Flow cytometric analysis of nuclear DNA content in Musa

 Nuclear genome size variation was studied in Musa acuminata (A genome), Musa balbisiana (B genome) and a range of triploid clones differing in genomic constitution (i.e. the relative number of A and B genomes). Nuclear DNA content was estimated by flow cytometry of nuclei stained by propidium iodide. The A and B genomes of Musa differ in size, the B genome being smaller by 12% on average. No variation in genome size was found among the accessions of M. balbisiana (average genome size 537 Mbp). Small, but statistically significant, variation was found among the subspecies and clones of M. acuminata (ranging from 591 to 615 Mbp). This difference may relate to the geographical origin of the individual accessions. Larger variation in genome size (8.8%) was found among the triploid Musa accessions (ranging from 559 to 613 Mbp). This variation may be due to different genomic constitutions as well as to differences in the size of their A genomes. It is proposed that a comparative analysis of genome size in diploids and triploids may be helpful in identifying putative diploid progenitors of cultivated triploid Musa clones. Statistical analysis of data on genome size resulted in a grouping which agreed fairly well with the generally accepted taxonomic classification of Musa. Received: 11 May 1998 / Accepted: 29 September 1998     

Relationships between nuclear DNA content and seed and leaf size in soybean

 A correlation between genome size and agronomically important traits has been observed in many plant species. The goal of the present research was to determine the relationship between genome size, seed size, and leaf width and length in soybean [Glycine max (L.) Merr.] Twelve soybean strains, representing three distinct seed size groups, were analyzed. Flow cytometry was used to estimate their 2C nuclear DNA contents. Data on seed size and leaf size of the 12 strains were obtained from 1994 and 1995 field experiments. Variation of 2C nuclear DNA among the 12 soybean strains was 4.6%, ranging from 2.37 pg for a small-seed strain to 2.48 pg for a large-seed strain. Strain seed size was positively associated with leaf width (r=0.92) and leaf length (r=0.93). Genome size was highly correlated with seed size (r=0.97), leaf width (r=0.90) , and leaf length (r=0.93). The results of our study indicate that there is a significant correlation between genome size and leaf and seed size in soybean. It is possible that selection for greater seed size either leads to, or results from, greater genome size. If so, this relationship might be worth exploring at a more fundamental level. Received: 5 April 1997 / Accepted: 9 January 1998     

Quantification of nuclear DNA and G-C content in marine macroalgae by flow cytometry of isolated nuclei

Summary The amounts of nuclear DNA in ten species of seaweeds belonging to the Rhodophyceae, Phaeophyceae, and Chlorophyceae were determined by flow cytometric analysis of nuclei isolated from protoplasts. Genome size was determined from the fluorescence of the nuclei stained with ethidium bromide. The size of the nuclear genome ranged from 0.13 pg per cell in the 1 C population ofUlva rigida to 3.40 pg per cell in the 2 C population ofSphacelaria sp. GC% analysis was based on staining with either Hoechst 33342 or mithramycin A, two fluorochromes specific for the bases A-T and G-C, respectively. Two models were used for the estimation of the proportion of guanine plus cytosine in the nuclear genome. The first one was based on the linear relationships mithramycin A fluorescence/G-C content and ethidium bromide fluorescence/total DNA content. The second model, based on the curvilinear relationships Hoechst 33342 fluorescence/A-T content and mithramycin A fluorescence/G-C content, resulted in comparatively more homogenous and consistent data and appears more accurate. Comparison with previous reports from other methods for the physical investigation of nuclear genomes shows that flow cytometry of nuclei isolated from protoplasts is an accurate, convenient and robust technique to assay for genome sizes and base pair composition in marine macroalgae.Abbreviations A-T nucleic bases adenine and thymine - CRBC chicken red blood cell - FALS forward-angle light scatter - G-C nucleic bases guanine and cytosine - SEIM sorbitol enzymatic incubation medium - SWIM sea water incubation medium - Tm thermal denaturation temperature of DNA     

