Search

This thesis is published on Munin with a CC BY license. Cuscuta is a genus of parasitic plants that form cross-species bridges and connect to the xylem and phloem of a wide variety of host plants. Because of their unique lifestyle, research into processes like the formation of connections in between plant cells and plant immunity would benefit from Cuscuta as a model system. However, a method to study gene functions in a Cuscuta species is still missing. Its development was therefore chosen as the goal of this thesis. In vitro culture techniques for Cuscuta campestris were developed and Agrobacterium transformation of Cuscuta tissue was successfully performed in two different systems, the adhesive disk area close to the parasitic infection organ, and seedlings of C. campestris. As an alternative method for gene function studies in Cuscuta, virus induced gene silencing was tested in C. campestris. Mineral transport across the host-parasite border was studied and the potential use of interspecies plasmodesmata between Cuscuta and its hosts was investigated. The results presented in this thesis allow for first gene function studies in Cuscuta and are good foundations for the development of pipelines for transgenic plant production and gene silencing techniques. Challenges and opportunities with this system are outlined. The conclusions of this thesis will help to “tame” Cuscuta and use its unique physique and strategies for host plant infection to establish a model species for studying plant-cell connections, transport processes and plant immunity.

Infectious diseases have been a problem for humans since the beginning of human existence. The “golden age” of antibiotics started at the end of the 1920s, with the discovery of penicillin by Sir Alexander Fleming. This was followed by the discovery of several life-saving antibiotics. The number of new marketed antibiotics has declined, and most pharmaceutical companies are no longer working in antibiotic development. This in itself would be unproblematic, had it not been for the rapid ability of bacteria to become resistant towards previously debilitating agents. The need for new antibiotics is therefore eminent. Natural products have been important contributors for antibiotic drug discovery and development. Microorganisms have been a particularly proliferative source of antibiotics, providing us with among others the penicillins, aminoglycosides, tetracyclines and polymyxins. Most naturally derived pharmaceuticals, including antibiotics, originate from terrestrial organisms. This is mainly because the terrestrial environment historically has been easier to access compared to the marine environment below the intertidal zone. The marine environment is highly diverse, and there is still a huge biodiversity that is yet to be explored. In this project, Arctic and sub-Arctic marine bacteria and fungi were cultivated and studied for their production of natural products. The cultures were extracted and fractionated, and the fractions were tested for bioactivity, mainly focusing on antibacterial activity. Using bioactivity-guided isolation, compounds were isolated and structurally characterized. Finally, the bioactivity of the isolated compounds was broadly evaluated. In paper I, a known siderophore, serratiochelin A, was isolated from a co-culture of two bacteria, Serratia sp. and Shewanella sp. The compound was not detected in axenic cultures, indicating that cocultivation triggered production. The acid-catalyzed degradation of serratiochelin A into serratiochelin C was also observed. Serratiochelin A had weak activity against Staphylococcus aureus, melanoma cells and non-malignant lung fibroblasts. No activity was observed for the degradation product serratiochelin C, indicating that the oxazoline moiety in the original compound is essential for the bioactivity. In paper II, a marine bacterium Lacinutrix sp. was cultivated and studied for its ability to produce bioactive natural products. Through bioactivity-guided isolation, two new lyso-ornithine lipids were isolated, and their structures elucidated, showing that they only differed by the length of the hydrocarbon tail. Analysis by UHPLC-HR-MS indicate that the purified solutions are mixtures of isomers, but these were not possible to separate by preparative HPLC-MS. The compounds were evaluated for antibacterial activity and antiproliferative activity against human cells. Compound 1 displayed weak activity against Streptococcus agalactiae, while compound 2 had weak activity against melanoma cells. In paper III, a new dimeric naphthopyrone substituted with a sulphate group was isolated in high yields from cultures of an obligate marine fungus in the family Lulworthiaceae. The compound was tested against an extended panel of clinical bacterial isolates and showed potent antibacterial activity against several clinical methicillin-resistant Staphylococcus aureus isolates, with MICs down to 1.56 μg/mL. Acid-catalyzed degradation was also observed. The compound also displayed moderate activities against three human cell lines: melanoma, hepatocellular carcinoma, and non-malignant lung fibroblast. In paper IV, a new chlovalicin variant, chlovalicin B, was isolated from cultures of the obligate marine fungus Digitatispora marina. The fungus has previously been studied for its distribution in the marine environment but has not been extensively studied for its biosynthetic potential. The compound was isolated in low yields, and the structure was elucidated by NMR and HRMS experiments. The compound was assessed for a range of bioactivities and had weak antiproliferative activity against human melanoma cells.

