Interspecific hybridisation in woody ornamentals

Interspecific hybridisation and polyploidisation offer great opportunities to increase variation within existing cultivars and species. Molecular and cytogenetic tools are available to determine the hybrid status of obtained seedlings. However, these modern breeding techniques are rarely used for woody ornamentals. In this study the possibilities of interspecific hybridisation and ploidy breeding for woody ornamentals are investigated. Also the optimal strategies to analyse the hybrid nature of seedlings are determined. Application of morphological, molecular (AFLP) and cytogenetic (GISH, flow cytometry) techniques is evaluated for the case-study genera Hydrangea, Hibiscus and Buddleja. Parental species within these 3 genera were (cyto)genetically very different. This was confirmed by AFLP, flowcytometry and karyotyping. This information gave indications about the crossability of different species within the 3 genera. When performing interspecific crosses there were no prezygotic barriers observed for the 3 genera. However, interspecific hybridisation was hampered due to postzygotic barriers, such as fruit abortion, albinism and growth aberrations in the seedlings. Therefore, in vitro embryo rescue was used for Hydrangea, Hibiscus and Buddleja. Within the 3 genera different hybrid populations were generated. Morphological studies, AFLP and genome size measurements confirmed the hybrid nature of the seedlings. Genomic in situ hybridisation (GISH), is rarely used in woody ornamentals for the identification of the origin of chromosomes in hybrids resulting from breeding programs. In this study, a GISH protocol was optimised for Buddleja and Hibiscus, characterised by small genomes and chromosomes. GISH was successfully used for B. x weyeriana, F1 and F2 hybrids of B. davidii x B. x weyeriana and for hybrids of H. syriacus x H. paramutabilis. The parental genomes could be distinguished and recombinant chromosomes were detected. Polyploidisation by application of mitotic inhibitors on seed(ling)s, was obtained for diploid B. globosa. By using a mitotic polyploidisation protocol a new hexaploid H. syriacus cultivar ‘DVPazurri’ was created and commercialised. Also the application of unreduced gametes offers opportunities to create new genetic variation in a breeding program. In F1 and F2 hybrids of H. syriacus x H. paramutabilis and in B. globosa and B. lindleyana unreduced gametes were observed. This study demonstrated that the creation of interspecific hybrids, along with the chromosome doubling technology offers opportunities for woody ornamental breeding programs. A good understanding of the phenomena associated with allopolyploids and interspecific hybrids lead to important innovations in the commercial assortment of many ornamental genera. In the future these will allow a more efficient and straightforward breeding and selection