Do polyploids require proportionally less rDNA loci than their corresponding diploids? Examples from Artemisia subgenera Absinthium and Artemisia (Asteraceae, Anthemideae)

Fluorescent in situ hybridisation (FISH) of 5S and 18S-5.8S-26S ribosomal DNA was carried out in two species of the genus Artemisia, belonging to the subgenera Artemisia (A. medioxima) and Absinthium (A. lagocephala), each one showing both low and high ploidy levels (2x, 4x and 16x, and 2x and 6x, respectively). Both species have a base chromosome number of x = 9. Linkage of both rDNA genes has been observed confirming previous results. Diploid A. lagocephala (2n = 18) shows three rDNA loci, and the hexaploid six. Also in A. medioxima, the number of rDNA loci does not increase in the proportion given by the ploidy level, and a relative loss is found. In this species, the diploid population shows two rDNA loci, the tetraploid four, and the hexaidecaploid has around 20. The results evidence a relative loss of rDNA loci and heterochromatin, a phenomenon that is more pronounced at higher ploidy levels. Nevertheless, the DAPI banding pattern of A. lagocephala does not follow this trend, as it shows a spectacular increase of heterochromatic bands at the hexaploid level. These results are discussed in the light of possible chromosome restructuring and gene silencing mechanisms that take place during polyploidy, and more especially allopolyploid formation.This work was subsidized by DGICYT (Spanish Government; projects BOS2001-3041-C02-01, CGL2004-04563-C02-02/BOS, CGL2007-64839-C02-02/BOS and CGL2007-64839-C02-01/BOS). One of the authors (J. P.) has been awarded a predoctoral grant from the Spanish Ministry of Education and Science, and another one (O. H.) benefited from a MICINN (Spanish Ministry of Science and Innovation) postdoctoral grant.Peer reviewe