Einar Árnason

Mitochondrial cytochrome b DNA sequence variation of Atlantic cod Gadus morhua, from Norway

We studied sequence variation of a fragment of the mitochondrial cytochrome b gene using polymerase chain reaction (PCR) and direct sequencing among 85 Norwegian Atlantic cod Gadus morhua, from nine sampling localities representing three overall areas: arctic, coastal and middle. In the analysis we include an additional 15 cod studied by Carr & Marshall (1991a). Nine base changes were found among the 100 sequences defining 11 haplotypes which differ from each other by one to five mutations. Two sites have been hit twice. All but one mutation are at third position silent sites. The variation passes several neutral‐theory tests and is thus suitable as marker for studying population differentiation. Coefficients of coancestry or intraclass correlation coefficient are negative both at the level of individuals within localities and localities within areas. This implies greater differences among individuals within populations than between populations. Intralocality nucleotide diversity is high and masks interlocality nucleotide divergence such that the net interlocality nucleotide divergence is nil. Similarly the net interarea nucleotide divergence is nil.

Genetic diversity and population history of two related seabird species based on mitochondrial DNA control region sequences

Geographical variation in two related seabird species, the razorbill (Alca torda) and common guillemot (Uria aalge), was investigated using sequence analysis of mitochondrial DNA (mtDNA) control regions. We determined the nucleotide sequence of the variable 5′ segment of the control region in razorbills and common guillemots from breeding colonies across the Atlantic Ocean. The ecology and life history characteristics of razorbill and common guillemot are in many respects similar. They are both considered highly philopatric and have largely overlapping distributions in temperate and subarctic regions of the North Atlantic, yet the species were found to differ widely in the extent and spatial distribution of mtDNA variation. Moreover, the differences in genetic differentiation and diversity were in the opposite direction to that expected from a consideration of traditional classifications and current population sizes. Indices of genetic diversity were highest in razorbill and varied among colonies, as did genotype frequencies, suggestive of restrictions to gene flow. The distribution of genetic variation suggests that razorbills originated from a refugial population in the south‐western Atlantic Ocean through sequential founder events and subsequent expansion in the east and north. In common guillemots, genetic diversity was low and there was a lack of geographical structure, consistent with a recent population bottleneck, expansion and gene flow. We suggest that the reduced level of genetic diversity and differentiation in the common guillemot is caused by an inherent propensity for repeated population bottlenecks and concomitantly unstable population structure related to their specialized feeding ecology.

Intense Habitat-Specific Fisheries-Induced Selection at the Molecular Pan I Locus Predicts Imminent Collapse of a Major Cod Fishery

Predation is a powerful agent in the ecology and evolution of predator and prey. Prey may select multiple habitats whereby different genotypes prefer different habitats. If the predator is also habitat-specific the prey may evolve different habitat occupancy. Drastic changes can occur in the relation of the predator to the evolved prey. Fisheries exert powerful predation and can be a potent evolutionary force. Fisheries-induced selection can lead to phenotypic changes that influence the collapse and recovery of the fishery. However, heritability of the phenotypic traits involved and selection intensities are low suggesting that fisheries-induced evolution occurs at moderate rates at decadal time scales. The Pantophysin I (Pan I) locus in Atlantic cod (Gadus morhua), representing an ancient balanced polymorphism predating the split of cod and its sister species, is under an unusual mix of balancing and directional selection including current selective sweeps. Here we show that Pan I alleles are highly correlated with depth with a gradient of 0.44% allele frequency change per meter. AA fish are shallow-water and BB deep-water adapted in accordance with behavioral studies using data storage tags showing habitat selection by Pan I genotype. AB fish are somewhat intermediate although closer to AA. Furthermore, using a sampling design covering space and time we detect intense habitat-specific fisheries-induced selection against the shallow-water adapted fish with an average 8% allele frequency change per year within year class. Genotypic fitness estimates (0.08, 0.27, 1.00 of AA, AB, and BB respectively) predict rapid disappearance of shallow-water adapted fish. Ecological and evolutionary time scales, therefore, are congruent. We hypothesize a potential collapse of the fishery. We find that probabilistic maturation reaction norms for Atlantic cod at Iceland show declining length and age at maturing comparable to changes that preceded the collapse of northern cod at Newfoundland, further supporting the hypothesis. We speculate that immediate establishment of large no-take reserves may help avert collapse.

Genetic Heterogeneity of Icelanders

Recently statements have been made about a special ‘genetic homogeneity’ of the Icelanders that are at variance with earlier work on blood groups and allozymes. To validate these claims an extensive reanalysis was undertaken of mtDNA variation by examining primary data from original sources on 26 European populations. The results show that Icelanders are among the most genetically heterogeneous Europeans by the mean number of nucleotide differences as well as by estimates of θ parameters of the neutral theory. The distribution of pairwise differences in general has the same shape as European populations and shows no evidence of bottlenecks of numbers in Iceland. The allelic frequency distribution of Iceland is relatively even with a large number of haplotypes at polymorphic frequencies contrasting with other countries. This is a signature of admixture during the founding or history of Iceland. Assumptions of models used to simulate number of haplotypes at sampling saturation for comparing populations are violated to different degrees by various countries. Anomalies identified in data in previous reports on Icelandic mtDNA variation appear to be due to errors in publicly accessible databases. This study demonstrates the importance of basing analyses on primary data so that errors are not propagated. Claims about special genetic homogeneity of Icelanders are not supported by evidence.