Cold seeps are locations on the seafloor where CH4 migrates from reservoirs below sediments towards the atmosphere, sustaining thereby a high microbial and macrofaunal biomass and a diversity contrasting from the surrounding seafloor. The oxidation of methane and sulphide are typically the main sources of primary productivity of these ecosystems and have therefore gained a particular attention in the global oceans. Yet, despite the ubiquitous presence of these seeping sites and the presence of gas hydrates in the Arctic Ocean and its adjacent shelves, the impact of methane on benthic and pelagic microbial communities in this region have remained limited. Recently, five gas hydrate bearing mounds with ongoing methane seeping activity were discovered south of Svalbard, in the northern Barents Sea. In this PhD project, I studied changes in the structure of microbial communities, including both prokaryotes and eukaryotes, and geochemical profiles at these mounds to highlight key microbial groups and to provide insights on their ecological roles. Different niches were addressed, including: deep anaerobic sediments (Paper I and II); niches at the sediment surface at gas flare locations and within bacterial mats and siboglinid fields (Paper III); and above gas flares in the shallow shelf water column (Paper IV). The microbial biodiversity and the structure of communities were successfully identified for each of the habitats listed above. Our investigations revealed a microbial composition similar to other cold seeps: a predominance of archaeal anaerobic methanotrophs (ANME) and sulphate-reducing bacteria (SRB) in CH4-rich sediments, a higher abundance of methane oxidizing bacteria associated to the Methylococcales in the surface sediments and water column; and a co-occurrence of other commonly found prokaryotic groups. Yet, uncommon biological traits were also uncovered at these methane seeping sites: the anaerobic oxidation of methane was merely only driven by ANME-1 without the co-occurrence of a specific SRB clade; an abundant methanotroph with little genetic similarity in databases was detected; and a strong niche differentiation of sulphide-oxidizing bacteria within the different bacterial mats. This project has thereby extended our knowledge on the microbial biodiversity at Arctic cold seeps and opened further future research perspective toward microbial activity and metabolism at these high latitudes.

Marine fungi comprise a group of organisms that have been overlooked for a long time. Research interest has increased with the realization of the important ecological role and rich chemistry of marine fungi. Marine fungi have yielded thousands of new natural products the last decade, but many taxa remain unstudied. Marine fungi from the Arctic have not been reported in literature in regard to bioprospecting campaigns and represent a novel source of natural products. The aim of this thesis is to assess the potential of Arctic marine fungi to produce bioactive secondary metabolites by fermentation and genome analysis. This was achieved in three steps. First, fungi were isolated from the Svalbard archipelago. The 20 isolates obtained were characterized based on molecular markers and their antibacterial activity was tested using an agar diffusion assay (Paper 1). Secondly, three distinct marine fungi were whole genome sequenced and characterized. One of the fungi represented a putatively novel species which was circumscribed based on morphology and phylogenetic inference (Paper 2). Finally, a metabolite from one fungus among the 20 obtained around Svalbard was isolated and the bioactivities characterized (Paper 3). In Paper 1, half of the fungal isolates showed activity against pathogenic bacteria and every third isolate represents potentially new species of fungi. Five of these isolates are strictly marine fungi belonging to the order of Lulworthiales. The