Evaluation of reproductive barriers and realisation of interspecific hybridisations depending on genetic distances between species in the genus Helleborus.

The genus Helleborus comprises 22 species, which are allocated to six sections. H. x hybridus and H. niger, which belong to different Helleborus sections, are economically important ornamentals. Several other species with minor impact exhibit interesting features, e.g. flower size, flower colour, foliage, scent and disease resistance, which should be introgressed into H. x hybridus or H. niger through interspecific hybridisation. The aims of this study were to investigate whether and which kind of hybridisation barriers occur in crosses between Helleborus species and if they differ in their manifestations, depending on the genetic distance of the respective partners. In order to obtain interspecific hybrids despite crossing barriers, a method to overcome these barriers should be developed. Crossing barriers in Helleborus were localised as predominantly post-zygotic according to in situ pollen tube staining with aniline blue. For certain crosses, pre-zygotic barriers could also be assumed, but pollen tube growth was not totally inhibited. Therefore, embryo rescue techniques via ovule culture were established to overcome the post-zygotic barriers. Ovules were isolated from maternal plants 5-7 weeks after pollination in most cases and then cultured in vitro. Overall, 219 hybrids were successfully obtained, of which 16 were derived from inter-sectional crosses. Hybrids were verified by flow cytometry and/or by molecular DNA markers.