Distribution and prevalence of the myxozoan parasite Tetracapsuloides bryosalmonae in northernmost Europe: analysis of three salmonid species

Global climate change is altering the abundance and spread of many aquatic parasites and pathogens. Proliferative kidney disease (PKD) of salmonids caused by the myxozoan Tetracapsuloides bryosalmonae is one such emerging disorder, and its impact is expected to increase with rising water temperature. Yet, the distribution and prevalence of T. bryosalmonae in Northern Europe remain poorly characterized. Here, we studied 43 locations in 27 rivers in northernmost Norway and Finland to describe T. bryosalmonae infection frequency and patterns in 1389 juvenile salmonids. T. bryosalmonae was discovered in 12 out of 27 rivers (44%) and prevalence ranged from 4.2 to 55.5% in Atlantic salmon and from 5.8 to 75% in brown trout among infected rivers. In sympatric populations, brown trout was more frequently infected with T. bryosalmonae than was salmon. Age-specific parasite prevalence patterns revealed that in contrast to lower latitudes, the infection of juvenile fish predominantly occurs during the second summer or later. Temperature monitoring over 2 yr indicated that the mean water temperature in June was 2.1 to 3.2°C higher in rivers containing T. bryosalmonae compared to parasite-free rivers, confirming the important role of temperature in parasite occurrence. Temporal comparison in T. bryosalmonae prevalence over a 10 yr period in 11 rivers did not reveal any signs of contemporary parasite spread to previously uninfected rivers. However, the wide distribution of T. bryosalmonae in rivers flowing to the Barents Sea indicates that climate change and heat waves may cause new disease outbreaks in northern regions.

Muscular microsporidian infection in Arctic char Salvelinus alpinus from two lakes in Nunavik, Quebec, Canada

Arctic char Salvelinus alpinus is an important cultural and subsistence resource for Inuit communities. Muscular infections by microsporidia were diagnosed for the first time in Arctic char originating from 2 different lakes in Nunavik (Quebec, Canada). The consumption of these infected fish was associated with digestive tract disorders in people. To better characterize microsporidiosis in these char populations, a cross-sectional study was conducted on 91 fish. The microsporidium was classified as a member of the Microsporidium collective genus by morphological evaluation and phylogenetic analysis using small subunit ribosomal DNA sequence data. The presence and severity of infection were determined histologically. Microsporidian infection occurred in 61% of the fish (56/91) and was significantly associated with an increase in their age, length and weight. The severity of infection (percentage of muscle area affected by microsporidia) was mild in most cases (<1% of the total muscle area). Based on multiple linear regression modeling, the severity of infection was significantly greater in females and negatively correlated with the body condition. Despite a high prevalence, the low pathogenicity of the infection suggests that microsporidiosis has little impact on these char populations. Moreover, since digestive-tract disorders following ingestion of fish infected by microsporidia have never been reported in humans, it seems unlikely that it was responsible for the reported clinical signs. Anisakid larvae are occasionally observed in these char populations. Digestive-tract infection associated with ingestion of these larvae should thus be considered as a potential differential diagnosis in these Inuit communities.

Genotyping Strategies Using ddRAD Sequencing in Farmed Arctic Charr (Salvelinus alpinus)

Incorporation of genomic technologies into fish breeding programs is a modern reality, promising substantial advances regarding the accuracy of selection, monitoring the genetic diversity and pedigree record verification. Single nucleotide polymorphism (SNP) arrays are the most commonly used genomic tool, but the investments required make them unsustainable for emerging species, such as Arctic charr (Salvelinus alpinus), where production volume is low. The requirement to genotype a large number of animals for breeding practices necessitates cost effective genotyping approaches. In the current study, we used double digest restriction site-associated DNA (ddRAD) sequencing of either high or low coverage to genotype Arctic charr from the Swedish national breeding program and performed analytical procedures to assess their utility in a range of tasks. SNPs were identified and used for deciphering the genetic structure of the studied population, estimating genomic relationships and implementing an association study for growth-related traits. Missing information and underestimation of heterozygosity in the low coverage set were limiting factors in genetic diversity and genomic relationship analyses, where high coverage performed notably better. On the other hand, the high coverage dataset proved to be valuable when it comes to identifying loci that are associated with phenotypic traits of interest. In general, both genotyping strategies offer sustainable alternatives to hybridization-based genotyping platforms and show potential for applications in aquaculture selective breeding.

