Distribution and chromosomal organization of 18S-5.8S-25S and 5S rDNA in Petunia species

We have analyzed the interspecific variation of the 5S rDNA spacer by the polymerase chain reaction in seven Petunia species. The species studied could be separated into two groups with regard to the size of the amplified 5S rDNA variants: one group, with a 460-bp repeat unit, including P linearis (2n = 18) and P integrifolia (2n = 14) and the second group, with a 350-bp repeat unit, including all the other wild species (2n = 14) studied and the P hybrida lines. The amplified fragments have been cloned, and used in FISH experiments to determine the number and the location of the 5S rDNA units. The chromosomal organization of the 5S rDNA enabled us to distinguish a group of coloured flowered species with one locus adjacent to the major 18S-5.8S-25S rDNA locus on chromosome II, and a group of white flowered species with an additional locus in the centromeric region of a metacentric chromosome (IV/VII). FISH analysis also revealed four hybridization sites of 18S-5.8S-25S rDNA in the majority of the Petunia species studied. Only P linearis and P parviflora (2n = 18) showed two hybridization sites. The four sites of 18S-5.8S-25S rDNA are transcriptionally active as shown by their expression pattern in interphase nuclei. Our FISH results combined with PCR amplification and RFLP studies of the rDNA clusters give a new insight into the phylogenetic relationships between wild species and the P hybrida lines.