Hordeum marinum (sea barley) is a complex species that is usually recognized as having two subspecies, marinum (2n = 2x = 14) and gussoneanum (2n = 2x =14 and 2n = 2x = 28). Carmona et al. use comparative physical mapping of repetitive DNA to determine the genome structure of several H. marinum accessions representing all taxa and cytotypes. They detect two subgenomes in the tetraploids, which appear to have come about through a cross between a diploid gussoneanum progenitor and a second, related – but unidentified – diploid ancestor. They conclude that the allopolyploids are segmental in nature with two closely related homeologous genomes, and merit taxonomic recognition as separate species.