A seasonal comparison of trace metal concentrations in the tissues of Arctic charr (Salvelinus alpinus) in Northern Québec, Canada

Ecotoxicological research detailing trace metal contamination and seasonal variation in the tissues of northern fishes such as Arctic charr (Salvelinus alpinus) has been poorly represented in the literature beyond examination of mercury. In an effort to address this, anadromous Arctic charr were collected from the Deception River watershed in the late summer and post-winter season, before quantifying seasonal and organotropic variations in dorsal muscle and liver concentrations of arsenic, cadmium, chromium, copper, nickel, lead, and zinc. Potential linkages with biological variables (fork length, age, and somatic condition) and indicators of feeding behavior (δ¹³C and δ¹⁵N) were also assessed. Trace metal organotropism favouring elevation in liver tissue concentrations was exhibited by cadmium, copper, nickel and zinc, while arsenic, chromium and lead exhibited no significant organotropic variation. Seasonal differences in concentrations were metal and tissue dependent, but generally increased in tissues collected from post-winter sampled Arctic charr. Significant correlations with biological and trophic descriptors were also determined to be element and tissue dependent. These parameters, in addition to season, were incorporated into multi-predictor variable models, where variations in trace metal concentration data were often best explained when season, somatic condition, and trophic descriptors were included. These variables were also of greatest relative importance across all considered trace metals and tissue types. These findings suggest that seasonally linked processes have the greatest influence on trace metal concentrations in anadromous Arctic charr. Future metal-related research on Arctic charr and other northern fish species should further consider these variables when evaluating elemental accumulation.

Complete mitochondrial genomes of the Arctic charr Salvelinus alpinus alpinus Linnaeus (Salmoniformes: Salmonidae)

The complete mitochondrial genomes were sequenced in three individuals of Arctic charr Salvelinus alpinus alpinus. The genome sequences were 16,655 and 16,657 bp in length, and the gene arrangement, composition, and size are very similar to the charr genomes published previously. The difference between the three genomes studied is low, 0.09%. The level of sequence divergence between S. alpinus alpinus and S. alpinus oquassa inferred from the complete mitochondrial genomes is relatively low (0.36%), indicating recent divergence of the taxa and/or potential historical hybridization between different lineages upon secondary contact.

"And if you gaze long into an abyss, the abyss gazes also into thee": four morphs of Arctic charr adapting to a depth gradient in Lake Tinnsjøen

The origin of species is a central topic in biology. Ecological speciation might be a driver in adaptive radiation, providing a framework for understanding mechanisms, level, and rate of diversification. The Arctic charr Salvelinus alpinus L. is a polymorphic species with huge morphological and life‐history diversity in Holarctic water systems. We studied adaptive radiation of Arctic charr in the 460‐m‐deep Lake Tinnsjøen to (a) document eco‐morphology and life‐history traits of morphs, (b) estimate reproductive isolation of morphs, and (c) illuminate Holarctic phylogeography and lineages colonizing Lake Tinnsjøen. We compared Lake Tinnsjøen with four Norwegian outgroup populations. Four field‐assigned morphs were identified in Lake Tinnsjøen: the planktivore morph in all habitats except deep profundal, the dwarf morph in shallow‐moderate profundal, the piscivore morph mainly in shallow‐moderate profundal, and a new undescribed abyssal morph in the deep profundal. Morphs displayed extensive life‐history variation in age and size. A moderate‐to‐high concordance was observed among morphs and four genetic clusters from microsatellites. mtDNA suggested two minor endemic clades in Lake Tinnsjøen originating from one widespread colonizing clade in the Holarctic. All morphs were genetically differentiated at microsatellites (F_ST: 0.12–0.20), associated with different mtDNA clade frequencies. Analyses of outgroup lakes implied colonization from a river below Lake Tinnsjøen. Our findings suggest postglacial adaptive radiation of one colonizing mtDNA lineage with niche specialization along a depth–temperature–productivity–pressure gradient. Concordance between reproductive isolation and habitats of morphs implies ecological speciation as a mechanism. Particularly novel is the extensive morph diversification with depth into the often unexplored deepwater profundal habitat, suggesting we may have systematically underestimated biodiversity in lakes. In a biological conservation framework, it is imperative to protect endemic below‐species‐level biodiversity, particularly so since within‐species variation comprises an extremely important component of the generally low total biodiversity observed in the northern freshwater systems.

Seasonal dietary shifts enhance parasite transmission to lake salmonids during ice cover

Changes in abiotic and biotic factors between seasons in subarctic lake systems are often profound, potentially affecting the community structure and population dynamics of parasites over the annual cycle. However, few winter studies exist and interactions between fish hosts and their parasites are typically confined to snapshot studies restricted to the summer season whereas host-parasite dynamics during the ice-covered period rarely have been explored. The present study addresses seasonal patterns in the infections of intestinal parasites and their association with the diet of sympatric living Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in Lake Takvatn, a subarctic lake in northern Norway. In total, 354 Arctic charr and 203 brown trout were sampled from the littoral habitat between June 2017 and May 2018. Six trophically transmitted intestinal parasite taxa were identified and quantified, and their seasonal variations were contrasted with dietary information from both stomachs and intestines of the fish. The winter period proved to be an important transmission window for parasites, with increased prevalence and intensity of amphipod-transmitted parasites in Arctic charr and parasites transmitted through fish prey in brown trout. In Arctic charr, seasonal patterns in parasite infections resulted mainly from temporal changes in diet toward amphipods, whereas host body size and the utilization of fish prey were the main drivers in brown trout. The overall dynamics in the community structure of parasites chiefly mirrored the seasonal dietary shifts of their fish hosts.

