[show abstract][hide abstract] ABSTRACT: The amount of DNA comprising the genome of an organism (its genome size) varies a remarkable 40 000-fold across eukaryotes, yet most groups are characterized by much narrower ranges (e.g. 14-fold in gymnosperms, 3- to 4-fold in mammals). Angiosperms stand out as one of the most variable groups with genome sizes varying nearly 2000-fold. Nevertheless within angiosperms the majority of families are characterized by genomes which are small and vary little. Species with large genomes are mostly restricted to a few monocots families including Orchidaceae.
A survey of the literature revealed that genome size data for Orchidaceae are comparatively rare representing just 327 species. Nevertheless they reveal that Orchidaceae are currently the most variable angiosperm family with genome sizes ranging 168-fold (1C = 0.33-55.4 pg). Analysing the data provided insights into the distribution, evolution and possible consequences to the plant of this genome size diversity.
Superimposing the data onto the increasingly robust phylogenetic tree of Orchidaceae revealed how different subfamilies were characterized by distinct genome size profiles. Epidendroideae possessed the greatest range of genome sizes, although the majority of species had small genomes. In contrast, the largest genomes were found in subfamilies Cypripedioideae and Vanilloideae. Genome size evolution within this subfamily was analysed as this is the only one with reasonable representation of data. This approach highlighted striking differences in genome size and karyotype evolution between the closely related Cypripedium, Paphiopedilum and Phragmipedium. As to the consequences of genome size diversity, various studies revealed that this has both practical (e.g. application of genetic fingerprinting techniques) and biological consequences (e.g. affecting where and when an orchid may grow) and emphasizes the importance of obtaining further genome size data given the considerable phylogenetic gaps which have been highlighted by the current study.
Annals of Botany 02/2009; 104(3):469-81. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Wollemi Pine (Wollemia nobilis, Araucariaceae) was discovered in 1994 in a remote area of the Wollemi National Park (New South Wales, Australia). With only around 40 adult trees known to be growing in the wild, it is one of the worlds rarest plant species. The results of the first cytological study are reported, including a chromosome count (2n = 2x = 26), karyotype illustration and nuclear DNA C-value (4C = 55.74 pg), obtained using Feulgen microdensitometry. The results are compared with data available on species in the other two genera of the Araucariaceae, Araucaria and Agathis, and with other gymnosperms
Botanical Journal of the Linnean Society 06/2008; 135(3):271 - 274. · 2.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: In studies looking at individual polyploid species, the most common patterns of genomic change are that either genome size in the polyploid is additive (i.e. the sum of parental genome donors) or there is evidence of genome downsizing. Reports showing an increase in genome size are rare. In a large-scale analysis of 3008 species, genome downsizing was shown to be a widespread biological response to polyploidy. Polyploidy in the genus Nicotiana (Solanaceae) is common with approx. 40 % of the approx. 75 species being allotetraploid. Recent advances in understanding phylogenetic relationships of Nicotiana species and dating polyploid formation enable a temporal dimension to be added to the analysis of genome size evolution in these polyploids.
Genome sizes were measured in 18 species of Nicotiana (nine diploids and nine polyploids) ranging in age from <200,000 years to approx. 4.5 Myr old, to determine the direction and extent of genome size change following polyploidy. These data were combined with data from genomic in situ hybridization and increasing amounts of information on sequence composition in Nicotiana to provide insights into the molecular basis of genome size changes.
By comparing the expected genome size of the polyploid (based on summing the genome size of species identified as either a parent or most closely related to the diploid progenitors) with the observed genome size, four polyploids showed genome downsizing and five showed increases. There was no discernable pattern in the direction of genome size change with age of polyploids, although with increasing age the amount of genome size change increased. In older polyploids (approx. 4.5 million years old) the increase in genome size was associated with loss of detectable genomic in situ hybridization signal, whereas some hybridization signal was still detected in species exhibiting genome downsizing. The possible significance of these results is discussed.
