Dynamics of the spatial organization of the chromosome set in cells of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> imaginal disks normally and under the action of the tumor-inducing mutation <Emphasis Type="Italic">Merlin</Emphasis>

Fluorescence of H3-p histone and DAPI was studi ed at different stages of interphase and mitosis in cells of imaginal disks of third-instar Drosophila melanogaster larvae. Three stages differing in the spatial organization of the chromosome set in mitosis were revealed. At the first stage (prophase, prometaphase), the histone 3 phosphorylation level rises, and the volume occupied by the chromosome set in the nucleus increases. The distinctive features of the second stage (metaphase) are a gradual decrease in the histone 3 phosphorylation (the density of phosphorylation remaining constant) and a reduction of the volume occupied by the chromosome set. At the third stage (anaphase, telophase), the intensity and density of the signal from H3-p histone decrease, and the volume occupied by the chromosome set reduces. At this stage, in Mer ⁴ larvae, in contrast to the control strain, the cells prematurely pass from anaphase into telophase. In addition, a subpopulation of cells with an abnormally large volume of nuclear DNA during the G₁ period was revealed in Mer ⁴ larvae. The cells of this subpopulation do not enter into the DNA synthesis and quit the cycle.     

Molecular organisation of the plant genome: Its relation to structure,recombination and evolution of chromosomes

Large scale changes in nuclear DNA amount accompany the evolution of species of higher plants. Much of the nuclear DNA accrued during the evolution of species does not encode genetic information and is selectively neutral. Nonetheless, the pattern of distribution of the excess DNA within and between chromosome complements suggests that there are rigid constraints underlying evolutionary changes in genome organisation. A five-fold increase in the amount of nuclear DNA has occurred in the evolution ofLathyrus species. Not withstanding this massive DNA variation, species show consistently similar patterns in base sequence proliferation, divergence and DNA distribution within and between chromosome complements. Within chromosome complements, the excess DNA is distributed evenly in all chromosomes irrespective of the large differences in chromosome size and, between complements, DNA distribution is discontinuous; species cluster into DNA groups with remarkably regular intervals. Similar constraints govern the frequency and distribution of chiasmata in the chromosome complements. Between species chiasma frequency and nuclear DNA amounts are not correlated but within complements it is positively correlated with the amount of DNA contained in each chromosome.     

Genome size variations within Dasypyrum villosum: correlations with chromosomal traits, environmental factors and plant phenotypic characteristics and behaviour in reproduction

 Feulgen/DNA cytophotometric determinations carried out in the root meristem of seedlings showed that substantial quantitative alterations in the nuclear genome are present between and within 15 natural populations of Dasypyrum villosum in Italy. When the most variant values are considered, there is a 17.6% difference between the mean genome size of the populations, and a 66.2% difference between the genome size of individual plants within a population. A highly significant, positive correlation was found to exist between the genome size of D. villosum plants and the altitude of their stations, and differences in DNA contents between individual plants were greater in populations from mountain sites. Karyological analyses showed all chromosome pairs to differ largely in size between plants with differing DNA contents. A highly significant, positive correlation was found to exist between genome size and both the length of the chromosome complement at metaphase and the length and arm ratio of pair VII. Significant correlations were also found between DNA content and certain phenotypic characteristics of the plants. The mean genome size of the populations was negatively correlated with the mean leaf length and width. In contrast, the genome size of individual plants was positively correlated with the weight of the seed from which they originated and their flowering interval. A large range of genome sizes was found in the half-sib progeny of a plant having a relatively large genome. In contrast, in the half-sib progeny of a plant having a small genome, the genome sizes of the individual plants were less divergent and similar to that of the mother plant. All siblings from crosses between plants with differing genome sizes had similar DNA contents, which were intermediate between those of the parental plants, even if closer to the DNA content of the parent plant having the smaller genome size. Size polymorphism within pairs was never observed in plants obtained from these crosses or in half-sibs whose genome size differed from that of the mother plant. The intraspecific alterations observed in the nuclear genome and their effects on plant development and phenotype are briefly discussed as evolutionary factors which allow D. villosum populations to withstand different environmental conditions as well as the variability of conditions in a given environment. Received: 6 October 1997 / Accepted: 28 October 1997     

