Diversity among strains causing bacterial kidney disease in salmonid fish

A study was made of the biochemical, cultural, morphological, physiological and serological characters of 25 Gram-positive bacterial isolates of bacterial kidney disease in salmonid fish. Two distinct homogenous phena and seven single-member clusters were defined as a result of overall similarity based on analyses with the Jaccard coefficient. One phenon was equated withCorynebacterium pyogenes, but the second represents a novel taxon.     

Proliferative kidney disease of salmonid fish

Proliferative kidney disease (PKD) is one of the most economically important diseases among commercially reared rainbow trout (Oncorhynchus mykiss) in Europe and causes significant losses among Pacific salmon and rainbow trout populations in western North America. The parasite that causes PKD is a poorly understood myxosporean (PKX), presumed but not yet proven to be a member of the family Sphaerosporidae, genus Sphaerospora. The disease occurs in both cultured and feral populations of salmonids that come into contact with a 20–25 μm waterborne infective stage. The disease is often seasonally dependent occurring at water temperatures above 15 °C in the summer and fall months of the year. At permissive water temperatures, the first recognizable vegetative or extrasporogonic stages appear about 10–14 days after exposure to the infective stage. They are initially prominent in the blood sinuses of the kidney, are accompanied by a mild hyperplasia of the interstitial hematopoietic cell populations, and multiply via binary fission, endogeny, and plasmotomy, provoking a strong inflammatory response. This diffuse granulomatous response occurs principally in the kidney but is also seen in the spleen and other organs. The major host cell type involved in the lesion is the macrophage, but lymphocytes are also abundant in close proximity to PKX. As the inflammation progresses (6–8 weeks postexposure to the infective stage), the gross renal swelling becomes evident as do other clinical signs including anemia. Mortality in uncomplicated cases of PKD is generally 20% or less but often secondary pathogens or unfavorable environmental conditions coincide with peak periods of PKD and mortalities can reach 95–100%. Compromised renal functions such as macromolecule adsorption and divalent cation excretion are also impaired and this may contribute to the mortality observed among fish with PKD. Generally, by 12–20 weeks postexposure, fish begin, or are in the process of, recovery and the renal hematopoietic and excretory functions return to normal. In fish that have experienced a full clinical episode of the disease, a strong acquired immunity develops. The basis of the immunity is unknown but circulating antiparasite antibodies can be detected as early as six weeks postexposure to the infective stage. A strong cellular component to the immunity is also suspected. No vaccines have yet been developed to control PKD, and only recently have experimental therapies been applied. Both fumagillin DCH, an antibiotic effective against certain microsporidia and myxosporidia, and the arylmethane dye malachite green, have shown some promise as treatments for PKD. Unfortunately, neither drug is licensed for use in the U.S. and both treatments suffer from potential difficulties with drug toxicity, tissue residues, or durg discharges in hatchery effluent waters.     

Comparative phylogenetic analysis of the evolution of semelparity and life history in salmonid fishes

The selective pressures involved in the evolution of semelparity and its associated life-history traits are largely unknown. We used species-level analyses, independent contrasts, and reconstruction of ancestral states to study the evolution of body length, fecundity, egg weight, gonadosomatic index, and parity (semelparity vs. degree of iteroparity) in females of 12 species of salmonid fishes. According to both species-level analysis and independent contrasts analysis, body length was positively correlated with fecundity, egg weight, and gonadosomatic index, and semelparous species exhibited a significantly steeper slope for the regression of egg weight on body length than did iteroparous species. Percent repeat breeding (degree of iteroparity) was negatively correlated with gonadosomatic index using independent contrasts analysis. Semelparous species had significantly larger eggs by species-level analysis, and the egg weight contrast for the branch on which semelparity was inferred to have originated was significantly larger than the other egg weight contrasts, corresponding to a remarkable increase in egg weight. Reconstruction of ancestral states showed that egg weight and body length apparently increased with the origin of semelparity, but fecundity and gonadosomatic index remained more or less constant or decreased. Thus, the strong evolutionary linkages between body size, fecundity, and gonadosomatic index were broken during the transition from iteroparity to semelparity. These findings suggest that long-distance migrations, which increase adult mortality between breeding episodes, may have been necessary for the origin of semelparity in Pacific salmon, but that increased egg weight, leading to increased juvenile survivorship, was crucial in driving the transition. Our analyses support the life-history hypotheses that a lower degree of repeat breeding is linked to higher reproductive investment per breeding episode, and that semelparity evolves under a combination of relatively high juvenile survivorship and relatively low adult survivorship.     

