Meiosis in interspecific Hordeum hybrids

36 hybrid families representing 29 interspecific and one intraspecific combinations between tetra- and hexa-ploid Hordeum species were analyzed for meiotic M I pairing. H. bulbosum, H. murinum, and H. marinum have quite separate genomes not related to the other species in the genus. H. bulbosum (with the I genome) and tetra- and hexaploid H. brevisubulatum (with the H genome) are autoploids. H. murinum (with the Y genome) and H. marinum (X genome) may also have had an autoploid origin, but have developed strong bivalent-forming genetic mechanisms. The pairing in the pentaploid hybrids including the five tetraploid species H. jubatum, H. brachyantherum, H. roshevitzii, H. tetraploidum, and H. fuegianum fitted throughout best with a 2:2:1 genomic model. They are segmental alloploids with the basic H genome together with a modification of it. The same modifications of the H genome are found in five hexaploid North and South American species. The pairing in hybrids including H. secalinum and H. capense fitted best with a 2:1:1:1 genomic model. Together with H. depressum they are alloploids with two unrelated genomes, one of which being the H genome. The other genome has not been detected among the hexaploid species. The interpretation of meiotic data in pentaploid hybrids is difficult, mainly because of the action of pairing-regulating genes and autosyndesis.