Chromosomal DNA variation in Cucumis

Summary Variation in nuclear DNA amounts found in different species of Cucumis was surveyed. The DNA amounts varied from 1.373 to 2.483 pg in diploids and from 2.846 to 3.886 pg in tetraploids. DNA amount was not correlated with chromosome number and periodicity. Tetraploids were found to have double the quantity of nuclear DNA of diploids. A positive linear relationship was established between the nuclear DNA amounts and volume of chromosomes. The botanical varieties within a particular species do not differ significantly for 2C DNA amounts. A comparison of the distribution of DNA amounts among different chromosomes of haploid complement in different species revealed that the quantitative DNA changes associated with speciation affected all chromosomes. DNA changes were not however, of the same magnitude in all chromosomes of the complement. Speciation in Cucumis thus seems to have occurred through amplification or diminution of DNA proportionate to the size of chromosomes. The relationship between the basic numbers, x=7 and x=12, will have to be considered relative to the high DNA amount noticed in some species with x=12.     

Ploidy-dependent genomic stability in the tissue cultures of ornamental Phlox drummondii Hook

Comparisons of the chromosome numbers, 2C nuclear DNA amounts and karyomorphology were made in explant cultures of diploid (2n = 2x = 14) and autotetraploid (2n = 4x = 28) Phlox drummondii. In 6–36 week old calli derived from diploid internodal segment explants, and in cells of root tips regenerated from such callus, marked differences were observed in chromosome number. The chromosome numbers ranged from 2n = 14 to 2n = 100 and DNA amounts from 8.20 to 63.20 pg in the diploid derived callus, while the extent of variation was much reduced in the regenerated roots. In contrast, the autotetraploid cultures were characterised by the maintenance of the same chromosome number and DNA amounts as the mother plant. Modified chromosome structures were not apparent in any of the cultures. The possible reasons for the chromosomal instability at the diploid level and stability at the tetraploid level are discussed.     

Determination of ploidy level and nuclear DNA content in blueberry by flow cytometry

The technique of DNA flow cytometry was used to study variation in DNA content among different ploidy levels, as well as among diploid species, of Vaccinium section Cyanococcus. In a sample of plants of varying ploidy level, the relative fluorescence intensity (RFI) of nuclei stained with propidium iodide was a function of the number of chromosome sets (x), as represented by the linear equation RFI=3.7x-2.3 (r²=95%). The data indicated that DNA flow cytometry could be useful for the determination of ploidy level at the seedling stage in blueberry. They also suggest that conventional polyploid evolution has occurred in this section of the genus Vaccinium with an increase in nuclear DNA content concurrent with the increase in chromosome number. The nuclear DNA content of diploid species of Vaccinium section Cyanococcus was estimated from the relationship of the observed RFI to an internal known DNA standard (trout red blood cells). A nested analysis of variance indicated significant variation among species, as well as among populations within species, in nuclear DNA content, although this variation was small compared to the variation among ploidy levels. The variation in nuclear DNA content corresponded to the phylogenetic relationships among species determined from previous studies.     

Quantitative karyology of some species ofLuzula

The dimensions of metaphase chromosomes and nuclear DNA contents were measured in eight species ofLuzula. The 2 C DNA contents ranged from 8.51 pg inL. purpurea to 0.55 pg inL. pilosa. Total chromosome volume shows a linear relationship with DNA content; however, the total chromosome length of the complement of the different species is approximately constant. Nucleolar volume and the number of chromocentres in the different species also show a relationship with DNA content. Taken together, these data suggest that while chromosome fragmentation could have generated the present-day range of chromosome numbers in the genus, there have also been changes in the total quantity of DNA with the result that species with similar chromosome numbers have different DNA contents. The relationships of DNA content with chromosome volume inLuzula and other genera are compared and the differences discussed.     

