Arnar Pálsson

The native terrestrial biota of Iceland required long oceanic journeys to colonize the island. Human colonization of Iceland, however, has allowed many less vagile species, such as ants, to reach the island. We compiled published, unpublished, and new records of ants from Iceland and evaluated the probable origin and current status of each species. We analyzed 565 records compiled by the Icelandic Institute of Natural History (1974-2020) and 84 new records (2019-2023) representing 25 species. Only five species were found living outdoors: Hypoponera ergatandria (507x indoors and 9x outdoors, mostly in geothermal areas), Lasius niger (24x indoors and 12x outdoors away from heated buildings), Hypoponera eduardi (5x only in geothermal areas), Lasius platythorax (1x near a small greenhouse and 1x in imported plant material), and Myrmica rubra (1x in an urban garden). Three ant species were exclusively found inhabiting heated buildings: Monomorium pharaonis (41x), Tapinoma melanocephalum (10x), and Monomorium nr. pharaonis (3x). Records of the remaining 17 species all appear to be based on temporary infestations, found on recently imported plant material. Previous records from Iceland of Hypoponera punctatissima appear to be based on misidentifications. Although it is possible that some ants were present before human arrival, all 25 ant species may have arrived accompanying humans. It remains to be seen whether any of these ant species are having or will have a significant impact on the native Icelandic biota. The arrival of Myrmica rubra in Iceland is particularly worrisome, given the impact this species is having in other high latitude areas it has invaded.

Loss of anadromy in Salmonids frequently follows postglacial colonization and habitat change, a process driven by selective pressures where benefits of residency outweigh migration costs. At the end of the last Ice Age ca . 10,000 years ago, anadromous brown trout arrived in Iceland and invaded newly formed habitats. Some populations became isolated in headwaters early after the ice retreated, presumably accelerating local adaptation and the evolution of residency. In this study, we assessed the genetic relatedness and diversity of brown trout on a countrywide scale to decipher the number of unique populations and their connectivity. We tested for gene flow from headwaters and assessed effects of isolation and adaptation. We genotyped 600 individuals from 40 locations using ddRADseq, identifying 2946 putative neutral SNPs. We identified 16 groups of genetically similar individuals (genetic clusters), five of which were primarily composed of anadromous individuals. On a large scale, the genetic variation and relatedness of trout populations reflect the geography of waterways while within watersheds the topography, for example, the presence of waterfalls and lakes, is key. Counterintuitively, downstream gene flow from isolated headwater populations was only seen in small river populations whereas gene flow from large lake populations appears negligible. Using outlier SNPs, we identified several hundred loci associated with temperature and habitat type (i.e., lake vs. river). Temperature‐associated markers mapped to loci associated with immune response, lipid metabolism and transmembrane protein genes. Our results highlight the effects of isolation on genetic diversity in headwater populations and the importance of identifying unique populations in conservation efforts.

Circulating proteins provide insights into an individual's health and aging trajectory. However, little is known about their long-term temporal stability. We evaluated the five-year temporal stability of 7288 serum proteins in 3093 participants (mean age 75 years) of the AGES-Reykjavik study. We observe wide variability in protein temporal stability, independent of transcriptomic stability in tissues. Temporally stable proteins tend to be extracellular, secreted, and tissue-specific, while temporally variable proteins are typically involved in intracellular housekeeping functions. Generally, protein levels become more stable with age, and for some proteins, temporal stability differs by sex. Similar patterns and consistent temporal stability were observed for a subset of plasma proteins in the Cardiovascular Health Study. Genetic factors and disease state have widespread effects on the temporal stability of the proteome, while the co-regulatory structure of the proteome is largely retained over time. Our findings underscore the protein-specific differences in long-term temporal stability, which are particularly important to consider in the context of biomarker development and precision medicine.

Sympatric morphs provide valuable systems for studying incipient divergence despite incomplete reproductive isolation. In connected waterbodies with spatially heterogeneous habitats, one or more morphs may form metapopulation structures, generating eco‐evolutionary dynamics unlike those in single lakes. We studied the phenotypic and genome‐wide differentiation in Arctic charr ( Salvelinus alpinus ) in two Icelandic lakes: Thingvallavatn, known to harbour four distinct morphs, and a smaller downstream lake, Ulfljotsvatn. Our analyses confirm a single origin of charr polymorphism in this system, with all morphs present in both lakes. Relative morph abundances differ between the lakes: PL‐charr dominate in Thingvallavatn, whereas LB‐charr are most abundant in Ulfljotsvatn. Three morphs, large‐ (LB‐), small (SB‐) benthivorous and planktivorous (PL‐) charr, are genetically distinct. The system likely forms a source‐sink structure for both PL‐ and SB‐charr, though migration rates from Thingvallavatn vary remarkably, resulting in distinct population dynamics. Conversely, LB‐charr exhibit genetic differentiation between the lakes, suggesting the presence of a separate population in Ulfljotsvatn. While piscivorous (PI‐) charr appear genetically similar to PL‐charr, evidence suggests hybridisation between PI‐ and LB‐charr in both lakes. Moreover, the higher hybridisation in the downstream lake likely contributes to the observed erosion of genetic separation between LB‐ and PL‐charr in Ulfljotsvatn. These findings suggest that the complex interplay of habitat heterogeneity and morph‐specific migrations shapes the coexistence and eco‐evolutionary dynamics of sympatric charr morphs in the connected lakes. Our study highlights the importance of investigating early divergence in spatially complex systems to advance eco‐evolutionary research.

