Variation In Nuclear DNA Amount Research Articles

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.

Genome size in natural and synthetic autopolyploids and in a natural segmental allopolyploid of several Triticeae species

Nuclear DNA amount (1C) was determined by flow cytometry in the autotetraploid cytotype of Hordeum bulbosum, in the cytologically diploidized autotetraploid cytotypes of Elymus elongatus, Hordeum murinum subsp. murinum and Hordeum murinum subsp. leporinum, in Hordeum marinum subsp. gussoneanum, in their progenitor diploid cytotypes, and in a newly synthesized autotetraploid line of E. elongatus. Several lines collected from different regions of the distribution area of every taxon, each represented by a number of plants, were analyzed in each taxon. The intracytotype variation in nuclear DNA amount of every diploid and autotetraploid cytotype was very small, indicating that no significant changes have occurred in DNA amount either after speciation or after autopolyploid formation. The autotetraploid cytotypes of H. bulbosum and the cytologically diploidized H. marinum subsp. gussoneanum had the expected additive amount of their diploid cytotypes. On the other hand, the cytologically diploidized autotetraploid cytotypes of E. elongatus and H. murinum subsp. murinum and H. murinum subsp. leporinum had considerably less nuclear DNA (10%-23%) than the expected additive value. Also, the newly synthesized autotetraploid line of E. elongatus showed similar reduction in DNA as its natural counterpart, indicating that the reduction in genome size occurred in the natural cytotype during autopolyploidization. It is suggested that the diploid-like meiotic behavior of these cytologically dipolidized autotetraploids is caused by the instantaneous elimination of a large number of DNA sequences, different sequences from different homologous pairs, leading to differentiation of the constituent genomes. The eliminated sequences are likely to include those that participate in homologous recognition and initiation of meiotic pairing. A gene system determining exclusive bivalent pairing by utilizing the differentiation between the two groups of homologues has been presumably superimposed on the DNA reduction process.

A century of B chromosomes in plants: so what?

Supernumerary B chromosomes (Bs) are a major source of intraspecific variation in nuclear DNA amounts in numerous species of plants. They favour large genomes, and create polymorphisms for DNA variation in natural populations. By studying Bs we can gain useful knowledge about the organization, function and evolution of genomes. There are also significant biological questions concerning the origin and structural organization of Bs, and the way in which these selfish elements can establish themselves by exploiting the replicative machinery of their host genome nucleus. It is a sine qua non that Bs originate from the A chromosomes, in a variety of ways. We can study their modes of drive and ask how it is that chromosomes which apparently lack genes can have control over their own drive process which leads to their survival in natural populations. Molecular cytogenetic studies are opening up new avenues of investigation. Population equilibria for B frequencies are determined by a balance between accumulation and harmful effects. Bs are also subject to meiotic loss due to polysomy and to elimination at meiosis as univalents. These balancing forces can be seen in the context of host/parasite interaction, based on a dissection of the genetic elements in both As and Bs (in maize) which interact to bring about a stable equilibrium, at least for a snapshot in time. Aside from their intrinsic enigmatic properties, B chromosomes make useful experimental tools to study genome organization. Thus far they have not been exploited for their applications, other than through the use of A-B translocations used for gene mapping in maize; but there are opportunities to use them to modulate the frequency and distribution of recombination, to diploidize allopolyploids, to study centromeres and to be developed as plant artificial chromosomes; given that they can be structurally modified and their inheritance stabilized.

Genome size and genome evolution in diploid Triticeae species

One of the intriguing issues concerning the dynamics of plant genomes is the occurrence of intraspecific variation in nuclear DNA amount. The aim of this work was to assess the ranges of intraspecific, interspecific, and intergeneric variation in nuclear DNA content of diploid species of the tribe Triticeae (Poaceae) and to examine the relation between life form or habitat and genome size. Altogether, 438 plants representing 272 lines that belong to 22 species were analyzed. Nuclear DNA content was estimated by flow cytometry. Very small intraspecific variation in DNA amount was found between lines of Triticeae diploid species collected from different habitats or between different morphs. In contrast to the constancy in nuclear DNA amount at the intraspecific level, there are significant differences in genome size between the various diploid species. Within the genus Aegilops, the 1C DNA amount ranged from 4.84 pg in A. caudata to 7.52 pg in A. sharonensis; among genera, the 1C DNA amount ranged from 4.18 pg in Heteranthelium piliferum to 9.45 pg in Secale montanum. No evidence was found for a smaller genome size in annual, self-pollinating species relative to perennial, cross-pollinating ones. Diploids that grow in the southern part of the group's distribution have larger genomes than those growing in other parts of the distribution. The contrast between the low variation at the intraspecific level and the high variation at the interspecific one suggests that changes in genome size originated in close temporal proximity to the speciation event, i.e., before, during, or immediately after it. The possible effects of sudden changes in genome size on speciation processes are discussed.

