Liver ultrastructure of juvenile Atlantic salmon (Salmo salar)

Light and transmission electron microscopy of the liver of juvenile Atlantic salmon (Salmo salar) reveals a tubular arrangement of parenchymal cells, with biliary passages typically located at the center of tubules. Hepatocytes generally contain a single nucleus surrounded by a cuff of rough endoplasmic reticulum (RER), with many round to elongate mitochondria associated with the perinuclear RER. Whereas glycogen deposits are common and usually lie at the cell periphery, parenchymal cells seldom contain lipid droplets. Golgi complexes and heterogeneous dense bodies also occur in many hepatocytes, often in close proximity to bile canaliculi. Numerous microvilli from hepatocytes extend into the subendothelial space of Disse, which is also the location of stellate fat-storing cells. Interhepatocytic macrophages, sometimes containing prominent phagolysosomes and residual bodies, are common in the liver. The intrahepatic biliary system consists of intercellular canaliculi, bile pre-ductules, ductules, and ducts. In contrast to some other teleosts, the liver of the Atlantic salmon contains no intracellular bile canaliculi or Kupffer cells. The hepatic endothelium, arterioles, and perivenous regions are also described.     

Cloning of the Atlantic salmon (Salmo salar) IL-1 receptor associated protein

In mammals, the pro-inflammatory cytokine interleukin-1 signals through a receptor complex containing a type I interleukin-1 receptor (IL-1RI) and a receptor associated protein (IL-1RAcP). Previously, we have described a cDNA from Atlantic salmon encoding a molecule with homology to the mammalian IL-RI. This molecule was named IL-1 receptor like protein (IL-1RLP) in the absence of functional data to support its proposed role as the salmon IL-1RI. Here, we describe the cloning and characterisation of a cDNA encoding salmon IL-1RAcP. Like other members of the IL-1R family, the salmon IL-1RAcP encodes three extracellular immunoglobulin-like domains and a cytoplasmic Toll/Interleukin-1 receptor (TIR) domain involved in signalling. Specific binding of salmon IL-1RAcP to IL-1RLP was shown by co-immunoprecipitation studies.     

Physiological seawater adaptation in juvenile Atlantic salmon (Salmo salar) autumn migrants

Summary

• 1 About 25 % of juvenile Atlantic salmon (Salmo salar) migrating downstream in the River Frome in southern England do so in the autumn rather than in the spring. Here, we examine the physiological status of these fish with regard to those features that adapt them to sea water during the parr–smolt transformation (i.e. gill Na⁺K⁺ ATPase activity; the number, size and type of chloride cells on the gill lamellae; salinity tolerance and relative plasma thyroid levels).
• 2 Autumn migrants, and those fish which subsequently reside in the tidal reaches during the winter, are not sufficiently physiologically adapted to permit permanent or early, entry into the marine environment.
• 3 It is not known what proportion of autumn migrating fish survive and return to spawn as adults. If significant numbers do return, however, the production from tidal reach habitats must be taken into account in the development of salmon stock management strategies, especially monitoring and assessment programmes, and in the evaluation of factors affecting stocks.

     

A lymphosarcoma in an Atlantic salmon (Salmo salar)

A lymphosarcoma that appeared to be of thymic origin and of lymphoblastic type was found in a 3.5-yr-old Atlantic salmon (Salmo salar). The fish was from a population of 60 broodfish maintained at a research fish laboratory. A large tumor mass was found under the left operculum. Small tumor nodules were found on the swim bladder and in the abdominal adipose tissue. The location of this neoplasm differed from those of previously described tumors in this fish species.     

Cloning of the Atlantic salmon (Salmo salar) estrogen receptor-alpha gene

A cDNA clone encoding most of an Atlantic salmon (Salmo salar) estrogen receptor (ER) was obtained from a liver cDNA library and the remainder of the coding sequence from the gene was isolated from a genomic library. Sequence comparisons showed that the cloned gene represents ER-alpha. Expression of the ER-alpha gene in male and female salmon parr was analysed by RT-PCR. Highest expression was found in brain and liver, with lower levels of ER-alpha mRNA present in all other tissues tested. There was little difference in expression of ER-alpha between male and female.     