First nuclear DNA amounts in more than 300 angiosperms

BACKGROUND AND AIMS: Genome size (DNA C-value) data are key biodiversity characters of fundamental significance used in a wide variety of biological fields. Since 1976, Bennett and colleagues have made scattered published and unpublished genome size data more widely accessible by assembling them into user-friendly compilations. Initially these were published as hard copy lists, but since 1997 they have also been made available electronically (see the Plant DNA C-values database http://www.kew.org/cval/homepage.html). Nevertheless, at the Second Plant Genome Size Meeting in 2003, Bennett noted that as many as 1000 DNA C-value estimates were still unpublished and hence unavailable. Scientists were strongly encouraged to communicate such unpublished data. The present work combines the databasing experience of the Kew-based authors with the unpublished C-values produced by Zonneveld to make a large body of valuable genome size data available to the scientific community. METHODS: C-values for angiosperm species, selected primarily for their horticultural interest, were estimated by flow cytometry using the fluorochrome propidium iodide. The data were compiled into a table whose form is similar to previously published lists of DNA amounts by Bennett and colleagues. KEY RESULTS AND CONCLUSIONS: The present work contains C-values for 411 taxa including first values for 308 species not listed previously by Bennett and colleagues. Based on a recent estimate of the global published output of angiosperm DNA C-value data (i.e. 200 first C-value estimates per annum) the present work equals 1.5 years of average global published output; and constitutes over 12 % of the latest 5-year global target set by the Second Plant Genome Size Workshop (see http://www.kew.org/cval/workshopreport.html). Hopefully, the present example will encourage others to unveil further valuable data which otherwise may lie forever unpublished and unavailable for comparative analyses.     

The relationship between nuclear DNA content and leaf strategy in seed plants

BACKGROUND AND AIMS: Species' 2C-values (mass of DNA in G(1) phase 2n nuclei) vary by at least four orders of magnitude among seed plants. The 2C-value has been shown to be co-ordinated with a number of other species traits, and with environmental variables. A prediction that species 2C-values are negatively related to leaf life span (LL) and leaf mass per area (LMA) is tested. These leaf traits are components of a major dimension of ecological variation among plant species. METHODS: Flow cytometry was used to measure the 2C-values for 41 Australian seed plant species, 40 of which were new to the literature. Where possible, LL and LMA data from the global literature were combined with 2C-values from our data set and online C-value databases. KEY RESULTS: Across all species, weak positive relationships were found between 2C-values and both LL and LMA; however, these did not reflect the relationships within either angiosperms or gymnosperms. Across 59 angiosperm species, there were weak negative relationships between 2C-values and both LL (r2 = 0.13, P = 0.005) and LMA (r2 = 0.15, P = 0.002). These relationships were the result of shifts to longer LL and greater LMA in woody compared with herbaceous growth forms, with no relationships present within growth forms. It was not possible to explain a positive relationship between 2C-values and LMA (r2 = 0.30, P = 0.024) across 17 gymnosperm species. The 2C-value was not related to LL or LMA either across species within orders (except for LMA among Pinales), or as radiation divergences in a model phylogeny. CONCLUSIONS: Gymnosperms appear to vary along a spectrum different from angiosperms. Among angiosperms, weak negative cross-species relationships were associated with growth form differences, and traced to a few divergences deep in the model phylogeny. These results suggest that among angiosperms, nuclear DNA content and leaf strategy are unrelated.     

Evolution of genome size in fishes: a phylogenetic test of the Hinegardner and Rosen hypothesis

Despite remarkable advances in genomic studies over the past few decades, surprisingly little is known about the processes governing genome evolution at macroevolutionary timescales. In a seminal paper, Hinegardner and Rosen (Am Nat 106:621–644, 1972) suggested that taxa characterized by larger genomes should also display disproportionately stronger fluctuations in genome size. Therefore, according to the Hinegardner and Rosen (HR) hypothesis, there should be a negative correlation between average within-family genome size and its corresponding coefficient of variation (CV), a prediction that was supported by their analysis of the genomes of 275 species of fish. In this study we reevaluate the HR hypothesis using an expanded dataset (2050 genome size records). Moreover, in addition to the use of standard linear regression techniques, we also conducted modern comparative analyses that take into account phylogenetic non-independence. Our analyses failed to confirm the negative relationship detected in the original study, suggesting that the evolution of genome size in fishes might be more complex than envisioned by the HR hypothesis. Interestingly, the frequency distribution of fish genome sizes was strongly skewed, even on a logarithmic scale, suggesting that the dynamics underlying genome size evolution are driven by multiplicative phenomena, which might include chromosomal rearrangements and the expansion of transposable elements.     