Evolution of genome size across some cultivated Allium species

Allium L. (Alliaceae), a genus of major economic importance, exhibits a great diversity in various morphological characters and particularly in life form, with bulbs and rhizomes. Allium species show variation in several cytogenetic characters such as basic chromosome number, ploidy level, and genome size. The purpose of the present investigation was to study the evolution of nuclear DNA amount, GC content, and life form. A phylogenetic approach was used on a sample of 30 Allium species, including major vegetable crops and their wild allies, belonging to the 3 major subgenera Allium, Amerallium, and Rhizirideum and 14 sections. A phylogeny was constructed using internal transcribed spacer (ITS) sequences of 43 accessions representing 30 species, and the nuclear DNA amount and the GC content of 24 Allium species were investigated by flow cytometry. For the first time, the nuclear DNA content of Allium cyaneum and Allium vavilovii was measured, and the GC content of 16 species was measured. We addressed the following questions: (i) Is the variation in nuclear DNA amount and GC content linked to the evolutionary history of these edible Allium species and their wild relatives? (ii) How did life form (rhizome or bulb) evolve in edible Allium? Our results revealed significant interspecific variation in the nuclear DNA amount as well as in the GC content. No correlation was found between the GC content and the nuclear DNA amount. The reconstruction of nuclear DNA amount on the phylogeny showed a tendency towards a decrease in genome size within the genus. The reconstruction of life form history showed that rhizomes evolved in the subgenus Rhizirideum from an ancestral bulbous life form and were subsequently lost at least twice independently in this subgenus.

Plant Classification for Ecological Purposes: is there a Role for Genome Size?

Genome size is a strong candidate for inclusion in the list of traits needed to devise a functional classification of plants. To facilitate modelling and prediction of vegetation responses to regional and global changes in landuse and climate, a distinctively ecological agenda is recommended for future research on inter-specific variation in nuclear DNA amount.

Variation in Karyotype and 4C DNA Content in Six Species of Melocactus of the Family Cactaceae.

Analysis of karyotype including somatic chromosome number determination, total chromosome length and volume, estimation of 4C DNA content and Interphase Nuclear Volume (INV) were carried out in 6 species of Melocactus of the family Cactaceae. Somatic chromosome number recorded 2n=22 in Melocactus bahiensis, M curvispinus, M robustispinus, M. townsendii and 2n=44 in M longispinus, M. matanzanus for the first time. Significant interspecific variation in nuclear DNA amount was recorded. The 4C DNA content varied from 20.55 pg in M. townsendii to 40.80 pg in M. longispinus. The correlation coefficient studies showed that the genomic chromosome length, chromosome volume and INV were possitively correlated. No interdependent was found between 4C DNA content and chromosome length or volume and INV. The structural alterations in the chromosomes as well as loss or addition of highly repetitive sequences in the genome showed variations in nuclear DNA at interspecific level indicating a macro- and micro-evolution of the species.

New report on chromosome number, karyotype and 4C DNA content in three species of Pachypodium Lindley

SUMMARYExtensive karyotype analysis including determination of somatic chromosome number, total chromosome length and volume and estimation of 4C DNA content were carried out on three species of Pachypodium namely P. lamerei, P. namaquanum and P. rosulatum ‘horomhense’ for the first time. A significant variation in nuclear DNA amount was recorded at interspecific level. The somatic chromosome number 2n=18 was recorded in all the species. The 4C DNA amount varied with 8.425 pg in P. lamerei, to 10.918 pg in P. rosulatum ‘horomhense’ and 11.928 pg in P. namaquanum. The correlation coefficient studies showed that the 4C DNA content and genomic chromosome volume were interdependent. The structural alteration of chromosomes as well as loss or addition of highly repetitive sequences in the genome showed a variation in the DNA amount at species level indicated a close relationship between them.

Estimation of 4C DNA and Karyotype Analysis in Ginger (Zingiber officinale Rosc.). I.

