Adapting through glacial cycles: Insights from a long-lived tree (Taxus baccata)

Mayol M., Riba M., Gonzalez-Martinez S.C., Bagnoli F., de Beaulieu J.-L., Berganzo E., Burgarella C., Dubreuil M., Krajmerova D., Paule L., Romsakova I., Vettori C., Vincenot L., Vendramin G.G. (2015) Adapting through glacial cycles: Insights from a long-lived tree (Taxus baccata). New Phytologist. : 0-0.
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Doi: 10.1111/nph.13496

Resum:

Despite the large body of research devoted to understanding the role of Quaternary glacial cycles in the genetic divergence of European trees, the differential contribution of geographic isolation and/or environmental adaptation in creating population genetic divergence remains unexplored. In this study, we used a long-lived tree (Taxus baccata) as a model species to investigate the impact of Quaternary climatic changes on genetic diversity via neutral (isolation-by-distance) and selective (isolation-by-adaptation) processes. We applied approximate Bayesian computation to genetic data to infer its demographic history, and combined this information with past and present climatic data to assess the role of environment and geography in the observed patterns of genetic structure. We found evidence that yew colonized Europe from the East, and that European samples diverged into two groups (Western, Eastern) at the beginning of the Quaternary glaciations, c. 2.2 Myr before present. Apart from the expected effects of geographical isolation during glacials, we discovered a significant role of environmental adaptation during interglacials at the origin of genetic divergence between both groups. This process may be common in other organisms, providing new research lines to explore the effect of Quaternary climatic factors on present-day patterns of genetic diversity. © 2015 New Phytologist Trust.

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Recent population decline and selection shape diversity of taxol-related genes

Burgarella C., NavascuÉs M., Zabal-Aguirre M., Berganzo E., Riba M., Mayol M., Vendramin G.G., González-MartÍnez S.C. (2012) Recent population decline and selection shape diversity of taxol-related genes. Molecular Ecology. 21: 3006-3021.
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Doi: 10.1111/j.1365-294X.2012.05532.x

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Taxanes are defensive metabolites produced by Taxus species (yews) and used in anticancer therapies. Despite their medical interest, patterns of natural diversity in taxane-related genes are unknown. We examined variation at five main genes of Taxus baccata in the Iberian Peninsula, a region where unique yew genetic resources are endangered. We looked at several gene features and applied complementary neutrality tests, including diversity/divergence tests, tests solely based on site frequency spectrum (SFS) and Zeng's compound tests. To account for specific demography, microsatellite data were used to infer historical changes in population size based on an Approximate Bayesian Computation (ABC) approach. Polymorphism-divergence tests pointed to positive selection for genes TBT and TAT and balancing selection for DBAT. In addition, neutrality tests based on SFS found that while a recent reduction in population size may explain most statistics' values, selection may still be in action in genes TBT and DBAT, at least in some populations. Molecular signatures on taxol genes suggest the action of frequent selective waves with different direction or intensity, possibly related to varying adaptive pressures produced by the host-enemy co-evolution on defence-related genes. Such natural selection processes may have produced taxane variants still undiscovered. © 2012 Blackwell Publishing Ltd.

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Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: insights on evolutionary processes leading to diversification in continental islands.

Mayol M, Palau C, Rosselló JA, González-Martínez SC, Molins A, Riba M (2012) Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: insights on evolutionary processes leading to diversification in continental islands. Annals of Botany 109: 429-441.

Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: Insights on evolutionary processes leading to diversification in continental islands

Mayol M., Palau C., Rosselló J.A., González-Marítnez S.C., Molins A., Riba M. (2012) Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: Insights on evolutionary processes leading to diversification in continental islands. Annals of Botany. 109: 429-441.
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Doi: 10.1093/aob/mcr298

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Background and Aims Archipelagos are unique systems for studying evolutionary processes promoting diversification and speciation. The islands of the Mediterranean basin are major areas of plant richness, including a high proportion of narrow endemics. Many endemic plants are currently found in rocky habitats, showing varying patterns of habitat occupancy at different spatial scales throughout their range. The aim of the present study was to understand the impact of varying patterns of population distribution on genetic diversity and structure to shed light on demographic and evolutionary processes leading to population diversification in Crepis triasii, an endemic plant from the eastern Balearic Islands. Methods Using allozyme and chloroplast markers, we related patterns of genetic structure and diversity to those of habitat occupancy at a regional (between islands and among populations within islands) and landscape (population size and connectivity) scale. Key Results Genetic diversity was highly structured both at the regional and at the landscape level, and was positively correlated with population connectivity in the landscape. Populations located in small isolated mountains and coastal areas, with restricted patterns of regional occupancy, were genetically less diverse and much more differentiated. In addition, more isolated populations had stronger fine-scale genetic structure than well-connected ones. Changes in habitat availability and quality arising from marine transgressions during the Quaternary, as well as progressive fragmentation associated with the aridification of the climate since the last glaciation, are the most plausible factors leading to the observed patterns of genetic diversity and structure. Conclusions Our results emphasize the importance of gene flow in preventing genetic erosion and maintaining the evolutionary potential of populations. They also agree with recent studies highlighting the importance of restricted gene flow and genetic drift as drivers of plant evolution in Mediterranean continental islands. © 2011 The Author.