Evolution of DNA amounts across land plants (embryophyta)

BACKGROUND AND AIMS: DNA C-values in land plants (comprising bryophytes, lycophytes, monilophytes, gymnosperms and angiosperms) vary approximately 1000-fold from approx. 0.11 to 127.4 pg. To understand the evolutionary significance of this huge variation it is essential to evaluate the phylogenetic component. Recent increases in C-value data (e.g. Plant DNA C-values database; release 2.0, January 2003; http://www.rbgkew.org.uk/cval/homepage.html) together with improved consensus of relationships between and within land plant groups makes such an analysis timely. METHODS: Insights into the distribution of C-values in each group of land plants were gained by superimposing available C-value data (4119 angiosperms, 181 gymnosperms, 63 monilophytes, 4 lycophytes and 171 bryophytes) onto phylogenetic trees. To enable ancestral C-values to be reconstructed for clades within land plants, character-state mapping with parsimony and MacClade was also applied. KEY RESULTS AND CONCLUSIONS: Different land plant groups are characterized by different C-value profiles, distribution of C-values and ancestral C-values. For example, the large ( approximately 1000-fold) range yet strongly skewed distribution of C-values in angiosperms contrasts with the very narrow 12-fold range in bryophytes. Further, character-state mapping showed that the ancestral genome sizes of both angiosperms and bryophytes were reconstructed as very small (i.e. < or =1.4 pg) whereas gymnosperms and most branches of monilophytes were reconstructed with intermediate C-values (i.e. >3.5, <14.0 pg). More in-depth analyses provided evidence for several independent increases and decreases in C-values; for example, decreases in Gnetaceae (Gymnosperms) and heterosperous water ferns (monilophytes); increases in Santalales and some monocots (both angiosperms), Pinaceae, Sciadopityaceae and Cephalotaxaceae (Gymnosperms) and possibly in the Psilotaceae + Ophioglossaceae clade (monilophytes). Thus, in agreement with several focused studies within angiosperm families and genera showing that C-values may both increase and decrease, it is apparent that this dynamic pattern of genome size evolution is repeated on a broad scale across land plants.     

New reports of nuclear DNA content for 407 vascular plant taxa from the United States

Background and Aims

The amount of DNA in an unreplicated haploid nuclear genome (C-value) ranges over several orders of magnitude among plant species and represents a key metric for comparing plant genomes. To extend previously published datasets on plant nuclear content and to characterize the DNA content of many species present in one region of North America, flow cytometry was used to estimate C-values of woody and herbaceous species collected in Wisconsin and the Upper Peninsula of Michigan, USA.

Methods

A total of 674 samples and vouchers were collected from locations across Wisconsin and Michigan, USA. From these, C-value estimates were obtained for 514 species, subspecies and varieties of vascular plants. Nuclei were extracted from samples of these species in one of two buffers, stained with the fluorochrome propidium iodide, and an Accuri C-6 flow cytometer was used to measure fluorescence peaks relative to those of an internal standard. Replicate extractions, coefficients of variation and comparisons to published C-values in the same and related species were used to confirm the accuracy and reliability of our results.

Key Results and Conclusions

Prime C-values for 407 taxa are provided for which no published data exist, including 390 angiosperms, two gymnosperms, ten monilophytes and five lycophytes. Non-prime reports for 107 additional taxa are also provided. The prime values represent new reports for 129 genera and five families (of 303 genera and 97 families sampled). New family C-value maxima or minima are reported for Betulaceae, Ericaceae, Ranunculaceae and Sapindaceae. These data provide the basis for phylogenetic analyses of C-value variation and future analyses of how C-values covary with other functional traits.     

Patterns of maximum body size evolution in Cenozoic land mammals: eco-evolutionary processes and abiotic forcing

There is accumulating evidence that macroevolutionary patterns of mammal evolution during the Cenozoic follow similar trajectories on different continents. This would suggest that such patterns are strongly determined by global abiotic factors, such as climate, or by basic eco-evolutionary processes such as filling of niches by specialization. The similarity of pattern would be expected to extend to the history of individual clades. Here, we investigate the temporal distribution of maximum size observed within individual orders globally and on separate continents. While the maximum size of individual orders of large land mammals show differences and comprise several families, the times at which orders reach their maximum size over time show strong congruence, peaking in the Middle Eocene, the Oligocene and the Plio-Pleistocene. The Eocene peak occurs when global temperature and land mammal diversity are high and is best explained as a result of niche expansion rather than abiotic forcing. Since the Eocene, there is a significant correlation between maximum size frequency and global temperature proxy. The Oligocene peak is not statistically significant and may in part be due to sampling issues. The peak in the Plio-Pleistocene occurs when global temperature and land mammal diversity are low, it is statistically the most robust one and it is best explained by global cooling. We conclude that the macroevolutionary patterns observed are a result of the interplay between eco-evolutionary processes and abiotic forcing.     

