Changes In Genome Size Research Articles

Editorial [Hot Topic: Comparative Genomics and Genome Evolution (Guest Editors: Sabyasachi Das and Masayuki Hirano)

It has become a fundamental need to have genome sequence data due to the fact that the basic process of evolution is the change in DNA sequence and genome size. Recent advances in DNA sequencing technology have facilitated the availability of quite a large number of complete genome sequences from simplest prokaryotes to higher eukaryotes. The availability of whole genome sequence data at our fingertips can provide critical insight into how various genomic compositions have contributed to a contemporary understanding of molecular evolution. For this reason, comparative evolutionary genomics has become one of the most rapidly advancing disciplines in the biological sciences. The new generation of comparative genomics offers a powerful aid to studying evolutionary changes among organisms and identifying the genes that are conserved among species, and also the genes that give each organism its own specific characteristics. Informatics related to structural and functional genomics is potentially important in understanding the emergence of new phenotypic characters necessary for the adaptation of organisms. In addition, the evolutionary perspective of genes and genomes is also helpful in understanding disease susceptibility. A large number of scientists from all over the world are involved in determining the genomic underpinnings of morphological, physiological, and behavioral changes. A lot of research has been done to find selection signals on a genome-wide scale, and also focusing on specific gene families to learn more about evolutionary mechanisms. Therefore, it is necessary to compile various current aspects of genomic and evolutionary research at some point. This prompts us to devote a special issue we have named “Comparative Genomics and Genome Evolution”. The series of review articles in this issue of Current Genomics present current advances in multiple areas of comparative genomics and molecular evolutionary studies. This special issue is comprised of eight high quality articles which have been selected through a rigorous reviewing process. Several years ago, with the ground-breaking work of Max D. Cooper and colleagues, an adaptive immune system was discovered in a representative jawless vertebrate (lamprey). The first review article describes concordance and divergence in the immunogenetic architecture between two alternative adaptive immune systems of jawed and jawless vertebrates, and discusses in detail the evolution of the jawless vertebrate immune system on the basis of currently available lamprey genomic resources. In the second review paper, Nikolas Nikolaidis and colleagues discuss the current knowledge on the genomics and evolution of the immunoglobulin mulitigene family in vertebrates. To provide further coverage and discussion of the evolutionary dynamics of multigene families, we commissioned Yoshihito Niimura to review the evolutionary genomics of the olfactory receptor multigene family on a genome-wide scale. The fourth review article presents the interesting scenario of the evolution of mammalian sex chromosomes. Yoko Satta and colleagues reviewed evolutionary aspects related to genomic rearrangements and structures of the sex chromosomes. The next review article, by Arnab Gupta and Svetlana Lutsenko, focuses on the origin and evolution of copper transporting ATPases in eukaryotic organisms. In the sixth article, Mariko Kondo and Koji Akasaka highlight the phylogenetic relationships among echinoderms and the current status of echinoderm genome analysis. The seventh article is related to the evolution of microRNAs. In this article, Zhumur Ghosh and Bibekanand Mallick describe advances of genomics, evolution, and biogenesis of microRNAs. Finally, we conclude with an interesting review article by Chitra Dutta and Sandip Paul on microbial genome signature related to different lifestyles. The authors describe how closely related microbial species are diverged on the basis of the genomic signature of ecological kinship throughout microbial evolution. Recognizing the growing interest in evolutionary genomics, we are happy to present this collection of high quality review articles for this special issue. We would like to thank all of the reviewers for their valuable comments meant to improve the quality of articles. In addition, we offer special thanks to the Editor-in-Chief, and the Current Genomics editorial/production staff, who have contributed invaluably to this project. We hope you find the issue timely, scholarly, and interesting.

Megacycles of atmospheric carbon dioxide concentration correlate with fossil plant genome size

Tectonic processes drive megacycles of atmospheric carbon dioxide (CO(2)) concentration, c(a), that force large fluctuations in global climate. With a period of several hundred million years, these megacycles have been linked to the evolution of vascular plants, but adaptation at the subcellular scale has been difficult to determine because fossils typically do not preserve this information. Here we show, after accounting for evolutionary relatedness using phylogenetic comparative methods, that plant nuclear genome size (measured as the haploid DNA amount) and the size of stomatal guard cells are correlated across a broad taxonomic range of extant species. This phylogenetic regression was used to estimate the mean genome size of fossil plants from the size of fossil stomata. For the last 400 Myr, spanning almost the full evolutionary history of vascular plants, we found a significant correlation between fossil plant genome size and c(a), modelled independently using geochemical data. The correlation is consistent with selection for stomatal size and genome size by c(a) as plants adapted towards optimal leaf gas exchange under a changing CO(2) regime. Our findings point to the possibility that major episodes of change in c(a) throughout Earth history might have selected for changes in genome size, influencing plant diversification.

