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. 2003 Aug;92(2):265–272. doi: 10.1093/aob/mcg141

Variation in DNA‐ploidy Levels of Reynoutria Taxa in the Czech Republic

BOHUMIL MANDÁK 1,*, PETR PYŠEK 1, MARTIN LYSÁK 2, JAN SUDA 1,3, ANNA KRAHULCOVÁ 1, KATEŘINA BÍMOVÁ 1,4
PMCID: PMC4243663  PMID: 12876190

Abstract

The genus Reynoutria is represented by four taxa in the Czech Republic: Reynoutria japonica var. japonica, R. japonica var. compacta, R. sachalinensis and R. ×bohemica. By using flow cytometry, cytological variability within the genus is described based on 257 Reynoutria samples. The varieties of R. japonica are cytologically uniform, var. japonica is exclusively octoploid (2n = 8x = 88) and var. compacta occurs only as a tetraploid (2n = 4x = 44), but R. sachalinensis and R. ×bohemica exhibit some variation in chromosome numbers. Reynoutria sachalinensis is predominantly tetraploid (2n = 4 x = 44), but also occurs occasionally as hexaploid and octoploid cytotypes. The most common ploidy level in R. ×bohemica is hexaploid (2n = 6x = 66), but tetraploid and octoploid clones were also found. The four taxa occurring in the Czech Republic are described briefly and the possible origins of the cytotypes discussed.

Key words: Clonality, Fallopia, flow cytometry, hybridization, invasion, Polygonaceae, polyploidy, Reynoutria japonica var. japonica, R. japonica var. compacta, R. sachalinensis, R. ×bohemica.

INTRODUCTION

The spread of an alien species and its ecological impact is sometimes positively correlated with the presence of polyploidy (Barrett, 1982; Thompson, 1991). Polyploidy in general allows the plant to diversify its genome due to genome reorganization (Soltis and Soltis, 1999) conferring adaptive plasticity (Roose and Gotlieb, 1976; Thompson and Lumaret, 1992) to hide deleterious alleles through an increase in frequency of heterozygous loci (Parsons, 1959), and subsequently to reduce the incidence of inbreeding. Polyploidy is frequently linked to vegetative reproduction (Gibby, 1981), which allows new taxa to buffer the disadvantages of small population size (Levin, 1975). Vegetative reproduction can help a plant of one sex to await the arrival of an individual of the opposite sex (Baker, 1986). Relatively high numbers of invasive plants possess diaspores adapted to asexual reproduction (Vogt Andersen, 1995), and some highly invasive plants, such as Reynoutria japonica var. japonica, are restricted exclusively to this method of reproduction in their adventive range (Hollingsworth et al., 1998).

Hybridization of related invasive taxa in the territory of their secondary distribution is a relatively common event (Vilà et al., 2000). Compared with their parents, the hybrids can possess various levels of fitness (Rieseberg, 1995). Examples of hybridization generating new taxa more highly invasive than their parents have been known (Vilà et al., 2000). Fast evolution of new taxa by coincidence of hybridization and polyploidization and subsequent spread of newly evolved species have been documented in Senecio (Abbott, 1992; Lowe and Abbott, 1996), Tragopogon (Roose and Gotlieb, 1976; Novak et al., 1991), Spartina (Marchant, 1967, 1968; Ayres and Strong, 2001) and Carpobrotus (Vilà and D’Antonio, 1998).

In members of the genus Reynoutria the combination of hybridization, the presence of various ploidy levels and pronounced clonality are probably the main determinants of their successful spread in the countryside. The basic scheme occurring within the genus has been described for the British Isles (Bailey and Stace, 1992; Bailey, 1994; Bailey et al., 1995; Hollingsworth et al., 1998; Hollingsworth and Bailey, 2000a, b), but little information is available for other parts of Europe. To understand better the determinants of invasiveness within the genus, it is useful to know the degree of cytological variation within the Czech Republic. Hence, the main aim of this paper is to describe the number of cytotypes and discuss their possible origins.

MATERIALS AND METHODS

Study species

Representatives of the genus Reynoutria Houtt. (syn. Fallopia Adans.) (Polygonaceae) are herbaceous perennials with robust erect stems, an extensive system of thick rhizomes, deeply three‐parted styles with fimbriate stigmas, and a functionally dioecious breeding system. There are two distinct opinions on the classification at the generic level. Some authors treat the group as the distinct genus Reynoutria (Holub, 1971), others as a section of the genus Fallopia [i.e. Fallopia sect. Reynoutria (Houtt.) Ronse Decr.] (Ronse Decraene and Akeroyd, 1988; Bailey and Stace, 1992). In this paper, we follow the former approach represented by the taxonomy of Holub (1971).