Comparative structures of the apopolysialoglycoproteins from unfertilized and fertilized eggs of salmonid fishes

The complete amino acid sequence of the major polysialoglycoproteins (PSGPs) from two genera of salmonid fish eggs, Salvelinus and Oncorhynchus, has been determined. The occurrence of tandem repeats of a genus-specific dodeca- and tridecapeptide was found for the apoPSGP of Salvelinus leucomaenis pluvius (Slp) and Oncorhynchus masou ishikawai (Omi), respectively, their amino acid sequences being highly homologous with that of rainbow trout [Salmo gairdneri (Sg)] apoPSGP (*denotes the glycosylation site; mean value of N = approximately 25): H-PSGP(Slp): (Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-)N H-PSGP(Omi): (Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Ser-)N H-PSGP(Sg): (Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Gly-)N Within 5-7 min following fertilization H-PSGP is converted to the low-molecular-mass PSGP (L-PSGP) by a specific protease (PSGPase). We have purified L-PSGP from the fertilized eggs of S. leucomaenis pluvius and Oncorhynchus keta (chum salmon) and compared it with rainbow trout egg L-PSGP(Sg) by analysis of their amino acid sequence: L-PSGP(Slp): Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Asp L-PSGP(Ok): Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Ser L-PSGP(Sg): Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser-Gly The data support the conclusion that H-PSGP is degraded in vivo 5-7 min after fertilization to L-PSGP by proteolytic cleavage at the position two residues C-terminally to the Pro residue, i.e., -Pro-Ser-Xaa-Asp-(Xaa = either Gly, Ser, or Asp) by the action of PSGPase.     

Complex evolution of vitellogenin genes in salmonid fishes

Vitellogenins (Vtg) are usually encoded by small multigene families containing up to six genes. With 20 tandemly arranged genes, the rainbow trout ( Oncorhynchus mykiss) is an exception to this rule. PCR amplification, cloning and sequence analysis of Vtg genes in other salmonid species revealed the existence of two paralogous gene clusters, designated Vtg-A and Vtg-B. Southern hybridization showed that the number of genes varies from 2 to 30 copies from one species to another, as well as between the two gene clusters. All Coregonus, Thymallus, Salmo and Salvelinus species studied have both gene clusters, while Oncorhynchus species possess only the Vtg-A locus. Phylogenetic trees constructed from Vtg sequences revealed conflicting nodes with the consensus tree based on morphological and anatomical data. Vtg sequences support the grouping ( Salmo, ( Salvelinus, Oncorhynchus)) instead of the accepted consensus ( Salvelinus, ( Salmo, Oncorhynchus)). Structural data on gene organization also support the contention that Salvelinus and Oncorhynchus are sister taxa. Evolutionary implications for the Vtg gene clusters in salmonids are discussed.     

The evolution of microsatellite loci in salmonid fishes

In ten species of salmonid fishes, sequences of five microsatellite loci were determined. Considerable differences in the structure of the same microsatellites in different species were found. It was demonstrated that the evolution of microsatellites was a complex process, including changes in the copy number, point mutations, and extended deletions and insertions, leading either to the formation of microsatellites or to their loss.     

PKX, the causative agent of proliferative kidney disease (PKD) in Pacific salmonid fishes and its affinities with the Myxozoa

Proliferative kidney disease (PKD), caused by an unclassified protozoan (PKX), is reported from Pacific salmon, Oncorhynchus tshawytscha (Walbaum) and O. kisutch (Walbaum), and steelhead trout, Salmo gairdneri Richardson, held at the Mad River Hatchery in California, USA. The cumulative mortality attributed to the disease was 95, 13, and 18% respectively. The mortalities were greatest at mean water temperatures of 12-14 degrees C during July 1983. The ultrastructure of the PKX organism and its associated pathology during clinical disease in all three species were consistent with those of the parasite in rainbow trout (Salmo gairdneri) as described in European outbreaks. Significant mortalities did not occur after August, at which time the parasite could no longer be detected in the salmon species. The steelhead continued to exhibit parasites in the kidney interstitium and epithelium and lumens of the tubules. Myxosporidan trophozoites and developing spores were also observed in the lumens of the kidney tubules of these fish. Although a mixed infection with another parasite may have occurred, evidence suggests that the myxosporidans are later stages of PKX. They were only observed in fish exposed to water with the infective stage and were particularly prominent in recovering fish. The PKX organism is similar to UBO, an unclassified protozoan of carp suspected to be an early stage of Sphaerospora renicola Dykovà & Lom. Both parasites infect the blood and kidney, divide by endogeny, and are released by disintegration of the primary mother cell. The intraluminal myxosporean forms show similarities to Sphaerospora spp. in that they are monosporous and sporoblasts are formed within pseudoplasmodia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tetracapsula renicola n. sp. (Myxozoa : Saccosporidae); the PKX myxozoan--the cause of proliferative kidney disease of salmonid fishes

