AOBPreview originally published online on February 19, 2008
Annals of Botany 2008 101(7):909-918; doi:10.1093/aob/mcn023
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Karyotype Diversification and Evolution in Diploid and Polyploid South American Hypochaeris (Asteraceae) Inferred from rDNA Localization and Genetic Fingerprint Data
1 Department of Systematic and Evolutionary Botany, University of Vienna, Rennweg 14, A-1030 Vienna, Austria
2 Departamento de Biología Vegetal y Ecología (Botánica), Facultad de Biología, Universidad de Sevilla, Avenida Reina Mercedes, 41080 Sevilla, Spain
3 Department of Biogeography and Botanical Garden, University of Vienna, Rennweg 14, A-1030 Vienna, Austria
4 Department of Plant Sciences, Cambridge University, Downing Street, Cambridge CB2 3EA, UK
* For correspondence. E-mail hanna.schneeweiss{at}univie.ac.at
Received: 20 November 2007 Returned for revision: 11 January 2008 Accepted: 21 January 2008 Published electronically: 19 February 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Background and Aims: Changes in chromosome structure and number play an important role in plant evolution. A system well-suited to studying different modes of chromosome evolution is the genus Hypochaeris (Asteraceae) with its centre of species' diversity in South America. All South American species uniformly have a chromosome base number of x = 4 combined with variation in rDNA number and distribution, and a high frequency of polyploidy. The aim of this paper is to assess directions and mechanisms of karyotype evolution in South American species by interpreting both newly obtained and previous data concerning rDNA localization in a phylogenetic context.
Methods: Eleven Hypochaeris species from 18 populations were studied using fluorescence in situ hybridization (FISH) with 35S and 5S rDNA probes. A phylogenetic framework was established from neighbour-net analysis of amplified fragment length polymorphism (AFLP) fingerprint data.
Key Results: A single 5S rDNA locus is invariably found on the short arm of chromosome 2. Using 35S rDNA loci, based on number (one or two) and localization (interstitial on the long arm of chromosome 2, but sometimes lacking, and terminal or interstitial on the short arm of chromosome 3, only very rarely lacking), seven karyotype groups can be distinguished; five of these include polyploids. Karyotype groups with more than one species do not form monophyletic groups.
Conclusions: Early evolution of Hypochaeris in South America was characterized by considerable karyotype differentiation resulting from independent derivations from an ancestral karyotype. There was marked diversification with respect to the position and evolution of the 35S rDNA locus on chromosome 3, probably involving inversions and/or transpositions, and on chromosome 2 (rarely 3) concerning inactivation and loss. Among these different karyotype assemblages, the apargioides group and its derivatives constitute by far the majority of species.
Key words: Asteraceae, fluorescence in situ hybridization, Hypochaeris, karyotype evolution, polyploidy, rDNA, South America
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Karyotypic changes involving chromosome structure, such as inversions, translocations and deletions, and chromosome number by aneuploidy/dysploidy or polyploidy play an important role in plant evolution and speciation (Stebbins, 1971; Lim et al., 2000; Levin, 2002; Lysak et al., 2006; Schubert, 2007). The in situ hybridization techniques of genomic in situ hybridization (GISH) and, particularly, fluorescence in situ hybridization (FISH) allow the tracing of subtle chromosomal changes, and have been successfully applied in numerous plant groups (Zhang and Sang, 1998; Adams et al., 2000; Lim et al., 2000, 2007; Weiss-Schneeweiss et al., 2003, 2007a; Clarkson et al., 2005). In an evolutionary context, the markers most often used, especially in non-model organisms, are the 5S and 35S ribosomal genes due to their abundance as ‘house-keeping genes’ and their relatively conserved nature (Ma
uszy
ska et al., 1998). Interpreting changes in rDNA loci numbers and localizations in related species within a phylogenetic framework (usually derived from molecular data such as DNA sequences or AFLP fingerprints) can be a powerful approach towards obtaining a clearer understanding of mechanisms and directions of chromosomal changes and their impact on plant evolution (Clarkson et al., 2005; Lim et al., 2006; Weiss-Schneeweiss et al., 2007a). A particularly well-suited system to study different modes of chromosome evolution is provided by the genus Hypochaeris (Asteraceae, Lactuceae), which includes 50–60 species found in northern Africa, Europe, Asia and South America. Although the Old World group comprises only about 15 species, it shows considerable cytological diversity (x = 3, 4, 5, 6; summarized in Cerbah et al., 1998), which agrees well with the current sectional classification based on morphological and molecular data (Samuel et al., 2003, and references therein). By contrast, the more species-rich South American group of 40–50 species (depending on the taxonomic treatment; Lack, 1979; Tremetsberger et al., 2006, and references therein) is uniform with respect to its chromosome base number (x = 4; summarized in Weiss-Schneeweiss et al., 2007b). This agrees with its relatively young age and rapid diversification (Samuel et al., 2003; Tremetsberger et al., 2005). It has only recently been shown that the South American clade is not a member of section Achyrophorus, as previously suggested (Hoffmann, 1893), but is instead sister to the Moroccan H. angustifolia, a species with x = 4 and as yet not assigned to any section (Tremetsberger et al., 2005). These findings have not only allowed inference of a putative origin of the New World species from north-western Africa, but have also suggested a cytological link between x = 5 and x = 4 karyotypes (Tremetsberger et al., 2005).