All species present in the Czech Republic were introduced into Europe as garden ornamentals from eastern Asia in the 19th century (Conolly, 1977; Bailey and Conolly, 2000). In the Czech Republic, the genus is represented by R. japonica Houtt. var. japonica, R. japonica Houtt. var. compacta Moldenke, R. sachalinensis (F. Schmidt) Nakai and the hybrid between R. sachalinensis and R. japonica, namely R. ×bohemica Chrtek et Chrtková described from the Czech Republic by Chrtek and Chrtková (1983). All invade riparian and various human‐made habitats and often spread into semi‐natural vegetation (Brabec and Pyšek, 2000; Pyšek et al., 2001). The spread of Reynoutria taxa in the Czech Republic is mainly vegetative through regeneration from rhizome and stem segments (Mandák and Pyšek, 1996; Bímová et al., 2001, 2003) resulting from the unusual form of sexual reproduction where members of the genus Reynoutria have been reported to sexually reproduce only rarely within the adventive distribution area due to inefficient seedling establishment (Bailey et al., 1995).

In R. sachalinensis, hermaphrodite and female tetraploid (2n = 4x = 44) clones have been recorded in Europe (Bailey and Stace, 1992). All European plants of R. japonica var. japonica recorded to date have been octoploid (2n = 8x = 88) and those of R. japonica var. compacta exclusively tetraploid (2n = 4x = 44) (Bailey and Stace, 1992). As yet, only female clones of R. japonica var. japonica have been recorded in the Czech Republic. However, despite the absence of pollen, the plants produce seeds because they are fertilized by pollen from Fallopia aubertii (L. Henry) Holub to give either Fallopia ×conollyana J. P. Bailey (Bailey, 2001) or R. sachalinensis (Mandák and Pyšek, 1996). In the latter case, the hybrid Reynoutria ×bohemica is produced (2n = 6x = 66) (Bailey and Stace, 1992).

Material collection

In total, 257 Reynoutria plants were collected from the wild in the Czech Republic between 1998 and 2001. Sampling locations were chosen such that they were representative of the whole of the Czech Republic and to obtain a sound basis for sampling the cytological variability [Fig. 1 and see Supplementary data (http://www.aob. oupjournals.org) for list of localities]. Samples were transplanted to the experimental garden of the Institute of Botany in Průhonice, Czech Republic. Rhizomes were planted in 12 l plastic pots filled with garden soil, and regenerated plants were used to ascertain chromosome numbers.

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Fig. 1. Distribution of the 257 Reynoutria clones used in this study. A, Reynoutria japonica var. japonica (white circles) and R. japonica var. compacta (black circles). B, R. ×bohemica; C, R. sachalinensis. Black circles indicate tetraploids (2n = 44), grey circles hexaploids (2n = 66), and open circles octoploids (2n = 88). For localities see Supplementary data: (http://www.aob.oupjournals.org).

Chromosome counts

A modified method of Bailey and Stace (1992) was used to prepare slides for chromosome counting. Actively growing roots were pretreated in 0·002 m 8‐hydroxyquinoline for 22–24 h at 4 °C, fixed overnight in 3 : 1 ethanol : acetic acid and stored at 4 °C in 70 % ethanol until used. Root‐tips were hydrolysed in 2n HCl for 10 min at 60 °C, rinsed in water and the meristematic tissue excised and squashed in a drop of lacto‐propionic orcein (Dyer, 1963). Chromosomes were counted using a phase‐contrast microscope.

Ploidy level estimation

All 257 clones were subjected to flow cytometric analysis of nuclear DNA content. A two‐step procedure of sample preparation was employed (Otto, 1990). Approximately 2 cm2 of fresh young leaf tissue of an analysed plant, together with approx. 1 cm2 of leaf tissue of an internal standard with known chromosome number [R. sachalinensis 2n = 4x = 44, R. ×bohemica 2n = 6x = 66 or R. japonica 2n = 8x = 88 (Supplementary data: http://www.aob. oupjournals.org)] were chopped with a new razor blade in a Petri dish containing 1 ml ice‐cold Otto I buffer (0·1 m citric acid, 0·5 % Tween 20). The sample was filtered through a nylon mesh (42 µm) and centrifuged at 150g for 5 min. The supernatant was removed and the nuclei resuspended in 100 µl of ice‐cold Otto I buffer. After 30 min incubation at room temperature, 1 ml of Otto II buffer (0·4 m Na2HPO4·12H2O) with DAPI at a final concentration of 4 µg ml–1 was added. Samples were stained for 30 min at room temperature. The relative fluorescence of isolated nuclei was analysed using a Partec PA II flow cytometer (Partec GmbH, Münster, Germany). The cytometer was adjusted so that the fluorescence of G1 nuclei of tetraploid plants (internal standard) was localized on channel 200.