Species and populations with parallel evolution of specific traits can help illuminate how predictable adaptations and divergence are at the molecular and developmental level. Following the last glacial period, dwarfism and specialized bottom feeding morphology evolved rapidly in several landlocked Arctic charr Salvelinus alpinus populations in Iceland. To study the genetic divergence between small benthic morphs and limnetic morphs, we conducted RNA-sequencing charr embryos at four stages in early development. We studied two stocks with contrasting morphologies: the small benthic (SB) charr from Lake Thingvallavatn and Holar aquaculture (AC) charr. The data reveal significant differences in expression of several biological pathways during charr development. There was also an expression difference between SB- and AC-charr in genes involved in energy metabolism and blood coagulation genes. We confirmed differing expression of five genes in whole embryos with qPCR, including lysozyme and natterin-like which was previously identified as a fish-toxin of a lectin family that may be a putative immunopeptide. We also verified differential expression of 7 genes in the developing head that associated consistently with benthic v.s.limnetic morphology (studied in 4 morphs). Comparison of single nucleotide polymorphism (SNP) frequencies reveals extensive genetic differentiation between the SB and AC-charr (~1300 with more than 50% frequency difference). Curiously, three derived alleles in the otherwise conserved 12s and 16s mitochondrial ribosomal RNA genes are found in benthic charr. The data implicate multiple genes and molecular pathways in divergence of small benthic charr and/or the response of aquaculture charr to domestication. Functional, genetic and population genetic studies on more freshwater and anadromous populations are needed to confirm the specific loci and mutations relating to specific ecological traits in Arctic charr.

Circulating proteins reflect diverse biological processes and can offer critical insights into an individual‘s overall health and aging trajectory. The circulating proteome is shaped by a complex interplay of genetic, biological, and environmental factors across the lifespan. However, little is known about which factors influence its long-term temporal stability. Here we used SomaScan proteomics to evaluate the five-year temporal stability of 7,288 proteins measured in serum from 3,093 participants (mean age 76 years) of the Age, Gene/Environment Susceptibility (AGES)-Reykjavik study. We observed a wide variability in the temporal stability of individual proteins, with temporally stable proteins more often being extracellular and associated with diseases, while temporally variable proteins are typically involved in intracellular housekeeping functions. We demonstrate that temporal stability of circulating proteins does not reflect that of transcriptomic stability in tissues, and that genetic effects and disease stage are two major contributors to protein temporal stability in the circulation. Our findings underscore the protein-specific differences in long-term temporal stability, and the genetic and biological factors influencing them, which are particularly important to consider in the context of biomarker development and precision medicine.

Sympatric morphs provide valuable systems for studying incipient divergence despite incomplete reproductive isolation. In connected waterbodies with spatially heterogeneous habitats, one or more morphs may form metapopulation structures, generating eco-evolutionary dynamics unlike those in single lakes. We studied the phenotypic and genome-wide differentiation in Arctic charr (Salvelinus alpinus) in two Icelandic lakes: Thingvallavatn, known to harbour four distinct morphs, and a smaller downstream lake, Ulfljotsvatn. Our analyses confirm a single origin of charr polymorphism in this system, with all morphs present in both lakes. Relative morph abundances differ between the lakes: PL-charr dominate in Thingvallavatn, whereas LB-charr are most abundant in Ulfljotsvatn. Three morphs, large- (LB-), small (SB-) benthivorous and planktivorous (PL-) charr, are genetically distinct. The system likely forms a source-sink structure for both PL- and SB-charr, though migration rates from Thingvallavatn vary remarkably, resulting in distinct population dynamics. Conversely, LB-char exhibit genetic differentiation between the lakes, suggesting the presence of a separate population in Ulfljotsvatn. While piscivorous (PI-) charr appear genetically similar to PL-charr, evidence suggests hybridization between PI- and LB-charr in both lakes. Moreover, the higher hybridization in the downstream lake likely contributes to the observed erosion of genetic separation between LB- and PL-charr in Ulfljotsvatn. These findings suggest that the complex interplay of habitat heterogeneity and morph-specific migrations shapes the coexistence and eco-evolutionary dynamics of sympatric charr morphs in the connected lakes. Our study highlights the importance of investigating early divergence in spatially complex systems to advance eco-evolutionary research.