CYP3A4 and pregnane X receptor humanized mice

Marked species differences exist in P450 expression and activities. In order to produce mouse models that can be used to more accurately predict human drug and carcinogen metabolism, P450- and xenobiotic receptor humanized mice are being prepared using bacterial artificial chromosomes (BAC) and P1 phage artificial chromosomes (PAC) genomic clones. In some cases, transgenic mice carrying the human genes are bred with null-mice to produce fully humanized mice. Mice expressing human CYP1A1, CYP1A2, CYP2E1, CYP2D6, CYP3A4, and CYP3A7 were generated and characterized. Studies with the CYP3A4-humanized (hCYP3A4) mouse line revealed new information on the physiological function of this P450 and its role in drug metabolism in vivo. With this mouse line, CYP3A4, under certain circumstances, was found to alter the serum levels of estrogen resulting in deficient lactation and low pup survival as a result of underdeveloped mammary glands. This hCYP3A4 mouse established the importance of intestinal CYP3A4 in the pharmacokinetics of orally administered drugs. The hCYP3A4 mice were also used to establish the mechanisms of potential gender differences in CYP3A4 expression (adult female > adult male) that could account for human gender differences in drug metabolism and response. The pregnane X receptor (PXR) is also involved in induction of drug metabolism through its target genes including CYP3A4. Since species differences exist in ligand specificity between human and mice, a PXR-humanized mouse (hPXR) was produced that responds to human PXR activators such as rifampicin but does not respond to the rodent activator pregnenalone 16alpha-carbonitrile.     

Seasonal variation in the thermal performance of juvenile Atlantic salmon (Salmo salar)

1. Experimental data on the maximum growth and food consumption of winter‐acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70°N were compared with predictions from published models of growth and food consumption of summer‐acclimatised fish from the same populations. 2. All winter‐acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1–6 °C). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 °C. 3. Growth and food consumption varied significantly among populations. Winter‐acclimatised fish from a Northern population had a higher mass‐specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer‐acclimatised salmon, where fish from the Northern population had the lowest growth efficiency. 4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.     

A microsatellite linkage map for Atlantic salmon (Salmo salar)

A linkage map of the Atlantic salmon is described here consisting of 15 linkage groups containing 50 microsatellite loci with a 14 additional unlinked markers (including three allozymes). The map shows the largest sex-specific recombination rate differences so far found in any vertebrate species (3.92:1 female:male). Homologies with previous linkage mapping studies of Atlantic salmon and rainbow trout are described. An in silico search of the Genbank database carried out using the microsatellites used in the mapping process identified significant matches between the flanking regions of the microsatellite SS11 and the calcium-binding mitochondrial carrier protein, 'Aralar1'.     

Molecular characterisation of a putative Atlantic salmon (Salmo salar) odorant receptor

The olfactory system of fish is extremely important as it is able to recognise and distinguish a vast array of odorous molecules that are involved in behaviours paramount to survival. This is achieved by the activation of a diverse multigene family of G-protein coupled receptors through odorous ligand binding. Using molecular techniques, the nucleotide sequence of the cDNA coding for an Atlantic salmon (Salmo salar) odorant receptor (ASOR1) has been determined. The full-length cDNA (1260 nt) encodes a protein of 320 amino acid residues, including one potential N-linked glycosylation site, within the short extracellular amino terminal of the receptor. Hydrophobicity analysis revealed seven hydrophobic regions within the amino acid sequence, corresponding to possible positions of the transmembrane domains characteristic of the G-protein coupled receptor superfamily. Several conserved motifs unique to odorant receptors were also present. Through characterisation of this receptor, we hope to increase the understanding of the mechanisms underlying olfaction in salmonid species.     

The ontogeny and extrahepatic expression of complement factor C3 in Atlantic salmon (Salmo salar)

Fish embryos and hatchlings are exposed to pathogens long before maturation of their lymphoid organs. Little is known about defence mechanisms during the earliest stages of life, but innate mechanisms may be essential for survival. The complement system in fish is well developed and represents a major part of innate immunity. Complement factor 3 (C3) is central subsequent to activation of all pathways of the complement system, leading to inflammatory reactions, such as chemotaxis, opsonisation and lysis of pathogens. Hepatocytes represent the major source of C3, but modern molecular biological methods have confirmed that C3 is synthesised at multiple sites. Our main objective was to study the ontogeny of C3 in Atlantic salmon by mapping the commencement of synthesis and localisation of proteins. Eggs, embryos, hatchlings and adult fish were analysed for the presence of C3 mRNA and proteins. From immunohistochemical studies, C3 proteins were detected at several extrahepatic sites, such as the skeletal muscle, developing notochord and chondrocytes of the gill arch. Immunoblotting revealed presence of C3 proteins in the unfertilised egg, but C3 mRNA was only detected after fertilisation by real-time RT-PCR. Taken together, the results implicated the maternal transfer of C3 proteins as well as novel non-immunological functions during development.     