Annals of Botany 05/2008; 101(6):805-14. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: Most angiosperms possess small genomes (mode 1C = 0.6 pg, median 1C = 2.9 pg). Those with truly enormous genomes (i.e. > or = 35 pg) are phylogenetically restricted to a few families and include Liliaceae - with species possessing some of the largest genomes so far reported for any plant as well as including species with much smaller genomes. To gain insights into when and where genome size expansion took place during the evolution of Liliaceae and the mode and tempo of this change, data for 78 species were superimposed onto a phylogenetic tree and analysed. Results suggest that genome size in Liliaceae followed a punctuated rather than gradual mode of evolution and that most of the diversification evolved recently rather than early in the evolution of the family. We consider that the large genome sizes of Liliaceae may have emerged passively rather than being driven primarily by selection.
Journal of Evolutionary Biology 11/2007; 20(6):2296-308. · 3.48 Impact Factor
[show abstract][hide abstract] ABSTRACT: A key target set at the second Plant Genome Size Workshop, held at the Royal Botanic Gardens, Kew in 2003, was to produce first DNA C-value data for an additional 1 % of angiosperm species, and, within this, to achieve 75 % familial coverage overall (up from approx. 50 %) by 2009. The present study targeted eudicot families for which representation in 2003 (42.5 %) was much lower than monocot (72.8 %) and basal angiosperm (69.0 %) families.
Flow cytometry or Feulgen microdensitometry were used to estimate nuclear DNA C-values, and chromosome counts were obtained where possible.
First nuclear DNA C-values are reported for 20 angiosperm families, including 18 eudicots. This substantially increases familial representation to 55.2 % for angiosperms and 48.5 % for eudicots.
The importance of targeting specific plant families to improve familial nuclear DNA C-value representation is reconfirmed. International collaboration will be increasingly essential to locate and obtain material of unsampled plant families, if the target set by the second Plant Genome Size Workshop is to be met.
Annals of Botany 01/2006; 96(7):1315-20. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: Within Oncidiinae, there are several groups of species that are effectively annuals, and we wished to see if these species had smaller genome sizes than average for the subtribe.
Fifty-four genome size estimates (50 of which are new) for species in subtribe Oncidiinae (Orchidaceae) were examined for the first time in a phylogenetic context to evaluate hypotheses concerning genome sizes and life history traits.
Within the limits of still relatively sparse sampling, the species that are effectively annuals do appear to have smaller genome sizes than average. However, the genome sizes of their immediate sister group are also small, indicating that changes in genome size preceded the change in life history traits. Genome sizes and chromosome numbers also do not correlate; some slowly growing species have lower chromosome numbers but large genomes and vice versa. Based on a survey of the literature on orchids, it is also clear that epiphytic species have smaller genome sizes than do terrestrial species, which could be an effect of different water relations or the fact that most terrestrial orchids are geophytic or have distinct growth and dormancy phases.
Annals of Botany 02/2005; 95(1):191-9. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: Andropogon is a pantropical grass genus comprising 100-120 species and found mainly in the grasslands of Africa and the Americas. While the genomic relationships between many Andropogon species have been resolved by studying chromosome behavior in interspecific hybrids, relationships between the North and South American diploids have remained elusive. Further, the genome composition of two hexaploid species (including the important forage grass Andropogon lateralis Nees) has been unclear because of the strong hybridization barriers that exist between species. Consequently, genomic in situ hybridization was applied to shed light on these issues. The results confirmed that (i) both the South American (Andropogon selloanus (Hack.) Hack., Andropogon macrothrix Trin.) and North American (Andropogon gyrans Michx.) diploid species shared a common S genome and (ii) the S genome comprises just one of the three genomes in the hexaploids A. lateralis Nees and Andropogon bicornis L. The evolutionary and taxonomic implications of these findings are discussed.
[show abstract][hide abstract] ABSTRACT: This paper reports first DNA C-values for 28 angiosperm genera. These include first DNA C-values for 25 families, of which 16 are monocots. Overall familial representation is 47.2 % for angiosperms, but is now much higher for monocots (75 %) and basal angiosperms (73.1 %) than for eudicots (38.7 %). Chromosome counts are reported for 22 taxa, including first records for six genera plus seven species. Unrepresented families will become increasingly enriched for monotypic taxa from obscure locations that are harder to access. Thus, completing familial representation for genome size for angiosperms may prove impossible in any short period, and progress towards this goal will become slower.
