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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...

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