Whereas the European and Mediterranean Hypochaeris taxa are well known cytologically (Parker, 1971, 1976; Mugnier and Siljak-Yakovlev, 1987; Barghi et al., 1989; Cerbah et al., 1995, 1998; Hall and Parker, 1995), such detailed information is still lacking for many South American taxa (Weiss-Schneeweiss et al., 2007b). Chromosome numbers in the New World Hypochaeris have been established for 39 species and two hybrids, with karyotypes and microphotographs available for 30 (Stebbins et al., 1953; Siljak-Yakovlev et al., 1994; Ruas et al., 1995, 2005; Cerbah et al., 1998; Weiss et al., 2003; Weiss-Schneeweiss et al., 2003, 2007b). Only 15, however, have been analysed so far with FISH (Cerbah et al., 1998; Weiss-Schneeweiss et al., 2003; Ruas et al., 2005). All the South American taxa examined have very similar bimodal and asymmetrical karyotypes with two large and two small chromosome pairs. The application of molecular cytogenetic techniques, however, has allowed discrimination of seven distinct karyotypic groups, as well as generating hypotheses concerning their relationships (Cerbah et al., 1998; Weiss-Schneeweiss et al., 2003; Ruas et al., 2005).
The aim of this paper is to assess directions and mechanisms of karyotype evolution in South American Hypochaeris species. To this end, we have investigated a further eleven species using FISH with 35S and 5S rDNA probes. These findings have been combined with the data already available for 15 other species (Weiss-Schneeweiss et al., 2003; Ruas et al., 2005), all set within a phylogenetic context established from AFLP fingerprint data. Specifically, we (1) test and refine the previously suggested karyotype groups; (2) compare the dynamics of change in chromosome structure and rDNA localization in diploids and tetraploids; and (3) identify the mechanisms involved in karyotype changes and interpret them in a phylogenetic context, allowing us to infer trends and directions of chromosome evolution in this speciating group.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Plant material
Eleven species and 18 populations of South American Hypochaeris were analysed (for localities, voucher numbers and chromosome numbers see Table 1). Surface-sterilized seeds were germinated on wet filter paper in Petri dishes. Two to three days after germination, seedlings were pre-treated with 0·1 % colchicine for 2 h at room temperature and 2 h at 4 °C, fixed in ethanol:acetic acid (3:1) for at least 12 h at room temperature, and stored at –20 ºC.
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Fluorescence in situ hybridization
Chromosomes were prepared by enzymatic digestion/squashing as described in Weiss-Schneeweiss et al. (2003). Fluorescence in situ hybridization (FISH) was carried out according to the methods of Schwarzacher and Heslop-Harrison (2000) and Weiss-Schneeweiss et al. (2003, 2007a) with minor modifications. Probes used for FISH were 35S (= 18S/25S) rDNA from Arabidopsis thaliana in plasmid pSK + , and 5S rDNA from Beta vulgaris in plasmid pBx1-2, labelled with biotin or digoxygenin (Roche, Vienna, Austria), respectively. Probes were labelled either directly by PCR (5S rDNA) or using a nick translation kit (35S rDNA; Roche, Vienna, Austria). Digoxygenin was detected with antidigoxygenin conjugated with FITC (1 µg mL–1; Roche, Vienna, Austria) and biotin with ExtrAvidin conjugated with Cy3 (3 µg mL–1; Sigma-Aldrich, Vienna, Austria). Analyses of preparations were made with an Axioplan2 epifluorescent microscope (Carl Zeiss, Vienna, Austria), images acquired with a CCD camera and files processed using Axiovision ver. 3·5 (Carl Zeiss, Vienna, Austria). For rDNA localization, a minimum of 30 well-spread metaphases and prometaphases was analysed in each species. Basic karyotype and idiogram construction followed Weiss-Schneeweiss et al. (2007b).
Phylogenetic analysis
AFLP data were generated following the protocols described previously (Tremetsberger et al., 2006). In each species the products of the restriction–ligation reactions of three individuals, chosen to encompass the whole geographic distribution of the species based on available collections (see Supplementary Information, available online), were pooled together and used as a single sample in the preselective amplification. Following results of Tremetsberger et al. (2006), two samples were included for H. palustris, one from the Coastal Cordillera of Chile and one from the Andes. The Moroccan endemic H. angustifolia was used as the outgroup. To obtain better resolution than in the study of Tremetsberger et al. (2006), we added three additional AFLP primer combinations [MseI-CAAG/EcoRI-ACT (Fam), MseI-CAAG/EcoRI-ACC (Ned), MseI-CTCG/EcoRI-AGG (Hex)] to the six already selected [MseI-CAG/EcoRI-ACT (Fam), MseI-CAG/EcoRI-AGC (Ned), MseI-CTC/EcoRI-ACG (Hex), MseI-CTGA/EcoRI-ACT (Fam), MseI-CTCG/EcoRI-ATC (Ned), MseI-CTTC/EcoRI-ACG (Hex)], giving a total of nine selective primer pairs (preselective MseI-primers had two base pairs fewer). Genographer 1·6 (available from http://hordeum.oscs.montana.edu/genographer) was used for scoring band presence and absence. A total of 1415 unambiguously scorable bands, of which 1385 (98 %) were polymorphic, were revealed in the ingroup. This total increased to 1492, of which 1469 (98 %) were polymorphic, when H. angustifolia was also included. Phylogenetic relationships were inferred using the distance-based neighbour-net method (implemented in SplitsTree 4, available from www.splitstree.org; Huson and Bryant, 2006) based on Nei–Li distances (Nei and Li, 1979) calculated with TreeCon 1·3b (van der Peer and de Wachter, 1997). In contrast to commonly used tree-building methods, a network method allows the visualization of potentially conflicting signals, such as may be caused by homoplasy or hybridization (Huson and Bryant, 2006). Group support was assessed via a neighbour-joining bootstrap analysis in TreeCon 1·3b using 1000 pseudo-replicates and Nei–Li-distances. All clades with bootstrap support (hereafter abbreviated to BS) >50 are indicated, irrespective of whether these are supported by splits or not, thus allowing direct comparison of incongruences arising from methodological differences.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Fluorescence in situ hybridization
All newly analysed diploid accessions have the single 5S rDNA locus localized on the short arm of chromosome 2 (Fig. 1). By contrast, four different karyotypes can be distinguished based on the localization of 35S rDNA. Thus Hypochaeris alba, H. elata, H. hookeri, H. incana (2x cytotype), H. parodii and H. pinnatifida all possess two loci of 35S rDNA, one on the long arm of chromosome 2 and the other interstitial on the short arm close to the centromere of chromosome 3 (Fig. 1A–E, H). The karyotype of H. sessiliflora is similar but differs in the terminal position of the locus on the short arm of chromosome 3 (Fig. 1I). Only one locus is present in H. patagonica and H. petiolaris, and this is located on the short arm of chromosome 3 in a terminal or interstitial position, respectively (Fig. 1F, G).
