Natural Genetic Variation of Arabidopsis Thaliana Is Geographically Structured in the Iberian Peninsula
Picó, F. Xavier, Méndez-Vigo, Belén, Martínez-Zapater, José M., Alonso-Blanco, Carlos, Genetics
To understand the demographic history of Arabidopsis thaliana within its native geographical range, we have studied its genetic structure in the Iberian Peninsula region. We have analyzed the amount and spatial distribution of A. thaliana genetic variation by genotyping 268 individuals sampled in 100 natural populations from the Iberian Peninsula. Analyses of 175 individuals from 7 of these populations, with 20 chloroplast and nuclear microsatellite loci and 109 common single nucleotide polymorphisms, show significant population differentiation and isolation by distance. In addition, analyses of one genotype from 100 populations detected significant isolation by distance over the entire Iberian Peninsula, as well as among six Iberian subregions. Analyses of these 100 genotypes with different model-based clustering algorithms inferred four genetic clusters, which show a clear-cut geographical differentiation pattern. On the other hand, clustering analysis of a worldwide sample showed a west-east Eurasian longitudinal spatial gradient of the commonest Iberian genetic cluster. These results indicate that A. thaliana genetic variation displays significant regional structure and consistently support the hypothesis that Iberia has been a glacial refugium for A. thaliana. Furthermore, the Iberian geographical structure indicates a complex regional population dynamics, suggesting that this region contained multiple Pleistocene refugia with a different contribution to the postglacial colonization of Europe.
THE annual wild weed species Arabidopsis thaliana is a model organism not only for molecular biology but also for ecological and evolutionary genetics, and hence, revealing the geographical structure of its genetic variationhasbecomeofparamountrelevance( MITCHELL-OLDS and SCHMITT 2006).Quantificationof geneticdiversity within and among populations of A. thaliana and analysis of its spatial distribution pattern across the species geographical range is the basis for elucidating demographic (historical) and ecological influences (MITCHELL-OLDS and SCHMITT 2006). Furthermore, identification of genetic structure is relevant tomapping the causal genes responsible for the natural variation of adaptive traits by genomewide association analysis. In these assays, knowledge of the genetic structure willreducespuriouscorrelationsbetweengenotypeandphenotype due to historical relationships affecting the genetic background (CARDON and PALMER 2003; ZHAO et al. 2007).
A. thaliana shows a worldwide geographical distribution, although its native range spans mainly Europe and central Asia, while it is mostly naturalized elsewhere (reviewed in HOFFMANN 2002). It remains unknown if the mainA. thaliana center of origin is centralAsia or Europe/North Africa, both areas showing the highest diversity of related species (HOFFMANN 2002). Currently, there are wild genotypes (accessions) collected from >500 populations across its world distribution, which have been used to estimate the amount and patterns of genetic variation on a worldwide scale. These analyses have found significant population structure on a global scale, as well as long-range isolationby distanceamong differentworld regions (SHARBEL et al. 2000; NORDBORG et al. 2005; OSTROWSKI et al. 2006; SCHMID et al. 2006; BECK et al. 2008). In addition, several laboratories have recently initiated the development of new A. thaliana collections for genetic variation studies on a regional scale in regions of the native distribution area such as northern Europe (STENØIEN et al. 2005; BAKKER et al. 2006), France (LE CORRE 2005), central Asia (SCHMID et al. 2006), and China (HE et al. 2007), as well as in regions of presumed recent introduction and expansion such as Japan (TODOKORO et al. 1996) and North America (JØRGENSEN and MAURICIO 2004; BAKKER et al. 2006). Thus far, a significant regional correlation between genetic and geographical distances has been observed only in China, which supports a progressive natural dispersal under strong anthropogenic influence. …