Annals of Botany 02/2003; 91(1):31-8. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nuclear DNA C-values and genome size are important biodiversity characters with fundamental biological significance. Yet C-value data for pteridophytes, a diverse group of vascular plants with approx. 9000 extant species, remain scarce. A recent survey by Bennett and Leitch (2001, Annals of Botany 87: 335-345) found that C-values were reported for only 48 pteridophyte species. To improve phylogenetic representation in this group and to check previously reported estimates, C-values for 30 taxa in 17 families were measured using flow cytometry for all but one species. This technique proved generally applicable, but the ease with which C-value data were generated varied greatly between materials. Comparing the new data with those previously published revealed several large discrepancies. After discounting doubtful data, C-values for 62 pteridophyte species remained acceptable for analysis. The present work has increased the number of such species' C-values by 93 %, and more than doubled the number of families represented (from 10 to 21). Analysis shows that pteridophyte C-values vary approx. 450-fold, from 0-16 pg in Selaginella kraussiana to 72.7 pg in Psilotum nudum var. gasa. Superimposing C-value data onto a robust phylogeny of pteridophytes suggests some possible trends in C-value evolution and highlights areas for future work.
Annals of Botany 09/2002; 90(2):209-17. · 3.45 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nuclear DNA C-value is an important genomic biodiversity character with many uses. An international workshop sponsored by Annals of Botany and held at the Royal Botanic Gardens, Kew, UK, in 1997 identified major gaps in our knowledge of plant DNA C-values and recommended targets for new work. Improved taxonomic coverage was highlighted as a key need for angiosperms, especially at the familial level. In 1997 C-values were known for only approx. 32% of angiosperm families; a goal of complete familial representation by 2002 was recommended. A review published in 2000 (Bennett et al. ; Annals of Botany 86 : 859–909) noted poor progress towards this aim: of the 691 first C-values for species only 12 (1.7%) were for unrepresented families. We began new work to address this in 1999, reporting first DNA C-values for 25 angiosperm families in 2001 (Hanson et al. ; Annals of Botany 87 : 251–258). Here we report first DNA C-values for a further 25 angiosperm families, increasing familial coverage in angiosperms to approx. 45%. Such targeting remains essential to approach the goal set by the 1997 workshop of familial coverage for angiosperms within 5 years. The 4C DNA amounts presented here range from 0.76 pg (similar to Arabidopsis thaliana ) in Roridula gorgonias (Roridulaceae) to 29.74 pg in Gunnera manicata (Gunneraceae). 1C values were < 3.5 pg in 23 of the 25 families; these data provide further support for the view that ancestral angiosperms almost certainly had small genomes (defined as 1C ⩽ 3.5 pg). Chromosome counts are reported for 20 taxa, including first records for one genus and five species. Copyright 2001 Annals of Botany Company
[show abstract][hide abstract] ABSTRACT: The gymnosperms are a monophyletic yet diverse group of woody trees with approx. 730 extant species in 17 families. A recent survey showed that DNA C-values were available for approx. 16% of species, but for only 12 of the 17 families. This paper completes familial representation reporting first C-values for the five remaining families: Boweniaceae, Stangeriaceae, Welwitschiaceae, Cephalotaxaceae and Sciadopityaceae. C-values for nine Ephedra and two Gnetum species are also reported. C-values are now available for 152 (21%) species. Analysis confirms that gymnosperms are characterized by larger C-values than angiosperms (modal 1C of gymnosperms = 15.8 pg compared with 0.6 pg in angiosperms) although the range (1C = 2.25–32.20 pg) is smaller than that in angiosperms (1C = 0.05–127.4 pg). Given complete familial coverage for C-values and increasing consensus in gymnosperm phylogeny, the phylogenetic component of C-value variation was also investigated by comparing the two datasets. This analysis revealed that ancestral gymnosperms (represented by cycads and/or Ginkgo ; mean genome size = 14.71 pg) probably had larger genomes than ancestral angiosperms. Copyright 2001 Annals of Botany Company
[show abstract][hide abstract] ABSTRACT: After selective enrichment and differential hybridisation of Cot-1 DNA fractions of plants with and without polymorphic heterochromatic segments, a repetitive sequence (called Bds1) specific
to the polymorphic chromosome segments of Brachycome dichromosomatica (Brachyscome dichromosomatica) was isolated. A single repeat unit of Bds1 is 92 bp long and is organised in tandem arrays at three different polymorphic
segment sites on the chromosomes of cytodeme A2. Although all three sites showed extensive polymorphism between plants, the
karyotypes of all analysed mitotic root cells were stable within a single plant. Electron microscopy revealed heavily condensed
chromatin structures at the most obvious polymorphic site. The mechanisms that generate and maintain the observed chromosome
structure polymorphisms are discussed.