DNA sequence variation of wild barley Hordeum spontaneum (L.) across environmental gradients in Israel

Wild barley Hordeum spontaneum (L.) shows a wide geographic distribution and ecological diversity. A key question concerns the spatial scale at which genetic differentiation occurs and to what extent it is driven by natural selection. The Levant region exhibits a strong ecological gradient along the North–South axis, with numerous small canyons in an East–West direction and with small-scale environmental gradients on the opposing North- and South-facing slopes. We sequenced 34 short genomic regions in 54 accessions of wild barley collected throughout Israel and from the opposing slopes of two canyons. The nucleotide diversity of the total sample is 0.0042, which is about two-thirds of a sample from the whole species range (0.0060). Thirty accessions collected at ‘Evolution Canyon'' (EC) at Nahal Oren, close to Haifa, have a nucleotide diversity of 0.0036, and therefore harbor a large proportion of the genetic diversity. There is a high level of genetic clustering throughout Israel and within EC, which roughly differentiates the slopes. Accessions from the hot and dry South-facing slope have significantly reduced genetic diversity and are genetically more distinct from accessions from the North-facing slope, which are more similar to accessions from other regions in Northern Israel. Statistical population models indicate that wild barley within the EC consist of three separate genetic clusters with substantial gene flow. The data indicate a high level of population structure at large and small geographic scales that shows isolation-by-distance, and is also consistent with ongoing natural selection contributing to genetic differentiation at a small geographic scale.     

Allopolyploid origin of Cardamine asarifolia (Brassicaceae): incongruence between plastid and nuclear ribosomal DNA sequences solved by a single-copy nuclear gene

Interspecific hybridization and polyploidization have played central roles in plant diversification. However, technical difficulties in the analyses of low-copy genes have limited the study of the origins of hybrid and polyploid plants. Here, we present a phylogenetic analysis of the hexaploid Cardamine asarifolia, distributed in the southern European Alps and northern Apennines. Our study included all relevant taxa of the genus found in Europe. A marked discrepancy was revealed between the trnL-trnF region of cpDNA and internal transcribed spacer (nrDNA ITS) sequences. To solve the incongruence, we sequenced a single-copy nuclear CHS gene (chalcone synthase) using a novel method to design homoeologue-specific PCR primers to bypass artefacts caused by artificial recombination of homoeologues during PCR and/or cloning. Three homoeologues were isolated from C. asarifolia, providing evidence for its allopolyploid origin. One homoeologue, showing the same phylogenetic position as the ITS sequences, most likely originated from an extinct parent. Furthermore, we documented recurrent polytopic hybridizations between C. asarifolia and diploid C. amara. The allohexaploidization and the following hybridization with a diploid species exemplify the ongoing dynamic processes of speciation in the genus Cardamine.     

Variation in macronuclear genome content of three ciliates with extensive chromosomal fragmentation: a preliminary analysis

The genome architecture of ciliates, including features such as nuclear dualism and large-scale genome rearrangements, impacts gene and genome evolution in these organisms. To better understand the structure of macronuclear chromosomes in ciliates with extensively processed chromosomes, a sample of complete macronuclear chromosomes was sequenced from three ciliate species: Metopus es (Class [Cl]: Armophorea), Nyctotherus ovalis (Cl: Armophorea), and Chilodonella uncinata (Cl: Phyllopharyngea). By cloning whole macronuclear chromosomes into a plasmid vector, we generated nine clones from each of M. es and C. uncinata, and 37 clones from N. ovalis. Analysis of these macronuclear chromosomes provides insight into the evolution of genome features such as chromosome content, gene structure, and genetic code. Phylogenetic patterns can be found in telomere structure and codon usage, which are both more similar in M. es and N. ovalis than C. uncinata. In addition, we provide evidence of lateral transfer of a bacterial endo-beta-mannanase gene onto a M. es chromosome and report the discovery of a 42-bp conserved sequence motif within N. ovalis untranslated regions.     

