Post glactial colonization


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Post glactial colonization

  1. 1. IntroductionNorway spruce Picea abies is a broadly distributed European conifer treeIntensively studied by means of fossil records to infer the location of glacial refugia and the main routes of postglacial colonizationIn this study they combine genetic data against fossil pollen data as a templateVariation assessed in two minisatellite regions of the mitochondrial nad 1 gene, among the 369 populations (4876 trees)
  2. 2. Glacial & Interglacial CyclesRepeated glacial and interglacial cycles are believed to have played an important role in shaping genetic diversityFossil pollen record imply that the entire northern range was colonized from a single glacial refugium in European RussiaThe timing and mode of colonization by Norway spruce as revealed by fossil pollen shows both similarities and differences between the northern and southern rangeSimilarities westward colonization during early Holocene 9000 ybpDifferences westward colonization continued in the northern range not in the south Difference may have been caused by differences in climate, topography, and presence of other competitive tree sp’s
  3. 3. Populations of Picea abies analysed for mitochondrial minisatellite variation Green shaded areas represent the natural range of Norway spruce Light green areas indicate zones where Norway spruce populations of different origin came into contact during the Holocene Grey shading illustrates the topography; the darker the tone, the higher the altitude
  4. 4. Molecular MarkersStudies of nuclear DNA markers have revealed a clear separation between the northern and southern populationsNuclear and chDNA markers shown that there are differences among the southern populations but with non-sufficient resolution to locate genetic boundaries among different refugial expansions or postglacial colonizationExceptionally mtDNA markers provided sufficient resolutionIn this study, researchers apply the same mtDNA marker as used in two earlier, geographically more restricted studies (Gugerli et al. 2001; Sperisen et al. 2001)
  5. 5. mtDNA markerThis marker comprises a fragment of the second intergenic spacer of nad1 gene which includes 2 highly variable minisatellitesVariation in the minisatellite- flanking sequences separates northern populations from the southernCopy number variation in the two minisatellites estimates diversity and population dynamics regionally or locally
  6. 6. Core Idea of the Study In areas where a clear relationship betweenrange expansion inferred from fossil pollen and genetic structure could be established,genetic change in space and time is analysed byrelating genetic diversity with the estimatedage of first spruce occurrence and withgeographic distance to the putative sourcearea of Holocene range expansion
  7. 7. Phylogenetic tree for the 28 haplotypes of the mt nad 1 gene in Picea abies Stepwise Mutation Model Haplotypes are indicated according their size in bp Grey boxes insertions/deletions of 1–69 bp Blue boxes insertions/deletions of the 34- bp repeat Red boxes insertions/deletions of the 32- bp repeat
  8. 8. Population-genetic analysesTo investigate patterns of genetic subdivision, they applied spatial analysis of molecular variance (samova) and a clustering approach based on principal component analysis (PCA) samova based on a computer-generated procedure that aims to maximize the proportion of total genetic variance due to differences between groups of populations (Fct)
  9. 9. Population-genetic analysesPCA in combination with clustering was used to delineate genetically similar populations based solely on population means and variances without using any geographic informationGenetic relationship between groups delineated by samova or PCA was inferred from a minimum -evolution (ME) tree based on pairwise differentiation among groups (Fct) estimated by analysis of molecular variance (amova)
  10. 10. Geographic distribution of groups of Picea abies populations outlined by SAMOVA
  11. 11. Geographic distribution of groups of Picea abies populations outlined by PCA clustering
  12. 12. Combined maps of genetic groups and fossil pollen for the inference of Holocene expansion regions in Picea abies samova pop. groups for the southern fossil pollen map range shown on the fossil pollen map
  13. 13. Combined maps of genetic groups and fossil pollen for the inference of Holocene expansion regions in Picea abiesPCA pop. groups for the southern range PCA pop. groups for the northern range shown on the fossil pollen map shown on the fossil pollen map
  14. 14. Combined maps of fossil pollen and average within-population gene diversity (hs) Most regions of early Holocene spruce expansion were characterized by high gene diversity (hs) Exceptions were parts of West Carpathians and of the eastern Alps, and the south- west Bulgarian mountains
  15. 15. Combined maps of fossil pollen and average genetic differentiation (Gst) In contrast, genetic differentiation (Gst) was low in most regions with early Holocene A high Gst was found in the southwest Bulgarian mountains
  16. 16. Concluding RemarksCombined genetic and pollen data suggest that during the last glaciation, Norway spruce (Picea abies) survived in at least seven refugial areas from which it expanded during the HoloceneThis study demonstrates that, in contrast to general expectation, within-population diversity is low in some refugial areasThe maintenance of within-population diversity observed in parts of the southern range is probably the consequence of the mixing of colonization routes
  17. 17. ConclusionDeep divergence observed between the mitochondriallineages of the northern and the southern rangeDemonstrating that surviving populations in the two regions were separated for a substantial period of time possibly over several glacial–interglacial cyclesDemographic changes associated with quaternary climatic fluctuations and natural selection are the most important factors shaping genetic variation, both in refugial and in expanding populations