Karyotype analysis and physical mapping of 45S rRNA genes in Hydrangea species by fluorescence in situ hybridization

Detailed karyotypes of Hydrangea macrophylla, H. paniculata and H. quercifolia were constructed on the basis of arm lengths and centromeric index, together with fluorescence in situ hybridisation (FISH) with the 45S rDNA sequence as probe. Although the chromosomes were very small, they were well distinguishable for all 3 species. Chromosome morphology and karyotypes were different for the 3 species. H. macrophylla had 6 metacentric (M), 8 submetacentric (SM) and 4 subtelocentric (ST) chromosomes. The karyotype of H. paniculata contained 7M, 10 SM and 1 ST chromosomes and for H. quercifolia 6M, 10 SM and 2 ST chromosomes were identified. The variability between the 3 Hydrangea species also was expressed by the 45S rDNA signals. H. macrophylla had a nucleolar organisation region (NOR) on chromosome 2, H. paniculata had 45S rDNA signals on chromosomes 2, 5 and 11 and on H. quercifolia a NOR was observed on chromosomes 3 and 8. Hybridisation signal always was distally on the short arm but the strength of the signals was different for the 3 species. The chromosome portraits made in this study will be used to trace chromosome behaviour in interspecific hybrids resulting from breeding work between H. macrophylla, H. paniculata and H. quercifolia.