Causes and consequences of genome expansion in fungi.

Fungi display a large diversity in genome size and complexity, variation that is often considered to be adaptive. But because nonadaptive processes can also have important consequences on the features of genomes, we investigated the relationship of genetic drift and genome size in the phylum Ascomycota using multiple indicators of genetic drift. We detected a complex relationship between genetic drift and genome size in fungi: genetic drift is associated with genome expansion on broad evolutionary timescales, as hypothesized for other eukaryotes; but within subphyla over smaller timescales, the opposite trend is observed. Moreover, fungi and bacteria display similar patterns of genome degradation that are associated with initial effects of genetic drift. We conclude that changes in genome size within Ascomycota have occurred using two different routes: large-scale genome expansions are catalyzed by increasing drift as predicted by the mutation-hazard model of genome evolution and small-scale modifications in genome size are independent of drift.

Widespread Interspecific Divergence in Cis-Regulation of Transposable Elements in the Arabidopsis Genus

Transposable elements (TEs) are so abundant and variable that they count among the most important mutational sources in genomes. Nonetheless, little is known about the genetics of their variation in activity or silencing across closely related species. Here, we demonstrate that regulation of TE genes can differ dramatically between the two closely related Arabidopsis species A. thaliana and A. lyrata. In leaf and floral tissues of F1 interspecific hybrids, about 47% of TEs show allele-specific expression, with the A. lyrata copy being generally expressed at higher level. We confirm that TEs are generally expressed in A. lyrata but not in A. thaliana. Allele-specific differences in TE expression are associated with divergence in epigenetic modifications like DNA and histone methylation between species as well as with sequence divergence. Our data demonstrate that A. thaliana silences TEs much better than A. lyrata. For long terminal repeat retrotransposons, these differences are more pronounced for younger insertions. Interspecific differences in TE silencing may have a great impact on genome size changes.

Co-evolution between transposable elements and their hosts: a major factor in genome size evolution?

Most models of genome size evolution emphasize changes in relative rates of and/or the efficacy of selection on insertions and deletions. However, transposable elements (TEs) are a major contributor to genome size evolution, and since they experience their own selective pressures for expansion, genome size changes may in part be driven by the dynamics of co-evolution between TEs and their hosts. Under this perspective, predictions about the conditions that allow for genome expansion may be altered. In this review, we outline the evidence for TE-host co-evolution, discuss the conditions under which these dynamics can change, and explore the possible contribution to the evolution of genome size. Aided partly by advances in our understanding of the mechanisms of TE silencing via small RNAs, there is growing evidence that the evolution of transposition rates can be important in driving genome expansion and contraction. Shifts in genome size and transposon abundance associated with interspecific hybridization and changes in mating system are consistent with an important role for transposition rate evolution, although other possible explanations persist. More understanding of the potential for the breakdown of host silencing mechanisms and/or the potential for TEs to evade host immune responses will improve our understanding of the importance of changes in TE activity in driving genome size evolution.

NONADAPTIVE EVOLUTION OF MITOCHONDRIAL GENOME SIZE

Genomes vary greatly in size and complexity, and identifying the evolutionary forces that have generated this variation remains a major goal in biology. A controversial proposal is that most changes in genome size are initially deleterious and therefore are linked to episodes of decrease in effective population sizes. Support for this hypothesis comes from large-scale comparative analyses, but vanishes when phylogenetic nonindependence is taken into account. Another approach to test this hypothesis involves analyzing sequence evolution among clades where duplications have recently fixed. Here we show that episodes of fixation of duplications in mitochondrial genomes of the gecko Heteronotia binoei (two independent clades) and of mantellid frogs (five distinct branches) coincide with reductions in the ability of selection to purge slightly deleterious mutations. Our results support the idea that genome complexity can arise through nonadaptive processes in tetrapods.

