Genome sizes in a breeders’ collection of azalea (Rhododendron spp.)

Nuclear DNA amounts or genome sizes mark biodiversity in plants. Within a genus, differences in genome size can indicate variation in chromosome numbers, ploidy levels and/or chromosome sizes. From a breeders’ point of view, genome size differences influence cross compatibility especially in interspecific hybridisation.
In this study the nuclear DNA-content was estimated for different Rhododendron species and cultivars. The Institute for Agricultural and Fisheries Research (ILVO) hosts the largest in vivo germplasm collection for pot azaleas in the world. Besides 350 accessions of Rhododendron simsii, also around 250 related Rhododendron species and varieties are present. This collection is used as a genepool for a pot azalea breeding program. Pot azalea is the most important ornamental plant grown in Belgium.
Flow cytometric genome size estimation based on propidium iodide DNA-staining was used with soybean (Glycine max ‘Polanka’ 1C = 1,25 pg) as internal standard. In the commercial Rhododendron simsii cultivars genetic diversity related to differences in genome sizes was small. Almost all commercial azaleas are diploid (2n = 26). The genome size of most diploid azalea genotypes in our study was approximately 0.56 pg/1C. In other Rhododendron spp. more variability in ploidy numbers is present according to literature data based on chromosome studies. Rododendron spp. include diploids, triploids (2n = 39), tetraploids (2n = 52), hexaploids (2n = 78), octaploids (2n = 104) and a dodecaploid (2n = 156). Also for genome sizes we found more variation when other Rhododendron species in the collection were analysed. Variations in genome size don’t seem to correlate to the known chromosome numbers for these species. Based on the genome sizes a selection was made of genotypes for chromosome counts and further analysis. Results on the variation in genome size and ploidy levels in the genus Rhododendron will be discussed.