study showed that the Arctic can yield novel marine fungal diversity that can be utilized in bioprospecting. For Paper 2, three marine fungi were whole genome sequenced and their biosynthetic gene clusters were characterized. Mapping of the biosynthetic gene clusters (BGCs) within the Emericellopsis genome confirmed the detection of the secondary metabolite helvolic acid produced during fermentation. The study revealed numerous unknown biosynthetic gene clusters and a range of carbohydrate active enzymes. Each of the three genomes provides the first genome of their respective taxa and can contribute to understanding their evolutionary adaption to the marine environment. In Paper 3, a novel compound from the fermentation broth of Mytilinidion sp. was isolated and its bioactivity was characterized using seven different bioactivity assays. The compound turned out to be a modified medium component with IC50 of 43 µM in an ACE-inhibitory assay. The compound was novel and this is the first report of its bioactivity. Molecular networking could perhaps have provided early indications that the compound was a modified medium component. Marin sopp er en gruppe av organismer som har vært oversett i lang tid. Interessen for marin sopp har steget i takt med økt forståelse for dens økologi og rike kjemi. Mange tusen nye naturprodukter har blitt beskrevet fra marin sopp de siste tiårene, men det er fremdeles mange grupper som ikke er godt studert. Marin sopp fra Arktis er ikke godt undersøkt i forbindelse med bioprospektering og representerer en ny kilde til naturprodukter. Målet med avhandlingen var å vurdere potensialet til Arktisk marin sopp for å produsere bioaktive metabolitter ved å bruke fermentering og genomanalyse. Dette ble gjennomført i tre steg. Først ble 20 marine sopp isolert i løpet av ett forskningstokt rundt Svalbard. Disse soppene ble karakterisert ved hjelp av molekylære markører og evnen til å produsere antibakterielle forbindelser ble undersøkt ved å bruke agar-diffusjons analyse (Artikkel 1). Det neste som ble gjort var at tre forskjellige marine sopp ble helgenomsekvensert og genomene ble karakterisert. En av soppene er antageligvis ny for vitenskapen og ble beskrevet basert på morfologisk og fylogenetisk analyser (Artikkel 2). Til sist ble ett stoff fra en av soppene fra Artikkel 1 isolert og bioaktiviteten til stoffet ble beskrevet (Artikkel 3). I den første artikkelen viste halvparten av de isolerte soppene antibakteriell aktivitet mot sykdomsfremkallende bakterier. Hvert tredje isolat representerer muligens nye arter basert på tilgjengelige referansesekvenser. Fem av de potensielt nye artene tilhører ordenen Lulworthiales som kun finnes i havet. Studien har vist at det er stort potensiale for å avdekke nye arter og at mange av disse viser evne til å produsere antibakterielle forbindelser. Dette viser at de kan utnyttes i bioprospektering. I Artikkel 2 ble tre ulike marine sopper helgenomsekvenseert og genomet ble karakterisert med tanke på hvilke biosyntetiske genklynger og karbohydrat-aktive enzymer som ble detektert. Karakteriseringen av genklyngene i Emericellopsis bekreftet deteksjonen av metabolitten helvolsyre som soppen produserte under fermentering. Studien avslørte mange ukjente genklynger og en rekke karbohydrat-aktive enzymer. De tre genomene er de første helsekvenserte genomene i deres respektive slekter og kan bidra til å gi ny kunnskap om de evolusjonære tilpasningene de har til det marine miljø. I den siste artikkelen ble ett ukjent stoff fra fermenteringen av Mytilinidion isolert. Bioaktiviteten til stoffet ble karakterisert ved hjelp av syv forskjellige bioaktivitetstester. Stoffet viste seg å være en delvis nedbrutt og modifisert mediekomponent som hadde en IC50 verdi på 42.3 µM i en ACE-inhiberingstest. Analyse ved hjelp av molekylært nettverk kunne kanskje avdekket at stoffet var relatert til dyrkningsmediumet.

