| 1. |
- Ardehed, Angelica, et al.
(author)
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Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitment
- 2015
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In: Ecology and Evolution. - : Wiley. - 2045-7758. ; 5:19, s. 4233-4245
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Journal article (peer-reviewed)abstract
- In dioecious species with both sexual and asexual reproduction, the spatial distribution of individual clones affects the potential for sexual reproduction and local adaptation. The seaweed Fucus radicans, endemic to the Baltic Sea, has separate sexes, but new attached thalli may also form asexually. We mapped the spatial distribution of clones (multilocus genotypes, MLGs) over macrogeographic (>500km) and microgeographic (<100m) scales in the Baltic Sea to assess the relationship between clonal spatial structure, sexual recruitment, and the potential for natural selection. Sexual recruitment was predominant in some areas, while in others asexual recruitment dominated. Where clones of both sexes were locally intermingled, sexual recruitment was nevertheless low. In some highly clonal populations, the sex ratio was strongly skewed due to dominance of one or a few clones of the same sex. The two largest clones (one female and one male) were distributed over 100-550km of coast and accompanied by small and local MLGs formed by somatic mutations and differing by 1-2 mutations from the large clones. Rare sexual events, occasional long-distance migration, and somatic mutations contribute new genotypic variation potentially available to natural selection. However, dominance of a few very large (and presumably old) clones over extensive spatial and temporal scales suggested that either these have superior traits or natural selection has only been marginally involved in the structuring of genotypes.
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| 2. |
- Ardehed, Angelica, et al.
(author)
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Divergence within and among Seaweed Siblings (Fucus vesiculosus and F. radicans) in the Baltic Sea
- 2016
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In: Plos One. - : Public Library of Science (PLoS). - 1932-6203. ; 11:8
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Journal article (peer-reviewed)abstract
- Closely related taxa provide significant case studies for understanding evolution of new species but may simultaneously challenge species identification and definition. In the Baltic Sea, two dominant and perennial brown algae share a very recent ancestry. Fucus vesiculosus invaded this recently formed postglacial sea 8000 years ago and shortly thereafter Fucus radicans diverged from this lineage as an endemic species. In the Baltic Sea both species reproduce sexually but also recruit fully fertile new individuals by asexual fragmentation. Earlier studies have shown local differences in morphology and genetics between the two taxa in the northern and western Bothnian Sea, and around the island of Saaremaa in Estonia, but geographic patterns seemin conflict with a single origin of F. radicans. To investigate the relationship between northern and Estonian distributions, we analysed the genetic variation using 9 microsatellite loci in populations from eastern Bothnian Sea, Archipelago Sea and the Gulf of Finland. These populations are located in between earlier studied populations. However, instead of bridging the disparate genetic gap between N-W Bothnian Sea and Estonia, as expected from a simple isolation-by-distance model, the new populations substantially increased overall genetic diversity and showed to be strongly divergent from the two earlier analysed regions, showing signs of additional distinct populations. Contrasting earlier findings of increased asexual recruitment in low salinity in the Bothnian Sea, we found high levels of sexual reproduction in some of the Gulf of Finland populations that inhabit extremely low salinity. The new data generated in this study supports the earlier conclusion of two reproductively isolated but very closely related species. However, the new results also add considerable genetic and morphological complexity within species. This makes species separation at geographic scales more demanding and suggests a need for more comprehensive approaches to further disentangle the intriguing relationship and history of the Baltic Sea fucoids.
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| 3. |
- Ardehed, Angelica
(author)
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Patterns of evolution in a young species, the Baltic seaweed Fucus radicans
- 2015
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Doctoral thesis (other academic/artistic)abstract
- To be able to interpret patterns of biodiversity it is important to understand the processes by which new species evolve and how closely related species remain reproductively isolated and ecologically differentiated. This thesis centers on the evolution of the two brown algae Fucus radicans and Fucus vesiculosus in the Baltic Sea, where Fucus radicans very recently diverged from F. vesiculosus, and where both species have aquired the unique character of asexual recruitment forming clones. Speciation is one of the most fundamental processes in biology, yet poorly understod. It is generally argued to build on a slow accumulation of differences between populations living in allopatry, but the formation of F. radicans most likely occurred without geographic separation and provides a unique opportunity to study the differentiation and evolution of a new species following a recent, rapid and sympatric speciation event. We investigated the spatial patterns of clonality in F. radicans, to understand how clones affect the potential for sexual reproduction, recombination and local adaptation. Results showed that rare sexual events, occasional long-distance migration and somatic mutations of large clones contribute new genotypic variation in populations of F. radicans. We further analysed the genetic structure of Fucus from areas not earlier investigated along the Finnish and Russian coasts of the Baltic Sea, comparing the results with previous data from Bothnian Sea and Estonia. The results indicated a genetic structure strongly affected by the geographic isolation of populations, but in most cases separating the two taxa at a local scale. In addition, we found some cases of strong genetic asymmetries with populations having fewer rare alleles than others, either as a consequence of directional gene flow or founder effects during colonization. Interestingly, in contrast to earlier findings from the Bothnian Sea, sexual reproduction remained important in the lowest salinities (<3‰) in eastern Gulf of Finland. Nevertheless, in F. radicans a few old clones have dominated much of the species' distribution over extensive periods of time suggesting that the distribution of genotypes cannot solely be ascribed to natural selection. Using a spatially explicit model, we tested a neutral scenario of colonization of a new environment by a species with the potential of both sexual and asexual reproduction. We found that a pattern with spatially and temporally dominant clones mixed with areas of sexual reproduction is quite unlikley during and thousands of generations after colonization of a new environment even so sexual reproduction eventually takes over. One hypothesis connected to the speciation process is that herbivores may have had different co-evolutionary effects on the two species. We assessed differences in inducible herbivory defense using an ecological experiment complemented by gene expression analysis. We found that F. vesiculosus produced deterrent chemicals faster than F. radicans, and that patterns of gene expression were different between the species under grazing by isopods. The evolutionary changes observed in transcriptional regulation of grazing resistance in these two macro algae has likely evolved under differential selection, with F. vesiculosus mainly distributed in areas of strong grazing pressure, while F. radicans is not found in such areas. My investigations have contributed with further understanding of the Baltic Sea Fucus system and its intriguingly rapid and recent speciation. In current times, climate change threatens Baltic Sea species and biodiversity, and increased knowledge of processes generating and maintaining biodiversity in this ecosystem seems particularly important and needed. Key words: brown algae, Fucus radicans, sympatric speciation, clonality, somatic mutations, microsatellites, reproductive barriers, asymmetric migration, transcriptomic differences
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| 4. |
- Rafajlović, Marina, 1983, et al.
(author)
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Neutral processes forming large clones during colonization of new areas
- 2017
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In: Journal of Evolutionary Biology. - : Wiley. - 1010-061X .- 1420-9101. ; 30:8, s. 1544-1560
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Journal article (peer-reviewed)abstract
- © 2017 The Authors. Journal of Evolutionary Biology Published by John Wiley & Sons ltd on Behalf of European Society for Evolutionary Biology. In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species’ range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonization starts with an asexual wave. After colonization is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long-distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a post-glacially established seaweed species (Fucus radicans) shows that in this case, a neutral mechanism is strongly supported.
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