Resolving fine-scale population structure and fishery exploitation using sequenced microsatellites in a northern fish

The resiliency of populations and species to environmental change is dependent on the maintenance of genetic diversity, and as such, quantifying diversity is central to combating ongoing widespread reductions in biodiversity. With the advent of next-generation sequencing, several methods now exist for resolving fine-scale population structure, but the comparative performance of these methods for genetic assignment has rarely been tested. Here, we evaluate the performance of sequenced microsatellites and a single nucleotide polymorphism (SNP) array to resolve fine-scale population structure in a critically important salmonid in north eastern Canada, Arctic Charr (Salvelinus alpinus). We also assess the utility of sequenced microsatellites for fisheries applications by quantifying the spatial scales of movement and exploitation through genetic assignment of fishery samples to rivers of origin and comparing these results with a 29-year tagging dataset. Self-assignment and simulation-based analyses of 111 genome-wide microsatellite loci and 500 informative SNPs from 28 populations of Arctic Charr in north-eastern Canada identified largely river-specific genetic structure. Despite large differences (~4X) in the number of loci surveyed between panels, mean self-assignment accuracy was similar with the microsatellite loci and the SNP panel (>90%). Subsequent analysis of 996 fishery-collected samples using the microsatellite panel revealed that larger rivers contribute greater numbers of individuals to the fishery and that coastal fisheries largely exploit individuals originating from nearby rivers, corroborating results from traditional tagging experiments. Our results demonstrate the efficacy of sequence-based microsatellite genotyping to advance understanding of fine-scale population structure and harvest composition in northern and understudied species.

Activation of oxygen-responsive pathways is associated with altered protein metabolism in Arctic char exposed to hypoxia

Fish exposed to fluctuating oxygen concentrations often alter their metabolism and/or behaviour to survive. Hypoxia tolerance is typically associated with the ability to reduce energy demand by supressing metabolic processes such as protein synthesis. Arctic char is amongst the most sensitive salmonid to hypoxia, and typically engage in avoidance behaviour when faced with lack of oxygen. We hypothesized that a sensitive species will still have the ability (albeit reduced) to regulate molecular mechanisms during hypoxia. We investigated the tissue-specific response of protein metabolism during hypoxia. Little is known about protein degradation pathways during hypoxia in fish and we predict that protein degradation pathways are differentially regulated and play a role in the hypoxia response. We also studied the regulation of oxygen-responsive cellular signalling pathways [hypoxia inducible factor (HIF), unfolded protein response (UPR) and mTOR pathways] since most of what we know comes from studies on cancerous mammalian cell lines. Arctic char were exposed to cumulative graded hypoxia trials for 3 h at four air saturation levels (100%, 50%, 30% and 15%). The rate of protein synthesis was measured using a flooding dose technique, whereas protein degradation and signalling pathways were assessed by measuring transcripts and phosphorylation of target proteins. Protein synthesis decreased in all tissues measured (liver, muscle, gill, digestive system) except for the heart. Salmonid hearts have preferential access to oxygen through a well-developed coronary artery, therefore the heart is likely to be the last tissue to become hypoxic. Autophagy markers were upregulated in the liver, whereas protein degradation markers were downregulated in the heart during hypoxia. Further work is needed to determine the effects of a decrease in protein degradation on a hypoxic salmonid heart. Our study showed that protein metabolism in Arctic char is altered in a tissue-specific fashion during graded hypoxia, which is in accordance with the responses of the three major hypoxia-sensitive pathways (HIF, UPR and mTOR). The activation pattern of these pathways and the cellular processes that are under their control varies greatly among tissues, sometimes even going in the opposite direction. This study provides new insights on the effects of hypoxia on protein metabolism. Adjustment of these cellular processes is likely to contribute to shifting the fish phenotype into a more hypoxia-tolerant one, if more than one hypoxia event were to occur. Our results warrant studying these adjustments in fish exposed to long-term and diel cycling hypoxia.

