rDNA loci and heterochromatin positions support a distinct genome type for ‘ x = 9 species’ of section Arachis ( Arachis , Leguminosae)

Most species of the genus Arachis (Leguminosae; 80 spp.) are diploid with x = 10 and only four species have x = 9 chromosomes. Three of these x = 9 species belong to section Arachis and are morphologically and chromosomally similar. To study the homeology of the genomes of x = 9 species and their relation to other genomes in section Arachis , we applied fluorescence in situ hybridization (FISH) of 18S–26S and 5S rDNA and 4′,6-diamidino-2-phenylindole (DAPI) banding. FISH revealed for these three species one pair of 5S rDNA sites interstitially within the short arm of the metacentric pair 6 and one pair of 18S–26S rDNA sites in the proximal region of the long arm of the SAT chromosomes. Conspicuous DAPI+ bands were detected pericentromerically in all nine chromosome pairs of A. decora and A. praecox and in all but one pair of A. palustris . Our results suggest that all three species with x = 9 of section Arachis share the same genome type and are different from the other genome types A, B, D, F, and K described for this section. Apparently, the x = 9 species of section Arachis form a monophyletic group characterized by a genome type, that we propose to call G genome.