Extent of mitochondrial DNA sequence variation in Atlantic cod from the Faroe Islands: a resolution of gene genealogy

Variation in a 250 base pair (bp) fragment of the mitochondrial cytochrome b (cyt b) has been used extensively for population studies in Atlantic cod Gadus morhua. To study the shape of the gene genealogy and the nature of the polymorphism, sequences of another region of the cyt b gene and the TP intergenic spacer were added, making a total of 566 bp from 74 cod from the Faroe Islands. A total of 44 segregating sites defined 41 haplotypes, many at frequencies greater than 5%. Haplotype diversity was 0.97 and nucleotide diversity 0.73% per base. A topology referred to as a constellation gene genealogy was observed with four major haplotypes at high frequencies, from each of which a number of rare variants were derived. A young relative age of the haplotypes was gauged from the structure of the genealogy. The variation was mostly at synonymous sites within the coding region and thus likely to be neutral or under weak purifying selection. By comparative analysis this also applies to the TP spacer. Applying the locus to study population variation in the Faroe Islands by AMOVA revealed that the overall areas and localities within areas accounted for none of the variation, and all the variation was due to differences among individuals.

Nucleotide variation and balancing selection at the Ckma gene in Atlantic cod: analysis with multiple merger coalescent models

High-fecundity organisms, such as Atlantic cod, can withstand substantial natural selection and the entailing genetic load of replacing alleles at a number of loci due to their excess reproductive capacity. High-fecundity organisms may reproduce by sweepstakes leading to highly skewed heavy-tailed offspring distribution. Under such reproduction the Kingman coalescent of binary mergers breaks down and models of multiple merger coalescent are more appropriate. Here we study nucleotide variation at the Ckma (Creatine Kinase Muscle type A) gene in Atlantic cod. The gene shows extreme differentiation between the North (Canada, Greenland, Iceland, Norway, Barents Sea) and the South (Faroe Islands, North-, Baltic-, Celtic-, and Irish Seas) with FST > 0.8 between regions whereas neutral loci show no differentiation. This is evidence of natural selection. The protein sequence is conserved by purifying selection whereas silent and non-coding sites show extreme differentiation. The unfolded site-frequency spectrum has three modes, a mode at singleton sites and two high frequency modes at opposite frequencies representing divergent branches of the gene genealogy that is evidence for balancing selection. Analysis with multiple-merger coalescent models can account for the high frequency of singleton sites and indicate reproductive sweepstakes. Coalescent time scales vary with population size and with the inverse of variance in offspring number. Parameter estimates using multiple-merger coalescent models show that times scales are faster than under the Kingman coalescent.

An assessment of mitochondrial variation in Arctic gadoids

Climatic changes during the quaternary history in Arctic regions have shaped the genetic variation and genealogies of Arctic species. Several studies have been conducted in recent years on genetic diversity of Arctic organisms, but marine fishes are largely underrepresented in these studies. Here, we present a study on mitochondrial variation in three Arctic gadoids: Arctic cod (Arctogadus glacialis), Greenland cod (Gadus ogac), and Polar cod (Boreogadus saida). In addition, geographic variation in Polar cod is presented. The sequence variation at the mtDNA presents similar patterns as observed for other related marine fishes. Variation in these three species reflects rather different historic processes, due to colonization and climatic changes than differences in life histories. In Polar cod, a deeper genealogy is observed and variation is dependent on both latitude and longitude. The deep genealogy indicates either admixture of separate lineages or a population, which has been stable in size during alternating cold and warm periods of the pleistocene.

Sequence variation for cytochrome b genes of three salmonid species from Iceland

Abstract Sequence variation was detected by polymerase chain reaction (PCR) and direct sequencing of a 295-nucleotide region of the mitochondrial cytochrome b gene of Arctic charr ( Salvelinus alpinus ), Atlantic salmon ( Salmo salar ) and brown trout ( Salmo trutta ) sampled around Iceland. Four haplotypes were found within Atlantic salmon, no variation was found within Arctic charr or brown trout. Synonymous substitution rates among the species were according to a molecular clock. There was a significant transition/transversion bias but apparent transitional bias among purines and pyrimidines was not significant, given permissible mutations which would not alter the amino acid sequence. The amino acid sequences were identical among the species where intraspecific variation did not exist.

Organization of a β and α globin gene set in the teleost Atlantic cod, Gadus morhua.

Developmental globin gene expression and gene switching in vertebrates have been extensively studied. Globin gene regions have been characterized in some fish species and show linked α and β loci. Understanding coordinated expression between α and β globin genes in fish is of importance for further insights into globin gene regulation in teleosts and higher vertebrates. We characterize linked β and α globin genes in Atlantic cod, pulled from the Atlantic cod genome with a PCR research strategy, by screening a genomic λ library and primer walking. The genes are oriented tail-to-head (5′–3′), differing from the head-to-head orientation in transcriptional polarity characteristic of teleostean globin genes. Four tandem repeats are found in an intergenic region of 1500 base pairs. One microsatellite, which consists primarily of atg tandem repeats, has an open reading frame. The globin genes and open reading frame have a CCAAT promoter element and TATA boxes. The promoters of the open reading frame and the β gene share an 89-bp block (with 100% identity) that probably regulates transcription.

Macromolecular Interaction and the Electrophoretic Mobility of Esterase-5 from Drosophila pseudoobscura

Esterase-5 is one of the most polymorphic loci in Drosophila pseudoobscura. Some variants reportedly produce a dimeric enzyme, while a few produce a monomeric form. This paper reports the finding that during electrophoresis ESTERASE-5 exists in a dynamic equilibrium between monomers and dimers, an equilibrium that is dependent on the running temperature of the gels. This is shown by a series of analytical electrophoresis experiments in which the apparent molecular weights of several variants are determined at four different temperatures. Increasing temperatures result in a linear decrease in the logarithm of apparent molecular weights. Macromolecular interactions thus are a significant determinant of EST-5 electrophoretic mobility.