Microbial eukaryotes, including protists and fungi, play diverse functions in virtually all ecosystems. In the High Arctic, their high biomass and diversity reflects crucial ecological importance and the performance of key ecological processes. Protists are the main primary producers in arctic seas, whereas fungi are an important group of decomposers and symbiotic partners of plants in terrestrial habitats. During the last decade, along with the development of new high-throughput sequencing methods, our knowledge regarding arctic microbial eukaryotes has expanded. Previous studies have identified the major groups of microbial eukaryotes present in Svalbard and how their richness and abundance may vary along various temporal and spatial scales. Those studies used high-throughput sequencing to reveal the dynamics, biodiversity patterns and community composition of diverse microbial eukaryotes such as marine protists, soil and root-associated fungi. However, altogether these studies have just scratched the surface of disentangling the biodiversity and its drivers. Basic questions regarding taxonomic diversity, community composition and their drivers are addressed in a limited manner, often leaving most of the observed variation unexplained. Regarding functionality of these organisms, even less is known. At the same time, these findings have also increased the amount of questions about microbial eukaryotes, their life histories, strategies, seasonality, sensitivity to changes in environmental conditions, as well as functional importance of these organisms at different scales. Previously unexplained variation and other emerging knowledge gaps regarding microbial eukaryotes formed a backstage for this thesis. The main focus was to look at these organisms from a functional angle regarding variation related to methodology, seasonality and biotic factors through case studies addressing the following knowledge gaps. Firstly, to understand the functionality of biodiversity in a temporal and spatial context of cold soils, we need to determine if our methods estimate biodiversity of the active community of microbial eukaryotes. In other words, does DNA-based detection of species provide good enough approximation to continue or is a different methodology needed? We found that the choice of marker gene template influenced diversity measures and read numbers in abundant fungal groups such as Helotiales and Agaricales. However, it did not impact the community structure. Secondly, the aim was to understand the role of biodiversity and functionality of plant root-associated fungi in relation to host plant performance. We explored putative effects of fungal diversity on plant morphology and the interplay between functional diversity and abiotic factors in a spatial context. Our results revealed the importance of fungal richness and functional diversity, but no impact of community structure on plant morphometrics. Moreover, we showed that temperature affects fungal richness, below- and aboveground parts of the plant in different ways, making it difficult to predict its impact on the biological outcomes in natural systems. The third aim was to address the lack of polar night investigations of microbial eukaryotes in general, especially with a strong focus on their functions. Here, we investigated the impact of strong seasonality on functions of microbial eukaryotes in the marine environment. Community-level gene expression was driven primarily by seasonal patterns of light availability. Among the most expressed transcripts, nearly ⅔ transcripts were not functionally annotated, providing further evidence for distinct genetic makeup of the Arctic Ocean. Through these three case studies, this thesis contributed some building blocks to close important knowledge gaps, but also revealed that there are more unknowns to be addressed. This thesis aimed to increase awareness of the importance of functional understanding of the roles of microbial eukaryotes in the High Arctic ecosystems. Finally, it highlights further possibilities and developments that could improve the understanding of possible future responses of these organisms and processes that they control.

During the last deglaciation, one clade of European whitefish (Coregonus lavaretus L.) colonized the lakes in the three contemporary watercourses of northern Fennoscandia (Pasvik, Tana and Alta) and diverged thereafter in sympatry. This thesis investigates the interplay between evolutionary processes and standing genetic variation in shaping phenotypic and genetic divergence, and how reproductive isolation is established and maintained in whitefish. In this thesis, I demonstrated how selection has shaped the whitefish population across a large spatial scale despite the clear effect of random genetic drift - presumably arising from the colonization history. The diversification of European whitefish follows a (non)parallel trajectory, as morphs in the different watercourses were characterised with various levels of phenotypic and genetic divergence, and signatures of both within and between lakes divergence were observed in all studies conducted within this thesis. As the reproductive isolation between whitefish eco-morph is not complete, hybridisation between morphs occurs frequently, which may refer relatively weak postzygotic barriers to gene flow between morphs. When whitefish morphs adapt to a specific niche, they encounter different environment conditions, like water temperature and prey availability that are important factors determining size at sexual maturity, somatic growth, and gonad development. Consequently, allochrony together with spatial differences in spawning may reinforces the ecological divergence and help to build up barriers to gene flow between the whitefish morphs. Indeed, allochrony seems to be working towards stronger reproductive isolation, as the advancement of the maturity, as well as the proportion of mature individuals differed between morphs. The results of this thesis are in line with the theory of ecological speciation and show how selection has shaped the European whitefish morphs towards stronger phenotype-environment correlation and more diversified populations, despite the decreasing amount of standing genetic variation between the three contemporary watercourses. Thus, the occurrence of whitefish morphs in the three watercourses may be an outcome of convergent evolution, where similar phenotypes have arisen from different genetic backgrounds.