Extensive genetic differentiation between recently evolved sympatric Arctic charr morphs

The availability of diverse ecological niches can promote adaptation of trophic specializations and related traits, as has been repeatedly observed in evolutionary radiations of freshwater fish. The role of genetics, environment, and history in ecologically driven divergence and adaptation, can be studied on adaptive radiations or populations showing ecological polymorphism. Salmonids, especially the Salvelinus genus, are renowned for both phenotypic diversity and polymorphism. Arctic charr (Salvelinus alpinus) invaded Icelandic streams during the glacial retreat (about 10,000 years ago) and exhibits many instances of sympatric polymorphism. Particularly, well studied are the four morphs in Lake Þingvallavatn in Iceland. The small benthic (SB), large benthic (LB), planktivorous (PL), and piscivorous (PI) charr differ in many regards, including size, form, and life history traits. To investigate relatedness and genomic differentiation between morphs, we identified variable sites from RNA-sequencing data from three of those morphs and verified 22 variants in population samples. The data reveal genetic differences between the morphs, with the two benthic morphs being more similar and the PL-charr more genetically different. The markers with high differentiation map to all linkage groups, suggesting ancient and pervasive genetic separation of these three morphs. Furthermore, GO analyses suggest differences in collagen metabolism, odontogenesis, and sensory systems between PL-charr and the benthic morphs. Genotyping in population samples from all four morphs confirms the genetic separation and indicates that the PI-charr are less genetically distinct than the other three morphs. The genetic separation of the other three morphs indicates certain degree of reproductive isolation. The extent of gene flow between the morphs and the nature of reproductive barriers between them remain to be elucidated.

Arctic charr brain transcriptome strongly affected by summer seasonal growth but only subtly by feed deprivation

Background

The Arctic charr (Salvelinus alpinus) has a highly seasonal feeding cycle that comprises long periods of voluntary fasting and a short but intense feeding period during summer. Therefore, the charr represents an interesting species for studying appetite-regulating mechanisms in fish.

Results

In this study, we compared the brain transcriptomes of fed and feed deprived charr over a 4 weeks trial during their summer feeding season. Despite prominent differences in body condition between fed and feed deprived charr at the end of the trial, feed deprivation affected the brain transcriptome only slightly. In contrast, the transcriptome differed markedly over time in both fed and feed deprived charr, indicating strong shifts in basic cell metabolic processes possibly due to season, growth, temperature, or combinations thereof. The GO enrichment analysis revealed that many biological processes appeared to change in the same direction in both fed and feed deprived fish. In the feed deprived charr processes linked to oxygen transport and apoptosis were down- and up-regulated, respectively. Known genes encoding for appetite regulators did not respond to feed deprivation. Gene expression of Deiodinase 2 (DIO2), an enzyme implicated in the regulation of seasonal processes in mammals, was lower in response to season and feed deprivation. We further found a higher expression of VGF (non-acronymic) in the feed deprived than in the fed fish. This gene encodes for a neuropeptide associated with the control of energy metabolism in mammals, and has not been studied in relation to regulation of appetite and energy homeostasis in fish.

Conclusions

In the Arctic charr, external and endogenous seasonal factors for example the increase in temperature and their circannual growth cycle, respectively, evoke much stronger responses in the brain than 4 weeks feed deprivation. The absence of a central hunger response in feed deprived charr give support for a strong resilience to the lack of food in this high Arctic species. DIO2 and VGF may play a role in the regulation of energy homeostasis and need to be further studied in seasonal fish.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5874-z) contains supplementary material, which is available to authorized users.

Screening-level risk assessment of methylmercury for non-anadromous Arctic char (Salvelinus alpinus)

Non-anadromous forms of Arctic char (Salvelinus alpinus), those that are restricted to lakes and rivers, typically have higher mercury (Hg) concentrations than anadromous forms, which migrate to and from the sea. Using tissue burden data from the literature and our own analyses, we performed a screening-level risk assessment of methylmercury (MeHg) for non-anadromous Arctic char. Our assessment included 1569 fish distributed across 83 sites. Site-specific mean total Hg concentrations in non-anadromous Arctic char muscle varied considerably from 0.01 to 1.13 µg/g wet weight, with 21% (17 of 83 sites) meeting or exceeding a threshold-effect level in fish of 0.33 µg/g wet weight, and 13% (11 of 83 sites) meeting or exceeding a threshold-effect level in fish of 0.5 µg/g wet weight. Of the sites in exceedance of the 0.33-µg/g threshold, 7 were located in Greenland and 10 in Canada (Labrador, Nunavut, and Yukon). All but one of these sites were located in interfrost or permafrost biomes. Maximum total Hg concentrations exceeded 0.33 µg/g wet weight at 53% of sites (40 of the 75 sites with available maximum Hg values), and exceeded 0.5 µg/g wet weight at 27% (20 of 75 sites). Collectively, these results indicate that certain populations of non-anadromous Arctic char located mainly in interfrost and permafrost regions may be at risk for MeHg toxicity. This approach provides a simple statistical assessment of MeHg risk to non-anadromous Arctic char, and does not indicate actual effects. We highlight the need for studies that evaluate the potential toxic effects of MeHg in non-anadromous Arctic char, as well as those that aid in the development of a MeHg toxic-effect threshold specific to this species of fish. Environ Toxicol Chem 2019;38:489-502. © 2018 SETAC.

