Physical locations of 5S and 18S‐25S rDNA in Asian and American diploid Hordeum species with the I genome

The physical locations of 5S and 18S‐25S rDNA sequences in 15 diploid Hordeum species with the I genome were examined by double‐target in situ hybridization with pTa71 (18S‐25S rDNA) and pTa794 (5S rDNA) clones as probes. All the three Asian species had a species‐specific rDNA pattern. In 12 American species studied, eight different rDNA types were found. The type reported previously in H. chilense (the ‘chilense’ type) was observed in eight American species. The chilense type had double 5S rDNA sites — two sites on one chromosome arm separated by a short distance — and two pairs of major 18S‐25S rDNA sites on two pairs of satellite chromosomes. The other seven types found in American species were similar to the chilense type and could be derived from the chilense type through deletion, reduction or addition of a rDNA site. Intraspecific polymorphisms were observed in three American species. The overall similarity in rDNA patterns among American species indicates the close relationships between North and South American species and their derivation from a single ancestral source. The differences in the distribution patterns of 5S and 18S‐25S rDNA between Asian and American species suggest differentiation between the I genomes of Asian and American species. The 5S and 18S‐25S rDNA sites are useful chromosome markers for delimiting Asian species, but have limited value as a taxonomic character in American species. On the basis of rDNA patterns, karyotype evolution and phylogeny of the I‐genome diploid species are discussed.