Salmonids have a remarkable ability to form sympatric morphs after postglacial colonisation of freshwater lakes. These morphs often differ in morphology, feeding and spawning behaviour. Here, we explored the genetic basis of morph differentiation in Arctic charr (n = 283) by first establishing a high‐quality reference genome and then using this in whole genome sequencing of distinct morphs present in two Norwegian and two Icelandic lakes. The four lakes represent the spectrum of genetic differentiation between morphs from one lake with no genetic differentiation between morphs, implying phenotypic plasticity, to two lakes with locus‐specific genetic differentiation, implying incomplete reproductive isolation, and one lake with strong genome‐wide divergence consistent with complete reproductive isolation. As many as 12 putative inversions ranging from 0.45 to 3.25 Mbp in size segregated among the four morphs present in one lake, Thingvallavatn, and these contributed significantly to the genetic differentiation among morphs. None of the putative inversions were found in any of the other lakes, but there were cases of partial haplotype sharing in similar morph contrasts in other lakes. Our findings are consistent with a highly polygenic basis of morph differentiation with population‐specific selection on alleles linked to the development of similar morph phenotypes. The results support a model where morph differentiation is first established through phenotypic plasticity, leading to niche expansion and separation. This may be followed by gradual development of reproductive isolation, locus‐specific differentiation and eventually complete reproductive isolation and genome‐wide divergence.

Salmonids have a remarkable ability to form sympatric morphs after postglacial colonization of freshwater lakes. These morphs often differ in morphology, feeding, and spawning behaviour. Here, we explored the genetics of morph differentiation by establishing a high-quality, annotated reference genome for the Arctic charr and using this for population genomic analysis of morphs from two Norwegian and two Icelandic lakes. The four lakes represent the spectrum of genetic differentiation between morphs from one lake with no genetic differentiation between morphs, implying phenotypic plasticity, to two lakes with locus-specific genetic differentiation, implying incomplete reproductive isolation, and one lake with strong genome-wide divergence consistent with complete reproductive isolation. As many as 12 putative inversions ranging from 0.45 to 3.25 Mbp in size segregated among the four morphs present in one lake, Thingvallavatn, and these contributed significantly to the genetic differentiation among morphs. None of the putative inversions was found in any of the other lakes, but there were cases of partial haplotype sharing in similar morph contrasts in other lakes. The results are consistent with a highly polygenic basis of morph differentiation with limited genetic parallelism between lakes. The results support a model where morph differentiation is first established through phenotypic plasticity, leading to niche expansion and separation. This is followed by gradual development of reproductive isolation, locus-specific differentiation, and eventually complete reproductive isolation and genome-wide divergence. A major explanation for salmonids' ability to diversify into multiple sympatric morphs is likely their genome complexity from ancient whole genome duplication, which enhances evolvability.

Salmonids have a remarkable ability to form sympatric morphs after postglacial colonization of freshwater lakes. These morphs often exhibit differences in morphology, feeding, and spawning behaviour. Here we explore the genetics of morph differentiation by establishing a high-quality, annotated reference genome for the Arctic charr and use this as a resource for population genomic analysis of morphs from two Norwegian and two Icelandic lakes. The four lakes represent the spectrum of genetic differentiation between morphs from one lake with no genetic differentiation between morphs, implying phenotypic plasticity only, to two lakes with locus-specific genetic differentiation, implying incomplete reproductive isolation, and one lake with strong genome-wide divergence consistent with complete reproductive isolation. As many as 12 putative inversions ranging from 0.45 to 3.25 Mbp in size segregated among the four morphs present in one lake, Thingvallavatn, and these contributed significantly to the genetic differentiation among morphs. None of the putative inversions was found in any of the other lakes, but there were cases of partial haplotype sharing in similar morph contrasts in other lakes. The results are consistent with a highly polygenic basis of morph differentiation with limited genetic parallelism between lakes. The results support a model where morph differentiation is usually first established due to phenotypic plasticity that results in niche expansion and separation, followed by gradual development of reproductive isolation and locus-specific differentiation and eventually complete reproductive isolation and genome-wide divergence. A major explanation for salmonids ability to diversify into multiple sympatric morphs is likely the genome complexity caused by their ancient whole genome duplication that enhances evolvability.