Extensive karyotype analysis including determination of somatic chromosome number, total chromosome length and volume and estimation of 4C DNA amount were carried out on 9 different cultivars of ginger (Zingiber officinale Rose.) for the first time. A significant variation in nuclear DNA amount was recorded at the cultivar level. The 4C DNA amount varied from 19.663-24.102 pg in the cultivars studied. The correlation coefficient studies showed that the 4C DNA content and genomic chromosome volume were interdependent. The structural alteration of chromosomes as well as loss or addition of highly repetitive sequences in the genome showed variation in the DNA amount at cultivar level, but a marginal variation in nuclear DNA content at the cultivar level indicated a close relationship between them.

Variation in nuclear DNA amount between wild and cultivated populations of Milium effusum (2n = 28)

Mean 2C DNA amounts varied by 35.6%, ranging from 7.52 to 10.20 pg, between 10 populations of the grass Milium effusum L. Such intraspecific variation occurred despite a constant chromosome number (2n = 28) and no obvious differences in karyotype. Plants originating from botanic garden populations growing in cultivation had significantly (P < 0.001) larger DNA amounts than plants collected from wild populations. Moreover, variation in DNA amount within either the "cultivated" or the "wild" groups was not significant. As the environment in which plants are kept in botanic gardens is clearly different to the natural habitat for M. effusum, it seems likely that the difference in nuclear DNA amount is causally related, perhaps through its nucleotypic effects, to microclimate adaptation. These results suggest that at least some genotypes of M. effusum are fluid and sensitive to environmental change. Such data may have broad practical importance regarding plant responses to various environmental changes such as a nuclear winter and global warming, and implications for plant conservation and reintroduction. Milium effusum is a potentially useful plant material for studying the nature of intraspecific variation in DNA amount.Key words: Milium effusum, nuclear DNA amounts, nucleotype, environmental adaptation.

Intra-Population Variation in Nuclear DNA Amount, Cell Size and Growth Rate in Poa annua L.

An 80% variation in nuclear DNA amount at constant chromosome number has been found between families of seed progeny derived from established plants of Poa ann ua L. taken from a single pasture population in North Wales. The 2C DNA amount varied from 2 9pg to 5 2pg. This variation was positively associated with variation in cell size, negatively correlated with variation in seedling growth rate, but was not associated with variation in life-history. We suggest that this variation may enable individuals to exploit different temporal, micro-climatic niches within the grassland habitat. Key-words: Intra-population variation, nuclear DNA amount, cell size, growth rate, temporal niche, grassland

Variation in 4C DNA content and chromosome characteristics in different varieties of Coriandrum sativum L.

Extensive karyological analysis including determination of somatic chromosome number, total chromosome length and volume and estimation of 4C DNA amount were carried out on 8 different varieties of Coriandrum sativum L. A significant variation in nuclear DNA amount was recorded in the varietal level. The structural alteration of chromosome as well as loss or addition of highly repetitive sequences in the genome have played a vital role in intervarietal DNA variation. The absence of wide differences in nuclear DNA content in the varietal level indicate a close relationship between them.

The distribution and divergence during evolution of families of repetitive DNA sequences inLathyrus species

Evolution and divergence among, species within the genusLathyrus have involved an approximately fivefold increase in the amounts of nuclear DNA. Most species inLathyrus are diploids with the same chromosome number, 2n=14. Significant changes in the amounts of repetitive sequences have accounted for much of the evolutionary DNA variation between species. Seven diploidLathyrus species with a twofold variation in nuclear DNA amounts between them were investigated. Using higher derivative analysis of the thermal denaturation profiles of the reassociated repetitive DNA, the reiteration frequency and divergence of repetitive families were compared. Much variation in the reiteration frequency was observed within and between species. In species with larger 2C DNA amounts repetitive families had on average greater amounts of DNA. Despite the massive differences in DNA amounts, six species were consistently similar in the number of repetitive families in their genomes, and they showed a similar pattern in base sequence divergence. In terms of base sequence relationships the repetitive families appeared to be heterogeneous. The evolutionary significance is discussed.

Constraints upon the organisation and evolution of chromosomes in Allium

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.