Genome size evolution in Sapindaceae at subfamily level: a case study of independence in relation to karyological and palynological traits

Sapindaceae s.l. is a moderately large family of trees, shrubs and lianas. Current knowledge on genome size and how it varies in this family is scarce. This research aims to characterize the DNA content in 39 species of Sapindaceae, mainly in tribe Paullinieae s.s., by the analysis of the variation in genome size relative to karyotypic and palynological features. Nuclear DNA amount was measured by flow cytometry, and linear regression analyses were conducted to analyse the relationship between genome size variation and various karyotypic and palynological features. Genome size varied nine‐fold among species, ranging from 1C = 0.305 pg (Lophostigma plumosum) to 2.710 pg (Cardiospermum heringeri). The low regression coefficients obtained suggest that genome size mainly varies independently of karyotypic and palynological features. With regard to karyotype evolution, the constant chromosome number but variable genome size in Houssayanthus, Paullinia and Serjania suggest that structural changes mainly caused by changes in the amounts of repetitive DNA are more important than numerical change. In contrast, in Cardiospermum and Urvillea, variation in chromosome number and genome size supports the suggestion that numerical and structural changes are important in the karyotype evolution of these genera. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 589–600.     

Quantitative nuclear DNA differences associated with genome evolution in Guizotia (Compositae)

The present communication deals with 2C nuclear genome size variation in a fairly small genus Guizotia. Twenty-four accessions belonging to six species, out of seven known, were analysed in order to elucidate the extent of DNA variation both at an intra—as well as interspecific level. At the intraspecific level none of the species exhibited significant differences in their genome size. Between the species, the 2C DNA amounts ranged from 3.61 pg in G. reptans to 11.37 pg in G. zavattarii; over three-fold DNA variation is evident. Apparently these interspecific DNA differences have been achieved independent of the numerical chromosomal change(s), as all the Guizotias share a common chromosome number 2n=2x=30. The cultivated oilseed crop, G. abyssinica (7.57 pg), has accommodated nearly 78% extra DNA in its chromosome complement during the evolutionary time scale of its origin and domestication from the wild progenitor G. schimperi (4.25 pg). The extent of genomic DNA difference(s) between the species has been discussed in the light of their interrelationships and diversity.     

Nuclear genetic activity in B-chromosome rye,in terms of the quantitative interrelationships between nuclear protein,nuclear RNA and histone

The nuclear basis of B-chromosome genetic activity in rye has been investigated using quantitative cytochemical techniques on isolated root tip nuclei. Nuclear DNA amount was found to be directly proportional to B-chromosome number. Relative amounts of total nuclear protein and nuclear RNA however, decreased with increasing numbers of B's but not in a strictly linear fashion. The values were disproportionately low for odd numbered B-classes of plants. Histone protein was found to increase as the number of B's went up, and in this case the values were disproportionately high for odd numbered B-classes. A negative correlation was found between histone and total nuclear protein and histone and nuclear RNA amounts.     

Analysis of transposable elements and organellar DNA in male and female genomes of a species with a huge Y chromosome reveals distinct Y centromeres

Few angiosperms have distinct Y chromosomes. Among those that do are Silene latifolia (Caryophyllaceae), Rumex acetosa (Polygonaceae) and Coccinia grandis (Cucurbitaceae), the latter having a male/female difference of 10% of the total genome (female individuals have a 0.85 pg genome, male individuals 0.94 pg), due to a Y chromosome that arose about 3 million years ago. We compared the sequence composition of male and female C. grandis plants and determined the chromosomal distribution of repetitive and organellar DNA with probes developed from 21 types of repetitive DNA, including 16 mobile elements. The size of the Y chromosome is largely due to the accumulation of certain repeats, such as members of the Ty1/copia and Ty3/gypsy superfamilies, an unclassified element and a satellite, but also plastome‐ and chondriome‐derived sequences. An abundant tandem repeat with a unit size of 144 bp stains the centromeres of the X chromosome and the autosomes, but is absent from the Y centromere. Immunostaining with pericentromere‐specific markers for anti‐histone H3Ser10ph and H2AThr120ph revealed a Y‐specific extension of these histone marks. That the Y centromere has a different make‐up from all the remaining centromeres raises questions about its spindle attachment, and suggests that centromeric or pericentromeric chromatin might be involved in the suppression of recombination.     

