AOBPreview published online on May 12, 2008
Annals of Botany, doi:10.1093/aob/mcn074
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Nuclear DNA Microsatellites Reveal Genetic Variation but a Lack of Phylogeographical Structure in an Endangered Species, Fraxinus mandshurica, Across North-east China
1 Laboratory of Forest Ecosystem Studies, Department of Ecosystem Studies, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
2 Key Laboratory of Forest Tree Genetic Improvement and Biotechnology, Ministry of Education, School of Forestry, Northeast Forestry University, Hexing Road 26, Harbin 150040, China
3 Tree Genetics Laboratory, Department of Forest Genetics, Forestry and Forest Products Research Institute, Matsunosato 1, Tsukuba, Ibaraki 305-8687 Japan
* For correspondence. E-mail hu{at}es.a.u-tokyo.ac.jp
Received: 29 January 2008 Returned for revision: 4 March 2008 Accepted: 15 April 2008
Background and Aims: The widely accepted paradigm that the modern genetic structure of plant species in the northern hemisphere has been largely determined by recolonization from refugia after the last glacial maximum fails to explain the presence of cold-tolerant species at intermediate latitudes. Another generally accepted paradigm is that mountain ridges act as important barriers causing genetic isolation of species, but this too has been challenged in recent studies. The aims of the work reported here were to determine the genetic diversity and distribution patterns of extant natural populations of an endangered cool temperate species, Faxinus mandshurica, and to examine whether these two paradigms are appropriate when applied to this species over a wide geographical scale.
Methods: 1435 adult individuals were sampled from 30 natural populations across the main and central range of the species, covering major mountain ranges across North-east China (NEC). Genetic variation was estimated based on nine polymorphic nuclear microsatellite loci. Phylogeographical analyses were employed using various approaches, including Bayesian clustering, spatial analysis of molecular variance, Monmonier's algorithm, neighbor-joining trees, principal co-ordinate analysis and isolation by distance.
Key Results: Genetic diversity within populations was relatively high, and no significant recent bottlenecks were detected in any of the populations. A significant negative correlation between intra-population genetic diversity and latitude was identified. In contrast, genetic differentiation among all the populations examined was extremely low and no clear geographic genetic structure was identified, with the exception of one distinct population.
Conclusions: The modern genetic structure in this species can be explained by extensive gene flow, an absence of mountains acting as barriers, and the presence of a wide refuge across NEC rather than multiple small refugia. Intra-population genetic variation along latitudes is probably associated with the systematically northward shifts of forest biomes in eastern China during the mid-Holocene. To determine important genetic patterns and identify resources for conservation, however, it will be necessary to examine differentially inherited genetic markers exposed to selection pressures (e.g. chloroplast DNA) and to investigate different generations.
Key words: Fraxinus mandshurica, nuclear microsatellites, latitude variation, historical migration, fossil pollen, spatial genetic structure, genetic barriers