Genomic DNA base composition (GC content) is predicted to significantly affect genome functioning and species ecology. Although several hypotheses have been put forward to address the biological impact of GC content variation in microbial and vertebrate organisms, the biological significance of GC content diversity in plants remains unclear because of a lack of sufficiently robust genomic data. Using flow cytometry, we report genomic GC contents for 239 species representing 70 of 78 monocot families and compare them with genomic characters, a suite of life history traits and climatic niche data using phylogeny-based statistics. GC content of monocots varied between 33.6% and 48.9%, with several groups exceeding the GC content known for any other vascular plant group, highlighting their unusual genome architecture and organization. GC content showed a quadratic relationship with genome size, with the decreases in GC content in larger genomes possibly being a consequence of the higher biochemical costs of GC base synthesis. Dramatic decreases in GC content were observed in species with holocentric chromosomes, whereas increased GC content was documented in species able to grow in seasonally cold and/or dry climates, possibly indicating an advantage of GC-rich DNA during cell freezing and desiccation. We also show that genomic adaptations associated with changing GC content might have played a significant role in the evolution of the Earth's contemporary biota, such as the rise of grass-dominated biomes during the mid-Tertiary. One of the major selective advantages of GC-rich DNA is hypothesized to be facilitating more complex gene regulation.plant genome | genome size evolution | Poaceae | phylogenetic regression | geographical stratification D eep insights into the genomic architecture of model plants are rapidly accumulating, especially because of advances being made in high-throughput next generation and third generation sequencing techniques (1). However, the genomic constitution of the vast majority of nonmodel plants still remains unknown (2), impeding our understanding of the relationship between particular genomic architectures and evolutionary fitness in various environments. One of the important qualitative aspects of genomic architecture is the genomic nucleotide composition, which is usually expressed as the proportion of guanine and cytosine bases in the DNA molecule (GC content). In prokaryotes, the GC content is a well-studied and widely used character in taxonomy (3), and numerous studies have shown both the impact of GC content on microbial ecology and the influence of the environment in shaping the DNA base composition of microbial communities (4-7). The DNA base composition is also frequently discussed in relation to the evolution of the isochore structure in humans and other homeothermic (warm-blooded) vertebrates (i.e., birds and mammals) (8-10). In contrast, considerably less attention has been paid to the biological relevance of genomic GC content variation in plants (11), with genomic GC co...

Genome sizes and genomic guanine+cytosine (GC) contents of the Czech vascular flora with new estimates for 1700 species.-Preslia 91: 117-142. The content of DNA in the somatic, unreplicated cell nuclei (genome size) and DNA base composition (GC content) are the basic genomic parameters that can be measured by flow cytometry. Genome size, or ploidy level, can affect many plant properties and are therefore included as important features in modern biological floras and plant trait databases. However, genomic data are still largely measured mainly for taxonomic and biosystematic purposes, and despite the popularity of flow cytometry in the Czech Republic, this information is still missing for many of the vascular plants in the Czech flora. The biological significance of the GC content is less clear compared to the genome size, which, along with the greater difficulty in measuring it, results in the absence of such information for the vast majority of species. Here, we measure these two genomic parameters for 1908 samples of 1700 species in the Czech vascular flora. Here for the first time are reported the genome sizes of more than 1000 species and GC contents of more than 1500 species, which more than doubles the amount of information on the GC content of vascular plants. Together with the published data obtained in our laboratory using the same methods and flow cytometers, this information is now available for 1910 species that occur in the Czech Republic (~83% of this country's permanent flora, excluding apomicts). They are summarized in a table, accompanied by information about assumed chromosome number, ploidy level and an estimate of the monoploid genome size. We further provide a descriptive analysis of this dataset, list extreme values and comment on some cytogeographically interesting findings. This dataset is the largest and most comprehensive set of genomic data covering almost the entire flora of a country. It will serve as the basis of the karyological traits section of the Czech plant trait database Pladias (www.pladias.cz) and for testing of hypotheses about the evolution and biological relevance of these genomic parameters.

The capacity to generate variation in ploidy and reproductive mode was compared in facultatively apomictic versus sexual maternal plants that coexist in two model populations. The population structure was studied in polyploid hybrid swarms comprised of Hieracium pilosella (usually sexual, less commonly apomictic), H. bauhini (apomictic), and their hybrids (sexual, apomictic, or sterile). Relationships among established biotypes were proposed on the basis of their DNA ploidy level/ chromosome number, reproductive mode and morphology. Isozyme phenotypes and chloroplast DNA haplotypes were assayed in the population that was richer in hybrids. The reproductive origin of seed progeny was identified in both sexual and apomictic mothers, using alternative methods: the karyological, morphological and reproductive characters of the cultivated progeny were compared with those of respective mothers, or flow cytometric seed screening was used. In both populations, the progeny of sexual mothers mainly retained a rather narrow range of ploidy level/ chromosome number, while the progeny of facultatively apomictic mothers was more variable. The high-polyploid hybrids, which had arisen from the fertilization of unreduced egg cells of apomicts, mainly produced aberrant non-maternal progeny (either sexually and/or via haploid parthenogenesis). Apparently, such versatile reproduction resulted in genomic instability of the recently formed high-polyploid Folia Geobot (2009) 44:281-306 hybrids. While the progeny produced by both true apomictic and sexual mothers mostly maintained the maternal reproductive mode, the progeny of those 'versatile' mothers was mainly sexual. Herein, we argue that polyploid facultative apomicts can considerably increase population diversity.

Analysis of double-peaks showed that genome size varied up to 1.188-fold within all 562 seedlings, 1.119-fold within the progeny of a single maternal plant and 1.117-fold in seedlings from grains of a single inflorescence. Generally, genome sizes of seedlings and their mothers were highly correlated. However, in maternal plants with both larger and smaller genomes, genome sizes of seedlings were shifted towards the population median. This was probably due to the frequency of available paternal genomes (pollen grains) in the population. There was a stabilizing selection on genome size during the development of seedlings into adults, which may be important for stabilizing genome size within species. Furthermore, a positive correlation was found between genome size and the development rate of seedlings. A larger genome may therefore provide a competitive advantage, perhaps explaining the higher proportion of plants with larger genomes in the population studied. The reason for the observed variation may be the recent induction of genome size variation, e.g. by activity of retrotransposons, which may be preserved in the long term by the segregation of homeologous chromosomes of different sizes during gametogenesis.

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