| 1. |
- Johannesson, Kerstin, 1955, et al.
(author)
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Oceanographic barriers to gene flow promote genetic subdivision of the tunicate Ciona intestinalis in a North Sea archipelago
- 2018
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In: Marine Biology. - : Springer Science and Business Media LLC. - 0025-3162 .- 1432-1793. ; 165:8
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Journal article (peer-reviewed)abstract
- Pelagic larval development has the potential to connect populations over large geographic distances and prevent genetic structuring. The solitary tunicate Ciona intestinalis has pelagic eggs and a swimming larval stage lasting for maximum a few days, with the potential for a homogenizing gene flow over relatively large areas. In the eastern North Sea, it is found in a geomorphologically complex archipelago with a mix of fjords and open costal habitats. Here, the coastal waters are also stratified with a marked pycnocline driven by salinity and temperature differences between shallow and deep waters. We investigated the genetic structure of C. intestinalis in this area and compared it with oceanographic barriers to dispersal that would potentially reduce connectivity among local populations. Genetic data from 240 individuals, sampled in 2 shallow, and 4 deep-water sites, showed varying degrees of differentiation among samples (F (ST) = 0.0-0.11). We found no evidence for genetic isolation by distance, but two distant deep-water sites from the open coast were genetically very similar indicating a potential for long-distance gene flow. However, samples from different depths from the same areas were clearly differentiated, and fjord samples were different from open-coast sites. A biophysical model estimating multi-generation, stepping-stone larval connectivity, and empirical data on fjord water mass retention time showed the presence of oceanographic barriers that explained the genetic structure observed. We conclude that the local pattern of oceanographic connectivity will impact on the genetic structure of C. intestinalis in this region.
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| 2. |
- Johannesson, Kerstin, 1955, et al.
(author)
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A Darwinian Laboratory of Multiple Contact Zones
- 2020
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In: Trends in Ecology & Evolution. - : Elsevier BV. - 0169-5347. ; 35, s. 1021-1036
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Research review (peer-reviewed)abstract
- © 2020 The Author(s) Barriers to gene flow between divergent populations result in contact (hybrid) zones. Locations where multiple contact zones overlap can be used in comparative studies asking: what mechanisms maintain barriers; what is the origin of the genetic variation involved; and do differences in life history affect the nature of barriers? In a review of 23 marine species’ genetic divergence over a postglacial salinity gradient, many showed steep genetic clines supported by divergent selection and/or temporal or spatial segregation. Contacts were primary or secondary and shaped by ancestral variation sometimes involving inversions. The dispersal potential of species seemed less important in shaping clines. Studies of multispecies contact zones will increase our understanding of speciation, but we need to address the taxonomic bias and focus more on postzygotic isolation.
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| 3. |
- Johannesson, Kerstin, 1955, et al.
(author)
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Allozyme variation in a snail (Littorina saxatilis) - Deconfounding the effects of microhabitat and gene flow
- 1997
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In: Evolution. - 0014-3820. ; 51:2, s. 402-409
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Journal article (peer-reviewed)abstract
- It is commonly observed that a restricted gene flow among populations of a species generates genetic differentiation in, for example, allozyme markers. However, recent studies suggest that microhabital-specific variation may contribute to the total differentiation. To appreciate the relative contributions of geographic variation and habitat-specific variation, we sampled 42 subpopulations of the intertidal snail Littorina saxatilis from three different microhabitats (boulders, low and high rocky intertidal) on five small islands within a distance of 15 km. We used a modified orthogonal version of Nei's gene diversity analysis with a modified analysis of variance (ANOVA) that estimated the significance of habitat and geographic separation and the interaction between them. Between subpopulation differentiation (GST) was usually in the range of 5% to 10% but was exceptionally high in one locus (Aat; 53%). Genetic differentiation attributable to different habitats accounted for 10% to 81% (mean 35%) of the between subpopulation variation and was significant (P < 0.05) in six loci. Differentiation doe to geographic separation accounted for 11% to 61% (mean 36%) and was significant (P < 0.05) in seven loci. Furthermore, three loci showed interactions between habitat and island, suggesting varying effects of habitat in different islands. microhabitat-specific variation, probably through spatially varying fitness, seems particularly likely in Aat and Pgm-2. Moderate levels of habitat associated variation added to the observed differentiation due to gene flow in Pgi, Pnp, and Pgm-1, whereas in the remaining three loci either the habitat effect was confused by strong habitat-island interaction (Ark) or was virtually absent (Pep and Mpi).
