Folia Geobotanica 40: 367–384, 2005 GENOME SIZE DISCRIMINATES BETWEEN CLOSELY RELATED TAXA ELYTRIGIA REPENS AND E. INTERMEDIA (POACEAE: TRITICEAE ) AND THEIR HYBRID Václav Mahelka 1) , Jan Suda 1,2) , Vlasta Jarolímová 1) , Pavel Trávníèek 1,2) & František Krahulec 1) 1) Institute of Botany, Academy of Sciences of the Czech Republic, Prùhonice 1, CZ-252 43, Czech Republic, fax +420 2 6775 0031, e-mail mahelka@ibot.cas.cz 2) Department of Botany, Charles University, Benátská 2, CZ-128 01, Prague, Czech Republic Abstract: Flow cytometric and karyological investigations were performed on the closely related taxa Elytrigia repens and E. intermedia (Poaceae: Triticeae) from the Czech Republic. DNA-hexaploids clearly prevailed among 238 examined plants and amounted to 96.2% of all samples. 2C-values ± s.d. for hexaploid Elytrigia repens and E. intermedia were estimated at 23.27 ± 0.20 pg and 27.04 ± 0.24 pg respectively. Genome size thus allowed reliable separation of the two species (difference ca. 16%) as well as the identification of hybrid individuals. Natural hybridization in E. repens – E. intermedia alliance seems to be quite a common phenomenon as indicated from a large proportion (one sixth) of hexaploid samples with intermediate 2C-values. Previously, the crosses were most probably overlooked or misidentified due to their weak morphological differentiation. New nonaploid cytotypes (2n=9x=63) were revealed for both species as well as for the hybrid (determined on the basis of morphological characters only), representing the first records from the field. Fusion of unreduced and reduced gametes of the hexaploids is the most plausible mode of nonaploid origin. Keywords: C-value, Chromosome number, Cytotype, DNA content, Flow cytometry, Hybridization, Nonaploid, Wheatgrass Nomenclature: LÖVE 1984 INTRODUCTION Elytrigia repens (L.) NEVSKI [Syn.: Agropyron repens (L.) P. BEAUV., Elymus repens (L.) GOULD] and Elytrigia intermedia (HOST)NEVSKI [Syn.: Agropyron intermedium (HOST) P. BEAUV., Thinopyrum intermedium (HOST) BARKWORTH et D.R. DEWEY] are representatives of the family Poaceae, tribe Triticeae (wheatgrasses). Triticeae is a large group comprising approximately 500 taxa divided into 37 genera (LÖVE 1984). The relationships among, and the taxonomy and phylogeny of members of Triticeae have triggered a long-term dispute and are still in need of further targeted investigation. Among others, the complexity of the situation is caused by frequent allopolyploid origin of the species that partly share the same genomes (up to dodecaploid plants are known, each combining up to four more-or-less different genomes). Reticulate evolution manifesting itself in the majority of characters is another source of problems. The complexity of the group can easily be distinguished from the taxonomy of the two taxa studied: 137 and 132 synonyms were found for E. intermedia and E. repens, respectively (CLAYTON &WILLIAMSON 2003). Both