Smolt Transformation Of Atlantic Salmon Research Articles

Transcriptional profiling of the parr–smolt transformation in Atlantic salmon

The parr–smolt transformation in Atlantic salmon (Salmo salar) is a complex developmental process that culminates in the ability to migrate to and live in seawater. We used GRASP 16K cDNA microarrays to identify genes that are differentially expressed in the liver, gill, hypothalamus, pituitary, and olfactory rosettes of smolts compared to parr. Smolts had higher levels of gill Na+/K+-ATPase activity, plasma cortisol and plasma thyroid hormones relative to parr. Across all five tissues, stringent microarray analyses identified 48 features that were differentially expressed in smolts compared to parr. Using a less stringent method we found 477 features that were differentially expressed at least 1.2-fold in smolts, including 172 features in the gill. Smolts had higher mRNA levels of genes involved in transcription, protein biosynthesis and folding, electron transport, oxygen transport, and sensory perception and lower mRNA levels for genes involved in proteolysis. Quantitative RT-PCR was used to confirm differential expression in select genes identified by microarray analyses and to quantify expression of other genes known to be involved in smolting. This study expands our understanding of the molecular processes that underlie smolting in Atlantic salmon and identifies genes for further investigation.

The effect of nonylphenol on gene expression in Atlantic salmon smolts

The parr–smolt transformation in Atlantic salmon (Salmo salar) is a complex developmental process that culminates in the ability to migrate to and live in seawater. Exposure to environmental contaminants like nonylphenol can disrupt smolt development and may be a contributing factor in salmon population declines. We used GRASP 16K cDNA microarrays to investigate the effects of nonylphenol on gene expression in Atlantic salmon smolts. Nonylphenol exposure reduced gill Na+/K+-ATPase activity and plasma cortisol and triiodothyronine levels. Transcriptional responses were examined in gill, liver, olfactory rosettes, hypothalamus, and pituitary. Expression of 124 features was significantly altered in the liver of fish exposed to nonylphenol; little to no transcriptional effects were observed in other tissues. mRNA abundance of genes involved in protein biosynthesis, folding, modification, transport and catabolism; nucleosome assembly, cell cycle, cell differentiation, microtubule-based movement, electron transport, and response to stress increased in nonylphenol-treated fish. This study expands our understanding of the effect of nonylphenol on smolting and provides potential targets for development of biomarkers.

Exposure to continuous light disrupts retinal innervation of the preoptic nucleus during parr–smolt transformation in Atlantic salmon

High quality salmon smolts are essential for aquaculture, enhancement programs and wild populations. However, intensification of aquaculture smolt production and changes in natural habitats can cause sub-optimal environmental conditions, which may result in poor smolt quality. The salmon brain, as the integrator of environmental information, plays a focal role in relaying this information through the light–brain–pituitary axis, which includes retinal and pineal innervation of the hypothalamus. Here we investigated the effect of rearing juvenile Atlantic salmon, Salmo salar, under constant light (LL) on optic nerve fiber growth into the hypothalamus. This was compared with the normal increased fiber growth in fish reared under a simulated-natural photoperiod (LDN). Parr were sampled from the LDN group in February and from the LDN and LL groups in May (peak smolt status for the LDN group). Retinohypothalamic projections to the preoptic area were traced using 1, 1′-dioctadecyl-3,3, 3′,3′-tetramethylindocarbocyanine perchlorate (DiI) and confocal laser scanning microscopy. Data showed that parr exposed to LL did not develop the same extensive retinal innervation to the preoptic nucleus (NPO) observed in control salmon smolts raised under LDN. Since the cells in NPO are central pituitary regulatory neurones, the increased retinohypothalamic innervation during normal smoltification may be responsible for the increased endocrine response to photoperiod information. The deprivation of photoperiod information, during continuous light exposure, may inhibit the natural developmental program to proceed during the parr–smolt transformation.

Pituitary gene expression of somatolactin, prolactin, and growth hormone during Atlantic salmon parr–smolt transformation

