Recurrent polyploid origins and chloroplast phylogeography in the Arabis holboellii complex (Brassicaceae)

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Abstract
Arabis holboellii is a North American member of the Brassicaceae that can reproduce via sex or apomixis. Previous studies have shown sexual individuals to be diploid, whilst apomictic individuals can be diploid (and aneuploid) or polyploid (typically 3x). Apomictic individuals can furthermore be facultative (i.e. both sexual and apomictic seed production in a single individual). Using flow cytometry, ploidy variation in 245 accessions of A. holboellii and A. drummondii from western North America and Greenland has been examined. Additionally, the chloroplast trnL intron region from each accession was sequenced for phylogenetic analysis of ploidy variation. Based upon 17 informative single nucleotide and insertion-deletion polymorphisms, we identified seven and 14 chloroplast haplotypes for A. drummondii and A. holboellii, respectively. Six of the haplotypes were found in both species. Ten of the chloroplast haplotypes were characterized by diploid, aneuploid, and triploid individuals, and thus we conclude that polyploidy has repeatedly and independently arisen within the species complex. As triploid individuals, which undergo normal meiosis, can only reproduce through apomixis, this may imply that the phenotype apomixis has also arisen multiple times. Arabis holboellii thus appears to have some predisposition to evolve apomictic reproduction.