Extensive ribosomal DNA (18S-5.8S-26S and 5S) colocalization in the North American endemic sagebrushes (subgenus Tridentatae, Artemisia, Asteraceae) revealed by FISH

Chromomycin A(3) banding and fluorescent in situ hybridization (FISH) have been performed for six Artemisia species with special emphasis on subgenus Tridentatae. Morphometrical data on karyotype characters were calculated and idiograms with the position of GC-rich regions and 18S-5.8S-26S and 5S sites of ribosomal DNA were constructed. These sites were all colocalized. To our knowledge, this is the first time in the large family Asteraceae, indeed in angiosperms in general, that colocalization of the two rDNA regions studied is found at every single marked locus. In addition, transcriptionally active nucleolar organizer regions were detected after silver nitrate staining. Tridentatae is a cytogenetically homogeneous subgenus, which suggests that evolution of these species has not been coupled with important karyotypic reorganization. However, a few species are taxonomically difficult and show substantial differences. A loss of rDNA loci has been detected in a tetraploid taxon with respect to the diploids studied. These data provide clarifying insight into interspecific relationships between the studied taxa and overall evolutionary and systematic relationships of the Tridentatae.