Search

A consortium of 16 laboratories have studied chloroplast DNA (cpDNA) variation in European white oaks. A common strategy for molecular screening, based on restriction analysis of four PCR-amplified cpDNA fragments, was used to allow comparison among the different laboratories. A total of 2613 oak populations (12,214 individual trees from eight species) were sampled from 37 countries, and analysed with the four fragments. They belong to eight related oak species: Quercus robur, Q. petraea, Q. pubescens, Q. frainetto, Q. faginea, Q. pyrenaica, Q. canariensis and Q. macranthera. During this survey, 45 chloroplast variants were detected and are described together with their phylogenetic relationships, but several of these haplotypes were pooled when there were some risks of confusion across laboratories during the survey, and finally 32 remained that were mapped and used in diversity analyses. A strong phylogeographic structure is apparent from the data, where related haplotypes have broadly similar geographic distributions. In total, six cpDNA lineages are identified, which have distinct geographic distributions, mainly along a longitudinal gradient. Most haplotypes found in northern Europe are also present in the south, whereas the converse is not true, suggesting that the majority of mutations observed were generated prior to postglacial recolonisation, corroborating the conclusions of earlier studies. The description of a new western European lineage constitutes a major finding, compared to earlier phylogenetic treatments. Although the eight oak species studied systematically share cpDNA variants when in sympatry, they partition cpDNA diversity differently, as a consequence of their different ecology and life history attributes. Regional differences in levels of differentiation also exist (either species-specific or general); these seem to be related to the intensity of past and present management of the forests across Europe but also to the level of fragmentation of the range within these regions.

The geographic distribution throughout Europe of each of 32 chloroplast DNA variants belonging to eight white oak species sampled from 2613 populations is presented. Clear-cut geographic patterns were revealed by the survey. These distributions, together with the available palynological information, were used to infer colonisation routes out of the glacial period refugia. In western Europe in particular, movements out of the Iberian and the Italian Peninsulas can be clearly identified. Separate refugia are also present in eastern Balkans, whereas further west in this peninsula similarities with Italy were evident. Movements resulting in the exchange of haplotypes between refugia both during the present interglacial and probably also during earlier glacial cycles were therefore inferred. The consequences of these past exchanges is that phylogenetically divergent haplotypes have sometimes followed very similar colonisation routes, limiting somewhat the phylogeographic structure. Cases of geographic disjunction in the present-day distribution of haplotypes are also apparent and could have been induced by the existence of rapid climatic changes at the end of the glacial period (specifically the Younger Dryas cold period), which resulted in range restriction following an early warm period during which oak first expanded from its primary refugia. This cold phase was followed by a new period of expansion at the outset of the Holocene, involving in some cases `secondary' refugia. It is expected that these short climate oscillations would have led to a partial reshuffling of haplotype distribution. Early association between haplotypes and oak species are also suggested by the data, although extensive introgression among species has ultimately largely blurred the pattern. This implies that colonisation routes may have been initially constrained by the ecological characteristics of the species hosting each chloroplast variant. We suggest for instance that two oak species distributed in the north of the Iberian Peninsula (Quercus petraea and Q. pubescens) are recent post-glacial immigrants there. When considered together, conclusions on the location of glacial period refugia and the colonisation routes derived from molecular information and fossil pollen data appear to be both largely compatible and complementary.