The Arabidopsis lyrata genome sequence and the basis of rapid genome size change

We present the 207 Mb genome sequence of the outcrosser Arabidopsis lyrata, which diverged from the self-fertilizing species A. thaliana about 10 million years ago. It is generally assumed that the much smaller A. thaliana genome, which is only 125 Mb, constitutes the derived state for the family. Apparent genome reduction in this genus can be partially attributed to the loss of DNA from large-scale rearrangements, but the main cause lies in the hundreds of thousands of small deletions found throughout the genome. These occurred primarily in non-coding DNA and transposons, but protein-coding multi-gene families are smaller in A. thaliana as well. Analysis of deletions and insertions still segregating in A. thaliana indicates that the process of DNA loss is ongoing, suggesting pervasive selection for a smaller genome.

Influence of a Heavy-Metal-Polluted Environment on Viola tricolor Genome Size and Chromosome Number

Intraspecific changes in genome size and chromosome number lead to divergence and species evolution. Heavy metals disturb the cell cycle and cause mutations. Areas contaminated by heavy metals (metalliferous sites) are places where microevolutionary processes accelerate; very often only a few generations are enough for a new genotype to arise. This study, which continues our long-term research on Viola tricolor (Violaceae), a species occurring on both metalliferous (Zn, Pb, Cd, Cu) and non-metalliferous soils in Western and Central Europe, is aimed at determining the influence of environments polluted with heavy metals on genome size and karyological variability. The genome size of V. tricolor ranged from 3.801 to 4.203 pg, but the differences between metallicolous and non-metallicolous populations were not statistically significant. Altered chromosome numbers were significantly more frequent in material from the polluted sites than from the non-polluted sites (43% versus 28%). Besides the standard chromosome number (2n = 26), aneuploid cells with lower (2n = 18–25) or higher (2n = 27, 28) chromosome numbers were found in plants from both types of site, but polyploid (2n = 42) cells were observed only in plants from the metalliferous locality. The lack of correlation between chromosome variability in root meristematic cells and genome size estimated from peduncle cells can be attributed to elimination of somatic mutations in generative meristem, producing chromosome-stable non-meristematic tissues in the peduncle.

Genome Size Study in the Valerianaceae: First Results and New Hypotheses

The purpose of this study is to provide a new focus to contribute, from the perspective of genomic evolution, towards a better understanding of the Valerianaceae evolutionary history. Chromosome numbers were determined by Feulgen staining in 24 populations of 18 species (first count for Valerianella multidentata, 2n=2x=14–16), and DNA contents were assessed by flow cytometry in 74 populations of 35 species (first assessments in all taxa but Centranthus ruber). A molecular phylogeny based on the trnL-trnF and including 41 new sequences was established, with the first DNA sequence for Centranthus nevadensis, Valeriana rotundifolia, V. saxatilis, Valerianella multidentata, and V. turgida. This work is the first large genome size study devoted to the Valerianaceae, showing a range of DNA amounts from 2C=0.39 pg (Valerianella turgida) to 2C=8.32 pg (Valeriana officinalis). At the family level, changes in basic chromosome number and genome size coincide with or precede major shifts in the evolutionary history of the group, such as those concerning stamen number and floral symmetry.

Genome Size in Diploids, Allopolyploids, and Autopolyploids of Mediterranean Triticeae

Nuclear DNA amount, determined by the flow cytometry method, in diploids, natural and synthetic allopolyploids, and natural and synthetic autopolyploids of the tribe Triticeae (Poaceae) is reviewed here and discussed. In contrast to the very small and nonsignificant variation in nuclear DNA amount that was found at the intraspecific level, the variation at the interspecific level is very large. Evidently changes in genome size are either the cause or the result of speciation. Typical autopolyploids had the expected additive DNA amount of their diploid parents, whereas natural and synthetic cytologically diploidized autopolyploids and natural and synthetic allopolyploids had significantly less DNA than the sum of their parents. Thus, genome downsizing, occurring during or immediately after the formation of these polyploids, provides the physical basis for their cytological diploidization, that is, diploid-like meiotic behavior. Possible mechanisms that are involved in genome downsizing and the biological significance of this phenomenon are discussed.

Recent Insights into Mechanisms of Genome Size Change in Plants

Genome sizes vary considerably across all eukaryotes and even among closely related species. The genesis and evolutionary dynamics of that variation have generated considerable interest, as have the patterns of variation themselves. Here we review recent developments in our understanding of genome size evolution in plants, drawing attention to the higher order processes that can influence the mechanisms generating changing genome size.

