Invasion of exotic species has caused the loss of biodiversity and imparts evolutionary and ecological changes in the introduced systems. In northern Fennoscandia, European whitefish (Coregonus lavaretus (L.)) is a highly polymorphic species displaying adaptive radiations into partially reproductively isolated and thus genetically differentiated sympatric morphs utilizing the planktivorous and benthivorous food niche in many lakes. In 1993, Lake Skrukkebukta was invaded by vendace (Coregonus albula (L.)) which is a zooplanktivorous specialist. The vendace displaced the densely rakered whitefish from its preferred pelagic niche to the benthic habitat harbouring the large sparsely rakered whitefish. In this study, we investigate the potential influence of the vendace invasion on the breakdown of reproductive isolation between the two whitefish morphs. We inferred the genotypic and phenotypic differentiation between the two morphs collected at the arrival (1993) and 15 years after (2008) the vendace invasion using 16 microsatellite loci and gill raker numbers, the most distinctive adaptive phenotypic trait between them. The comparison of gill raker number distributions revealed two modes growing closer over 15 years following the invasion. Bayesian analyses of genotypes revealed that the two genetically distinct whitefish morphs that existed in 1993 had collapsed into a single population in 2008. The decline in association between the gill raker numbers and admixture values over 15 years corroborates the findings from the Bayesian analysis. Our study thus suggests an apparent decrease of reproductive isolation in a morph-pair of European whitefish within 15 years (≃ 3 generations) following the invasion of a superior trophic competitor (vendace) in a subarctic lake, reflecting a situation of “speciation in reverse”.

The cold-adapted polar cod Boreogadus saida, a key species in Arctic ecosystems, is vulnerable to global warming and ice retreat. In this study, 1257 individuals sampled in 17 locations within the latitudinal range of 75–81°N from Svalbard to East Siberian Sea were genotyped with a dedicated suite of 116 single-nucleotide polymorphic loci (SNP). The overall pattern of isolation by distance (IBD) found was driven by the two easternmost samples (East Siberian Sea and Laptev Sea), whereas no differentiation was registered in the area between the Kara Sea and Svalbard. Eleven SNP under strong linkage disequilibrium, nine of which could be annotated to chromosome 2 in Atlantic cod, defined two genetic groups of distinct size, with the major cluster containing seven-fold larger number of individuals than the minor. No underlying geographic basis was evident, as both clusters were detected throughout all sampling sites in relatively similar proportions (i.e. individuals in the minor cluster ranging between 4 and 19% on the location basis). Similarly, females and males were also evenly distributed between clusters and age groups. A differentiation was, however, found regarding size at age: individuals belonging to the major cluster were significantly longer in the second year. This study contributes to increasing the population genetic knowledge of this species and suggests that an appropriate management should be ensured to safeguard its diversity.

Disease resistance of fish larvae may be improved by bath treatment in water containing immunostimulants. Pattern recognition receptors, such as TLR3, TLR7, and MDA5, work as an “early warning” to induce intracellular signaling and facilitate an antiviral response. A single bath of newly hatched larvae, with Astragalus, upregulated the expression of IFNα, IFNc, ISG15, MDA5, PKR, STAT1, TLR3, and TLR7 immune genes, on day 4 post treatment. Similar patterns were observed for Hyaluronic acid and Poly I:C. Increased expression was observed for ISG15, MDA5, MX, STAT1, TLR3, TLR7, and RSAD2, on day 9 for Imiquimod. Metabolic gene expression was stimulated on day 1 after immunostimulant bath in ULK1, MYC, SLC2A1, HIF1A, MTOR, and SIX1, in Astragalus, Hyaluronic acid, and Imiquimod. Expression of NOS2 in Poly I:C was an average fourfold above that of control at the same timepoint. Throughout the remaining sampling days (2, 4, 9, 16, 32, and 45 days post immunostimulant bath), NOS2 and IL1B were consistently overexpressed. In conclusion, the immunostimulants induced antiviral gene responses, indicating that a single bath at an early life stage could enable a more robust antiviral defense in fish. Additionally, it was demonstrated, based on gene expression data, that cell metabolism was perturbed, where several metabolic genes were co-regulated with innate antiviral genes.

As a result of ocean warming, the species composition of the Arctic seas has begun to shift in a boreal direction. One ecosystem prone to fauna shifts is the Northeast Greenland shelf. The dispersal route taken by boreal fauna to this area is, however, not known. This knowledge is essential to predict to what extent boreal biota will colonise Arctic habitats. Using population genetics, we show that Atlantic cod ( Gadus morhua ), beaked redfish ( Sebastes mentella ), and deep-sea shrimp ( Pandalus borealis ) specimens recently found on the Northeast Greenland shelf originate from the Barents Sea, and suggest that pelagic offspring were dispersed via advection across the Fram Strait. Our results indicate that boreal invasions of Arctic habitats can be driven by advection, and that the fauna of the Barents Sea can project into adjacent habitats with the potential to colonise putatively isolated Arctic ecosystems such as Northeast Greenland.

As Arctic Ocean temperatures rise, the composition and distribution of fish species are changing. A clear and accurate classification of species is therefore instrumental to the conservation and management of this region. Careproctus snailfishes (Liparidae) are common in the region, but suffer from controversial taxonomy, relying on morphological characters that are difficult to study due to fragile skin, soft bodies, and poorly ossified skeletons. In the present study, we provide a screening of the Careproctus species diversity in the northeast Atlantic and adjacent Arctic region. Using a combination of mitochondrial DNA and nuclear SNP data, we evaluate the taxonomic validity of certain proposed species, distinct from C. micropus and C. reinhardti. Our findings provide minimal support for more than these two species. Since the species unit forms the foundation for biodiversity research and communication, we highlight the importance of reliable and established morphological traits in taxonomy. We recommend supplementing morphology with genetic methods to minimize the risk of taxonomic inflation, especially in rare or understudied taxa. Furthermore, we confirm the occurrence of C. micropus from a coastal fjord system in northern Norway, extending its known distribution.

Background : Pancreas disease, caused by salmonid alphavirus subtype 3 (SAV3), remains a significant threat to Atlantic salmon (Salmo salar L.) aquaculture in Norway despite the widespread use of inactivated vaccines. Molecular adjuvants represent a promising strategy to improve vaccine efficacy by enhancing immune responses. Methods: In this study, we evaluated the immunological and protective effects of two transcription factors, gata3 and eomes encoded by plasmid DNA adjuvants in combination with a low-dose inactivated SAV3 vaccine. Atlantic salmon were immunized with inactivated virus alone or in combination with either gata3- or eomes-encoding plasmids, followed by SAV3 challenge. Vaccine performance was assessed by growth, antibody response (ELISA and virus neutralization assay), and histopathological scoring of heart and pancreas. Results: The effects of gata3 and eomes were context-dependent: eomes reduced pancreas lesions at 4 weeks post-challenge, while both plasmid-adjuvant groups showed improved growth at 10 weeks post-challenge compared to the plasmid control. Although antibody responses were increased in all vaccinated groups at 6 weeks post-immunization, the plasmid control performed comparably to the GATA3 or EOMES groups, and no significant differences were detected in the virus neutralization test. Interestingly, the empty plasmids given to control fish exhibited immunostimulatory activity, warranting further investigation. In vitro and in vivo fluorescence microscopy confirmed expression of the transcription factors in transfected cells and at the injection site. Conclusions: Our results suggest that transcription factor-encoding plasmids can provide selective adjuvant effects, while also highlighting the potential that plasmid DNA can have as adjuvants in fish vaccine development.