The goal of this thesis was to comprehend evolutionary patterns in different contexts. To get a broad spectrum of methods, three projects were included of which one is published and two are in preparation. In the first project, we looked into the fate of new mutations in protein regions that do not have a defined three-dimensional structure. Generally, the fate of a new mutation is governed by selection that can be either purifying, neutral, or positive. We could show that disordered protein regions tend to be under more positive and less purifying selection. For the second project, we looked at four different transporter proteins that are localised in the inner plastid membrane. Plastids are organelles that are found in plants and some closely related species. The best-known type of plastids is the chloroplast, where photosynthesis takes place. The four transporter proteins shuttle phosphorylated carbon compounds across the inner membrane to allow an exchange of the products of photosynthesis and other metabolic processes to be shared between the plastids and the host cell. In our work, comparing the sequences of those transporters from various plants and algae, we could identify a new subgroup of transporters. In the third project, we tried to understand the distribution of enzymes that regenerate the ubiquitous metabolite NAD from its breakdown product nicotinamide. There are two pathways that synthesise NAD from nicotinamide. Additionally, nicotinamide can be methylated and subsequently excreted. We found two predominant patterns in the distribution of these pathways in eukaryotes and could explain them with mathematical models and cell culture experiments. All three projects included sequences from extraordinarily many species. The comprehensive analyses gave insight into the evolution of the respective proteins and pathways and highlight the possibilities of phylogenetic analyses.

Paper III of this thesis is not available in Munin. Paper III: Salgado‐Flores, A., Tveit, A. T., Wright, A. D., Pope, P. B., Sundset, M. A.: “Characterization of the cecum microbiome from wild and captive rock ptarmigans from Svalbard and northern Norway in relation to diet composition”. (Manuscript). Enteric methane (CH4) from gut microbial fermentation of complex organic polymers in ruminants comprises an important source of the anthropogenic CH4 emissions and may also represent a significant loss of metabolic energy to the host animal depending on diet. CH4 is produced by specialized microbes called methanogens. Extensive research exists on enteric CH4 production and methanogens in domestic ruminants, but little is known about arctic herbivores. Reindeer (Rangifer tarandus tarandus), muskox (Ovibos moschatus), and rock ptarmigan (Lagopus muta), typically consume plants rich in toxic plant secondary metabolites (PSMs). PSMs may depress enteric CH4 production in high concentrations. The objective of this PhD project was to characterize the gut microbiome from these animals, with special emphasis on CH4 metabolism and related to diets high in PSMs. Investigating the microbiology of methanogenesis would help better understand their digestive physiology. Molecular biology techniques were used for these studies: quantitative real-time PCR, amplicon sequencing, and shotgun metagenomics. Reindeer fed lichens (high in PSMs) and muskoxen feeding on an autumn pasture presented increased proportions of methanogens (Methanobrevibacter ruminantium and Methanobrevibacter olleyae) that may be related to the low CH4 output previously described for these ruminants. Housing methanogens with expected low CH4 potentials might result in reduced energy losses in reindeer. Other factors apart from high PSMs contents may account for the presence of these methanogens. Rock ptarmigans mostly contained methanol-utilizing methanogens, potentially related to methanol production from pectin degradation. This thesis presents the first molecular study of the gut microbiota in muskoxen and rock ptarmigans. Muskoxen possessed bacteria mediating fiber degradation, which would allow them live off fibrous plants like graminoids. Rock ptarmigans presented a diverse microbiota with bacteria involved in PSMs degradation and a variable range of polysaccharides (e.g. hemicellulose, starch). In both cases, their gut microbiota would allow them utilizing the available food.

