synonyms

A revision of the genera Myxobolus and Myxosoma is presented. On the basis of Lom & Noble (1984), all species are designated as Myxobolus. A total of 444 species of Myxobolus, including three subspecies, are listed together with their hosts. All former Myxosoma species are assigned to the genus Myxobolus and those which were previously homonyms or synonyms have been assigned new names where valid. Synonyms and other species omitted from the definitive Myxobolus list are also given.

Phylogenetic analysis of complete small subunit ribosomal RNA coding region of Myxidium lieberkuehni: evidence that Myxozoa are Metazoa and related to the Bilateria.

Summary: The phylogenetic position of Myxozoa relative to other eukaryotes has been controversialDuring their complex life cycles they show both protist and metazoan charactersIn contrast to their general classification as protists, phylogenetic comparisons of the complete 16Slike rRNA sequence of the myxosporean Myxidium lieberkuehni with other, unicellular and metazoan sequences show that Myxozoa share a common evolutionary history with Metazoa and are most closely related to the Bilateria

Notes on the ultrastructure and sporoblast development in fish parasitizing myxosporidian of the genusSphaeromyxa

SummaryThe ultrastructure of trophic stage (plasmodium), spore and changes in fine structure during morphogenesis of the spore were studied by electron microscopy in two representatives of the genusSphaeromyxa. Plasmodium has a highly differentiated structure; there is an outer layer of homogeneous plasm, the endoplasm consisting of a vacuolated mass in which “float” generative cells and sporoblasts in different degree of development. Generative cells have well developed pseudopodia. Sporoblasts arise from the union of two cells, out of which the inner one forms all cells of the spore, the outer one has only an enveloping function. Polar capsule develops in a way identical with other myxosporidia; the wide filament, however, has a longitudinally folded structure and is located within the capsule in two loose loops. The mitochondria of an early sporoblast are characterized by a high content of DNA. The identity of polar capsule development with the nematocyst morphogenesis is too conspicuous to be taken for a mere convergence. Studies on ultrastructure give no evidence for a protozoan character of myxosporidia; together with other findings, they are in favor of the nonprotozoan nature of this group of organisms.

Perkinsiella amoebae-like endosymbionts of Neoparamoeba spp., relatives of the kinetoplastid Ichthyobodo

Eukaryotic endosymbionts (“parasomes”) were studied in Neoparamoeba strains of different origin including two strains of N. pemaquidensis (Page, 1970), the agent of amoebic gill disease (AGD). Comparative study of the endosymbionts revealed their mutual ultrastructural similarity and also similarity with the endosymbiont Perkinsiella amoebae Hollande, 1980 described from Janickina chaetognathi and J. pigmentifera. Morphological features supported Hollande’s hypothesis on the kinetoplastid origin of P. amoebae but the first conclusive results were obtained in this study using marker genes. The recognition of euglenozoan spliced leader RNA (SL RNA) gene sequences in the genomic DNA of endosymbionts from five Neoparamoeba strains together with the acquisition of one SSU RNA gene sequence allowed us to specify the relationship of the endosymbionts under study with kinetoplastids. Phylogenetic analyses of SSU rRNA gene sequence data currently available revealed close relationship of the first sequenced Perkinsiella amoebae-like organism with Ichthyobodo necator. Sequence comparisons disclosed that P. amoebae-like organisms possess SL RNA genes with a significant level of divergence from other kinetoplastids. However, the sequences are more closely related to kinetoplastids and Diplonema spp. than to euglenids. Three types of the SL RNA gene sequences obtained from P. amoebae-like organisms were congruent with phylogeny of their Neoparamoeba host strains.

Sphaerospora renicola n.sp., a myxosporean from carp kidney, and its pathogenicity

Sphaerospora renicola n.sp. is a common parasite of carp in Czechoslovakia. Its life cycle involves intracellular stages in the epithelial cells of renal tubuli and trophozoite stages proliferating in the tubular lumen, transforming ultimately into pansporoblasts, each having one pansporoblast nucleus and producing two spores. The spores are almost globular with an average size of 7.3×7.2 μ, with polar capsules of equal size, and may have two slightly protruding tubercles on their shell valves. Differential diagnosis from otherSphaerospora species infecting carp, as well as fromMitraspora cyprini Fujita, is made. Intracellular stages ofS. renicola cause swelling and hyperplasia of the epithelium in renal tubuli followed by dystrophic changes. Accumulation of developmental stages in the tubular lumen provokes pronounced regressive changes of the epithelium, which may be followed by necrosis.

