Professor Fenny Dane, PhD
Department of Horticulture
Auburn University
[click to enlarge]Figure 1. Distribution of vegetation on US continent 18,000 years ago during the last ice age
The geographical range and distribution of a plant species is dependent on its adaptation to ecological and environmental conditions and the distribution of the populations of trees, both in the past and present over the continents. In the case of Castanea species, the distribution has been especially influenced by the occurrence of ice ages during the Pleistocene and the susceptibility of especially the American chestnut species to chestnut blight.
[click to enlarge]Chestnut species with their characteristic 3 nuts per bur can now be found in Japan (C. crenata), China (C. mollissima and C. seguinii), Europe (C. sativa) and North America (C. dentata), while chinkapins with 1 nut per bur are distributed in China (C. henryi) and North America (C. pumila). Fossils of Castanea detected in Greenland and throughout western North America and Asia can be dated back to more than 85-60 million years ago (mya) and indicate that the species once had a much wider distribution than today.
Migration between Eurasia and North America might have occurred via the Bering Land Bridge which connected eastern Asia with western North American and via the North Atlantic Land Bridge which at one time connected eastern North America with Europe.
[click to enlarge]Figure 3. Plant cell with chloroplasts
Phylogeography is a field of study that analyses the geographical distribution of different plant populations. Plant phylogeographical studies predominantly use genetic variation at chloroplast DNA. Chloroplasts are small organelles within the plant cell responsible for photosynthesis. These organelles contain DNA which is effectively haploid and maternally inherited so we can study seed-mediated migration and colonization routes of populations.
[click to enlarge]Figure 4. Chloroplast DNA variation at short regions from different Castanea species
DNA variation at several chloroplast regions from Castanea species and tree populations collected from different geographical locations was analyzed. We are interested in point mutations or substitutions (top) and insertions or deletions (bottom)
[click to enlarge]Figure 5. Colonization routes of Castanea via the North Atlantic bridge, which was broken by the late Eocene (40 million years ago).
Each Castanea species has several distinct chloroplast types. More differences can be detected among Asian species than within European or North American species. Based on chloroplast sequence variation we hypothesize that the genus originated in Asia, moved to Europe and via Europe to North America as indicated in this figure (Lang et al., 2007).
[click to enlarge]Figure 6: Chloroplast type distribution in C. dentata populations.
Figure 6. Chloroplast DNA of trees in populations across the Appalachian mountain range was analyzed. Circles reflect location of population, fill effect different chloroplast types, circle size reflects frequency of chloroplast type.
From chloroplast DNA analysis we can hypothesize that C. dentata probably found refuge in the southern region of the Appalachian mountain range during glacial periods and that following the retreat of the ice sheet, one lineage indicated by green circles migrated into north easterly direction.
The lineage in the southern region of the range is closely related to the Allegheny chinkapin and there is evidence for sharing of the chloroplasts between the different species. This means that hybridization between the American chestnut and Allegheny chinkapin has occurred over time and will continue in the future.
We are continuously using not only chloroplast but also nuclear DNA in our studies to gain a better understanding of the evolution of Castanea species on the North American continent.
If you have samples or would like to have your tree leaves or nuts analyzed, please email danefen “at” auburn “dot” edu
