An area-resolved phylogeography of bulbous barley (Hordeum bulbosum; Poaceae)

Hordeum bulbosum, the closest relative of barley (H. vulgare), is an important source of resistance genes for cereals in the Triticeae. This perennial and mainly outcrossing species occurs with two cytotypes: diploids thrive in the western and central parts of the Mediterranean, while autotetraploids extend from Greece mainly eastwards to western Asia. To elucidate H. bulbosums origin, colonization patterns, population relationships, and distinctions between the two cytotypes, we determined ploidy and performed genotyping-by-sequencing analyses on 314 individuals from across the species distribution range. Our results revealed two distinct lineages within diploid H. bulbosum: individuals from Libya vs. all other diploids. Southeastern Italian populations were the origin for the species expansion eastward into Albania/Greece and westward into mainland Italy, Sicily/Sardinia, Tunisia, and Spain. The tetraploid cytotype originated early in the evolution of the species, thus retaining alleles found in extant Libyan diploids. Tetraploids underwent local introgression from diploids in Greece, where also a secondary origin of tetraploids was detected. We found that ecoclimatic conditions alone cannot account for the clear geographic separation of the cytotypes, as habitats across much of the central Mediterranean would be suitable for both. We conclude that minority cytotype exclusion is the most plausible explanation for the distinct distribution patterns observed. While both cytotypes are morphologically indistinguishable, diploid populations typically occur in scattered stands within their range, while tetraploids tend to form larger, often contiguous populations.