Proliferative kidney disease (PKD) of salmonid fishes is caused by the extrasporogonic stage of an enigmatic myxozoan, referred to as PKX. Sporogenesis occurs in the renal tubules, resulting in monosporous pseudoplasmodia. The spores are ovoid with indistinguishable valves and measure 12 microm in length and 7 microm in width. Two spherical polar capsules (2 microm diameter) with 4 coils occur at the anterior end of the spore. Prominent capsulogenic cell nuclei posterior to the polar capsules are evident in histological sections stained with hematoxylin and eosin. Regardless of the true nature of the valves (indistinguishable or absent), this myxozoan is morphologically distinct from all other described members of the phylum Myxozoa. Comparisons of small subunit rDNA sequences of PKX with other myxozoans demonstrated that it branches from all other members of the myxosporeans from fish examined thus far, including representatives of the phenotypically most closely related genera, Sphaerospora and Parvicapsula. Recent reports, based on rDNA comparisons, indicate that the alternate stage of PKX occurs in bryozoans, and that PKX clusters in a clade with Tetracapsula bryozoides. Our analyses and those of others, along with phenotypic observations, indicate that salmonids are the primary myxosporean hosts for PKX, that the cryptic spores of PKX in salmonids are the fully formed myxospores as they occur in the fish host, and that PKX represents distinct species that we previously place in the genus Tetracapsula in the family Saccosporidae. The latter 2 taxa were described based on stages from bryozoans, and the myxosporean stage in fish of the type species, T. bryozoides, has not been identified (if it exists). Thus, more complete resolution of the life cycle of both PKX and T. bryozoides, as well as more genetic data, are required to determine the precise relationship of these organisms.     

The phylogeography of salmonid proliferative kidney disease in Europe and North America

Salmonid proliferative kidney disease (PKD) is caused by the myxozoan Tetracapsuloides bryosalmonae. Given the serious and apparently growing impact of PKD on farmed and wild salmonids, we undertook a phylogeographic study to gain insights into the history of genealogical lineages of T. bryosalmonae in Europe and North America, and to determine if the global expansion of rainbow trout farming has spread the disease. Phylogenetic analyses of internal transcribed spacer 1 sequences revealed a clade composed of all North American sequences plus a subset of Italian and French sequences. High genetic diversity in North America and the absence of genotypes diagnostic of the North American clade in the rest of Europe imply that southern Europe was colonized by immigration from North America; however, sequence divergence suggests that this colonization substantially pre-dated fisheries activities. Furthermore, the lack of southern European lineages in the rest of Europe, despite widespread rainbow trout farming, indicates that T. bryosalmonae is not transported through fisheries activities. This result strikingly contrasts with the commonness of fisheries-related introductions of other pathogens and parasites and indicates that fishes may be dead-end hosts. Our results also demonstrate that European strains of T. bryosalmonae infect and induce PKD in rainbow trout introduced to Europe.     

Comparative phylogeography of salmonid fishes (Salmonidae) reveals late to post-Pleistocene exchange between three now-disjunct river basins in Siberia

Abstract. We use a comparative phylogeographical framework to evaluate the hypothesis of hydrological exchange during the Pleistocene among the now disjunct Lena, Amur, and Enisei basins in Siberia, and to provide evidence on the causal mechanism of their present day faunal dissimilarities. Approximately 600 bases of the mitochondrial control region were sequenced in five distinct lineages among three genera of salmonid fishes, Hucho, Brachymystax and Thymallus. All three basins were fixed for divergent (2–5.4%) lineages of Thymallus whereas a single shared haplotype was present in all three basins for Hucho taimen ( Pallas, 1773 ) and one shared haplotype between the Lena and Amur basins out of a total of five for blunt‐snouted and one out of five for sharp‐snouted Brachymystax lenok ( Pallas, 1773 ). For both blunt‐ and sharp‐snouted lenok the haplotypes found within each basin did not form clades, so no relationship between genotypes and geographical occurrence was found. Our data support relatively recent hydrological mixing of the major river drainage systems in eastern and far‐eastern Siberia, congruent with the hypothesis of large‐scale palaeo‐hydrological exchange stemming from glacial advance, retreat and melting during Pleistocene climate fluctuations. Furthermore, these results in conjunction with a comparison of overall faunal composition suggest that environmental differences rather than historical contingency may be responsible for the faunal dissimilarities of the Amur, Lena, and Enisei river basins.     