Different scales of Ty1/copia-like retrotransposon proliferation in the genomes of three diploid hybrid sunflower species

Activation of transposable elements in species' genomes represents an important mechanism of new mutation and of potential rapid change in genome size. Thus, it is increasingly recognized that transposable elements likely have played a significant role in shaping species' evolution. In an earlier report, we showed that the genomes of three sunflower species of ancient hybrid origin have experienced large-scale proliferation events of sequences within the Ty3/gypsy-like superfamily of long terminal repeat (LTR) retrotransposons. In this report, we investigate whether another superfamily of LTR retrotransposon (Ty1/copia-like elements) have experienced similar derepression and proliferation events in the genomes of these sunflower hybrid taxa. We show that Ty1/copia-like elements also have undergone copy number increases following or associated with the origins of these species, although the scale of proliferation is less than that for Ty3/gypsy-like elements. Surveys of sequence heterogeneity of Ty1/copia-like elements in the genomes of the three hybrid and two parental species' genomes reveal that a single sub-lineage of these elements exhibits characteristics of recent amplification, and likely served as the proliferative source lineage. These findings indicate that the genomic and/or environmental conditions associated with the origins of these sunflower hybrid taxa were conducive to derepression of at least two major groups of transposable elements.

Computational Method for Estimating DNA Copy Numbers in Normal Samples, Cancer Cell Lines, and Solid Tumors Using Array Comparative Genomic Hybridization

Genomic copy number variations are a typical feature of cancer. These variations may influence cancer outcomes as well as effectiveness of treatment. There are many computational methods developed to detect regions with deletions and amplifications without estimating actual copy numbers (CN) in these regions. We have developed a computational method capable of detecting regions with deletions and amplifications as well as estimating actual copy numbers in these regions. The method is based on determining how signal intensity from different probes is related to CN, taking into account changes in the total genome size, and incorporating into analysis contamination of the solid tumors with benign tissue. Hidden Markov Model is used to obtain the most likely CN solution. The method has been implemented for Affymetrix 500K GeneChip arrays and Agilent 244K oligonucleotide arrays. The results of CN analysis for normal cell lines, cancer cell lines, and tumor samples are presented. The method is capable of detecting copy number alterations in tumor samples with up to 80% contamination with benign tissue. Analysis of 178 cancer cell lines reveals multiple regions of common homozygous deletions and strong amplifications encompassing known tumor suppressor genes and oncogenes as well as novel cancer related genes.

Correlated evolution of LTR retrotransposons and genome size in the genus Eleocharis.

BackgroundTransposable elements (TEs) are considered to be an important source of genome size variation and genetic and phenotypic plasticity in eukaryotes. Most of our knowledge about TEs comes from large genomic projects and studies focused on model organisms. However, TE dynamics among related taxa from natural populations and the role of TEs at the species or supra-species level, where genome size and karyotype evolution are modulated in concert with polyploidy and chromosomal rearrangements, remain poorly understood. We focused on the holokinetic genus Eleocharis (Cyperaceae), which displays large variation in genome size and the occurrence of polyploidy and agmatoploidy/symploidy. We analyzed and quantified the long terminal repeat (LTR) retrotransposons Ty1-copia and Ty3-gypsy in relation to changes in both genome size and karyotype in Eleocharis. We also examined how this relationship is reflected in the phylogeny of Eleocharis.ResultsUsing flow cytometry, we measured the genome sizes of members of the genus Eleocharis (Cyperaceae). We found positive correlation between the independent phylogenetic contrasts of genome size and chromosome number in Eleocharis. We analyzed PCR-amplified sequences of various reverse transcriptases of the LTR retrotransposons Ty1-copia and Ty3-gypsy (762 sequences in total). Using real-time PCR and dot blot approaches, we quantified the densities of Ty1-copia and Ty3-gypsy within the genomes of the analyzed species. We detected an increasing density of Ty1-copia elements in evolutionarily younger Eleocharis species and found a positive correlation between Ty1-copia densities and C/n-values (an alternative measure of monoploid genome size) in the genus phylogeny. In addition, our analysis of Ty1-copia sequences identified a novel retrotransposon family named Helos1, which is responsible for the increasing density of Ty1-copia. The transition:transversion ratio of Helos1 sequences suggests that Helos1 recently transposed in later-diverging Eleocharis species.ConclusionsUsing several different approaches, we were able to distinguish between the roles of LTR retrotransposons, polyploidy and agmatoploidy/symploidy in shaping Eleocharis genomes and karyotypes. Our results confirm the occurrence of both polyploidy and agmatoploidy/symploidy in Eleocharis. Additionally, we introduce a new player in the process of genome evolution in holokinetic plants: LTR retrotransposons.

