Comparative physical mapping of the 5S and 18S-25S rDNA in nine wild Hordeum species and cytotypes

The physical locations of the 5S and 18S-25S rDNA sequences were examined i n nine wild Hordeum species and cytotypes by double-target in situ hybridiz ation using digoxigenin-labelled 5S rDNA and biotin-labelled 18S-25S rDNA a s probes. H. vulgare ssp. spontaneum (2n = 2x = 14; I-genome) had a similar composition of 5S and 18S-25S rDNA to cultivated barley (H. vulgare ssp. v ulgare, I-genome), with two major 18S-25S rDNA sites and minor sites on fou r of the other five chromosomes; three chromosomes had 5S rDNA sites. The c losely related H. bulbosum (2x; also I-genome) showed only one pair of 5S r DNA sites and one pair of 18S-25S rDNA sites on different chromosomes. Four wild diploid species, H. marinum (X-genome), H. glaucum and H. murinum (Y- genomes) and H. chilense (H-genome), differed in the number (2-3 pairs), lo cation, and relative order of 5S and the one or two major 18S-25S rDNA site s, but no minor 18S-25S rDNA sites were observed. H. murinum 4x had three c hromosome pairs carrying 5S rDNA, while the diploid had only a single pair. Two other tetraploid species, H. brachyantherum 4x and H. brevisubulatum 4 x (both considered to have H-type genomes), had minor 18S-25S rDNA sites, a s well as the major sites. Unusual double 5S rDNA sites - two sites on one chromosome arm separated by a short distance - were found in the American H -genome species, H. chilense and H. brachyantherum 4x. The results indicate that the species H. brachyantherum 4x and H. brevisubulatum 4x have a comp lex evolutionary history, probably involving the multiplication of minor rD NA sites (as in H. vulgare sensu late), or the incorporation of both I and H types of genome. The rDNA markers are useful for an investigation of chro mosome evolution and phylogeny.