Sigurdur S. Snorrason

The Arctic Charr Salvelinus alpinus in Thingvallavatn

The coexistence of four morphs of arctic charr in Thingvallavatn, Iceland, was demonstrated by investigations of morphology, habitat use, diet, endoparasitic fauna, life history, time and place of spawning, early ontogeny, and population genetics. Head morphology in embryos, juveniles and adults revealed the presence of two charr morphotypes, each including two morphs: (1) a benthic morphotype including large benthivorous (LB-) and small benthivorous (SB-) charr, featuring overshot mouth and relatively large pectoral fins, and (2) a pelagic morphotype including planktivorous (PL-) and piscivorous (PI-) charr, featuring termimal mouth and relatively small pectoral fins

Social relationships in a group of horses without a mature stallion

1. The social relationships in a group of Icelandic horses without a mature stallion were studied. The horses were all familiar to each other. Mutual grooming and play relationships, spatial associations, dominance-subordinate relations and the effect of kinship on these relationships were analysed. 2. The social structure was clearly dominated by the behaviour of the adult mares. The horses preferred to form bonds within their social class (sex/age) and they kept close proximity with their friends. The group was effectively divided into two social subgroups, adult mares as one group and adult geldings and sub-adults as another group. The sub-adults and adult geldings formed associations, which were based on mutual grooming and play, while the adult mares did not play. Differences between the sexes were evident. Males played more than the females, had more playing partners and were more popular as playmates. 3. Aggression rates were low. The dominance hierarchy was linear. Adult mares ranked higher than adult geldings, sub-adults and the foals. Rank was significantly correlated with age. The closer the adult mares were in rank, the more they groomed with each other. Such relationships were not found amongst the other social group. 4. Kinship was calculated between all pairs of animals for up to 4 or 5 generations. Allogrooming and play frequencies and proximity were all positively correlated with kinship. Adult mares, which were close in the dominance hierarchy, were on average more related than those further apart. 5. The social relationships in the Icelandic herd were, to some extent, different from relationships reported from unmanaged and feral horse-herds with mature stallions and bachelors. Our results suggest that adult mares groom more in groups without a stallion. Furthermore, they have more preferred partners than in natural harems and their partners are other adult mares, not their weaned offspring as seems to be the case in feral herds. The sub-adults also seem to be more socially active in the absence of stallions. Interestingly, in the Icelandic group, the adult mares showed stallion like behaviours, like mounting and protecting foals. Only by studying the behaviour and the nature of the relationships of horses in groups of different compositions, can we expect to gain a comprehensive understanding about individual social strategies and cognitive capabilities of the species. Such knowledge is valuable for management and welfare of the horse.

Biodiversity of macroinvertebrates on rocky substrate in the surf zone of Icelandic lakes

Sinee 1992 an EcoJogieal Survey of lcelandie Lakes (ESIL projeet) has been eondueted, where key bioJogieal, hydrogeoJogical and physieo-chemieal faetors are assessed and reeorded. The aim of the projeet is to estabJish a database with standardized eeoJogieal information for alJ major types of lcelandic Jakes. The database offers a multitude of uses, espeeialJy for pattern analysis and eomparative studies within the field of environmental and evoJutionary eeoJogy. Other appJieations relate to the inereasing demand for knowJedge on the status of water quality and biodiversity, e.g. for environmental impaet assessment. The eeoJogieal impJieations of bedroek geoJogy have, until reeentJy, not been studied systematieally in lcelandic freshwaters. The present study of the ESIL projeet and a study of lcelandie rivers (GfsLASON & ADALSTEINSSON 1996) are the first made in this eontext. However, GARDARSSON (1979) proposed an eeoJogieaJ classifieation of lcelandie freshwater systems, partly dedueed from hydrogeoJogieal features of eatehment areas. Also, the importanee of the struetural eompJexity of Java roeks and Java bottom has been advoeated in the eontext of the evoJutionary eeology of fish and invertebrates in Lake ThingvalJavatn (e.g. SNORRASON 1982, MALMQUIST et al. 1992, )óNASSON 1993, SNORRASON et al. 1994). In this paper we analyse the taxonomie diversity and abundanee of macroinvertebrates on roeky substrate in the Jittoral surf zone of 35 freshwater Jakes. This is done in relarion to Jake altitude, area, mean depth, eleetroJytie eonduetivity, temperature, and microhabitat strueture of the roeky substrate, a feature strongJy Jinked to bedroek geoJogy. By this we aim to identify bioJogical eommunity strueture in the surf zone o f Icelandie Jakes within the eontext o f key environmental variabJes.

Density, length distribution, and diet of age-0 arctic charrSalvelinus alpinus in the surf zone of Thingvallavatn, Iceland

SynopsisPopulation densities of age-0 arctic chaff in the surf zone averaged 1.83 and 4.70 fish m-2 in August 1984 and June 1985, respectively. Length variation of the littoral fish was low in early summer, increasing in late summer and autumn. Newly emerged charr, ∼ 20 mm long, appeared in the stony shallow water zone during both May and June. From length variation and variation in mouth position of the young charr, it is concluded that at least two of the four chaff morphs in the lake are present in the surf zone during spring and summer. In August, some of the larger age-0 charr had moved out from the surf zone, into the pelagic and the deeper epibenthic waters. The food of young littoral charr was dominated by large chironomid larvae (instar 3 and 4) and pupae

Alternative life-history styles in salmonine fishes with emphasis on arctic charr, Salvelinus alpinus

The widespread occurrence of local phenotypic differentiation within salmonine species, especially arctic charr, Salvelinus alpinus, is considered from the perspective of the evolution of life-history styles. Heterochronic shifts in ontogeny in relation to niche utilization are discussed as a possible proximate mechanism to account for this phenomenon. The relatively high degree of philopatry, especially homing to particular spawning locations, combined with localized differentiation of physical conditions on the spawning grounds predisposes the salmonines to the evolution of localized, differentiated spawning stocks. This differentiation ranges from relatively minor phenotypic distinctions, such as colour variants, through more distinct trophic morphs to sympatric sub-species or possibly even distinct species.