Whereas all tetraploid species analysed show the expected pattern of the 5S rDNA locus being present on the short arms of all four copies of chromosome 2 (Fig. 1J–M), the 35S rDNA loci number deviates from this strictly additive pattern. The putatively autotetraploid cytotype of H. incana has 35S rDNA signals on only two copies of chromosome 2 (Fig. 1K). Although H. taraxacoides has diploid and tetraploid cytotypes, only tetraploids were included in the FISH analysis due to under-representation of diploids in the population seed samples. All plants in the uniformly tetraploid H. taraxacoides population 18508 have four equally strong 35S rDNA signals at both the locus on the long arm of chromosome 2 and at a second locus localized interstitially on the short arm of chromosome 3 (Fig. 1L). By contrast, two individuals from the tetraploid population 18089 revealed a heteromorphic locus on chromosome 3, the signals being much stronger in two of the four homologues (Fig. 1M). In addition, a single individual from this population had three typical acrocentric plus one metacentric chromosome 1 and had lost the secondary constrictions from two of the four copies of chromosome 2.
Hypochaeris caespitosa possesses only a single locus of 35S rDNA, which, in contrast to all other species, whether diploid or tetraploid, is found on chromosome 2 (Fig. 1J).
Phylogenetic relationships
Here we use the term ‘clade’ for the phylogenetically defined groups delimited by Tremetsberger et al. (2006), to distinguish them from the groups based on karyotype features (Weiss-Schneeweiss et al., 2007b; this study).
Phylogenetic relationships of about two-thirds of South American Hypochaeris species are illustrated in Fig. 2. Although each species is well separated (as illustrated by heavily weighted splits), the basal relationships of clades, usually including a few species only, are poorly resolved with many contradicting, yet often only weakly weighted, splits in the centre of the network. Most of the nodes supported by BS > 50 in a neighbour-joining analysis are also supported by splits in the network. The few clades not supported by any split usually have weak bootstrap support (BS < 65), with the exception of a clade including, among others, H. petiolaris, H. pampasica, H. parodii and H. alba (BS 87; see Fig. 2).
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If we take H. angustifolia as the outgroup species (Tremetsberger et al., 2005), then H. patagonica and H. chondrilloides (chondrilloides clade, BS 90) are sister to the remainder of the South American taxa (BS 91). The apargioides clade (e.g. H. apargioides, H. gayana, H. spathulata; BS 100), the pampasica clade (e.g. H. pampasica; BS 76), the microcephala clade (e.g. H. microcephala, H. chillensis, H. alba; BS 95) with H. variegata and H. petiolaris together form a major clade with very strong bootstrap support (BS 98). The relationships of H. lutea (syn. H. rosengurttii), H. scorzonerae and H. caespitosa to each other, and to remaining species, are essentially unresolved.
Two more clades can be discriminated: one including H. elata, H. sessiliflora and H. taraxacoides (if we exclude H. elata this corresponds to the sessiliflora clade; BS 99) and the second consisting of H. acaulis, H. palustris, H. tenuifolia, H. incana and H. hookeri (all of them in the tenuifolia clade; BS 56).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Karyotype groups in South American Hypochaeris
As a result of this study of 18 populations from eleven species the number of cytogenetically FISH-characterized South American Hypochaeris is now 25, about half of all the New World species. The 5S rDNA studies have been uninformative, with a single locus in the short arm of chromosome 2 in all species. By contrast, analyses of basic chromosome morphology and 35S rDNA localization have enabled the definition of seven karyotypic groups (Weiss-Schneeweiss et al., 2007b; Fig. 3). These groups have been named here after one of their constituent species (Weiss-Schneeweiss et al., 2007b). This classification replaces the designations A, B, etc., introduced previously by Weiss-Schneeweiss et al. (2003), in order to avoid confusion in case new groups are found, and so as not to suggest any evolutionary directionality in their relationships.
