Genome size is an important feature of all living organisms. It is expressed either in DNA picograms (=10-9 g) or in megabase pairs (1 pg = 978 Mbp). Two terms should be distinguished (Greilhuber et al. 2005): genome size at holoploid level (referred to as C-value) and genome size at monoploid level (referred to as Cx-value). Although genome size varies over a broad range in angiosperms (exceeding 1000-fold difference - Bennett & Leitch 2004), its intraspecific variation is usually limited and species-specific values could have been delimited in several taxa or sections (Suda 2004). Inspecting the data in the phylogenetic context, they provide valuable information in modern molecular research. Studies on hybrids between closely related species offer further utilization of genome size values. If the difference between the parental genome sizes is sufficient, the hybrid plants, arisen from homoploid crosses, can be identified by flow cytometry.
Hieracium research benefits from both applications. The first study of this type (Bršutigam & Bršutigam 1996) was focused on genome size in several species of Hieracium subgen. Pilosella co-occurring at one German locality. It revealed differences among species, especially the lower DNA content in H. pilosella compared with other species at the same ploidy level. Our ongoing research extended the area of interest to the Central European region, as well as the number of species and cytotypes examined in this respect (Suda et al. 2003: poster presentation, Suda & KrahulcovŠ 2003-2004: project). The species H. pilosella, a putative parent of many wild hybrids including stabilized hybridogenous species, is characterized by distinctly lower genome size. This enables the flow-cytometric detection of such hybrid products, often close in morphology to one of the parents due to continuous variation. Moreover, the composition of the nuclear genome (i.e., the proportion of "H. pilosella genome") in some hybridogenous polyploid types can be estimated in this way.