Despite Arabidopsis thaliana's pre-eminence as a model organism, major questions remain regarding the geographic structure of its genetic variation due to the geographically incomplete sample set available for previous studies. Many of these questions are addressed here with an analysis of genome-wide variation at 10 loci in 475 individuals from 167 globally distributed populations, including many from critical but previously un-sampled regions. Rooted haplotype networks at three loci suggest that A. thaliana arose in the Caucasus region. Identification of large-scale metapopulations indicates clear east-west genetic structure, both within proposed Pleistocene refugia and post-Pleistocene colonized regions. The refugia themselves are genetically differentiated from one another and display elevated levels of within-population genetic diversity relative to recolonized areas. The timing of an inferred demographic expansion coincides with the Eemian interglacial (approximately 120,000 years ago). Taken together, these patterns are strongly suggestive of Pleistocene range dynamics. Spatial autocorrelation analyses indicate that isolation by distance is pervasive at all hierarchical levels, but that it is reduced in portions of Europe.

Gametophytic apomixis is a common form of asexual reproduction in plants.Virtually all gametophytic apomicts are polyploids, and some view polyploidy as a prerequisite for the transition to apomixis. However, any causal link between apomixis and polyploidy is complicated by the fact that most apomictic polyploids are allopolyploids, leading some to speculate that hybridization, rather than polyploidy, enables apomixis. Diploid apomixis presents a rare opportunity to isolate the role of hybridization, and a number of diploid apomicts have been documented in the genus Boechera (Brassicaceae). Here, we present the results of a microsatellite study of 1393 morphologically and geographically diverse diploid individuals, evaluating the hypothesis that diploid Boechera apomicts are hybrids. This genus-wide dataset was made possible by the applicability of a core set of microsatellite loci in 69 of the 70 diploid Boechera species and by our ability to successfully genotype herbarium specimens of widely varying ages. With few exceptions, diploid apomicts exhibited markedly high levels of heterozygosity resulting from the combination of disparate genomes. This strongly suggests that most apomictic diploid Boechera lineages are of hybrid origin, and that the genomic consequences of hybridization allow for the transition to gametophytic apomixis in this genus. K E Y W O R D S :Apomixis, "genomic-collision," heterozygosity, microsatellites, polyploidy.Sexual reproduction is a pervasive feature of biodiversity, and its preponderance over asexual alternatives is a longstanding evolutionary puzzle (Maynard Smith 1978;Bell 1982;Judson and Normark 1996; but see Gorelick and Heng 2010). Knowledge of the factors limiting asexuality therefore have clear implications for our understanding of the success of sexuality. In seed plants, asexuality includes both vegetative reproduction (Silvertown 2008) and apomixis, here defined as the production of a seed without reductive meiosis and fertilization (Van Dijk 2003;Bicknell and Koltunow 2004). Two major types of apomixis are recognized among seed plants. In sporophytic apomixis, no game-

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Boechera (Brassicaceae) has many features to recommend it as a model genus for ecological and evolutionary research, including species richness, ecological diversity, experimental tractability and close phylogenetic proximity to Arabidopsis. However, efforts to realize the full potential of this model system have been thwarted by the frequent inability of researchers to identify their samples and place them in a broader evolutionary context. Here we present the Boechera Microsatellite Website (BMW), a portal that archives over 55 000 microsatellite allele calls from 4471 specimens (including 133 nomenclatural types). The portal includes analytical tools that utilize data from 15 microsatellite loci as a highly effective DNA barcoding system. The BMW facilitates the accurate identification of Boechera samples and the investigation of reticulate evolution among the ±83 sexual diploid taxa in the genus, thereby greatly enhancing Boechera’s potential as a model system. Database URL: http://sites.biology.duke.edu/windhamlab/

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