Fertility barriers in interspecific crosses within Viburnum

Viburnum is an important genus in horticulture with great ornamental value. Interspecific hybridisations between Viburnum species are made with the aim to enlarge the assortment with more diverse flower colours. However, interspecific pre- and postzygotic crossing barriers hamper the success of breeding efforts. In this study we evaluated the occurrence of interspecific barriers in the cross between V. tinus and V. × bodnantense. Approximately 3000 interspecific crosses were performed between V. tinus and V. × bodnantense. Aniline blue staining revealed that the pollen tubes reached the ovary between 2 and 7 days after pollination, indicating the possibility of fertilisation. Indeed, one month after pollination we could observe about 11.72% swollen fruits. Of these, only 4.66% fruits remained 4 months after pollination, however, indicating the existence of post-zygotic barriers. Tetrazolium staining and flowcytometry showed a viable embryo in the analysed fruits 4 months after pollination. Application of 1 mg/ml 2,4-D prior to pollination showed a positive effect on the fruit set in crosses between V. tinus and V. × bodnantense. By tetrazolium staining and flowcytometry viable embryos were observed in V. tinus fruits, but not in V. × bodnantense fruits. Since knowledge about the type of incompatibilities is the first step in circumventing interspecific crossing barriers, our results fit into an integrated approach required for success in future breeding programs within Viburnum.