An upper limit of the ratio of DNA volume to nuclear volume exists in plants

The variations in nuclear DNA content from 2 x 10(2) to 2.5 x 10(5) Mbp are reported in higher plants. The major finding so far is that the genome size of plant species differs by three orders of magnitude, which are more variable than the other organisms. Investigations pertaining to the manner in which DNA is packaged in the nucleus provide us with basic information on the made of DNA existence in the plant nucleus. However, the fundamentals on nuclear DNA content and nuclear size, which underlie and enable the flexible containment of such large differences in nuclear DNA content, remain unknown. We analyzed the nuclear volumes of plants with 2C value DNA contents ranging from 3.2 x 10(2) to 1.0 x 10(5) Mbp. As a result, we obtained a constant ratio between the DNA volume and nuclear volume, which does not exceed 3%. Furthermore, we also demonstrate that the nuclear Rabl model of chromatin organisation is not a common 3-D structure, even in plants with large nuclear DNA contents. The existence of an upper limit of DNA volume ratio would present a basal parameter for the future insight into the nuclear organisation in higher plants.     

DNA Content Variation and Its Significance in the Evolution of the Genus Micrasterias (Desmidiales,Streptophyta)

It is now clear that whole genome duplications have occurred in all eukaryotic evolutionary lineages, and that the vast majority of flowering plants have experienced polyploidisation in their evolutionary history. However, study of genome size variation in microalgae lags behind that of higher plants and seaweeds. In this study, we have addressed the question whether microalgal phylogeny is associated with DNA content variation in order to evaluate the evolutionary significance of polyploidy in the model genus Micrasterias. We applied flow-cytometric techniques of DNA quantification to microalgae and mapped the estimated DNA content along the phylogenetic tree. Correlations between DNA content and cell morphometric parameters were also tested using geometric morphometrics. In total, DNA content was successfully determined for 34 strains of the genus Micrasterias. The estimated absolute 2C nuclear DNA amount ranged from 2.1 to 64.7 pg; intraspecific variation being 17.4–30.7 pg in M. truncata and 32.0–64.7 pg in M. rotata. There were significant differences between DNA contents of related species. We found strong correlation between the absolute nuclear DNA content and chromosome numbers and significant positive correlation between the DNA content and both cell size and number of terminal lobes. Moreover, the results showed the importance of cell/life cycle studies for interpretation of DNA content measurements in microalgae.     

Chromosome studies in Hippeastrum (Amaryllidaceae): variation in genome size

This paper presents the karyotype and DNA content of 12 diploid species of Hippeastrum from South America. The variation in genome size is compared with the karyotype and DNA content of Amaryllis belladonna from South Africa. The Hippeastrum species present a uniform and bimodal basic karyotype formula, but significant differences are found in the total chromosome volume (TCV) and nuclear DNA content. A positive correlation between the DNA content and TCV is also observed. The karyotype's constancy is a product of changes in DNA content occurring in the whole chromosome complement. The DNA addition to the long and short sets of chromosomes varies independently. In species with higher DNA contents, the short chromosomes add equal DNA amounts to both arms, maintaining their metacentric morphology, whereas the long chromosomes add DNA only to the short arm, increasing the chromosome symmetry. These data show that the evolutionary changes in DNA amount are proportional to chromosome length, maintaining the karyotypic uniformity. A. belladonna has a larger DNA content and possesses a karyotype different from that of Hippeastrum spp., supporting the distinction between the two genera and upholding the name Amaryllis for the South African entity against Hippeastrum for the South American genus.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 171–178.     