The papers I, II and III of this thesis are not available in Munin. Paper I: Bailey, A., Thor, P., Browman, H. I., Fields, D. M., Runge, J., Vermont, A., Bjelland, R., Thompson, C., Shema, S., Durif, C. M. F., Hop, H.: “Early life stages of the Arctic copepod Calanus glacialis are unaffected by increased seawater pCO2”. Available in ICES Journal of Marine Science 2016. Paper II: Bailey, A., De Wit, P., Thor, P., Browman, H. I., Bjelland, R., Shema, S., Fields, D. M., Runge, J. A., Thompson, C., Hop, H.: “Regulation of gene expression underlies tolerance of the Arctic copepod Calanus glacialis to CO2-acidified seawater”. (Manuscript). Paper IV: Thor, P., Bailey, A., Dupont, S., Calosi, P., Søreide, J., De Wit, P., Guscelli, E., Loubet-Sartrou, L., Deichmann, I., Candee, M., Svensen, C., King, A. L., Bellerby, R. G.J.: “Potential for rescue from future ocean acidification by extant physiological differences among distinct Arctic copepod populations”. (Manuscript). Uptake of anthropogenic carbon dioxide (CO2) into the world’s oceans is increasing seawater acidity in a process termed ocean acidification. This is predicted to have harmful effects on a variety of marine organisms. With the greatest and fastest changes in pH expected to occur the Arctic seas over the next 100 years, the need for understanding the effects of low pH on Arctic marine organisms is pressing. This thesis examines the effects of projected levels of ocean acidification on the physiology of a key component of the Arctic marine ecosystem, the copepod Calanus glacialis, using molecular, organismal, and evolutionary methodologies to investigate the effects of low pH throughout its lifespan, in combination with other stressors (food availability) and across geographically distant sub-populations (to predict their capacity for adaptation or acclimation in the future). Young stages of the copepod, potentially the most sensitive, were tolerant to realistic pH levels for future ocean acidification: the nauplii developed successfully from egg to naupliar stage N6 at all four pH treatments investigated (pH 7.5, 7,7, 7.9 and 8.05). Gene expression N6 nauplii supported the organismal-level tolerance observed, indicating only mild gene expression and no stress response to low pH. Detrimental effects were found, however, in the young copepodite stages (C2-C4), which indicated an energetic cost at low pH. However, differences in this response between geographically distant, and potentially genetically isolated sub-populations showed that those that lived in a low pH environment tolerated it better, indicating that C. glacialis may be able to alleviate these detrimental effects over time. C. glacialis populations face a myriad of environmental changes driven by global warming and ocean acidification in the Arctic, and while they may experience declines due to the interactions of these multiple stressors, they likely will not experience declines primarily due to ocean acidification.

The papers II and III of this thesis are not available in Munin. Paper II : Bhat, S., Bernatchez, L., Amundsen, P. –A., Fevolden, S. –E., Benestan, L., Præbel, K.: «Speciation reversal leads to unpredictable genomic outcomes in European whitefish”. (Manuscript). Paper III: Bhat, S., Lamaze, F. C., Bernatchez, L., Præbel, K.: “Effect of speciation reversal on genomic introgression in sympatric whitefish eco-morphs.” (Manuscript). Anthropogenic activities, such as introduction of non-native species, is considered as one of the most significant causes for declining biodiversity, since such introduction may destabilise ecological barriers to gene flow between native populations. In the 1960s a salmonid fish, vendace, was introduced to the upper parts of Pasvik watercourse and during the early 1990s it invaded the entire watercourse, including the lakes studied in this project/thesis. These lakes harbour an eco-morph pair of European whitefish that has diverged in sympatry in post-glacial times. The vendace, being a competitively superior planktivore to the densely rakered eco-morph, relegated this eco-morph from its native pelagic habitat into the non-native littoral habitat, which is mainly occupied by the large sparsely rakered eco-morph. These observations have inspired this PhD thesis to study the phenotypic, genetic, and genomic consequences of the vendace invasion on native sympatric European whitefish eco-morphs. Using neutral microsatellite markers, the study documented that the introduction and invasion of vendace have induced speciation reversal in the eco-morph pair. The comparison of the number of gill rakers from the pre- and post-invasion periods revealed merging of two peaks of distribution of number of gill rakers following the invasion suggesting reduction in discreteness of phenotypes. Population genomic analyses, using genome-wide coverage of SNP markers obtained by sequencing of restriction site associated DNA libraries (RADseq), revealed an unpredicted outcome of speciation reversal at the genomic and functional phenotypic levels. This was attributed to a change in the selective forces during speciation reversal. The introgression patterns at genomic regions of adaptive importance, showed that speciation reversal have unpredictable consequences on introgression. Overall, this PhD study concludes that anthropogenic activities have wide-ranging and stochastic effects for species undergoing speciation reversal.