- The apargioides group (formerly group B) is the largest and includes 17 species (table 3 in Weiss-Schneeweiss et al., 2007b). The species H. alba, H. elata, H. hookeri, H. incana, H. parodii and H. pinnatifida have been analysed for the first time. The species of the apargioides group have two interstitial 35S rDNA loci, one on the long arm of chromosome 2 and one on the short arm of chromosome 3, both forming secondary constrictions (Fig. 3). Interestingly, tetraploid cytotypes of H. incana and H. taraxacoides often show deviations from a strictly additive pattern (see below ‘Evolution of polyploids’).
- The species of the tenuifolia group (formerly group C) (usually) lack the secondary constriction on chromosome 2, but have the corresponding 35S rDNA locus intact. This is most likely due to locus inactivation.
- The sessiliflora group so far consists only of H. sessiflora. Its karyotype differs from the apargioides group by the terminal location of the 35S rDNA locus on chromosome 3. This species was erroneously assigned to the apargioides group by Weiss et al. (2003) due to the failure to detect the terminal satellite on chromosome 3 in Feulgen-stained preparations.
- The sole member of the chondrilloides group (Weiss-Schneeweiss et al., 2007b) is H. chondrilloides, which has a telocentric rather than submetacentric chromosome 3. However, the presence of a satellite on this chromosome, detected in Feulgen-stained chromosomes, still requires confirmation via FISH analysis.
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Two other small karyotypic groups have only a single 35S rDNA locus per genome, located on the short arm of chromosome 3 in both.
- (5) This is terminal in the patagonica group (formerly group A), including H. patagonica and H. lutea (Ruas et al., 2005; Weiss-Schneeweiss et al., 2007b),
- (6) or interstial in the acaulis group (formerly group D) including, among others, H. petiolaris.
- (7) The last distinct karyotype is characterized by having a single locus of 35S rDNA on chromosome 2 instead of chromosome 3. This has been found only in the tetraploid genome of H. caespitosa (caespitosa group).
- (6) or interstial in the acaulis group (formerly group D) including, among others, H. petiolaris.
The overall karyotypic similarity of South American Hypochaeris species suggests that large structural rearrangements have been of minor importance in the evolution of this group. However, analysis of F1 hybrids or application of a wide range of dispersed markers is necessary to establish this. It is also not possible to assess the evolutionary significance of spontaneous changes at the individual level, such as the centric shift in chromosome 1 of H. apargioides (Weiss-Schneeweiss et al., 2007b; H. Weiss-Schneeweiss, unpubl. res.). More detailed studies are clearly necessary.
Evolution in polyploids
Polyploidy is relatively frequent amongst South American Hypochaeris species, occurring in different karyotypic groups and in different phylogenetic clades (Figs 2 and 3), suggesting that polyploidization is an active process (Weiss-Schneeweiss et al., 2007b). Polyploid cytotypes are so far known only at the tetraploid level. They have been reported in H. caespitosa (caespitosa group), H. chondrilloides (chondrilloides group), H. sessiliflora (sessiliflora group), H. apargioides, H. incana, H. taraxacoides, H. meyeniana (all in apargioides group), H. tenuifolia (tenuifolia group) and in the unassigned species H. scorzonerae (Weiss et al., 2003). A summary of previous chromosome number reports is given by Weiss et al. (2003), Baeza et al. (2006) and Weiss-Schneeweiss et al. (2007b). Tetraploid cytotypes of Hypochaeris often co-occur with conspecific diploids within the same population (Weiss-Schneeweiss et al., 2007b), suggesting an autopolyploid origin.
The process of diploidization in newly formed polyploids does not follow a clear pattern in Hypochaeris (Adams and Wendel, 2005; Clarkson et al., 2005; Comai, 2005; Ma and Gustafson, 2005). The number of 5S rDNA increases additively with tetraploidy, as shown in other species' groups (Adams et al., 2000; de Melo and Guerra, 2005; Weiss-Schneeweiss et al., 2007a; but see also Mishima et al., 2002; Clarkson et al., 2005). The 35S rDNA locus number by contrast deviates from a strictly additive pattern on chromosome 2 of H. incana and chromosome 3 of some individuals of H. taraxacoides, and the locus is completely lost from chromosome 3 of H. caespitosa. Epigenetically regulated inactivation, asymmetrical amplification/reduction of loci via unequal recombination and eventual loss have been reported in other polyploids (e.g. Vaughan et al., 1993; Chen and Pikaard, 1997; Dadejová et al., 2007) and are likely also to be operating in Hypochaeris. The occurrence of the same patterns of locus asymmetry (H. spathulata and H. variegata of the apargiodes group: Ruas et al., 2005; H. Weiss-Schneeweiss, unpubl. res.), or inactivation and loss (tenuifolia and acaulis groups; Weiss-Schneeweiss et al., 2003) on the diploid as well as on the tetraploid level, suggests that 35S rDNA diploidization may be triggered by factors other than polyploidization itself.
Despite its relatively high frequency, there is no convincing evidence for a causal contribution of polyploidization and accompanying microstructural changes to the rapid morphological and genetic diversification of South American Hypochaeris species. The frequency of polyploidy in this group, however, was greatly underestimated until only a few years ago. Further studies across as wide a range of species as possible are needed to assess fully the role of polyploidy in differentiation and eventual speciation of this group.
