Cyto-evolution of Boronia genomes revealed by fluorescent in situ hybridization with rDNA probes

The physical location of the 25S-26S rDNA sequences was examined in 11 taxa of nine species of Boronia. In diploid species, two rDNA sites were detected in Boronia clavata (2n = 14), Boronia pinnata 'White' (2n = 22), and Boronia chartacea (2n = 32); four in Boronia megastigma (2n = 14) and Boronia denticulata (2n = 18); six in Boronia pinnata 'Pink' (2n = 22); and eight in Boronia molloyae (2n = 16). Eleven sites were found in Boronia heterophylla 'Red' and 'Near White' (2n = 15), but only two active nucleolar organizer regions (NORs) were observed. In polyploid species, Boronia pilosa (2n = 44) had four rDNA sites, while Boronia coerulescens (2n = 72) had six. Most of the rDNA sequences were terminal, but a few were interstitial. There were also differences in signal intensity indicating that the gene copies between and within rDNA sites might be different. The result suggests that considerable chromosome rearrangements have occurred during Boronia cyto-evolution, leading to variation among Boronia taxa in rDNA copy number, site number, and location. These changes together with dysploid reduction during cyto-evolution have made the Boronia genome considerably diverse in chromosome number, genome organization, and chromosome structure.