The geographic distribution throughout Europe of each of 32 chloroplast DNA variants belonging to eight white oak species sampled from 2613 populations is presented. Clear-cut geographic patterns were revealed by the survey. These distributions, together with the available palynological information, were used to infer colonisation routes out of the glacial period refugia. In western Europe in particular, movements out of the Iberian and the Italian Peninsulas can be clearly identified. Separate refugia are also present in eastern Balkans, whereas further west in this peninsula similarities with Italy were evident. Movements resulting in the exchange of haplotypes between refugia both during the present interglacial and probably also during earlier glacial cycles were therefore inferred. The consequences of these past exchanges is that phylogenetically divergent haplotypes have sometimes followed very similar colonisation routes, limiting somewhat the phylogeographic structure. Cases of geographic disjunction in the present-day distribution of haplotypes are also apparent and could have been induced by the existence of rapid climatic changes at the end of the glacial period (specifically the Younger Dryas cold period), which resulted in range restriction following an early warm period during which oak first expanded from its primary refugia. This cold phase was followed by a new period of expansion at the outset of the Holocene, involving in some cases secondary' refugia. It is expected that these short climate oscillations would have led to a partial reshuffling of haplotype distribution. Early association between haplotypes and oak species are also suggested by the data, although extensive introgression among species has ultimately largely blurred the pattern. This implies that colonisation routes may have been initially constrained by the ecological characteristics of the species hosting each chloroplast variant. We suggest for instance that two oak species distributed in the north of the Iberian Peninsula (Quercus petraea and Q. pubescens) are recent post-glacial immigrants there. When considered together, conclusions on the location of glacial period refugia and the colonisation routes derived from molecular information and fossil pollen data appear to be both largely compatible and complementary.

A consortium of 16 laboratories have studied chloroplast DNA (cpDNA) variation in European white oaks. A common strategy for molecular screening, based on restriction analysis of four PCR-amplified cpDNA fragments, was used to allow comparison among the different laboratories. A total of 2613 oak populations (12,214 individual trees from eight species) were sampled from 37 countries, and analysed with the four fragments. They belong to eight related oak species: Quercus robur, Q. petraea, Q. pubescens, Q. frainetto, Q. faginea, Q. pyrenaica, Q. canariensis and Q. macranthera. During this survey, 45 chloroplast variants were detected and are described together with their phylogenetic relationships, but several of these haplotypes were pooled when there were some risks of confusion across laboratories during the survey, and finally 32 remained that were mapped and used in diversity analyses. A strong phylogeographic structure is apparent from the data, where related haplotypes have broadly similar geographic distributions. In total, six cpDNA lineages are identified, which have distinct geographic distributions, mainly along a longitudinal gradient. Most haplotypes found in northern Europe are also present in the south, whereas the converse is not true, suggesting that the majority of mutations observed were generated prior to postglacial recolonisation, corroborating the conclusions of earlier studies. The description of a new western European lineage constitutes a major finding, compared to earlier phylogenetic treatments. Although the eight oak species studied systematically share cpDNA variants when in sympatry, they partition cpDNA diversity differently, as a consequence of their different ecology and life history attributes. Regional differences in levels of differentiation also exist (either species-specific or general); these seem to be related to the intensity of past and present management of the forests across Europe but also to the level of fragmentation of the range within these regions.

International audience

A total of 1113 oak trees from 222 populations originating from eight countries (Austria, Bosnia-Herzegovina, Croatia, Hungary, Romania, Slovakia, Slovenia, Yugoslavia) were sampled in natural populations or in provenance tests. The sampled trees belong to four different species (Quercus robur, Quercus petraea, Quercus pubescens, Quercus frainetto) and to several putative subspecies. Variation at four chloroplast DNA (cpDNA) fragments was studied using restriction enzymes, resulting in the detection of 12 haplotypes. One haplotype was present in 36% of the trees, and six were found in 6– 17% of the trees. The haplotypes are shared extensively between species and subspecies. They belong to three different lineages (A, C and E) and are phylogeographically structured in the region investigated. Haplotypes of lineage E dominate to the east of the Carpathian mountains in Romania, whereas the Carpathian Basin seems to have been colonised along several different colonisation routes, from the Balkan peninsula but also from Italy. The data support the possible role of climatic instability during the late glacial period in shaping this complex geographic structure. The presence of several secondary refugia could be inferred in the region, which have played a major role in the second step of recolonisation, at the onset of the Holocene period.