Mechanisms and directions of karyotype evolution
The following hypotheses depend on the phylogenetic hypothesis derived from the AFLP data. The use of AFLP data for phylogenetic purposes at the specific level is not uncontroversial (see discussion in Tremetsberger et al., 2006, and Dixon et al., 2008), yet in our case none of the employed sequence markers (nuclear ITS and plastid matK and rps16 intron: Samuel et al., 2003; Tremetsberger et al., 2005) was able to resolve relationships within this group.
The restriction of conflicting signals to the centre of the network constructed using AFLP data (Fig. 2) suggests a high level of homoplasy at the deepest nodes, which results in a poorly resolved backbone. This renders the current phylogenetic hypothesis conservative, in the sense that relationships will not be resolved at all rather than resolved wrongly. In the absence of detailed chromosome maps or analysis of meiotic configurations in hybrids, which would allow definition of the patterns of chromosome rearrangement (e.g. Hall and Parker, 1995), we present here a hypothesis of karyotype evolution following the parsimony principle (Fig. 3). Inevitably, more complex evolutionary patterns cannot be excluded.
The majority of South American Hypochaeris species analysed belong to the apargioides karyotypic group, which occurs in all phylogenetic clades except the chondrilloides clade (Fig. 2). The high frequency of this karyotype together with the presence of the maximal number of 35S rDNA loci (two per haploid genome) found in the South American species, and also characteristic of the Old World H. angustifolia (Cerbah et al., 1998; Weiss-Schneeweiss et al., 2003; Ruas et al., 2005; Tremetsberger et al., 2005), suggest this karyotype as the ancestral one. If the karyotype seen today in H. angustifolia, which is sister to the New World species (Tremetsberger et al., 2005), is the ancestral form of all the South American species, a number of mechanisms proposed for other groups may be invoked to derive the apargioides group, including inversions, translocations, rDNA transposition or rDNA dispersion (Jackson, 1973; Schubert and Wobus, 1985; Leitch and Heslop-Harrison, 1992; Dubcovsky and Dvo
ák, 1995; Weiss and Mauszyska, 2000; Shishido et al., 2000; Raskina et al., 2004; Vaio et al., 2005; Datson and Murray, 2006). Although some circumstantial evidence for or against any of these mechanisms may be put forward, for instance an enrichment in copia-type retroelements of the short arm of chromosome 3 and 4 (Ruas et al., 2008) in support of retroelement-mediated rDNA transposition, the available data do not allow any inferences to be made beyond mere speculation.
Previously, a karyotype corresponding to that now known in the sessiliflora group was suggested as being ancestral to all South American taxa (Weiss-Schneeweiss et al., 2003). This hypothesis was suggested by the (sub)terminal position of the 35S rDNA locus on the short arm of chromosome 3 characteristic of many Mediterranean and Eurasian Hypochaeris species, including H. angustifolia (Weiss-Schneeweiss et al., 2003), and was in accordance with the idea that a nucleolar organizer region (NOR) occurring at a (sub)terminal position on a short chromosome arm would be the primitive character state (Lima de Faria, 1976). However, phylogenetic studies show that H. sessiliflora is derived and groups with H. elata and other species in the sessiliflora clade (Fig. 2). Since these species all belong to the apargioides group, phylogeny thus supports a derived state for the sessiliflora group. The mechanisms resulting in this reversal to a terminal position of the 35S rDNA locus position on chromosome 3, the ancestral condition seen in H. angustifolia, require elucidation but presumably involve paracentric inversion, transposition or minor locus expansion/major locus reduction.
The unclear relationships of H. patagonica and H. lutea at the base of the South American radiation perhaps indicate that the patagonica group karyotype might be derived from the ancestral H. angustifolia-like karyotype by a single-step loss of the locus on chromosome 2. This might have happened independently in H. patagonica (chondrilloides clade) and in H. lutea, which is not assigned to any phylogenetic group yet.
The phylogenetic position of H. chondrilloides (in the chondrilloides clade) as sister to the remaining South American species of Hypochaeris suggests that its karyotype (based on chromosome morphology) might have been directly derived from the H. angustifolia karyotype. Mechanisms may include those suggested above for the apargioides group, followed by the loss of a fragment of the short arm of chromosome number 3. Only two individuals from a single population have been examined and more data are needed to establish if the observed karyotype is typical for the species, and to be able to trace effectively its evolutionary history.
Species possessing karyotypes of the tenuifolia or acaulis groups do not form monophyletic clades, but are nested in clades including species from the apargioides group (pampasica clade, microcephala clade, tenuifolia clade). This agrees well with the proposed cytological mechanisms involved, namely initial deactivation (tenuifolia group) followed by eventual loss of the whole terminal segment on chromosome 2 (acaulis group). Significantly, in H. chillensis and H. tenuifolia, a few individuals analysed still possessed a visible secondary constriction on chromosome 2 (H. Weiss-Schneeweiss, unpubl. res.), perhaps indicating that the silencing of this locus can be reversible or is still ongoing in this species (see also Hasterok and Mauszyska, 2000b). Silencing or loss of rDNA loci is known mainly in polyploids and hybrids (Vaughan et al., 1993; Chen and Pikaard, 1997; Hasterok and Mauszyska, 2000a; Dadejová et al., 2007), but has also been observed in diploids possessing more than one locus (Mauszyska et al., 1998).