Abstract Somatolactin (SL), prolactin (PRL) and growth hormone (GH) belong to the same family of adenohypophyseal hormones. In fish, these hormones have all been linked to several important physiological functions. During salmon smoltification, GH has an important role in stimulating growth and improving hypoosmoregulatory ability. The role of PRL in fish osmoregulation has mostly been linked to survival in fresh water (FW), whereas the physiological functions of SL is less clear, having been linked to reproduction, stress, Ca-regulation and acid–base balance. To obtain more information on the roles and interactions of these hormones during the parr–smolt transformation, juvenile Atlantic salmon in FW were sampled from early-February until late-June. Approximately every 2 weeks, fish were sacrificed and pituitaries sampled for assessment of SL, PRL, and GH mRNA. In addition, fish were transferred to seawater (SW) in late-May, and sampled after 1 and 4 weeks in SW. Somatolactin mRNA expression did not change during the parr–smolt transformation of the Atlantic salmon, but increased transiently during the first week in SW, a response which may be correlated to stress and/or Ca-balance. PRL and GH mRNA expression increased transiently during the latter part of the parr–smolt transformation of Atlantic salmon. Following SW entry, the PRL mRNA expression decreased, while GH mRNA expression increased further. This indicates that PRL is of importance during the parr–smolt transformation of the Atlantic salmon, probably by counteracting the hypoosmoregulatory effects of GH and cortisol, allowing the fish to maintain osmotic balance during the smoltification process.

Plasma levels of free and bound thyroid hormones during parr–smolt transformation in Atlantic salmon, Salmo salar L.

To determine if the marked spring surges that occur in plasma L-thyroxine (T4) concentrations during parr–smolt transformation are due to changes in the properties or levels of thyroid hormone binding plasma proteins, plasma levels of total T4 (TT4) and 3,5,3′-triiodo-L-thyronine (TT3), indices of the percentage of T4 or T3 in the free form (%FT4 or %FT3), and indices of free T4 (FT4) and free T3 (FT3) levels were measured on a population of Atlantic salmon undergoing parr–smolt transformation under a natural temperature and photoperiod regime in Brittany, France. Gill Na+–K+ ATPase increased to a peak in mid-May, whereas TT4 exhibited two major peaks in late April and mid-June, with small seasonal change in TT3. Indices of the %FT4 and %FT3 changed negligibly, resulting in high correlations between TT4 and FT4 (r = 0.97) and TT3 and FT3 (r = 0.96). Thus the high TT4 peaks encountered during smoltification are not due to changes in properties of plasma T4 binding proteins and reflect approximately proportionate changes in the putative physiologically relevant FT4 level. The TT4 and FT4 peaks coincided with basal levels of TT3 and FT3, implying no systemic increase in T4 to T3 conversion and reinforcing the concept that T4 may exert a local tissue-specific role during parr–smolt transformation.

Influence of the olfactory sense upon smolt transformation in salmonid fishes

The effect of olfactory deprivation on the process of smolt transformation in Atlantic salmon (Salmo salar L.) was investigated. Salmon parr were rendered anosmic by heat-cauterizing in November, and, together with controls, size, purine deposition (silvering), and mortality were monitored throughout spring in individuals within sibling groups. In May the fish were exposed to seawater (3.2% salinity) to determine levels of smoking in anosmic fish compared with control fish. Olfactory deprivation was found to induce a negative effect on survival in both anosmic fish as well as controls when reared within the same rearing tank. A 54% survival was metered in anosmic fish as opposed to only 26% survival in control fish during the experimental period. The low survival rates observed are suggested to result from abnormal behavior in anosmic fish due to lack of proper olfactory stimuli. Within each strain, the anosmic fish were found shorter and displayed less-silvering than their control fish at the end of the experimental period. Further, since control fish could withstand salinity exposure longer than anosmic fish, smolting is suggested to have been retarded among the anosmic fish. Olfactory influence on smolt transformation may be caused by conspecific pheromones secreted for the purpose of mediating effects of life-history patterns in salmonid fishes.

Purines in the Integument of the Atlantic Salmon (Salmo salar) During Parr–Smolt Transformation

During the parr–smolt transformation in Atlantic salmon, silvery materials were found beneath the scales (scale layer) and deep in the dermis adjacent to the muscle (skin layer). Both layers were thicker in smolts than in parr and both layers contained guanine and lesser amounts of hypoxanthine.Changes in levels of guanine, hypoxanthine, and the ratio between them (G/H) were measured from late December to late May on 2-year-old parr held in the laboratory under natural photoperiods and at temperatures varying from 6.0 in December to 9.5 C in late May. The change from parr to silvery parr during February and March accompanied sharp increases in guanine, hypoxanthine, and G/H values in the skin layer with less marked increases in the scale layer. The change from silvery parr to smolts during April and May accompanied sharp increases in guanine, hypoxanthine, and G/H values in the scale layer but no change occurred in the trend already established in the skin layer.Parr held all winter under stream conditions and examined in early April possessed purine levels and G/H ratios in the skin and scale layers comparable to those in laboratory fish during the previous December.Silvery parr captured migrating seaward in early June had purine levels and G/H ratios in the scale layer similar to those in laboratory smolts in late May but skin guanine values were much lower and the G/H ratio in the skin was extremely low.