Variation in abundance across a species' range predicts climate change responses in the range interior will exceed those at the edge: a case study with North American beaver

The absence of information about how abundance varies across species' ranges restricts most modeling and monitoring of climate change responses to the range edge. We examine spatial variation in abundance across the northeastern range of North American beaver ( Castor canadensis ), evaluate the extent to which climate and nonclimate variables explain this variation, and use a species–climate envelope model that includes spatial variation in abundance to predict beaver abundance responses to projected climate change. The density of beaver colonies across Québec follows a roughly logistic pattern, with high but variable density across the southern portion of the province, a sharp decline in density at about 49°N, and a long tail of low density extending as far as 58°N. Several climate and nonclimate variables were strong predictors of variation in beaver density, but 97% of the variation explained by nonclimate variables could be accounted for by climate variables. Because of the peak and tail density pattern, beaver climate sensitivity (change in density per unit change in climate) was greatest in the interior and lowest at the edge of the range. Combining our best density–climate models with projections from general circulation models (GCM) predicts a relatively modest expansion of the species' northern range limit by 2055, but density increases in the range interior that far exceed those at the range edge. Thus, some of the most dramatic responses to climate change may be occurring in the core of species' ranges, far away from the edge-of-the-range focus of most current modeling and monitoring efforts.     

The economic and environmental efficiency assessment in EU cross-country: Evidence from DEA and quantile regression approach

This article aims to estimate the efficiency of 26 different European Countries over 2001 and 2012 comparing their performance. Data Envelopment Analysis technique is used in a first step to evaluate the performance of each European country. The output-oriented model was used with two specifications (Variable and Constant Returns to Scale) including as inputs labour and capital productivity, the weight of fossil energy and the share of renewable energy in GDP (gross domestic product), being the output GDP per GHG (greenhouse gases) emissions. In a second step, the quantile regression technique was used, to explain different efficiency scores through variables as Environmental Taxes Revenues, Resources Productivity and Domestic Material Consumption. Results indicate that share of renewables and non-renewable energy sources are important to explain differences in emissions. They suggest a significant change in the trend of economic and environmental efficiency in European countries and put forward the high disparities existing among them. Policy recommendations point for the need of higher steps if the goal is to equal countries efficiency scores. Moreover, environmental tax revenue effects are negatively stronger in less efficient countries, whereas also exerting negative influence over those more eco-efficient. Transport taxes affect negatively more eco-efficient countries and positively less eco-efficient countries. Energy taxes only seem to positively influence the lower eco-efficient countries. There is also evidence for a negative premium of efficiency considering domestic materials consumption. Finally, resources productivity shows a positive and significant influence independently of the country technical eco-efficiency level.     

Histones and nucleosomes in Archaea and Eukarya: a comparative analysis

Archaeal histones from mesophilic, thermophilic, and hyperthermophilic members of the Euryarchaeota have primary sequences, the histone fold, tertiary structures, and dimer formation in common with the eukaryal nucleosome core histones H2A, H2B, H3, and H4. Archaeal histones form nucleoprotein complexes in vitro and in vivo, designated archaeal nucleosomes, that contain histone tetramers and protect approximately 60 base pairs of DNA from nuclease digestion. Based on the sequence and structural homologies and experimental data reviewed here, archaeal nucleosomes appear similar, and may be homologous in evolutionary terms and function, to the structure at the center of the eukaryal nucleosome formed by the histone (H3+H4)₂ tetramer. Received: January 22, 1998 / Accepted: February 16, 1998     

Variation in recombination rate across the genome: evidence and implications

Recent data from humans and other species provide convincing evidence of variation in recombination rate in different genomic regions. Comparison of physical and genetic maps reveals variation on a scale of megabases, with substantial differences between sexes. Recombination is often suppressed near centromeres and elevated near telomeres, but neither of these observations is true for all chromosomes. In humans, patterns of linkage disequilibrium and experimental measures of recombination from sperm-typing reveal dramatic hotspots of recombination on a scale of kilobases. Genome-wide variation in the amount of crossing-over may be due to variation in the density of hotspots, the intensity of hotspots, or both. Theoretical models of selection and linkage predict that genetic variation will be reduced in regions of low recombination, and this prediction is supported by data from several species. Heterogeneity in rates of crossing-over provides both an opportunity and a challenge for identifying disease genes: as associations occur in blocks, genomic regions containing disease loci may be identified with relatively few markers, yet identifying the causal mutations is unlikely to be achieved through associations alone.     