Multiple origins of the acaulis group karyotypes are suggested from genome size data (Ch. König, Department of Systematic and Evolutionary Botany, University of Vienna, unpubl. res.). Thus Hypochaeris pampasica and H. megapotamica, members of the large clade including H. apargioides, H. chillensis and H. alba, have considerably larger genomes (4·5 pg and 4·6 pg DNA [2C]) than H. acaulis and H. palustris (3·5 pg and 3·7 pg DNA), which group with H. tenuifolia and H. taraxacoides (Figs 2 and 3).
A third, probably early, derivative of the apargioides group is the caespitosa group (H. caespitosa). This evolved via loss of the rDNA locus on chromosome number 3 (Fig. 3). The karyotype is only known from the tetraploid condition, but so far only one population has been analysed, and the diploids might exist. It remains unclear if the loss is connected to polyploidization of the apargioides group (see ‘Evolution in polyploids’), or follows the same trend seen in the tenuifolia and acaulis groups.
In conclusion, the exact nature of chromosomal rearrangements relative to the putative ancestral karyotype seen in H. angustifolia remains unclear. However, interpretation of the karyotypic information in a phylogenetic context suggests that the early evolution of Hypochaeris in South America was characterized by considerable karyotypic differentiation resulting from independent derivations from the ancestral karyotype and marked diversification with respect to the position and evolution of the 35S rDNA locus on chromosome 3. By contrast, the congruent position in all South American species of the 35S rDNA locus on the long arm of chromosome 2 (compared to its short arm localization in H. angustifolia; Fig. 3), suggests that this change occurred only once, and pre-dated those of chromosome 3. Among the different karyotypes, the apargioides group and its clear derivatives (tenuifolia, acaulis, sessiliflora and probably also caespitosa group) are the most species-rich. Further data are required to unravel whether and how the karyotypic constitution has contributed to the diversification of these species. Additionally, there is also a need for meiotic studies of intraspecific hybrids to elucidate relationships between species and to unravel potential chromosomal rearrangements.
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SUPPLEMENTARY INFORMATION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Supplementary Information is available online at http://aob.oxfordjournals.org/ and lists the plant material of Hypochaeris analysed for AFLP in this study, including collector(s), and number and location of voucher specimens.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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We thank A. Tribsch, F. Essl, M. Staudinger, P. Schönswetter (University of Vienna, Austria), E. Urtubey, P. Simón (Museo de La Plata, Argentina) and H.A. Valdebenito (Universidad San Francisco de Quito, Ecuador) for help in collecting materials (seeds and vouchers) for the present study. We also thank Dr C. König for sharing unpublished data on genome size in Hypochaeris. The program for karyotyping (Autoidiogram) was provided by Dr Wolfgang Harand (University of Vienna, Austria). Financial support is appreciated from Austrian Science Foundation research grants (FWF P15225 [GenBank] and P18446 [GenBank] to T.F.S.), a research award from La Fundación Banco Bilbao Vizcaya Argentaria (BBVA to T.F.S.), a Hertha-Firnberg postdoctoral fellowship (FWF, Austria, T218 to H. W.-S.) and a Juan de la Cierva postdoctoral fellowship (Ministerio de Educación y Ciencia, Spain to K.T).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONSUPPLEMENTARY INFORMATIONACKNOWLEDGEMENTSLITERATURE CITED |
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Adams KL, Wendel JF. Polyploidy and genome evolution in plants. Current Opinion in Plant Biology (2005) 8:135–141.[CrossRef][Web of Science][Medline]
Adams SP, Leitch IJ, Bennett MD, Chase MW, Leitch AR. Ribosomal DNA evolution and phylogeny in Aloe (Asphodelaceae). American Journal of Botany (2000) 87:1578–1583.
Baeza CM, Jara S, Stuessy TF. Cytotaxonomical studies in populations of Hypochaeris apargiodes Hook. et Arn. (Asteraceae, Lactuceae) from Chile, VIII region. Gayana Botanica (2006) 63:99–105.
Barghi N, Mugnier C, Siljak-Yakovlev S. Karyological studies in some Hypochaeris species from Sicily. Plant Systematics and Evolution (1989) 168:49–57.[CrossRef][Web of Science]
Cerbah M, Coulaud J, Godelle B, Siljak-Yakovlev S. Genome size, fluorochrome banding, and karyotype evolution in some Hypochoeris species. Genome (1995) 38:689–695.[Medline]
Cerbah M, Coulaud J, Siljak-Yakovlev S. rDNA organization and evolutionary relationships in the genus Hypochaeris (Asteraceae). Journal of Heredity (1998) 89:312–318.
Chen ZJ, Pikaard CS. Transcriptional analysis of nucleolar dominance in polyploid plants: biased expression/silencing of progenitor rRNA genes is developmentally regulated in Brassica. Proceedings of the National Academy of Sciences,USA (1997) 94:3442–3447.[CrossRef]
Clarkson JJ, Lim KY, Kovaik A, Chase MW, Knapp S, Leitch AR. Long-term genome diploidization in allopolyploid Nicotiana section Repandae (Solanaceae). New Phytologist (2005) 168:241–252.[CrossRef][Web of Science][Medline]
Comai L. The advantages and disadvantages of being polyploid. Nature Reviews Genetics (2005) 6:836–846.[CrossRef][Web of Science][Medline]
Dadejová M, Lim KY, Soucková-Skalická K, Matyá
ek R, Grandbastien MA, Leitch A, Kovaík A. Transcription activity of rRNA genes correlates with a tendency towards intergenomic homogenization in Nicotiana allotetraploids. New Phytologist (2007) 174:658–668.[CrossRef][Web of Science][Medline]
Datson PM, Murray BG. Ribosomal DNA locus evolution in Nemesia: transposition rather than structural rearrangement as the key mechanism? Chromosome Research (2006) 14:845–857.[CrossRef][Medline]
Dixon CJ, Schönswetter P, Schneeweiss GM. Morphological and geographical evidence are misleading with respect to the phylogenetic position and origin of the narrow endemic polyploid Androsace cantabrica (Primulaceae). Systematic Botany (2008) in press.
