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 Horde um species with the I genome were examined by double-target in situ hybridi zation 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 America n species studied, eight different rDNA types were found. The type reported previously in H. chilense (the 'chilense' type) was observed in eight Amer ican species. The chilense type had double 5S rDNA sites-two sites on one c hromosome 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 fou nd in American species were similar to the chilense type and could be deriv ed 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 t he close relationships between North and South American species and their d erivation from a single ancestral source. The differences in the distributi on patterns of 5S and 18S-25S rDNA between Asian and American species sugge st differentiation between the I genomes of Asian and American species. The 5S and 18S-25S rDNA sites are useful chromosome markers for delimiting Asi an species, but have limited value as a taxonomic character in American spe cies. On the basis of rDNA patterns. karyotype evolution and phylogeny of t he I-genome diploid species are discussed.