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| 4. |
- Johannesson, Kerstin, 1955
(author)
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Are we analyzing speciation without prejudice?
- 2010
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In: Annals of the New York Academy of Sciences. - 0077-8923. ; 1206, s. 143-149
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Journal article (peer-reviewed)abstract
- Physical isolation has long been the null hypothesis of speciation, with exceptional evidence required to suggest speciation with gene flow. Following recent persuasive theoretical support and strong empirical examples of nonallopatric speciation, one might expect a changed view. However, a review of 73 recent empirical studies shows that when allopatric speciation is suggested, a nonallopatric alternative is rarely considered, whereas the opposite is true in studies suggesting sympatric speciation, indicating a biased treatment of different speciation models. Although increasing support for ecological speciation suggests natural selection as the most critical component of speciation, gene flow remains an issue. Methods for unbiased hypothesis testing are available, and the genetic and phylogeographic data required for appropriate tests can be generated. Focus on phylogenies and functions of individual genes have revealed strong idiosyncratic elements of speciation, such as single genes with possible allopatric origin that make significant contributions during nonallopatric phases of speciation. Hence a more complex picture of speciation is now emerging that will benefit from unbiased evaluation of both allopatric and sympatric mechanisms of speciation.
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| 5. |
- Johannesson, Kerstin, 1955
(author)
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Biologisk mångfald – inte bara arter.
- 2005
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In: Bevara arter – till vilket pris? Formas Fokuserar (red. B. Johansson), Formas, Stockholm..
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Journal article (other academic/artistic)
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| 6. |
- Johannesson, Kerstin, 1955, et al.
(author)
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Diverse pathways to speciation revealed by marine snails
- 2024
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In: Trends in Genetics. - 0168-9525 .- 1362-4555.
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Research review (peer-reviewed)abstract
- Speciation is a key evolutionary process that is not yet fully understood. Combining population genomic and ecological data from multiple diverging pairs of marine snails (Littorina) supports the search for speciation mechanisms. Placing pairs on a one-dimensional speciation continuum, from undifferentiated populations to species, obscured the complexity of speciation. Adding multiple axes helped to describe either speciation routes or reproductive isolation in the snails. Divergent ecological selection repeatedly generated barriers between ecotypes, but appeared less important in completing speciation while genetic incompatibilities played a key role. Chromosomal inversions contributed to genomic barriers, but with variable impact. A multidimensional (hypercube) approach supported framing of questions and identification of knowledge gaps and can be useful to understand speciation in many other systems.
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| 7. |
- Johannesson, Kerstin, 1955
(author)
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Evolution in Littorina: ecology matters
- 2003
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In: Journal of Sea Research. - 1385-1101. ; 49:2, s. 107-117
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Journal article (peer-reviewed)abstract
- Organisms of marine rocky shores are exposed to physical stress from abiotic factors, such as temperature, salinity and wave action. These factors vary over compressed temporal and spatial scales, producing an exceedingly heterogeneous habitat with steep gradients of selection, and it seems likely that this has a strong influence on the evolution of populations of rocky shore organisms. With the periwinkles (genus Littorina) as a model group, I review strategies for coping with small-scale heterogeneous environments and what implications these strategies have on the evolution of these species. Some species of Littorina have long-lived pelagic larvae and sites of various habitats are thus recruited from a common gene pool. This largely prevents local adaptation but minor adjustments are possible through a plastic phenotype. Other species of the genus are directly developing with no larval dispersal and among these there is evidence of strong local adaptation forming distinct ecotypes in contrasting habitats by parallel evolution. In at least one of the directly developing species (L. saxatilis) divergent selection among ecotypes has resulted in partial reproductive barriers that further impede gene flow among ecotypes. Furthermore, convergent evolution among species has produced superficially similar morphs in different habitats. Ecotype formation, ecological reproductive barriers and convergence among species all indicate that ecological processes are critical for evolution of Littorina species. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 8. |
- Johannesson, Kerstin, 1955, et al.
