'Missing link' species Capsella orientalis and Capsella thracica elucidate evolution of model plant genus Capsella (Brassicaceae)

To elucidate the evolutionary history of the genus Capsella, we included the hitherto poorly known species C.similar to orientalis and C.similar to thracica into our studies together with C.similar to grandiflora, C.similar to rubella and C.similar to bursa-pastoris. We sequenced the ITS and four loci of noncoding cpDNA regions (trnL F, rps16, trnH psbA and trnQ rps16). Sequence data were evaluated with parsimony and Bayesian analyses. Divergence time estimates were carried out with the software package BEAST. We also performed isozyme, cytological, morphological and biogeographic studies. Capsella orientalis (self-compatible, SC; 2n = 16) forms a clade (eastern lineage) with C.similar to bursa-pastoris (SC; 2n = 32), which is a sister clade (western lineage) to C.similar to grandiflora (self-incompatible, SI; 2n = 16) and C.similar to rubella (SC; 2n = 16). Capsella bursa-pastoris is an autopolyploid species of multiple origin, whereas the Bulgarian endemic C.similar to thracica (SC; 2n = 32) is allopolyploid and emerged from interspecific hybridization between C.similar to bursa-pastoris and C.similar to grandiflora. The common ancestor of the two lineages was diploid and SI, and its distribution ranged from eastern Europe to central Asia, predominantly confined to steppe-like habitats. Biogeographic dynamics during the Pleistocene caused geographic and genetic subdivisions within the common ancestor giving rise to the two extant lineages.