The genus Hordeum L. occurs with about 32 species in temperate climates mostly in Eurasia and the Americas. Four basic genomes were described within Hordeum: 'I' in barley and the closely related H. bulbosum, 'Y' in H. murinum, 'X' in H. marinum, and 'H' in all other species of the genus. The latter group contains several (tetra- and hexa-) polyploids. To reveal species history the nrDNA ITS region of all Hordeum species was PCR amplified, cloned, and several clones of each species were sequenced. The analysis of the resulting sequences allowed in many cases the reconstruction of the parental ancestors (or ancestor groups) of allopolyploids and a distinction between putative auto- and allopolyploid species. Phylogenetic analysis clusters 'I' and 'Y' genomes and 'X' and 'H' genomes, respectively, and reveals a SW Asian origin of the genus. A relatively fast radiation occurred in C Asia and S America possibly due to the evolution of salt tolerance and/or small seeds in a basal taxon of the X/H subgroup of the genus.

Key words: Hordeum, hybridization, internal transcribed spacer, phylogenetic analysis