Nuclear DNA changes, genome differentiation and evolution inNicotiana (Solanaceae)

A survey of 51 species fromNicotiana subgg.Tabacum, Rustica andPetunioides has shown that evolution was accompanied by a five-fold variation in nuclear DNA amounts. This variation, however, was not directly correlated with the changes in chromosome number. Drastic rearrangement of karyotypes is characteristic for the evolution ofNicotiana spp. Significant gain or loss in nuclear DNA has often accompanied such changes, but DNA variation has also occurred without significant changes in karyotype arrangements.—The distribution of nuclear DNA is discontinuous inNicotiana, species cluster into DNA groups with consistently regular increments in the mean DNA amounts. The discontinuities are viewed as “steady states” in terms of genomic balance and biological fitness.—Changes in the amount of nuclear DNA and in the heterochromatin are compared with the morphological, chromosomal and adaptive changes which accompanied speciation in 14 subgeneric sections. The evolutionary significance of DNA variation is discussed.

Chromosomal DNA variation in Cucumis

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.

Feulgen microspectrophotometric estimation of nuclear DNA of species and varieties of three different genera of Marantaceae

Karyological analysis including determination of somatic chromosome number, total chromosome length, volume and karyotype formula andin situ estimation of 4C-nuclear DNA amount were carried out on 14 different species and varieties of the generaCalathea, Maranta andStromanthe. The 4C nuclear DNA amount was estimated through Feulgen microspectrophotometry following single wavelength method and expressed in arbitrary units of relative absorbances. The variation in 4C DNA amounts between the species ofCalathea was not distinct but in two species ofMaranta, a notable variation in nuclear DNA amount was recorded. In addition, the amount of nuclear DNA did not show direct correlation with the total chromosome length and volume. The absence of wide difference in nuclear DNA content at an interspecific level might have some adaptive value.

Nuclear DNA divergence among Lathyrus species

Among diploid Lathyrus species a threefold variation in nuclear DNA amount is attributable to differences in the amount of repetitive DNA. Cross reassociation among repetitive and among non-repetitive DNA fractions from different species shows substantial divergence in DNA composition. The divergence in base composition is correlated with nuclear DNA amount. The degree of divergence is of the same order of magnitude in both the repetitive and nonrepetitive fractions.

Nuclear DNA amounts in populations of Picea and Pinus species

There are numerous reports of a very wide range of variation in nuclear DNA amounts among populations within species of the Pinaceae, including Picea glauca, the White Spruce. Our survey of 26 provenances, covering almost the entire range of White Spruce in North America showed, in contrast, no significant variation in nuclear DNA amount within the species except for minor fluctuations due to B chromosomes. The DNA estimates, throughout, fall within the range of 37·4 to 40·4 × 10−12 g. The areas of the 2C nuclei measured at G1 from 12 provenances were also highly uniform. In addition, there was no significant variation in the chromosome volume between and within provenances. Both facts reinforce the conclusion that nuclear DNA amounts within the species are constant. There are no detectable differences in the nuclear DNA content between Picea glauca and P. engelmannii. In Pinus contorta as in P. glauca, nuclear areas and DNA amounts do not vary significantly between or within provenances and the mean DNA value of 2C nuclei is 40·34 × 10−12 g. White Spruce in North America alone ranges over 3000 miles of longitude and some 1000 miles of latitude. The constancy of the nuclear DNA amounts among provenances of White Spruce and of the other species growing in widely differing environments is both surprising and impressive. It provides further testimony to the pronounced inertia to quantitative DNA change within species.

Tolerance of Lolium hybrids to quantitative variation in nuclear DNA.

THE genus Lolium contains six species, all diploids with 14 chromosomes. The chromosomes of the inbreeding species are larger than those of the outbreeders and carry about 30% more DNA1. Although the range of variation in nuclear DNA amount is not large, in comparison with other genera of flowering plants2, Lolium is exceptional in that crosses between diploid species with widely different amounts of DNA produce F1 hybrids which set viable seed on backcrossing or, alternatively, by intercrossing to produce F2s. Segregation of nuclear DNA amount among hybrid derivatives by independent chromosome assortment and crossing over at meiosis in the F1 makes feasible a genetic assay of the effects of the differential interspecific DNA component. This report describes the segregation of nuclear DNA amounts among F2 progenies (seedlings 5–7 d old) from F2 hybrids between L. temulentum with high DNA and L. rigidum with low DNA content. F1 seeds were obtained by allowing the F1 hybrids to intercross in isolation from foreign pollen. From the DNA distribution in F2 it is possible to draw conclusions about the influence of the differential DNA component on gamete and zygote viability. The results are given in Fig. 1.