No support for cryptic choice by ovarian fluid in an external fertilizer

Whether the ovarian fluid (OF) represents a selective environment influencing cryptic female choice was tested using an external fertilizer experiencing intense sperm competition and large effects of OF on sperm swimming behavior-the Arctic charr (Salvelinus alpinus). We physically separated the OF from the eggs of reproductively active females and reintroduced either their own OF or fluid from another female to the eggs. The eggs were then fertilized in vitro in a replicated split-brood design with sperm from two males under synchronized sperm competition trials, while also measuring sperm velocity of the individual males in the individual OFs. We found large effects of males, but no effect of females (i.e., eggs) on paternity, determined from microsatellites. More important, we found no effect of OF treatments on the relative paternity of the two competing males in each pair. This experimental setup does not provide support for the hypothesis that OF plays an important role as medium for cryptic female choice in charr. Power analyses revealed that our sample size is large enough to detect medium-sized changes in relative paternity (medium-sized effect sizes), but not large enough to detect small changes in relative paternity. More studies are needed before a conclusion can be drawn about OF's potential influence on paternity under sperm competition-even in charr.

First confirmation of salmonid alphavirus infection in Arctic char Salvelinus alpinus and in Austria

To date, sleeping disease (SD) caused by salmonid alphavirus 2 (SAV 2) has been reported in freshwater rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar. This study describes for the first time the occurrence of SD in farm-reared Arctic char Salvelinus alpinus and the occurrence of SAV in Austria. Clinical symptoms were indicative of the disease, and the diagnosis was confirmed by histopathology, infectivity in first passages of CHSE-214 cells and PCR. The phylogenetic analysis of the amplified SAV-nonstructural protein-3 (nsP3) fragment revealed the affiliation to the SAV 2 genotype.

Freezer on, lights off! Environmental effects on activity rhythms of fish in the Arctic

Polar regions are characterized by acute seasonal changes in the environment, with organisms inhabiting these regions lacking diel photoperiodic information for parts of the year. We present, to our knowledge, the first high-resolution analysis of diel and seasonal activity of free-living fishes in polar waters (74°N), subject to extreme variation in photoperiod, temperature and food availability. Using biotelemetry, we tracked two sympatric ecomorphs of lake-dwelling Arctic charr (Salvelinus alpinus n = 23) over an annual cycle. Charr activity rhythms reflected the above-surface photoperiod (including under ice), with diel rhythms of activity observed. During the dark winter solstice period, charr activity became arrhythmic and much reduced, even though estimated light levels were within those at which charr can feed. When twilight resumed, charr activity ensued as diel vertical migration, which continued throughout spring and with increasing day length, despite stable water temperatures. Diel activity rhythms ceased during the polar day, with a sharp increase in arrhythmic fish activity occurring at ice-break. Despite contrasting resource use, circannual rhythms were mirrored in the two ecomorphs, although individual variability in activity rhythms was evident. Our data support conclusions of functionally adaptive periods of arrhythmicity in polar animals, suggesting maintenance of a circannual oscillator for scheduling seasonal behavioural and developmental processes.

Determinants of parasite distribution in Arctic charr populations: catchment structure versus dispersal potential

Parasite distribution patterns in lotic catchments are driven by the combined influences of unidirectional water flow and the mobility of the most mobile host. However, the importance of such drivers in catchments dominated by lentic habitats are poorly understood. We examined parasite populations of Arctic charr Salvelinus alpinus from a series of linear-connected lakes in northern Norway to assess the generality of lotic-derived catchment-scale parasite assemblage patterns. Our results demonstrated that the abundance of most parasite taxa increased from the upper to lower catchment. Allogenic taxa (piscivorous birds as final host) were present throughout the entire catchment, whereas their autogenic counterparts (charr as final hosts) demonstrated restricted distributions, thus supporting the theory that the mobility of the most mobile host determines taxa-specific parasite distribution patterns. Overall, catchment-wide parasite abundance and distribution patterns in this lentic-dominated system were in accordance with those reported for lotic systems. Additionally, our study highlighted that upper catchment regions may be inadequate reservoirs to facilitate recolonization of parasite communities in the event of downstream environmental perturbations.