Dubcovsky J, Dvoák J. Ribosomal RNA multigene loci: nomads of the Triticeae genomes. Genetics (1995) 140:1367–1377.[Abstract]
Hall KJ, Parker JS. Stable chromosome fission associated with rDNA mobility. Chromosome Research (1995) 3:417–422.[CrossRef][Medline]
Hasterok R, Mauszyska J. Nucleolar dominance does not occur in root tip cells of allotetraploid Brassica species. Genome (2000) a 43:574–579.[Medline]
Hasterok R, Mauszyska J. Different rRNA gene expression in primary and adventitious roots of Allium cepa L. Folia Histochemica et Cytobiologica (2000) b 38:181–184.[Medline]
Hoffmann O. Hypochoeris L. In: Die natürlichen Pflanzenfamilien nebst ihren Gattungen und wichtigeren Arten, insbesondere den Nutzpflanzen IV Teil. 5. Abteilung—Engler A, Prantl K, eds. (1893) Leipzig: Wilhelm Engelmann. 361–363.
Huson D, Bryant D. Application of phylogenetic networks in evolutionary studies. Molecular Biology and Evolution (2006) 23:254–267.
Jackson RC. Chromosomal evolution in Haplopappus gracilis: a centric transposition race. Evolution (1973) 27:243–256.[CrossRef][Web of Science]
Lack HW. The subtribe Hypochoeridinae (Asteraceae, Lactuceae) in the tropics and the Southern Hemisphere. In: Tropical botany—Larsen K, Holm-Nielsen LB, eds. (1979) London: Academic Press. 265–276.
Leitch IJ, Heslop-Harrison JS. Physical mapping of the 18S–5·8S–26S rRNA genes in barley by in situ hybridization. Genome (1992) 35:1013–1018.
Levin DA. The role of chromosomal change in plant evolution (2002) New York: Oxford University Press.
Lim KY, Matyáek R, Lichtenstein CP, Leitch AR. Molecular cytogenetic analyses and phylogenetic studies in the Nicotiana section Tomentosae. Chromosoma (2000) 109:245–258.[CrossRef][Web of Science][Medline]
Lim KY, Kovaik A, Matyáek R, Chase MW, Knapp S, McCarthy E, Clarkson JJ, Leitch AR. Comparative genomics and repetitive sequence divergence in the species of diploid Nicotiana section Alatae. The Plant Journal (2006) 48:907–919.[CrossRef][Web of Science][Medline]
Lim KY, Kovarik A, Matyáek R, Chase MW, Clarkson JJ, Grandbastien MA, Leitch AR. Sequence of events leading to near-complete genome turnover in allopolyploid Nicotiana within five million years. New Phytologist (2007) 175:756–763.[CrossRef][Web of Science][Medline]
Lima de Faria A. The chromosome field. I Prediction of the location of ribosomal cistrons. Hereditas (1976) 83:1–22.[Web of Science]
Lysak MA, Berr A, Pecinka A, Schmidt R, McBreen K, Schubert I. Mechanisms of chromosome number reduction in Arabidopsis thaliana and related Brassicaceae species. Proceedings of the National Academy of Sciences, USA (2006) 103:5224–5229.
Ma XF, Gustafson JP. Genome evolution of allopolyploids: a process of cytological and genetic diploidization. Cytogenetics and Genome Research (2005) 109:236–249.[CrossRef]
Mauszyska J, Hasterok R, Weiss H. rRNA genes – their distribution and activity in plants. In: Plant cytogenetics—Mauszyska J, ed. (1998) Katowice: Silesian University Press. 75–95.
de Melo NF, Guerra M. Variability of the 5S and 45S rDNA sites in Passiflora L. species with distinct base chromosome numbers. Annals of Botany (2005) 92:309–316.
Mishima M, Ohmido N, Fukui K, Yahara T. Trends in site-number change of rDNA loci during polyploid evolution in Sanguisorba (Rosaceae). Chromosoma (2002) 110:550–558.[Web of Science][Medline]
Mugnier C, Siljak-Yakovlev S. Karyological study in some Yugoslavian populations of Hypochoeris (Compositae). Caryologia (1987) 40:319–325.[Web of Science]
Nei M, Li WH. Mathematical model for studying genetic variation in terms of restriction endonucleases. Proceedings of the National Academy of Sciences, USA (1979) 76:5269–5273.
Parker JS. The control of recombination (1971) UK: Oxford University. PhD Thesis.
Parker JS. The B-chromosome system of Hypochaeris maculata. I. B-distribution, meiotic behavior and inheritance. Chromosoma (1976) 59:167–177.[CrossRef][Web of Science]
van der Peer Y, de Wachter R. Construction of evolutionary distance trees with TREECON for Windows: accounting for variation in nucleotide substitution rate among sites. Computer Applications in the Biosciences (1997) 13:227–230.