The production of juvenile Atlantic salmon, Salmo salar in the upper Wye, Wales

The production of juvenile Atlantic salmon ( Sulmo salur L.) was investigated in 16 study sites in the upper Wye catchment during the period February 1975 to November 1976. The population structure was characterised by large numbers of 0+ fish whose abundance decreased with respect to time so that the numbers of each of the older year-classes was a function of the year-class strength of the original fry. The range of parr densities was similar to that recorded for other rivers: the estimates of 2+ smolts did not exceed about 0.04 m⁻². Production ranged from 0.3 to 11.0 gm⁻² a^−l; Of and 1+ fish contributed over 72% and 3+ fish less than 1 % of the total annug production. The relationship between production P (gm⁻² a⁻¹) and mean biomass B (gm⁻²) for all yeatclasses considered separately and in combination can be expressed by the relationship P=a^b . The values for a vary with age-composition and season but the values for b are not significantly different. Differences in growth rate of salmon between sites are small and differences in production result principally from differences in standing crop (biomass density). In most cases low values for salmon production can be attributed to poor spawning.     

Influence of exogenous thyroxine on plasma melatonin in juvenile Atlantic salmon (Salmo salar)

One of the most clearly defined endocrine changes during the parr-smolt transformation of anadromous salmonids is an increase in plasma levels of thyroid hormones. The role of pineal hormone melatonin in timing and synchronisation of smoltification is widely discussed. The effect of administration of exogenous thyroxine (T4) on plasma melatonin was investigated in juvenile Atlantic salmon (Salmo salar) at the early stages of parr-smolt transformation. Fish were kept in fresh water under simulated-natural photoperiod and exposed to exogenous T4. Fish were sampled at 12.00 and 24.00 h from treatment and control tanks, 2 and 14 days after treatment started. Plasma melatonin and L-thyroxine were measured using RIA and competitive enzyme immunoassay, respectively. After 2 days of T4 treatment, marked difference in plasma melatonin concentration measured at 12.00 and 24.00 h was still observed in both groups. However, 2-week exposure to T4 caused a reduction in night-time plasma melatonin level and thus, probably, inhibited melatonin related time-keeping system in juvenile salmon. Additional studies are needed to clarify the mechanism of the described phenomenon.     

Diel activity pattern of juvenile Atlantic salmon (Salmo salar) in early and late winter

Radiotelemetry was used to investigate the diel activity pattern of juvenile Atlantic salmon (Salmo salar) in early and late winter. Fish were active throughout the diel cycle. However, there was significantly less daytime than nighttime movement and movement declined significantly with increasing fork length. Maximizing winter growth rate, through an overall increase in foraging activity, may reduce the risk of starvation in smaller fish. The results of the present study provide evidence that the activity patterns of juvenile salmonids are quite complex and support the suggestion that individual variation in activity patterns are, at least, partially driven by body size.     

The effects of prior residence on behavior and growth rates in juvenile Atlantic salmon (Salmo salar)

It is well documented that prior residence confers advantagesin territorial disputes, but its impact on other aspects ofbehavior and fitness is less understood. We tested how priorresidence influences the subsequent feeding behavior and growthperformance of dispersing Atlantic salmon fry (Salmo salar)using experimental manipulations of residence in a seminaturalstream tank. In replicated trials, groups of seven "primary"fish were released into the stream tank 3 days ahead of seven"secondary" fish. Standardized behavioral observations were madeon each fish over the following 14 days, after which all fishwere removed and measured. Primaries and secondaries were initiallythe same size and body condition and exhibited the same degreeof site fidelity. However, primaries darted higher into thewater column to intercept prey items, fed at a higher rate,and subsequently grew faster. Larger fish (in terms of body length)tended to be more dominant, and dominants grew faster than subordinates.However, there was no difference in dominance between primaries andsecondaries. These results suggest that the well-documentedadvantage of early-emerging salmon fry over late-emerging frycannot be completely attributed to intrinsic differences andthat the advantage is partly mediated via a prior residenceeffect. Furthermore, prior residents gain foraging advantageswithout necessarily becoming more dominant.