Genome size analysis: Application to plant systematics

Research project P 14607 Genome size analysis: Application to plant systematics Johann GREILHUBER 09.10.2000 The vast majority of the genetic information of eukaryotes resides in the DNA of the cell nuclei, but there is in addition a comparatively huge amount of non-informative DNA, causing 800-fold genome size variation in plants, that nevertheless has important consequences on the lifestyle of the organism via its effect on cell size and mitotic and meiotic cell cycle time. A significant part of the non-coding DNA is contributed by intergene retrotransposons and other potentially mobile dispersed DNA elements. Closely related morphologically indistinguishable races or cryptic species can diverge widely (e.g., 60% plus within Scilla autumnalis), while in other cases there is world- wide intraspecific constancy (e.g. Pisurn. sativurn). The importance of such studies for nature conservation and the fight against genetic erosion is obvious. Many authors assume that there is adaptively caused variation within species, but there is no general agreement on that matter. In the present project we want to apply the methodologies of genome size measurement with flow cytometry and DNA image analysis to questions of plant systematics. Model species or higher taxa are selected to investigate specific problems relevant for the interpretation and taxonomic evaluation of genome size differences. Alpine endemic species with glaciation-caused N-S disjunctions that have lead already to subspecific morphological differentiation will be compared for genome size - has there any divergence occurred? In several plant groups as Centaurea, Hieracium, Scorzonerinae, Lupinus, Ornithogalum, and Scilla genome size will be used -as a criterion for taxonomic differentiation, and correlations with ecological characteristics will be sought. Grass species with wide ecological amplitude will be tested to obtain evidence for or against an adaptive role of intraspecific genome size variation. Another novel approach will be the analysis of higher taxa with known molecular phylogeny, the Hyacinthaceae, the genera Veronica and Lupinus, and major groups therein in detail. We will see here the direction in which genome size evolved and then can correlate these with other adaptive trends.

Most of the genetic information of eukaryotes resides in the DNA of the cell nucleus, but in addition there is a comparatively huge amount of noninformative DNA causing genome size variation in the range of three orders of magnitude. In the present project, 12 regular papers have been published hitherto and 23 congress presentations have been made to show, using flow cytometry and DNA image densitometry, the value of these methods for clarifying taxonomic questions, and to investigate the selective value - if any - of genome size dependent on climatological parameters, modes of reproduction and resistance against stress. For the first time, plants exhibiting high mutional rates were discoverd, whose genomes (C-values) are significantly smaller than in Arabidopsis thaliana. Angiosperms now vary 1800-fold instead of 800-fold before. This record established is important for understanding mechanisms of "genome downsizing". A bilateral study on the effect of heavy metal pollution on the genome size spectrum of the vegetation showed, that along a gradient of increasing pollution, caused by a lead smelter in Slovenia, species with larger genomes are progressively erased. This lends confirmation to the "large genome exclusion hypothesis", which indicates that plants with large genomes are at a selective disadvantage in extreme environments (supposedly due to longer cell cycles with large genomes). A comprehensive study in Veronica showed, that small genomes are even more strongly correlated with selfing breeding mechanism than with annual life form. This is remarkable, because in a study on Malvaceae the expected strong correlation with annuality emerged. The long-held theory of the "plastic genome" is again refuted, this time in the grass Dasypyrum villosum. A number of studies on polyploidy and hybridity in mosses and angiosperms (including apomicts) additionally testifyes the value of genome size measurements as a method in biosystematics.