Comparative Genomics on the Drosophila Phylogenetic Tree

With the sequencing of 12 complete euchromatic Drosophila genomes, the genus Drosophila is a leading model for comparative genomics. In this review, we discuss the novel insights into evolutionary processes afforded by the newly available genomic sequences when placed in the context of the phylogeny. We focus on three levels: insights into whole-genome content, such as changes in genome size and content across the phylogeny; insights into large-scale patterns of divergence and conservation, such as selective constraints on genes and chromosome-level evolution of sex chromosomes; and insights into finer-scale processes in individual lineages and genes, such as lineage-specific evolution in response to ecological context. As the field of comparative genomics is still young, we also discuss current challenges, such as the development of more sophisticated evolutionary models to capture nonequilibrium processes and the improvement of assembly and alignment algorithms to better capture uncertainty in the data.

Rapid DNA loss as a counterbalance to genome expansion through retrotransposon proliferation in plants.

Transposable elements, particularly LTR-retrotransposons, comprise the primary vehicle for genome size expansion in plants, while DNA removal through illegitimate recombination and intrastrand homologous recombination serve as the most important counteracting forces to plant genomic obesity. Despite extensive research, the relative impact of these opposing forces and hence the directionality of genome size change remains unknown. In Gossypium (cotton), the 3-fold genome size variation among diploids is due largely to copy number variation of the gypsy-like retrotransposon Gorge3. Here we combine comparative sequence analysis with a modeling approach to study the directionality of genome size change in Gossypium. We demonstrate that the rate of DNA removal in the smaller genomes is sufficient to reverse genome expansion through Gorge3 proliferation. These data indicate that rates of DNA loss can be highly variable even within a single plant genus, and that the known mechanisms of DNA loss can indeed reverse the march toward genomic obesity.

Plant genomes. Genome dynamics vol. 4

This book explores some of the latest research in the field of plant genomics with an emphasis on genome dynamics in Poaceae and the role of retroelements and polyploidy in establishing genome diversity. It comprises a collection of eleven short papers, including some by the finest practitioners of the subject. Most papers are of review style, although a few have included small amounts of new data. The book is not designed to be read from cover to cover as each paper stands alone, and whilst this leads to a little redundancy between chapters, the majority of papers are well worth the read. Several papers, such as those by Deragon et al. and Messing and Bennetzen, explore the contribution of transposable elements (especially retroelements) to angiosperm genome structure, diversity and evolution. The papers review a substantial body of evidence demonstrating that these elements are responsible for much of the genomic dynamism reported across angiosperms. At a more focused level, a review of the rice genome is provided by Sasaki, whilst an overview of current in-depth maize genomic research is given by Rafalski and Tingey. Further evidence of extensive genome fluidity and dynamism is also exhibited in terms of nuclear genome size. Grover et al.'s review clearly outlines our current understanding of the significant mechanisms that have contributed to the overall pace and direction of genome size change (both up and down) over evolution. The other substantial component of angiosperm genome structure that is explored is polyploidy (or whole-genome duplication), which has so characterized and shaped angiosperm divergence. The paper by Paterson describes the impact of paleopolyploidy on angiosperm diversity, while Sampedro and Cosgrove outline how the collinear organization of genes in duplicated syntenic blocks can be used to infer evolutionary relationships between species, and the particular pitfalls that may arise using such approaches. The fate of duplicated genes has been the focus of much research and the excellent paper by Freeling examines different hypotheses (i.e. gain-of-function, subfunctionalization, and balanced-gene-drive) that have been proposed to explain patterns of duplicate-gene retention following polyploidy. Analysing the increasing amounts of comparative sequence data being generated, Freeling concludes that balanced-gene-drive rather than subfunctionalization is the most likely mechanism responsible for duplicate-gene retention. In addition to these central foci in the book, there is a paper on plant sex chromosomes by Charlesworth and one on plant centromeres by Lamb et al. The latter explores exactly what the plant centromere is, with recent data clearly showing that this essential structure is largely the product of epigenetic modifications rather than genetic sequence. There is also a paper on small interfering RNAs by Meyers et al. that provides a valuable marker for the state-of-knowledge in this young, but undoubtedly important field in relation to genome dynamics. Unfortunately the paper is likely to age rapidly. Overall, the book presents us with state-of-the-art information in most areas of plant genome dynamics. We say ‘in most areas’, however, because there is nothing significant on tandem repeats, which comprise a substantial fraction of plant genomes, and it does not mention ribosomal DNA (rDNA) or telomeres, despite significant advances in our understanding of their evolutionary and functional dynamics over the last few years. Furthermore there is no allusion to the avalanche of high-throughput sequence data that is on the brink of arriving. The quality of the presentation is high, although there could have been a bit more editorial intervention in places to strengthen or remove a few weak sections to some manuscripts that crept through, and to enhance poorly explained figures. Nevertheless the book is free of minor errors and this small criticism should not deter the reader from buying this specialist book. This book is suitable for anyone with a research or high-level teaching interest in genome dynamics in angiosperms. In summary, this book effectively establishes that the angiosperm genome is highly dynamic, and provides insights into the key processes and mechanisms that contribute to that dynamism, particularly polyploidy and retroelements. It provides a good overall marker of our current knowledge in most areas. We are pleased to have read it and feel it would make a valuable addition to our collection or a library.

