Interspecific hybridisation in woody ornamentals: how to deal with barriers?

For diverse reasons, interspecific hybridization was attempted in several woody ornamental genera (Rhododendron, Hydrangea, Hibiscus, Buddleia and Ligustrum), and yielded information on the occurrence and the overcoming of multiple crossing barriers. Prezygotic barriers (inhibition of pollen tube growth) were absent in all genera except Rhododendron. The first signals of postzygotic incongruity were the malformation of endosperm and the inhibition of germination. In order to replace the endosperm and allow maturation of the hybrid embryo and thus solve the lack of endosperm, in vitro media were established for all genera mentioned, mostly resulting in a significant increase of germination rates compared to in vivo germination. After germination, incompatibility between the plastome and nuclear genome may inhibit chloroplast development and chlorophyll formation. As a result, albinism is observed, mainly in interspecific Rhododendron and Hibiscus hybrids. Many hybrids are not viable, and growth aberrations caused by ‘hybrid’ vigor have been observed in Rhododendron and Hibiscus. Although flowering hybrids have so far only been obtained in the genera Buddleia and Hibiscus, preliminary research efforts with regard to fertility restoring through polyploidization were already performed in all genera. Proof was found for the occurrence of 2n gametes in Rhododendron, that might be a more recommendable tool for allopolyploidization than somatic chromosome doubling.