Morphological and molecular evidence for hybridization and introgression in Central European Arctium (Asteraceae)

Three species of burdock (Arctium lappa, A. tomentosum, and A. minus; Asteraceae), which commonly occur in sympatry in Central Europe, are thought to hybridize frequently, hence raising questions about their integrity. We surveyed mixed, monotypic, and/or single plant stands of these species in Central Germany to assess their extent of hybridization and introgression, and also the likelihood of insect host shifts between them. Individual plants were characterized by multivariate analyses of morphological and RAPD variation in combination with RAPD-based character-index-scores. All analyses recognized the three species as distinct units, but also identified a limited number of first (or early) generation hybrids between sympatric A. lappa and either A. tomentosum or A. minus, respectively, as well as one introgressant of A. tomentosum into A. lappa. While pre-zygotic isolation may partly account for the rarity of first generation crosses (F₁'s), the near absence of later generation hybrid segregants or introgressants likely reflects post-zygotic isolation due to the disruption of co-adapted gene complexes and/or habitat-mediated superiority shared by the ruderal parentals. As the tephritid fly Tephritis bardanae oviposits on both A. tomentosum (T) and A. minus (M), but not on A. lappa (L), the occurrence of L × M and L × T hybrids and/or introgressants may have important evolutionary consequences regarding the acquisition of a new host, i.e. A. lappa, by these insects.