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

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