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Recent population decline and selection shape diversity of taxol-related genes.

Burgarella C, Navascués M, Zabal-Aguirre M, Berganzo E, Riba M, Mayol M, Vendramin GG, González-Martínez SC (2012) Recent population decline and selection shape diversity of taxol-related genes. Molecular Ecology 21: 3006-3021.

Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: insights on evolutionary processes leading to diversification in continental islands.

Mayol M, Palau C, Rosselló JA, González-Martínez SC, Molins A, Riba M (2011) Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: insights on evolutionary processes leading to diversification in continental islands. Annals of Botany doi: 10.1093/aob/mcr298.

Variabilidad genética de Taxus baccata en el Mediterráneo occidental: el papel de los procesos históricos y de la fragmentación del paisaje. En: A. Caritat (ed.), II Jornades sobre el teix a la Mediterrània occidental.

Mayol M, Dubreuil M, González-Martínez SC, Sebastiani F, Vendramin GG, Riba M (2011) Variabilidad genética de Taxus baccata en el Mediterráneo occidental: el papel de los procesos históricos y de la fragmentación del paisaje. En: A. Caritat (ed.), II Jornades sobre el teix a la Mediterrània occidental. Documents de la delegació de la Garrotxa de la Institució Catalana d’Història Natural, 1, Olot, pp. 103-106. ISBN: 978-84-9965-053-1.

Aspectos genéticos y demográficos de Taxus baccata en la Red de Parques Nacionales. En: L. Ramírez y B. Asensio (eds.), Proyectos de Investigación en Parques Nacionales 2007-2010.

Burgarella C, Berganzo E, Zabal-Aguirre M, Prada A, Iglesias S, Riba M, Mayol M, Vendramin GG, González-Martínez SC (2011) Aspectos genéticos y demográficos de Taxus baccata en la Red de Parques Nacionales. En: L. Ramírez y B. Asensio (eds.), Proyectos de Investigación en Parques Nacionales 2007-2010. Organismo Autónomo de Parques Nacionales, pp. 125-137. ISBN: 978-84-8014-805-4.

Genetic effects of chronic habitat fragmentation revisited: Strong genetic structure in a temperate tree, Taxus baccata (Taxaceae), with great dispersal capability

Dubreuil M., Riba M., González-Martínez S.C., Vendramin G.G., Sebastiani F., Mayol M. (2010) Genetic effects of chronic habitat fragmentation revisited: Strong genetic structure in a temperate tree, Taxus baccata (Taxaceae), with great dispersal capability. American Journal of Botany. 97: 303-310.
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Doi: 10.3732/ajb.0900148

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Tree species are thought to be relatively resistant to habitat fragmentation because of their longevity and their aptitude for extensive gene flow, although recent empirical studies have reported negative genetic consequences, in particular after long-term habitat fragmentation in European temperate regions. Yet the response of each species to habitat loss may differ greatly depending on their biological attributes, in particular seed dispersal ability. In this study, we used demographic and molecular data to investigate the genetic consequences of chronic habitat fragmentation in remnant populations of Taxus baccata in the Montseny Mountains, northeast Spain. The age structure of populations revealed demographic bottlenecks and recruitment events associated with exploitation and management practices. We found a strong genetic structure, both at the landscape and within-population levels. We also detected high levels of inbreeding for a strictly outcrossing species. Chronic forest fragmentation resulting from long-term exploitation in the Montseny Mountains seems the most plausible explanation for the strong genetic structure observed. Our results support the view that, contrary to some predictions, tree species are not buffered from the adverse effects of habitat fragmentation, even in the case of species with a high dispersal potential.

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Spatial genetic structure of Taxus baccata L. in the western Mediterranean Basin: Past and present limits to gene movement over a broad geographic scale

González-Martínez S.C., Dubreuil M., Riba M., Vendramin G.G., Sebastiani F., Mayol M. (2010) Spatial genetic structure of Taxus baccata L. in the western Mediterranean Basin: Past and present limits to gene movement over a broad geographic scale. Molecular Phylogenetics and Evolution. 55: 805-815.
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Doi: 10.1016/j.ympev.2010.03.001

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English yew (Taxus baccata L., Taxaceae), a Tertiary relict, provides a seminal example of a widespread albeit locally endangered (often close to extinction) tree species. In order to gain detailed insights into the evolutionary dynamics of the species on a broad geographical scale, over 1000 trees from 91 populations of English yew in the western Mediterranean were analyzed using seven nuclear microsatellite markers. Our results revealed contrasting patterns of genetic structure at different spatial scales: genetic variation was highly structured at the local scale, while only a low proportion of the observed variation was attributed to regional differences. We also found a geographic gradient of decreasing diversity and increasing population divergence from northwest (central Europe and northern Iberian Peninsula) to southeast (Mediterranean Iberia and North Africa). The patterns revealed in this study probably reflect the combined effects of Quaternary climatic changes and recent impact of human activities, and potentially also more ancient events dating back to the Tertiary. Both climatic and anthropogenic factors seem to have conducted to a long history of population isolation, which may have contributed significantly to enhance population divergence through restricted gene flow and genetic drift. © 2010 Elsevier Inc. All rights reserved.

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