Differential gene expression during early development in recently evolved and sympatric Arctic charr morphs

Phenotypic differences between closely related taxa or populations can arise through genetic variation or be environmentally induced, leading to altered transcription of genes during development. Comparative developmental studies of closely related species or variable populations within species can help to elucidate the molecular mechanisms related to evolutionary divergence and speciation. Studies of Arctic charr (Salvelinus alpinus) and related salmonids have revealed considerable phenotypic variation among populations and in Arctic charr many cases of extensive variation within lakes (resource polymorphism) have been recorded. One example is the four Arctic charr morphs in the ∼10,000 year old Lake Thingvallavatn, which differ in numerous morphological and life history traits. We set out to investigate the molecular and developmental roots of this polymorphism by studying gene expression in embryos of three of the morphs reared in a common garden set-up. We performed RNA-sequencing, de-novo transcriptome assembly and compared gene expression among morphs during an important timeframe in early development, i.e., preceding the formation of key trophic structures. Expectedly, developmental time was the predominant explanatory variable. As the data were affected by some form of RNA-degradation even though all samples passed quality control testing, an estimate of 3'-bias was the second most common explanatory variable. Importantly, morph, both as an independent variable and as interaction with developmental time, affected the expression of numerous transcripts. Transcripts with morph effect, separated the three morphs at the expression level, with the two benthic morphs being more similar. However, Gene Ontology analyses did not reveal clear functional enrichment of transcripts between groups. Verification via qPCR confirmed differential expression of several genes between the morphs, including regulatory genes such as AT-Rich Interaction Domain 4A (arid4a) and translin (tsn). The data are consistent with a scenario where genetic divergence has contributed to differential expression of multiple genes and systems during early development of these sympatric Arctic charr morphs.

Biomarker response and hypothalamus-pituitary-interrenal axis functioning in Arctic charr from Bjørnøya (74°30' N), Norway, with high levels of organohalogenated compounds

The populations of Arctic charr (Salvelinus alpinus) residing in Lake Ellasjøen at Bjørnøya Island in the Norwegian Arctic (74° 30'N, 19° 00'E) possess substantially higher levels of organohalogenated compounds (strongly dominated by polychlorinated biphenyls, PCBs) than conspecifics residing in other, proximate lakes on the island. In the present study we sampled large (<400g), immature charr from Lake Ellasjøen (high PCB levels) and Lake Laksvatn (reference lake, low PCB levels) by hook and line for an immediate blood sampling, and blood and tissue sampling after a 1h confinement stressor. This was done in order to investigate possible effects of pollutants on an acute stress performance in a high-latitude fish species by comparing muscle PCB levels, hepatic cytochrome P4501A (CYP1A) biomarker activation and functioning of the hypothalamus-pituitary-interrenal (HPI) axis between these two populations of Arctic charr. As expected sum PCB muscle levels were 8-fold higher on a wet weigh basis, and 19-fold higher on a lipid weight basis, in charr from Ellasjøen than in charr from Laksvatn. This was accompanied by a 3.5-fold higher liver cyp1a mRNA abundance in the Ellasjøen charr compared to Laksvatn charr. Brain transcript levels encoding glucocorticoid receptor 1 and 2 (GR2) and corticotropin-releasing factor, and pituitary transcript levels encoding GR2 and proopiomelanocortin A1 and A2 were higher in Ellasjøen charr than in Laksvatn charr, while interrenal transcript levels encoding melanocortin 2 receptor and steroidogenic acute regulatory protein were lower. There were no differences in plasma cortisol concentration between the two charr populations immediately after capture and one hour after confinement. The strong biomarker response to OHCs and altered mRNA abundances of key genes related to HPI axis functioning in the Ellasjøen charr suggest endocrine disruptive effects of OHCs in this charr population. Possible ecological implications are not known, but it cannot be excluded that a slower growth rate in Ellasjøen charr compared to Laksvatn charr due to an increased metabolic demand associated with the activation of xenobiotic defense and detoxification systems may have contributed to the lower body mass of Ellasjøen charr compared to Laksvatn charr.

Experimental challenges with Renibacterium salmoninarum in Arctic charr Salvelinus alpinus