(author)
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FREQUENT CLONALITY IN FUCOIDS (FUCUS RADICANS AND FUCUS VESICULOSUS; FUCALES, PHAEOPHYCEAE) IN THE BALTIC SEA
- 2011
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In: Journal of Phycology. - : Wiley. - 0022-3646 .- 1529-8817. ; 47:5, s. 990-998
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Journal article (peer-reviewed)abstract
- Asexual reproduction by cloning may affect the genetic structure of populations, their potential to evolve, and, among foundation species, contributions to ecosystem functions. Macroalgae of the genus Fucus are known to produce attached plants only by sexual recruitment. Recently, however, clones of attached plants recruited by asexual reproduction were observed in a few populations of Fucus radicans Bergstrom et L. Kautsky and F. vesiculosus L. inside the Baltic Sea. Herein we assess the distribution and prevalence of clonality in Baltic fucoids using nine polymorphic microsatellite loci and samples of F. radicans and F. vesiculosus from 13 Baltic sites. Clonality was more common in F. radicans than in F. vesiculosus, and in both species it tended to be most common in northern Baltic sites, although variation among close populations was sometimes extensive. Individual clonal lineages were mostly restricted to single or nearby locations, but one clonal lineage of F. radicans dominated five of 10 populations and was widely distributed over 550 x 100 km of coast. Populations dominated by a few clonal lineages were common in F. radicans, and these were less genetically variable than in other populations. As thalli recruited by cloning produced gametes, a possible explanation for this reduced genetic variation is that dominance of one or a few clonal lineages biases the gamete pool resulting in a decreased effective population size and thereby loss of genetic variation by genetic drift. Baltic fucoids are important habitat-forming species, and genetic structure and presence of clonality have implications for conservation strategies.
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| 9. |
- Johannesson, Kerstin, 1955, et al.
(author)
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Growth rate differences between upper and lower shore ecotypes of the marine snail Littorina saxatilis (Olivi) (Gastropoda)
- 1997
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In: Biological Journal of the Linnean Society. - 0024-4066. ; 61:2, s. 267-279
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Journal article (peer-reviewed)abstract
- Shell polymorphisms are widespread among those intertidal gastropods that lack a pelagic spreading stage. These polymorphisms may indicate diversifying selection in a heterogeneous habitat, but to do this the variation must be at least partly inherited. Galician populations of Littorina saxatilis (Olivi) living in exposed rocky shores are highly polymorphic in several shell traits, e.g. ornamentation, banding and size. Mature snails of the upper-shore ridged and banded (RB) morph is, for example, often twice as large as mature individuals of the lower-shore smooth and unbanded (SU) morph of the same shore. We investigated the hypothesis that lower-shore snails grow more slowly and that differences in growth rate were at least partly inherited and could be explained by diversifying selection. We released snails of different origin (upper, mid- and lower shore) and morph (RB, SU and hybrids) at different shore levels and compared their shell increment after one month of growth. We found that despite considerable variation among individuals and among replicate samples (together about 53% of the total variation), average rates of growth differed between morphs. RB snails both from the upper and mid-shores grew at a high rate at all shore levels, SU snails grew considerably less, and hybrids grew at intermediate rates, at all levels. Inherited difference among morphs explained about 34% of the total variation while effects of shore levels and the interaction morph x shore level explained only 5 and 7%, respectively. Thus a large part of the difference in growth rate leading to different adult sizes of the two morphs has probably evolved due to spatially varying selection favouring large sizes in upper-shore and small sizes in lower-shore environments. (C) 1997 The Linnean Society of London.
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