On the structure of the extruded microsporidian polar filament

SummaryThe structure of the polar filament of a species of Nosema (Protozoa, Microspora) invading a sea horse, Hippocampus erectus, has been studied with the electron microscope. Filaments extruded from spores were examined on ultrathin sections and on negatively stained preparations. The extruded filament appears as a hollow tube whose walls are composed of membranes; mostly there are three membranes separated or closely joint together. There was no positive evidence of fibrillar structures in the extruded filaments. In a resting spore, the membranous walls of the filament, although less distinct, are also discernible. A collar-like structure at the base of the filament provides for the anchorage of the extruded filament, being firmly wedged in the aperture pierced in the spores shell by the filament discharge. In resting state this structure attaches the basal part of the filament to the organelle known as polar cap. No evidence was found of a “terminal sac” at the loose end of the filament. During extrusion, sporoplasm passes through the cavity of the filament in an extremely elongated state. The empty space in the extruded spore shell is in fact the enlarged posterior vacuole. At the anterior end there is a separated space communicating with the filament cavity.

Studies on Protozoan Parasites of Australian Fishes: III. Species of the Genus Myxobolus Bütschli, 1882

Summary Six species of the genus Myxobolus were recorded from estuarine and marine fishes at the New South Wales coast. A species infecting the submucosa in the intestine, intrahepatic bile ducts and gall bladder of Mugil cepbalus was tentatively identified as M. cf. spina-curvatura Maeno, Sorimachi, Ogawa and Egusa, 1990. M. rohdei n. sp. was discovered in kidney interstitium of Mugil cephalus. M. purkynjei n. sp. was found in the gill filaments of Girella tricuspidata, while in the kidney and in pyloric caeca of the same host M. girellae n. sp. was detected. M. acantbopagri n. sp. was found to infect the intestine of Acanthopagrus australis. M. conei n. sp. is described from the lumen of bile ducts in the liver of Pseudocaranx dentex. Masses of plasmodia obstructing the lumen of biliary ducts elicit histopathological changes in the walls of the ducts and in the neighbouring liver tissue. Coelozoic plasmodia are rather unique in the genus Myxobolus, the species of which are tissue dwellers. This paper raises the number of species of the genus Myxobolus found in marine fishes to twenty-nine.

Sporogenesis and spore structure in Kudoa lunata (Myxosporea, Multivalvulida)

The multivalvulid myxosporean Kudoa lunata Lom, Dyková and Lhotáková, 1983 forms large polysporic trophozoites within the myocytes of scaldfish, Arnoglossus imperialis and A. laterna. The spores arise by the division of generative cells that produce a number of sporogonic cells necessary to complete a sporoblast. The development of some of the sporogonic cells can take place in isolation from other sporoblast cells, as shown by aberrant cases of polar capsule formation. Pansporoblast formation does not exist in Kudoa, at variance with large polysporic trophozoites of other myxosporean genera. The genus Kudoa also includes species with small trophozoites producing just one or two spores without pansporoblasts, as in Sphaerospora. Perhaps this type of sporogenesis was preserved in the species of Kudoa that have large trophozoites. Kudoa spores have a unique type of sporoplasm, comprising an outer cell enveloping an inner one; these differ in their contents of ribosomes and glycogen granules. This type of sporoplasm is reminiscent of the basic unit of all types of myxosporean development, i.e., the primary (vegetative) cell enclosing an inner (generative) cell. The canals for filament discharge extend through the apical spore projections and are of a length unmatched in other myxosporea.

Phylogenetic analysis of freshwater fish trypanosomes from Europe using ssu rRNA gene sequences and random amplification of polymorphic DNA

The taxonomy and phylogenetic relationships of fish trypanosomes are uncertain. A collection of 22 cloned trypanosome isolates from 14 species of European freshwater fish and 1 species of African freshwater fish were examined by molecular phylogenetic analysis. The small subunit ribosomal RNA (ssu rRNA) genes of 8 clones were sequenced and compared with ssu rRNA gene sequences from a wider selection of vertebrate trypanosome isolates by phylogenetic analysis. All trypanosomes from freshwater fish fell in a single clade, subdivided into 3 groups. This clade sits within a larger, robust clade containing trypanosomes from marine fish and various amphibious vertebrates. All 22 trypanosome clones were analysed by random amplification of polymorphic DNA. The resulting dendrogram shows 3 groups, which are congruent with the groups identified in the ssu rRNA gene phylogeny. Two of the groups contain the majority of trypanosome isolates and within-group variation is slight. These groups do not separate purported trypanosome species distinguished by morphology or host origin, and thus these criteria do not appear to be reliable guides to genetic relationships among fish trypanosomes. However, we suggest that the 2 groups themselves may represent different species of fish trypanosomes. The polymorphic DNA markers we have identified will facilitate future comparisons of the biology of these 2 groups of fish trypanosomes.