Progress in phylogenetic analysis and a new infrageneric classification of the barley genus Hordeum (Poaceae: Triticeae)

During the last decade several phylogenetic studies of Hordeum were published using a multitude of loci from the chloroplast and nuclear genomes. In many studies taxon sampling was not representative and, thus, does not allow the inference of relationships among species. Generally, chloroplast data seem not suitable for reliable phylogenetic results, as far reaching incomplete lineage sorting result in nearly arbitrary species relationships within narrow species groups, depending on the individuals included in the analyses. Nuclear loci initially resulted at contradictory phylogenetic hypotheses. However, combining at least three nuclear loci in total evidence analyses finally provides consistent relationships among Hordeum species groups, and supports data from earlier cytological and karyological studies. Thus, recently published phylogenies agree on the monophyly of the four Hordeum genome groups (H, I, Xa, Xu), monophyly of the H/Xu and I/Xa groups, and separation between Asian and American members within the I-genome group. A new infrageneric classification of Hordeum is proposed, dividing the genus in subgenus Hordeum comprising sections Hordeum and Trichostachys, and subgenus Hordeastrum with sections Marina, Nodosa, and Stenostachys. The latter consists of two series reflecting the geographical distribution of the taxa, i.e. series Sibirica with Central Asian taxa and series Critesion comprising native taxa from the Americas. In section Nodosa all allopolyploid taxa are grouped, which are characterized by I/Xa genome combinations.