Nuclear DNA changes within Helianthus annuus L.: cytophotometric,karyological and biochemical analyses

Summary Cytophotometric measurement of the root meristems of seedlings after Feulgen-staining reveals that large differences (up to 58.16%) in nuclear DNA content may occur in the thirty-one cultivated varieties or lines of Helianthus annuus tested. Significant variations (not exceeding 25%) in the amount of DNA, which does not differ between the root and the shoot meristems of a single seedling, are also found to exist within cultivars or lines; even seedlings obtained from seeds collected from different portions of single heads of plants belonging to a selfed line may vary one from the other in this respect. Variations in the number of chromosomes or alterations in the chromosome structure do not account for the differences observed in nuclear DNA content. Karyometric analyses demonstrate that the surface area of squashed interphase nuclei and metaphase chromosomes and the total length of the latter increase with the increase in Feulgen/DNA absorption. DNA thermal denaturation and reassociation kinetics indicate that a frequency variation in repeated DNA sequences goes hand in hand with changes in the size of the genome. These results, supporting the concept that a plant genome is highly flexible, are discussed in relation to other data to be found in the literature on the intraspecific variation in the nuclear DNA content and in relation to the way in which it is produced in H. annuus.     

Genome size dynamics in Artemisia L. (Asteraceae): following the track of polyploidy

Polyploidy is a key factor in the evolution of higher plants and plays an important role in the variation of plant genomes, leading to speciation in some cases. During polyploidisation, different balancing processes take place at the genomic level that can promote variation in nuclear DNA content. We estimated genome size using flow cytometry in 84 populations of 67 Artemisia species and one population of Crossostephium chinense. A total of 73 sequences of nrDNA ITS and 3′‐ETS were newly generated and analysed, together with previously published sequences, to address the evolution of genome size in a phylogenetic framework. Differences in 2C values were detected among some lineages, as well as an increase of genome size heterogeneity in subgenera whose phylogenetic relationships are still unclear. We confirmed that the increase in 2C values in Artemisia polyploids was not proportional to ploidy level, but 1Cx genome size tended to decrease significantly when high ploidy levels were reached. The results lead us to hypothesise that genome size in polyploids tends to a maximum as it follows saturation behaviour, in agreement with the Michaelis–Menten model. We tested different arithmetic functions with our dataset that corroborated a non‐linear relationship of genome size increase in polyploids, allowing us to suggest a theoretical upper limit for the DNA content of this genus.     

B chromosome ancestry revealed by histone genes in the migratory locust

In addition to the standard set of chromosomes (A), about 15% of eukaryote genomes carry B chromosomes. In most cases, B chromosomes behave as genomic parasites being detrimental for the individuals carrying them and prospering in natural populations because of transmission advantages (drive). B chromosomes are mostly made up of repetitive DNA sequences, especially ribosomal DNA (rDNA), satellite DNA and mobile elements. In only two cases have B chromosomes been shown to carry protein-coding genes. Although some B chromosomes seem to have derived from interspecific hybridisation, the most likely source of B chromosomes is the host genome itself, but the specific A chromosome being the B ancestor has not been identified in any B-containing species. Here, we provide strong evidence for B chromosome ancestry in the migratory locust, based on the location of genes for the H3 and H4 histones in the B chromosome and a single A chromosome pair (i.e. the eighth in order of decreasing size). The high DNA sequence similarity of A and B chromosome H3–H4 genes supports B-origin from chromosome 8. The higher variation shown by B sequences, compared to A sequences, suggests that B chromosome sequences are most likely inactive and thus less subjected to purifying selection. Estimates of time of divergence for histone genes from A and B chromosomes suggest that B chromosomes are quite old (>750,000 years), showing the B-chromosome ability to persist in natural populations for long periods of time.     

Constraints upon the organisation and evolution of chromosomes in Allium

Summary In terms of chromosome morphology, karyotype organisation, taxonomy and genetic relationship as judged from chromosome pairing in the Fl hybrid, A. cepa and A.fistulosum are two closely related species. But large variation in nuclear DNA amounts has occurred during the evolution of the two species. A comparison of the molecular composition of DNA in the two species has confirmed that the excess DNA acquired during evolution was predominantly repetitive sequences (sequences which do not encode genetic information). However, its distribution within the chromosome complements was equal in all chromosomes irrespective of the differences in chromosome size. The even distribution of the excess DNA within complements suggests strong constraints underlying evolutionary changes in genome organisation. The nature of the constraints is discussed, and it is shown that such constraints can influence the direction of karyotype evolution during speciation.