[show abstract][hide abstract] ABSTRACT: Allohexaploid wild oat, Avena fatua L. (Poaceae; 2n = 6x = 42), is one of the world's worst weeds, yet unlike some of the other Avena hexaploids, its genomic structure has been relatively little researched. Consequently, in situ hybridisation was carried out on one accession of A. fatua using an 18S-25S ribosomal DNA (rDNA) sequence and genomic DNA from A. strigosa (AA-genome diploid) and A. clauda (CC-genome diploid) as probes. Comparing these results with those for other hexaploids studied previously: (i) confirmed that the genomic composition of A. fatua was similar to the other hexaploid Avena taxa (i.e., AACCDD), (ii) identified major sites of rDNA on three pairs of A/D-genome chromosomes, in common with other Avena hexaploids, and (iii) revealed eight chromosome pairs carrying intergenomic translocations between the A/D- and C-genomes in the accession studied. Based on karyotype structure, the identity of some of these recombinant chromosomes was proposed, and this showed that some of these could be divided into two types, (i) those common to all hexaploid Avena species analysed (3 translocations) and (ii) one translocation in this A. fatua accession not previously observed in reports on other hexaploid Avena species. If this translocation is found to be unique to A. fatua, then this information, combined with more traditional morphological data, will add support to the view that A. fatua is genetically distinct from other hexaploid Avena species and thus should retain its full specific status.
[show abstract][hide abstract] ABSTRACT: Of the world's 250000 angiosperm species, only about 200 are recognized as important weeds. 4C nuclear DNA amounts were estimated for 39 such species. Success for many important weeds is suggested to reflect several traits known to correlate with low DNA C-value, so such weeds may have smaller DNA C-values than other species. Our work tests this hypothesis, comparing DNA amounts in 156 species recognized as important world weeds or British garden weeds, with 2685 other species. DNA amounts did not differ significantly between the two weed samples, but weeds showed highly significant differences from other species. For example, nuclear DNA amount in weeds (mean 11·74 pg) was smaller than in other species (mean 28·13 pg), and restricted to the lowest 20% of their range. Similarly, DNA amount per genome in weeds (mean 3·79 pg) was smaller than in other species (mean 12·14 pg), and restricted to the lowest 10% of their range. As significant differences between weeds and other species remain for almost all sub-samples tested, this contrast is widely distributed. So it is important to ask how selection against high nuclear DNA amount and genome size in weeds operates. The probability of a species being a weed fell significantly with increasing nuclear 4C DNA amount, and mean genome size, reaching zero just above 100 pg, and 19 pg, respectively. Moreover, polyploidy was significantly more frequent in weeds (51%) than in other species (27%), increasing with nuclear DNA amount in both, reaching 100% in weeds with the highest 4C DNA amounts, but only 41% in other species. Thus, selection for polyploidy in weeds may partly reflect their increased genetic variability, independent of DNA amount. However, such selection pressure grows strongly with rising nuclear DNA amount, and this may act mainly on correlated factors including faster development.
[show abstract][hide abstract] ABSTRACT: A detailed description of ovule development and embryology ofHemiphylacus alatostylus is given, together with a chromosome count and karyotype. Comparative data are presented forAsparagus andAnemarrhena, since earlier analysis ofrbcL sequence data indicated a relationship between them.Hemiphylacus should be included inAsparagaceae. A close relationship between them is strongly supported byrbcL analysis, and they are further linked by the shape and histology of the fertilised ovule. However, both morphological and molecular data indicate thatAnemarrhena is a more isolated taxon.
Plant Systematics and Evolution 01/1998; 211(3):181-199. · 1.31 Impact Factor