Relative sensitivities of environmental legionellae to selective isolation procedures

A survey of water samples to determine the efficacy of standard procedures for the isolation of environmental legionellae was conducted. Marked variations in intraspecies resistance to selective agents and treatments were observed, and in experiments with one of the isolates, the response was modified by culture conditions. Five selective procedures incorporating acid (pH 2.2) and heat (50 degrees C, 30 min) treatments, with and without plating on buffered charcoal-yeast extract agar supplemented with vancomycin (5 micrograms/ml), polymyxin B (60 U/ml), and cycloheximide (80 micrograms/ml), caused 5 to 99% decreases in viable counts of pure cultures in water suspensions. The differences in the responses of the cultures to the five treatments were statistically significant. Cells in retained samples of naturally contaminated water from which the original cultures had been isolated were significantly less sensitive than artificially grown isolates. The sensitivities of the laboratory-grown cells to the treatments were affected by the length of incubation on buffered charcoal-yeast extract agar. Whereas acid resistance increased after 24 h of incubation, resistance to the antibiotic mixture decreased.     

The evolutionary ecology of alternative migratory tactics in salmonid fishes

Extensive individual variation in spatial behaviour is a common feature among species that exhibit migratory life cycles. Nowhere is this more evident than in salmonid fishes; individual fish may complete their entire life cycle in freshwater streams, others may migrate variable distances at sea and yet others limit their migrations to larger rivers or lakes before returning to freshwater streams to spawn. This review presents evidence that individual variation in migratory behaviour and physiology in salmonid fishes is controlled by developmental thresholds and that part of the variation in proximal traits activating the development of alternative migratory tactics is genetically based. We summarize evidence that alternative migratory tactics co‐exist within populations and that all individuals may potentially adopt any of the alternative phenotypes. Even though intra‐specific genetic divergence of migratory tactics is uncommon, it may occur if female competition for oviposition sites results in spawning segregation of alternative phenotypes. Because of their polygenic nature, alternative migratory tactics are considered as threshold traits. Threshold traits have two characteristics: an underlying 'liability' trait that varies in a continuous fashion, and a threshold value which is responsible for the discreetness observed in phenotypic distribution. We review evidence demonstrating that body size is an adequate proxy for the liability trait controlling the decision to migrate, but that the same phenotypic outcome (anadromy or residency) may be reached by different developmental pathways. The evidence suggesting a significant heritable component in the development of alternative migratory tactics is subsequently reviewed, leading us to conclude that alternative migratory tactics have considerable potential to respond to selection and evolve. We review what is known about the proximal physiological mechanisms mediating the translation of the continuous value of the liability trait into a discontinuous migratory tactic. We conclude by identifying several avenues for future research, including testing the frequency‐dependent selection hypothesis, establishing the relative importance of adaptive phenotypic plasticity in explaining some geographic gradients in migratory behaviour and identifying the physiological and genetic basis of the switching mechanisms responsible for alternative migratory tactics.     

Immunochemical properties of erythrocyte-specific histone H5 in salmonid fishes

Comparative studies have been made on the immunochemical properties of histone H5 in Salmonidae species. High degree of homology between these histones and H5 histone from Oncorhynchus tschawytscha was demonstrated by microcomplement fixation. Properties of H5 histones in fish and birds are discussed.     

Relative sensitivities of environmental legionellae to selective isolation procedures.

A survey of water samples to determine the efficacy of standard procedures for the isolation of environmental legionellae was conducted. Marked variations in intraspecies resistance to selective agents and treatments were observed, and in experiments with one of the isolates, the response was modified by culture conditions. Five selective procedures incorporating acid (pH 2.2) and heat (50 degrees C, 30 min) treatments, with and without plating on buffered charcoal-yeast extract agar supplemented with vancomycin (5 micrograms/ml), polymyxin B (60 U/ml), and cycloheximide (80 micrograms/ml), caused 5 to 99% decreases in viable counts of pure cultures in water suspensions. The differences in the responses of the cultures to the five treatments were statistically significant. Cells in retained samples of naturally contaminated water from which the original cultures had been isolated were significantly less sensitive than artificially grown isolates. The sensitivities of the laboratory-grown cells to the treatments were affected by the length of incubation on buffered charcoal-yeast extract agar. Whereas acid resistance increased after 24 h of incubation, resistance to the antibiotic mixture decreased.