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Hybridization and Evolution in the Genus Pinus
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gene flow and hybridization are pervasive in nature, and can lead to different evolutionary outcomes. They can either accelerate divergence and promote speciation or reverse differentiation. The process of divergence and speciation are strongly influenced by both neutral and selective forces. Disentangling the interplay between these processes in natural systems is important for understanding the general importance of interspecific gene flow in generating novel biodiversity in plants. This thesis first examines the importance of introgressive hybridization in the evolution of the genus Pinus as a whole, and then focusing on specific pine species, investigates the role of geographical, environmental and demographical factors in driving divergence and adaptation.

By examining the distribution of cytoplasmic DNA variation across the wide biogeographic range of the genus Pinus, I revealed historical introgression and mtDNA capture events in several groups of different pine species. This finding suggests that introgressive hybridization was common during past species’ range contractions and expansions and thus has played an important role in the evolution of the genus. To understand the cause and process of hybrid speciation, I focused on the significant case of hybrid speciation in Pinus densata. I established the hybridization, colonization and differentiation processes that defined the origin of this species. I found P. densata originated via multiple hybridization events in the late Miocene. The direction and intensity of introgression with two parental species varied among geographic regions of this species. During the colonization on Tibetan Plateau from the ancestral hybrid zone, consecutive bottlenecks and surfing of rare alleles caused a significant reduction in genetic diversity and strong population differentiation. Divergence within P. densata started from the late Pliocene onwards, induced by regional topographic changes and Pleistocene glaciations. To address the role of neutral and selective forces on genetic divergence, I examined the association of ecological and geographical distance with genetic distance in Pinus yunnanensis populations. I found both neutral and selective forces have contributed to population structure and differentiation in P. yunnanensis, but their relative contributions varied across the complex landscape. Finally, I evaluated genetic diversity in the Vietnamese endemic Pinus krempfii. I found extremely low genetic diversity in this species, which is explained by a small ancestral population, short-term population expansion and recent population decline and habitat fragmentation.

These findings highlight the role of hybridization in generating novel genetic diversity and the different mechanisms driving divergence and adaptation in the genus Pinus

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2013. , 49 p.
Keyword [en]
Adaptation, biogeography, coalescent simulation, cytoplasmic genome, demographic history, genetic diversity, hybridization, migration, Pinus, population structure, selection, speciation
National Category
Evolutionary Biology
Research subject
evolutionär genetik
URN: urn:nbn:se:umu:diva-80998ISBN: 978-91-7459-702-8 (print)OAI: diva2:652236
Public defence
2013-10-25, KBC-huset, KB3B1, Stora Hörsalen, Umeå Universitet, Umeå, 13:00 (English)
Available from: 2013-10-04 Created: 2013-09-30 Last updated: 2013-09-30Bibliographically approved
List of papers
1. Colonization of the Tibetan Plateau by the homoploid hybrid pine Pinus densata
Open this publication in new window or tab >>Colonization of the Tibetan Plateau by the homoploid hybrid pine Pinus densata
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2011 (English)In: Molecular Ecology, ISSN 0962-1083, E-ISSN 1365-294X, Vol. 20, no 18, 3796-3811 p.Article in journal (Refereed) Published
Abstract [en]

Pinus densata is an intriguingly successful homoploid hybrid species that occupies vast areas of the southeastern Tibetan Plateau in which neither of its parental species are present, but the colonization processes involved are poorly understood. To shed light on how this species colonized and became established on the plateau, we surveyed paternally inherited chloroplast (cp) and maternally inherited mitochondrial (mt) DNA variation within and among 54 populations of P. densata and its putative parental species throughout their respective ranges. Strong spatial genetic structure of both cp and mtDNA were detected in P. densata populations. Mitotypes specific to P. densata were likely generated by complex recombination events. A putative ancestral hybrid zone in the northeastern periphery of P. densata was identified, and we propose that the species then colonized the plateau by migrating westwards. Along the colonization route, consecutive bottlenecks and surfing of rare alleles caused a significant reduction in genetic diversity and strong population differentiation. The direction and intensity of introgression from parental species varied among geographic regions. In western parts of its range, the species seems to have been isolated from seed and pollen flow from its parent species for a long time. The observed spatial distribution of genetic diversity in P. densata also appears to reflect the persistence of this species on the plateau during the last glaciation. Our results indicate that both ancient and contemporary population dynamics have contributed to the spatial distribution of genetic diversity in P. densata, which accordingly reflects its evolutionary history.