Raskina O, Belyayev A, Nevo E. Quantum speciation in Aegilops: molecular cytogenetic evidence from rDNA cluster variability in natural populations. Proceedings of the National Academy of Sciences, USA (2004) 101:14818–14823.
Ruas CF, Ruas PM, Matzenbacher NI, Ross G, Bernini C, Vanzela ALL. Cytogenetic studies of some Hypochoeris species (Compositae) from Brazil. American Journal of Botany (1995) 82:369–375.[CrossRef][Web of Science]
Ruas CF, Vanzela ALL, Santos MO, Fregonezi JN, Ruas PM, Matzenbacher NI, Aguiar-Perecin MLR. Chromosomal organization and phylogenetic relationships in Hypochaeris species (Asteraceae) from Brazil. Genetics and Molecular Biology (2005) 28:129–139.[Web of Science]
Ruas CF, Weiss-Schneeweiss H, Stuessy TF, Samuel MR, Pedrosa-Harand A, Tremetsberger K, et al. Characterization, genomic organization and chromosome distribution of Ty1-copia retrotransposons in species of Hypochaeris (Asteraceae). Gene in press. (2008).
Samuel R, Stuessy TF, Tremetsberger K, Baeza CM, Siljak-Yakovlev S. Phylogenetic relationships among species of Hypochaeris (Asteraceae, Lactuceae) based on ITS, plastid trnL intron, trnL-F spacer and matK sequences. American Journal of Botany (2003) 90:496–507.
Schubert I. Chromosome evolution. Current Opinion in Plant Biology (2007) 10:109–115.[CrossRef][Medline]
Schubert I, Wobus U. In situ hybridization confirms jumping nucleolus organizing regions in Allium. Chromosoma. (1985) 92:143–148.
Schwarzacher T, Heslop-Harrison P. Practical in situ hybridization (2000) 2nd edn. Oxford, UK: BIOS.
Shishido R, Sano Y, Fukai K. Ribosomal DNAs: an exception to the conservation of gene order in rice genomes. Molecular and General Genetics (2000) 263:586–591.[CrossRef][Web of Science]
Siljak-Yakovlev S, Bartoli A, Roitman G, Barghi N, Mugnier C. Etude caryologique de trois especes d'Hypochoeris originaires d'Argentine: H. chillensis, H. microcephala var. albiflora et H.megapotamica. Canadian Journal of Botany (1994) 72:1496–1502.
Stebbins GL. Chromosomal evolution in higher plants (1971) London: Edward Arnold.
Stebbins GL, Jenkins JA, Walters M. Chromosomes and phylogeny in the Compositae, tribe Cichorieae. University of California Publications in Botany (1953) 26:401–430.
Tremetsberger K, Weiss-Schneeweiss H, Stuessy TF, Samuel R, Kadlec G, Ortiz MÁ, Talavera S. Nuclear ribosomal DNA and karyotypes indicate a NW African origin of South American Hypochaeris (Asteraceae, Cichorieae). Molecular Phylogenetics and Evolution (2005) 35:102–116.[CrossRef][Web of Science][Medline]
Tremetsberger K, Stuessy TF, Kadlec G, Urtubey E, Baeza CM, Beck SG, et al. AFLP phylogeny of South American species of Hypochaeris (Asteraceae, Lactuceae). Systematic Botany (2006) 31:610–626.[Web of Science]
Vaio M, Speranza P, Vallis JF, Guerra M, Mazzella C. Localization of the 5S and 45S rDNA sites and cpDNA sequence analysis in species of the Quadrifaria group of Paspalum (Poaceae, Paniceae). Annals of Botany (2005) 96:191–200.
Vaughan HE, Jamilena M, Ruiz Rejón C, Parker JS, Garrido-Ramos MA. Loss of nucleolar-organizer regions during polyploid evolution in Scilla autumnalis. Heredity (1993) 71:574–580.[CrossRef][Web of Science]
Weiss H, Mauszyska J. Chromosomal rearrangement in autotetraploid plants of Arabidopsis thaliana. Hereditas (2000) 133:255–261.[CrossRef][Web of Science][Medline]
Weiss H, Stuessy TF, Grau J, Baeza CM. Chromosome reports from South American Hypochaeris (Asteraceae). Annals of the Missouri Botanical Garden (2003) 90:56–63.[CrossRef][Web of Science][Medline]
Weiss-Schneeweiss H, Stuessy TF, Siljak-Yakovlev S, Baeza CM, Parker J. Karyotype evolution in South American species of Hypochaeris (Asteraceae, Lactuceae). Plant Systematics and Evolution (2003) 241:171–184.[CrossRef][Web of Science]
Weiss-Schneeweiss H, Schneeweiss GM, Stuessy TF, Mabuchi T, Park JM, Jang CG, Sun BY. Chromosomal stasis in diploids contrasts with genome restructuring in auto- and allopolyploid taxa of Hepatica (Ranunculaceae). New Phytologist (2007) a 174:669–682.[CrossRef][Web of Science][Medline]
Weiss-Schneeweiss H, Stuessy TF, Tremetsberger K, Urtubey E, Valdebenito HA, Beck SG, Baeza CM. Chromosome numbers and karyotypes of South American species and populations of Hypochaeris (Asteraceae). Botanical Journal of the Linnean Society (2007) b 153:49–60.[CrossRef][Web of Science]
Zhang D, Sang T. Chromosomal structural rearrangement of Paeonia brownii and P. californica revealed by fluorescence in situ hybridization. Genome (1998) 41:848–853.[Medline]
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