Arctic charr Salvelinus alpinus L. is an important species in Icelandic aquaculture and the most common wild salmonid in Iceland. A study on the course of infection with the bacterium Renibacterium salmoninarum was conducted using 3 different challenge methods in brackish and fresh water. Bacterial isolation, ELISA and PCR tests were used for detection of the bacterium in multiple organ samples. In an experiment, run for 34 wk in brackish water, infection was established by intraperitoneal injection with 5 × 106 colony forming units (CFU) fish-1. There were external and internal symptoms of bacterial kidney disease (BKD) and mortalities between 6 and 13 wk after injection. A cohabitation trial was run simultaneously and infection was well established after 4 wk, as demonstrated by the detection methods applied. Symptoms of BKD were not seen and all but 1 cohabitant survived. In a separate experiment, infection was established by pumping a fixed amount of water from a tank with fingerlings infected by intraperitoneal injection into tanks with naïve fish, in fresh or brackish water, for 6 wk. Fish in the inflow tanks were reared for an additional 3 wk. There were neither macroscopic symptoms nor mortalities. ELISA and PCR tests showed that infection started to take hold after 3 wk. The challenge trials demonstrated that Arctic charr is susceptible to R. salmoninarum. Cohabitation and inflow of water from tanks with infected fish provide useful models for further studies on R. salmoninarum infection acquired in a natural way in Arctic charr.

Sequencing and characterization of a multi-organ Arctic charr transcriptome: A toolbox for investigating polymorphism and seasonal life in a high Arctic fish

The Arctic charr (Salvelinus alpinus L.) inhabits fresh water ecosystems of the high North. The species has developed a strong phenotypic plasticity and variability in life history characteristics which has made this species an attractive model for investigations on phenotype plasticity, morph formation and ecological speciation. Further, the extreme seasonal variations in environmental conditions (e.g. food availability) in the high North induce seasonal changes in phenotype, which require precise timing mechanisms and physiological preparations. Individual gating of life-history strategies (e.g. formation of resident and sea-migrating morphs) and transitions (e.g. maturation) depends on conditional traits (size/energy status) at specific assessment time windows, and complex neuroendocrine regulation, which so far is poorly understood. In the absence of a reference genome, and in order to facilitate the investigation of the complex biological mechanisms of this unique fish model, the present study reveals a reference transcriptome for the Arctic charr. Using Roche 454 GS FLX+, we targeted various organs being either at the crossroads of many key pathways (neuroendocrine, metabolic, behavioral), of different ontological origins or displaying complementary physiological functions. The assemblage yielded 34,690 contigs greater than 1000bp with an average length (1690bp) and annotation rate (52%) within the range, or even higher, than what has been previously obtained with other teleost de novo transcriptomes. We dramatically improve the publically available transcript data on this species that may indeed be useful for various disciplines, from basic research to applied aspects related to conservation issues and aquaculture.

Assessing the reproducibility of fractional rates of protein synthesis in muscle tissue measured using the flooding dose technique

The flooding dose technique of Garlick et al. (1980) has become the main method for measuring tissue and whole-animal rates of protein synthesis in ectotherms. However, single tissue samples are used to determine rates of protein synthesis and no studies have examined the pattern of flooding in large tissues such as the white muscle in fishes, which can comprise up to 55% of the wet body mass of a fish and which is poorly perfused. The present study has examined, for the first time, the patterns of flooding and measured rates of protein synthesis in five different regions of the white muscle in the Arctic charr Salvelinus alpinus ranging in size from 25g to 1.6kg following a flooding dose injection of L-[(3)H]-phenylalanine. The results indicate that the degree of flooding (i.e. free pool specific radioactivity relative to that of the injection solution) and elevation in free phenylalanine concentrations can vary between regions but the calculated fractional rates of protein synthesis were similar in four of the five regions studied. The variability in rates of protein synthesis increased with body size with greater variability observed between regions for fish >1kg in body mass. For consistency between studies, it is recommended that samples are taken from the epaxial muscle in the region below the dorsal fin when measuring fractional rates of white muscle synthesis in fishes.

A rapid and convenient method for measuring the fractional rate of protein synthesis in ectothermic animal tissues using a stable isotope tracer

A method was devised to measure the fractional rate of protein synthesis in fish using a stable isotope labelled tracer (ring-D5-phenylalanine) instead of radioactive phenylalanine. This modified flooding dose technique utilizes gas chromatography with mass spectrometry detection (GC-MS). The technique was validated by measuring the fractional rate of protein synthesis in the liver and white muscle of Arctic charr (Salvelinus alpinus) and then tested by comparing the fractional rate of protein synthesis of fed and starved Arctic charr. The modified technique met the assumptions of the flooding dose technique and was successfully used to detect alterations in the rate of protein synthesis in fed and starved fish. This modified technique allows for studies on protein metabolism to be carried out in situations where the use of radioactivity is difficult, if not impossible.

Transcriptional dynamics of a conserved gene expression network associated with craniofacial divergence in Arctic charr

Background

Understanding the molecular basis of craniofacial variation can provide insights into key developmental mechanisms of adaptive changes and their role in trophic divergence and speciation. Arctic charr (Salvelinus alpinus) is a polymorphic fish species, and, in Lake Thingvallavatn in Iceland, four sympatric morphs have evolved distinct craniofacial structures. We conducted a gene expression study on candidates from a conserved gene coexpression network, focusing on the development of craniofacial elements in embryos of two contrasting Arctic charr morphotypes (benthic and limnetic).