The papers of the thesis are not available in Munin: 1. Lynghammar, A., Christiansen, J.S., Mecklenburg, C.W., Karamushko, O.V., Møller, P.R. & Gallucci, V.F.: 'Species richness and distribution of chondrichthyan fishes in the Arctic Ocean and adjacent seas', Biodiversity 14(1): 57-66 (2013). Available at http://dx.doi.org/10.1080/14888386.2012.706198 2. Lynghammar, A., Christiansen, J.S., Griffiths, A.M., Fevolden, S-E., Hop, H. & Bakken, T.: 'DNA barcoding of the northern Northeast Atlantic skates (Chondrichthyes, Rajidae), with remarks on the widely distributed starry ray', Zoologica Scripta Volume 43, Issue 5, published online: 23 June 2014. Available at http://dx.doi.org/10.1111/zsc.12064 3. Lynghammar, A., Præbel, K., Fevolden, S-E. & Christiansen, J.S., 'Population genetic structure of an abundant skate (Amblyraja radiata) in the North Atlantic Ocean.' Manuscript The basal prerequisite for managing species and understanding ecosystems is correct species and population identification. Without knowing the species, we cannot with accuracy detect changes in distribution or abundance due to climate change, fisheries, diseases or any other types of impact. This thesis gives the first complete overview of chondrichthyan fishes (sharks, skates, rays and chimaeras), recorded in the Arctic Ocean and adjacent seas. A total of 49 species were found to occur, and the most specious regions were the Bering Sea, the Norwegian Sea and the Barents Sea, also known as the Arctic Gateways. The Bering Strait is an effective zoogeographic barrier, separating the Pacific from the Atlantic species. In particular, skates are prone to misidentifications. Species complexes are known to occur, and the group is commonly lumped as “Skates & Rays” in fishery statistics or in scientific publications. By use of genetic methods (DNA barcoding), the skate species reported from the northern Northeast (nNE) Atlantic was investigated. A total of 12 species were found to occur in the area, and three of those were new for the region. Skates are commonly encountered in the nNE Atlantic, but ~95 % are the smaller-sized Amblyraja radiata. Larger skates may be landed and they are necessarily a mix of the remaining 11 species, some of which are considered threatened. There were strong indications of the disappearance of Leucoraja fullonica, as well as over-reporting of the critically endangered common skate complex formerly known as Dipturus batis. Amblyraja radiata was suspected to be cryptic due to large differences in size-at-maturity across its distributional range. However, the mitochondrial gene cytochrome c oxidase subunit 1 (COI) did not support this hypothesis. To further elucidate this question, a higher resolution method was applied, and 10 microsatellite markers were investigated from specimens across the North (N) Atlantic. Pairwise fixation indexes (FST) indicated that there may be three major clusters in the N Atlantic: Northwest Atlantic, Mid Atlantic (Greenland) and NE Atlantic. However, more rigorous tests revealed high physical mixing of individuals over a large geographical area. The microsatellite results did not support the hypothesis of A. radiata being cryptic. Although the population structure of A. radiata could not be fully explained, baselines for occurrence of chondrichthyan fishes are established, and a new identification guide for skates in the nNE Atlantic is provided. Hopefully, this will pave the way for a better management of the skates by improving data quality.