Phylogenetic relationships of the grass family inferred from the sequences of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA (rDNA) are generally in accord with the boundaries of the six commonly recognised subfamilies: Bambusoideae, Pooideae, Arundinoideae, Centothecoideae, Chloridoideae and Panicoideae. ITS phylogeny recognises the herbaceous bamboo allies, Streptochaeta and Pharus, as the basal taxa to all grasses. The core grass lineage is resolved into three major clades: the basal Bambusoideae is sister to the monophyletic Pooideae and the Panicoideae–Arundinoideae–Centothecoideae–Chloridoideae (PACC) clade. Several genera with uncertain taxonomic affinities, Lygeum, Nardus, Brachyelytrum, Diarrhena, Anisopogon, Ampelodesmos, and the tribe Stipeae, are all clustered with a broadly defined Pooideae, and may be the ‘missing links’ between the Pooideae and the Bambusoideae. Relationships of the PACC clade indicate that C4 photosynthesis evolved independently among and within the PACC subfamilies. ITS phylogeny of the grass family, and evidence from the chloroplast genome, cytogenetics, fossil records, biogeography, and plate tectonic theory, suggest that the origin of the grasses is probably ‘out of South America’.

Phylogenetic relationships of the whole Poaceae family inferred from the sequence data of rDNA (ITS) support the hypothesis that the arundinoids are monophyletic, not polyphyletic as previously proposed. The ITS sequence phylogeny of 42 arundinoid species demonstrates an early divergence of the Aristideae and three major groupings, corresponding loosely to the tribes Aristideae, Arundineae and Danthonieae. The Arundineae are resolved into two paraphyletic clades. In one clade, Arundo, Monachather, and Dregeochloa are the sister group to Amphipogon and Diplopogon. In the other clade, Phragmites, Molinia, Gynerium, Thysanolaena, Spartochloa, and Cyperochloa are the sister group to Eriachne and Micraira. Arundo is not closely related to Phragmites. The Danthonieae clade, including taxa from Africa, Australia, New Guinea, New Zealand, the South American Cortaderia, and the North American Danthonia, appears to be monophyletic. However, the genus Merxmuellera is polyphyletic.

Hybrids of Elymus canadensis (2n = 28; SSHH) × Critesion californicum (2n = 14) and E. canadensis × C. bulbosum (2n = 14) were synthesized at relative frequencies of 11.8 and 0.3%, respectively, by the aid of embryo rescue techniques. A natural hybrid was identified as C. violaceum × C. bogdanii (2n = 14) by a combination of karytotype analysis and plant morphology. Gross spike morphology of the hybrids was intermediate to that of the parents. Meiotic chromosome pairings in these hybrids suggested that the genome of C. californicum and C. bogdanii is more or less homologous with one of the two genomes of E. canadensis. Genomes of C. violaceum and C. bulbosum appeared to be different from each other and from those in C. bogdanii and C. californicum. Based on the data in this study and others, the degree of genome differentiation among some Critesion and some Elymus species containing the H genome was examined and discussed. The symbol H is proposed for the genome in C. bogdanii and C. californicum, Hv for C. violaceum, Hb for C. bulbosum, and Hc for C. chilense.Key words: interspecific hybrids, synapsis, phylogeny, Hordeum, Elymus, Critesion.

Agropyron vaillantianum (Wulf. &Schreber) Trautv. and E. arizonicus (Scribn. &Smith) Gould were studied to describe their (i) reproductive characteristics, (ii) meiotic behavior, (iii) genomic constitution, and (iv) correct taxonomic alignment based on genomic relationships. Both species were found to be self-fertile tetraploids (2n = 28) and behaved as strict allotetraploids averaging 14.00 and 13.77 bivalents per cell, respectively. The hybrids A. vaillantianum × Pseudoroegneria spicata (Pursh) A. Love, 2n = 14, SS, A. vaillantianum × E. trachycaulus (Link) Gould ex Shinners, 2n = 28, SSHH, and E. arizonicus × E. canadensis L., 2n = 28, SSHH, averaged 6.21, 12.56, and 12.60 bivalents per cell, respectively. Chromosome pairing in this series of hybrids demonstrated that A. vaillantianum and E. arizonicus contain the S and H genomes, with each taxon having a slight modification resulting from evolutionary pressures under different ecological parameters. On the basis of chromosome pairing and mode of pollination it is proposed that A. vaillantianum be treated in the genus Elymus rather than in the genus Agropyron, with the following new name combination: Elymus vaillantianus (Wulf. &Schreb.) K. B. Jensen comb.nov., based on Triticum vaillantianum Wulfen &Schreber. Elymus typically encompasses those species that are self-fertile, and contain the SH genomes. Elymus arizonicus has been correctly classified.Key words: genome, meiosis, chromosome pairing, cytology, interspecific hybridization, Elymus, Agropyron, and Triticeae.