Comparative analysis of methylthioalkylmalate synthase (MAM) gene family and flanking DNA sequences in Brassica oleracea and Arabidopsis thaliana

Gene BoGSL-PRO is associated with presence of 3-carbon side-chain glucosinolates (GSL). This gene is a member of the methylthioalkylmalate synthase (MAM) gene family. A BAC clone of Brassica oleracea, B21F5, containing this gene, was sequenced, annotated and compared to its corresponding region in Arabidopsis thaliana. Twelve protein-coding genes and 10 transposable elements were found in this clone. The corresponding region in A. thaliana chromosome I has 14 genes and no transposable elements. Analysis of MAM gene family in both species, which also include genes controlling 4-carbon side-chain GSL, separated the genes in two groups based on exon numbers and function. Phylogenetic analysis of the amino acid sequences encoded by these genes suggest that these two groups were produced by a duplication that must have occurred before the divergence of the Rosid and Asterid lineages of angiosperms. Comparison with putative orthologs from several prokaryotes further suggest that the members of the gene family with 10 exons, which encode proteins involved in 4-carbon side-chain GSL biosynthesis, were derived via truncation of the 3′ end from ancestral genes more similar in length to those with 12 exons, which encode proteins involved in 3-carbon side-chain GSL biosynthesis. Lower gene density in B. oleracea compared to A. thaliana is due in part to presence of transposable elements (TE) mostly in inter-genic regions.     

Variation in queen size across a behavioral transition zone in the ant Messor pergandei

Summary. Ant colonies should be selected to optimally allocate resources to individual reproductive offspring so as to balance production costs with offspring fitness gains. Different modes of colony founding have different size-dependent fitness functions, and should thus lead to different optimal queen sizes. We tested whether a behavioral transition from solitary colony founding (haplometrosis) to group colony founding (pleometrosis) across the range of the ant Messor pergandei was associated with a difference in queen size or condition. Both winged gynes and founding queens were significantly smaller and lighter at pleometrotic than at haplometrotic sites, with an abrupt shift in these characters across the 8.5 km-wide behavioral transition zone. Both the mutualistic advantages of grouping and among-queen competition within associations are likely to be important in selecting for smaller queen size in pleometrotic populations.Received 16 January 2004; revised 13 August 2004; accepted 16 August 2004.     

Long Repeats in a Huge Genome: Microsatellite Loci in the Grasshopper Chorthippus biguttulus

It is commonly believed that both the average length and the frequency of microsatellites correlate with genome size. We have estimated the frequency and the average length for 69 perfect dinucleotide microsatellites in an insect with an exceptionally large genome: Chorthippus biguttulus (Orthoptera, Acrididae). Dinucleotide microsatellites are not more frequent in C. biguttulus, but repeat arrays are 1.4 to 2 times longer than in other insect species. The average repeat number in C. biguttulus lies in the range of higher vertebrates. Natural populations are highly variable. At least 30 alleles per locus were found and the expected heterozygosity is above 0.95 at all three loci studied. In contrast, the observed heterozygosity is much lower (≤0.51), which could be caused by long null alleles. [Reviewing Editor: Dr. Dmitri Petrov] Sequence data from this article have been deposited with the EMBL/GenBank databases under accession numbers AY532396–AY532400.     

Densitometric analysis of nuclear DNA content in lentil roots grown in space

Lentil seedlings were grown for 28 h in space, on board Spacelab (IML 1 Mission) and growth of the primary root was analysed. The length of cortical cells was less in near weightlessness than on the 1 g centrifuge (flight control) and mitotic index was lower but there was no apparent perturbation in the mitosis. To further investigate which phase of cell cycle was modified, densitometric analysis of nuclear DNA content in cortical cells was carried out by the mean of an image processing system (SAMBA). In microgravity there was a decrease in DNA synthesis and a promotion of the arrest in the G2 phase of cell cycle. These results, and other ones obtained elsewhere on a slowly rotating clinostat, led us to think that in microgravity the perturbation of the gravisensing cells and/or the absence of convection could account for the modification of cell growth registered in the primary root.