Place, publisher, year, edition, pages
Leicester: Blackwell Scientific Publications, 2011
bottleneck, colonization history, hybrid speciation, mtDNA recombination, population structure
National Category
Biological Sciences
urn:nbn:se:umu:diva-47950 (URN)10.1111/j.1365-294X.2011.05157.x (DOI)21689188 (PubMedID)
Available from: 2011-10-04 Created: 2011-10-04 Last updated: 2017-12-08Bibliographically approved
2. Demography and speciation history of the homoploid hybrid pine Pinus densata on the Tibetan Plateau
Open this publication in new window or tab >>Demography and speciation history of the homoploid hybrid pine Pinus densata on the Tibetan Plateau
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2012 (English)In: Molecular Ecology, ISSN 0962-1083, E-ISSN 1365-294X, Vol. 21, no 19, 4811-4827 p.Article in journal (Refereed) Published
Abstract [en]

Pinus densata is an ecologically successful homoploid hybrid that inhabits vast areas of heterogeneous terrain on the south-eastern Tibetan Plateau as a result of multiple waves of colonization. Its region of origin, route of colonization onto the plateau and the directions of introgression with its parental species have previously been defined, but little is known about the isolation and divergence history of its populations. In this study, we surveyed nucleotide polymorphism over eight nuclear loci in 19 representative populations of P. densata and its parental species. Using this information and coalescence simulations, we assessed the historical changes in its population size, gene flow and divergence in time and space. The results indicate a late Miocene origin for P. densata associated with the recent uplift of south-eastern Tibet. The subsequent differentiation between geographical regions of this species began in the late Pliocene and was induced by regional topographical changes and Pleistocene glaciations. The ancestral P. densata population had a large effective population size but the central and western populations were established by limited founders, suggesting that there were severe bottlenecks during the westward migration out of the ancestral hybrid zone. After separating from their ancestral populations, population expansion occurred in all geographical regions especially in the western range. Gene flow in P. densata was restricted to geographically neighbouring populations, resulting in significant differentiation between regional groups. The new information on the divergence and demographic history of P. densata reported herein enhances our understanding of its speciation process on the Tibetan Plateau.

Place, publisher, year, edition, pages
John Wiley & Sons, 2012
coalescent simulation, effective population size, gene flow, hybrid speciation, isolation history, nucleotide diversity
National Category
urn:nbn:se:umu:diva-61180 (URN)10.1111/j.1365-294X.2012.05712.x (DOI)000309234300014 ()
Available from: 2012-11-09 Created: 2012-11-07 Last updated: 2017-12-07Bibliographically approved
3. Impact of Geography and Climate on the Genetic Differentiation of the Subtropical Pine Pinus yunnanensis
Open this publication in new window or tab >>Impact of Geography and Climate on the Genetic Differentiation of the Subtropical Pine Pinus yunnanensis
2013 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 8, no 6, e67345- p.Article in journal (Refereed) Published
Abstract [en]

Southwest China is a biodiversity hotspot characterized by complex topography, heterogeneous regional climates and rich flora. The processes and driving factors underlying this hotspot remain to be explicitly tested across taxa to gain a general understanding of the evolution of biodiversity and speciation in the region. In this study, we examined the role played by historically neutral processes, geography and environment in producing the current genetic diversity of the subtropical pine Pinus yunnanensis. We used genetic and ecological methods to investigate the patterns of genetic differentiation and ecological niche divergence across the distribution range of this species. We found both continuous genetic differentiation over the majority of its range, and discrete isolated local clusters. The discrete differentiation between two genetic groups in the west and east peripheries is consistent with niche divergence and geographical isolation of these groups. In the central area of the species' range, population structure was shaped mainly by neutral processes and geography rather than by ecological selection. These results show that geographical and environmental factors together created stronger and more discrete genetic differentiation than isolation by distance alone, and illustrate the importance of ecological factors in forming or maintaining genetic divergence across a complex landscape. Our findings differ from other phylogenetic studies that identified the historical drainage system in the region as the primary factor shaping population structure, and highlight the heterogeneous contributions that geography and environment have made to genetic diversity among taxa in southwest China.

Place, publisher, year, edition, pages
plosone, 2013
National Category
urn:nbn:se:umu:diva-79435 (URN)10.1371/journal.pone.0067345 (DOI)000321424400097 ()
Available from: 2013-08-28 Created: 2013-08-19 Last updated: 2017-12-06Bibliographically approved

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