Allopolyploidy and the complex phylogenetic relationships within the Hordeum brachyantherum taxon

• Wide divergence exists between the chromosomes of subsp. californicum and H. roshevitzii.
• Chromosomes of tetraploids clearly differ from the sum of those present in the diploids.
• Subsp. gussoneanum, is one of the progenitors of subsp. brachyantherum 6×.
• Subsp. brachyantherum 4×, is unlikely the other progenitor of the 6× form.

Hordeum brachyantherum Nevski includes two subspecies: the diploid (2×) subsp. californicum, and subsp. brachyantherum, which itself includes a tetraploid (4×) and a hexaploid (6×) cytotype. The phylogenetic relationships between these taxa and the origin of the polyploids remain controversial. To provide additional information to the many molecular phylogenetic analyses conducted within Hordeum, FISH-based karyotypes were produced for all subspecies/cytotypes within H. brachyantherum. Chromosomes of H. roshevitzii and H. marinum subsp. gussoneanum were also analysed since these species are potentially involved in the origin of the polyploids. For karyotyping, ten repetitive DNA sequences were screened to indentify repeats showing sufficient diversity in terms of copy number and localisation that they might serve as physical markers for distinguishing between each mitotic chromosome pair in all accessions. Genomic in situ hybridisation (GISH) was used to distinguish between subgenomes in polyploids. The karyotype maps allowed the assessment of the chromosomal diversity within species/cytotypes and the identification of possibly homoeologous chromosomes. The results show a wide divergence between the chromosomes of subsp. californicum and H. roshevitzii, and with their supposed derivatives in subsp. brachyantherum 4×. One of the three subgenomes of subsp. brachyantherum 6× is derived from subsp. gussoneanum with no genomic reorganisation (i.e., neither amplification nor loss of the repetitive DNA sequences analysed). It is generally accepted that subsp. brachyantherum 4× is the other progenitor of subsp. brachyantherum 6×, but the present results suggest this to be unlikely. The present findings thus show the cytogenetic diversity and genomic structure of H. brachyantherum, and reveal its complex evolutionary history, in which chromosomal diversification and allopolyploidy have played important roles.

Schema representing the phylogenetic relationships within the Hordeum brachyantherum taxon as inferred from karyotyping. The diploid species (2×) are on the left, tetra- (4×) and hexaploid (6×) taxa are to the right. Dashed lines represent ambiguous parental affiliations. The metaphase cell from 6× subsp. brachyantherum shows the results of ND-FISH, with (AG)10 and (AAG)5 probes, and GISH, using total genomic DNA (gDNA) from subsp. gussoneanum as probe (see colour key). Arrows indicate the chromosomes derived from subsp. gussoneanum. Asterisks indicate a marker chromosome pair also presents in H. roshevitzii. These results indicate that subsp. gussoneanum was one of the progenitors of 6× subsp. brachyantherum.Figure optionsDownload as PowerPoint slide