Polyploid speciation has played a significant role in the production of flowering plant diversity. Despite its importance, several aspects of the ecology of the speciation process remain poorly understood. Reproductive interference from surrounding diploid plants is thought to be a major factor limiting the establishment of novel tetraploid populations of outcrossing plants. In this study I examined barriers reproduction in snow buttercups (Ranunculus adoneus: Ranunculaceae). I conducted hand pollinations in the field using plants from diploid or tetraploid buttercup populations. I compared seed mass and number among cross-ploidy, within-ploidy, and mixed ploidy pollinations to assess the potential barriers to tetraploid reproduction when in a diploid population, and scored the ploidy of the resulting seeds by flow cytometry. Cross-ploidy crosses resulted in fewer, lighter seeds than within-ploidy crosses, indicating an incomplete barrier to the formation of triploids. Mixed ploidy pollinations using both tetraploid and diploid plants as pollen donors produced fewer seeds but of the same mass as the within-ploidy crosses, indicating that cross-cytotype pollen interfered with fertility when within-cytotype pollen was present. Flow cytometry revealed that the seeds resulting from mixed-ploidy crosses were largely diploid (for diploid dams) or tetraploid (for tetraploid dams), not triploid. This confirms that interference from cross-ploidy pollinations could decrease the fitness of tetraploid plants in mixed populations.

Key words: Ranunculus adoneus , flow cytometry, pollination, Polyploidy, reproductive barriers