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

BackgroundUnderstanding 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).ResultsFour 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.ConclusionOur 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.

Differentiation at the MHCIIα and Cath2 Loci in Sympatric Salvelinus alpinus Resource Morphs in Lake Thingvallavatn

Northern freshwater fish may be suitable for the genetic dissection of ecological traits because they invaded new habitats after the last ice age (∼10.000 years ago). Arctic charr (Salvelinus alpinus) colonizing streams and lakes in Iceland gave rise to multiple populations of small benthic morphotypes, often in sympatry with a pelagic morphotype. Earlier studies have revealed significant, but subtle, genetic differentiation between the three most common morphs in Lake Thingvallavatn. We conducted a population genetic screen on four immunological candidate genes Cathelicidin 2 (Cath2), Hepcidin (Hamp), Liver expressed antimicrobial peptide 2a (Leap-2a), and Major Histocompatibility Complex IIα (MHCIIα) and a mitochondrial marker (D-loop) among the three most common Lake Thingvallavatn charr morphs. Significant differences in allele frequencies were found between morphs at the Cath2 and MHCIIα loci. No such signal was detected in the D-loop nor in the other two immunological genes. In Cath2 the small benthic morph deviated from the other two (FST = 0.13), one of the substitutions detected constituting an amino acid replacement polymorphism in the antimicrobial peptide. A more striking difference was found in the MHCIIα. Two haplotypes were very common in the lake, and their frequency differed greatly between the morphotypes (from 22% to 93.5%, FST = 0.67). We then expanded our study by surveying the variation in Cath2 and MHCIIα in 9 Arctic charr populations from around Iceland. The populations varied greatly in terms of allele frequencies at Cath2, but the variation did not correlate with morphotype. At the MHCIIα locus, the variation was nearly identical to the variation in the two benthic morphs of Lake Thingvallavatn. The results are consistent with a scenario where parts of the immune systems have diverged substantially among Arctic charr populations in Iceland, after colonizing the island ∼10.000 years ago.

Threespine Stickleback Gasterosteus aculeatus in Thingvallavatn: Habitat and Food in a Lake Dominated by Arctic Charr Salvelinus alpinus

In the large and deep Lake Thingvallavatn, Iceland, threespine stickleback were restricted to the Nitella opaca vegetation found in the deep littoral zone (10-20 m). The stickleback matured during their third year, and reached an average asymptotic length of 57 mm. The diet consisted mainly of benthic cladocerans, ostracods and chironomid larvae. Some copepods were also eaten by fish of age 1 and older in August and September. There were some differences in food choice between the size groups of stickleback present, as older fish ate more large zoobenthos, like snails and water bugs

Production of fish stocks in Thingvallavatn, Iceland

Annual net production of four morphs of 1+ and older arctic charr Salvelinus alpinus (L.) and threespined stickleback Gasterosteus aculeatus L. in Thingvallavatn, Ice-land, was estimated. Production of the dominant morph, planktivorous charr, was 28.3 KJ m #752 yr #751 , which constitutes 14.3% and 0.8% of the estimated zooplankton and phytoplankton production, respectively. Corresponding values for the benthic charr morphs, i.e. small and large benthivorous charr of the stony littoral zone (0-10 m depth), was 1.2 KJ m #752 yr #751 , which amounts to 1.8% and 0.23% of the estimated secondary and primary production in this zone, respectively

Differential expression of the aryl hydrocarbon receptor pathway associates with craniofacial polymorphism in sympatric Arctic charr

BackgroundThe developmental basis of craniofacial morphology hinges on interactions of numerous signalling systems. Extensive craniofacial variation in the polymorphic Arctic charr, a member of the salmonid family, from Lake Thingvallavatn (Iceland), offers opportunities to find and study such signalling pathways and their key regulators, thereby shedding light on the developmental pathways, and the genetics of trophic divergence.ResultsTo identify genes involved in the craniofacial differences between benthic and limnetic Arctic charr, we used transcriptome data from different morphs, spanning early development, together with data on craniofacial expression patterns and skeletogenesis in model vertebrate species. Out of 20 genes identified, 7 showed lower gene expression in benthic than in limnetic charr morphs. We had previously identified a conserved gene network involved in extracellular matrix (ECM) organization and skeletogenesis, showing higher expression in developing craniofacial elements of benthic than in limnetic Arctic charr morphs. The present study adds a second set of genes constituting an expanded gene network with strong, benthic–limnetic differential expression. To identify putative upstream regulators, we performed knowledge-based motif enrichment analyses on the regulatory sequences of the identified genes which yielded potential binding sites for a set of known transcription factors (TFs). Of the 8 TFs that we examined using qPCR, two (Ahr2b and Ap2) were found to be differentially expressed between benthic and limnetic charr. Expression analysis of several known AhR targets indicated higher activity of the AhR pathway during craniofacial development in benthic charr morphotypes.ConclusionThese results suggest a key role of the aryl hydrocarbon receptor (AhR) pathway in the observed craniofacial differences between distinct charr morphotypes.