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Intraspecific diversification in North American Boechera stricta (= Arabis drummondii), Boechera xdivaricarpa, and Boechera holboellii (Brassicaceae) inferred from nuclear and chloroplast molecular markers—an integrative approach¹

Christoph Dobe², Thomas Mitchell-Olds³ and Marcus A. Koch^2,4

²Heidelberg Institute of Plant Sciences, Dept. of Biodiversity and Plant Systematics, Im Neuenheimer Feld 345, D-69120 Heidelberg, Germany; ³Max Planck Institute of Chemical Ecology, Beutenberg Campus, Winzerlaer Str. 10, D-07745 Jena, Germany

We performed a combined evolutionary analysis of North American Boechera stricta, Boechera holboellii, and their hybrid Boechera xdivaricarpa using information on ploidy level estimators, allelic microsatellite variation, noncoding regions of the plastidic genome (cpDNA), and sequences of the internal transcribed spacers 1 and 2 of the nuclear ribosomal DNA (ITS). Somatic ploidy levels of herbarium specimens were estimated based on comparison of pollen size and the number of alleles per locus at seven microsatellites. Results indicate that B. stricta and B. holboellii are genetically distinct from each other, although we also find evidence for occasional introgression between both parental species. Microsatellite patterns for B. stricta from northeastern North America are genetically distinct from western populations, suggesting isolation in glacial refugia along the southeastern margin of the continuous ice shield. Microsatellites supported recent origin of B. xdivaricarpa. Correspondence of nrDNA with cpDNA genetic variation for the majority of diploid B. holboellii accessions suggests a basal, sexual evolutionary unit within a polymorphic B. holboellii group. Hybridization of genetically distinct lineage(s) evidently played an important role in the establishment of polyploid B. holboellii. Frequency of polyploid B. holboellii is substantially higher in the southern United States. This trend corresponds to a southerly distribution of derived chloroplast haplotypes, suggesting an evolutionary advantage of polyploidy and associated apomixis in the colonization of the Sierra Nevada and the Southern Rocky Mountains.

Key Words: Arabis • Boechera • Boechera stricta • biogeography • Brassicaceae • hybridization • microsatellites • North America • pollen

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