Results

Four Arctic charr morphs were studied: one limnetic and two benthic morphs from Lake Thingvallavatn and a limnetic reference aquaculture morph. The presence of morphological differences at developmental stages before the onset of feeding was verified by morphometric analysis. Following up on our previous findings that Mmp2 and Sparc were differentially expressed between morphotypes, we identified a network of genes with conserved coexpression across diverse vertebrate species. A comparative expression study of candidates from this network in developing heads of the four Arctic charr morphs verified the coexpression relationship of these genes and revealed distinct transcriptional dynamics strongly correlated with contrasting craniofacial morphologies (benthic versus limnetic). A literature review and Gene Ontology analysis indicated that a significant proportion of the network genes play a role in extracellular matrix organization and skeletogenesis, and motif enrichment analysis of conserved noncoding regions of network candidates predicted a handful of transcription factors, including Ap1 and Ets2, as potential regulators of the gene network. The expression of Ets2 itself was also found to associate with network gene expression. Genes linked to glucocorticoid signalling were also studied, as both Mmp2 and Sparc are responsive to this pathway. Among those, several transcriptional targets and upstream regulators showed differential expression between the contrasting morphotypes. Interestingly, although selected network genes showed overlapping expression patterns in situ and no morph differences, Timp2 expression patterns differed between morphs.

Conclusion

Our comparative study of transcriptional dynamics in divergent craniofacial morphologies of Arctic charr revealed a conserved network of coexpressed genes sharing functional roles in structural morphogenesis. We also implicate transcriptional regulators of the network as targets for future functional studies.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-5-40) contains supplementary material, which is available to authorized users.

Tissue-specific molecular immune response to lipopolysaccharide challenge in emaciated anadromous Arctic charr

Anadromous Arctic charr (Salvelinus alpinus) undergo voluntary winter fasting for months in the Arctic. We tested the hypothesis that extended fasting will compromise the ability of this species to evoke an immune response. Charr were either fed or fasted for 85 days and challenged with lipopolysaccharide (LPS), and the molecular immune response in the liver and spleen assessed at 8 and 96 h post-injection. LPS increased IL-1β, IL-8, and serum amyloid protein A (SAA) mRNA levels in both groups, but the liver IL-1β and IL-8, and spleen IL-8 responses were reduced in the fasted group. Fasting upregulated SOCS-1 and SOCS-2 mRNA abundance, while LPS stimulated SOCS-3 mRNA abundance and this response was higher in the fasted liver. Collectively, extended fasting and emaciation does not curtail the capacity of charr to evoke an immune response, whereas upregulation of SOCS may be a key adaptation to conserve energy by restricting the inflammatory response.

Contemporary effective population and metapopulation size (N e and meta-N e): comparison among three salmonids inhabiting a fragmented system and differing in gene flow and its asymmetries

We estimated local and metapopulation effective sizes ([Formula: see text] and meta-[Formula: see text]) for three coexisting salmonid species (Salmo salar, Salvelinus fontinalis, Salvelinus alpinus) inhabiting a freshwater system comprising seven interconnected lakes. First, we hypothesized that [Formula: see text] might be inversely related to within-species population divergence as reported in an earlier study (i.e., FST: S. salar> S. fontinalis> S. alpinus). Using the approximate Bayesian computation method implemented in ONeSAMP, we found significant differences in [Formula: see text] ([Formula: see text]) between species, consistent with a hierarchy of adult population sizes ([Formula: see text]). Using another method based on a measure of linkage disequilibrium (LDNE: [Formula: see text]), we found more finite [Formula: see text] values for S. salar than for the other two salmonids, in line with the results above that indicate that S. salar exhibits the lowest [Formula: see text] among the three species. Considering subpopulations as open to migration (i.e., removing putative immigrants) led to only marginal and non-significant changes in [Formula: see text], suggesting that migration may be at equilibrium between genetically similar sources. Second, we hypothesized that meta-[Formula: see text] might be significantly smaller than the sum of local [Formula: see text]s (null model) if gene flow is asymmetric, varies among subpopulations, and is driven by common landscape features such as waterfalls. One 'bottom-up' or numerical approach that explicitly incorporates variable and asymmetric migration rates showed this very pattern, while a number of analytical models provided meta-[Formula: see text] estimates that were not significantly different from the null model or from each other. Our study of three species inhabiting a shared environment highlights the importance and utility of differentiating species-specific and landscape effects, not only on dispersal but also in the demography of wild populations as assessed through local [Formula: see text]s and meta-[Formula: see text]s and their relevance in ecology, evolution and conservation.