Genome size as a predictor of guard cell length in Arabidopsis thaliana is independent of environmental conditions

The recent discovery of a strong positive relationship between angiosperm genome size and stomatal guard cell length (GCL) opens the possibility of using plant fossil guard cell size as a proxy for changes in angiosperm genome size over periods of environmental change. The responses of GCL to environmental stimuli are currently unknown and may obscure this predictive relationship. Here, we investigated the effects of environmental variables (atmospheric CO2, drought, relative humidity, irradiance, ultraviolet radiation and pathogen attack) on GCL in the model plant Arabidopsis thaliana to quantify environmentally induced variation. GCL responded to all variables tested, but the changes incurred did not significantly impinge on the predictive capability of the relationship.

The problem of the eukaryotic genome size

The current state of knowledge concerning the unsolved problem of the huge interspecific eukaryotic genome size variations not correlating with the species phenotypic complexity (C-value enigma also known as C-value paradox) is reviewed. Characteristic features of eukaryotic genome structure and molecular mechanisms that are the basis of genome size changes are examined in connection with the C-value enigma. It is emphasized that endogenous mutagens, including reactive oxygen species, create a constant nuclear environment where any genome evolves. An original quantitative model and general conception are proposed to explain the C-value enigma. In accordance with the theory, the noncoding sequences of the eukaryotic genome provide genes with global and differential protection against chemical mutagens and (in addition to the anti-mutagenesis and DNA repair systems) form a new, third system that protects eukaryotic genetic information. The joint action of these systems controls the spontaneous mutation rate in coding sequences of the eukaryotic genome. It is hypothesized that the genome size is inversely proportional to functional efficiency of the anti-mutagenesis and/or DNA repair systems in a particular biological species. In this connection, a model of eukaryotic genome evolution is proposed.

Genome size is a strong predictor of cell size and stomatal density in angiosperms

Across eukaryotes phenotypic correlations with genome size are thought to scale from genome size effects on cell size. However, for plants the genome/cell size link has only been thoroughly documented within ploidy series and small subsets of herbaceous species. Here, the first large-scale comparative analysis is made of the relationship between genome size and cell size across 101 species of angiosperms of varying growth forms. Guard cell length and epidermal cell area were used as two metrics of cell size and, in addition, stomatal density was measured. There was a significant positive relationship between genome size and both guard cell length and epidermal cell area and a negative relationship with stomatal density. Independent contrast analyses revealed that these traits are undergoing correlated evolution with genome size. However, the relationship was growth form dependent (nonsignificant results within trees/shrubs), although trees had the smallest genome/cell sizes and the highest stomatal density. These results confirm the generality of the genome size/cell size relationship. The results also suggest that changes in genome size, with concomitant influences on stomatal size and density, may influence physiology, and perhaps play an important genetic role in determining the ecological and life-history strategy of a species.