RESULTS

The most common taxon, R. japonica var. japonica, was represented by 99 samples, R. ×bohemica by 94 samples, R. sachalinensis by 61 samples and R. japonica var. compacta was found only in three localities [Fig. 1 and Supplementary data (http://www.aob.oupjournals.org)].

The analysis of DNA content of nuclei isolated from leaf tissue showed that most of the nuclei were in the G0/G1 phase of the cell cycle and thus formed a dominant peak in histograms of DNA content. This peak was localized at channel 205 for tetraploid plants (CV = 2·23), at channel 335 for hexaploid plants (CV = 1·71) and at channel 461 for octoploid plants (CV = 1·31). The peak ratios were 1 : 1·63 : 2·25 for R. sachalinensis, R. ×bohemica and R. japonica, respectively (Fig. 2).

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Fig. 2. Histogram of relative nuclear DNA content of particular Reynoutria species. The G0/G1 peak of tetraploid R. sachalinensis was on channel 205, that of hexaploid R. ×bohemica on channel 335 and that of octoploid R. japonica var. japonica on channel 461, i.e. the peak ratios are 1 : 1·63 : 2·25 for R. sachalinensis, R. ×bohemica and R. japonica, respectively.

Both varieties of R. japonica were cytologically uniform; all R. j. var. japonica plants were octoploid, and all those of R. j. var. compacta tetraploid (Fig. 3). Reynoutria sachalinensis was predominantly tetraploid. Their hybrid, R. ×bohemica, was hexaploid. In addition, ploidy levels deviating from the above pattern were found in R. ×bohemica and R. sachalinensis (Fig. 3). Their most likely origins are shown in Fig. 4.

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Fig. 3. Ploidy levels in particular Reynoutria taxa. Numbers indicate the number of samples analysed.

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Fig. 4. Schema of hybridization and polyploidization within the genus Reynoutria in the Czech Republic. More frequent ploidy levels are indicated by wider frames: those with 6–20 localities have intermediate frames, those with more than 20 localities have wide frames. Solid lines indicate origin of particular taxa by hybridization, dashed lines indicate expected fusion of reduced and unreduced gametes, and dot‐and‐dashed lines indicate autopolyploidization.

The distribution of particular taxa and ploidy levels in the area studied is shown in Fig. 1. Reynoutria taxa occur more frequently in human‐made and riparian habitats regardless of ploidy level of particular taxa in the Czech Republic (Fig. 5). The occurrence of octoploid R. sachalinensis and R. ×bohemica is restricted to several localities that closely correspond to the presence of parks or old gardens. However, there is no general geographical pattern, and particular localities of different taxa and ploidy levels are often far from each other (Fig. 1).

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Fig. 5. Contribution of particular habitat types to the total number of localities for particular taxa and ploidy levels. J88, Reynoutria japonica var. japonica (2n = 88); C44, R. japonica var. compacta (2n = 44); B44, R. ×bohemica (2n = 44); B66, R. ×bohemica (2n = 66); B88, R. ×bohemica (2n = 88); S44, R. sachalinensis (2n = 44); S66, R. sachalinensis (2n = 66); S88, R. sachalinensis (2n = 88).

DISCUSSION

Polyploidy is a common phenomenon in plants; it is estimated to occur in 47–70 % of angiosperm species (Ramsey and Schemske, 1998). Being of great importance for plant taxonomy, evolution and ecology, it is thus of interest to plant biologists. Formation of polyploid series of morphologically and ecologically different, but closely related, species allows us to understand the importance of polyploidy in nature using methods of comparative ecology.

Cytological variation of R. japonica var. japonica in its native regions is high and includes tetraploids, hexaploids, octoploids and decaploids (Table 1). This is in contrast to the situation in the adventive distribution area covered by the reported study where only octoploids were found. The frequency distribution of ploidy levels ascertained in R. japonica var. japonica does not differ from that reported from other European countries (Table 1), with the exception of Slovakia. However, the results of the reported study and of chromosome counts made by Bailey and Stace (1992) who only found octoploid plants in Europe, question the tetraploid chromosome numbers reported from Slovakia (Májovský et al., 1987), which are probably due to misidentification of the taxa. Reynoutria japonica var. japonica is exclusively octoploid in Europe and does not possess any variation at either cytological or genotype level (Bailey and Stace, 1992; Hollingsworth and Bailey, 2000a). The present distribution probably resulted from a single introduction to Europe in the 19th century (Bailey and Conolly, 2000).

Table 1.

Literature records of chromosome numbers found in Reynoutria japonica var. japonica, R. japonica var. compacta, R. sachalinensis and R. ×bohemica in both native and adventive territories

Reference Chromosome number (2n) Locality
Reynoutria japonica Houtt. var. japonica
 Native distribution area
  Bailey (1999) 88 Korea
  Bailey (2003) 44, 88 Japan
  Bailey (1999), Hollingsworth and Bailey (2000b) 88, 110 China
  Sugiura (1931), Doida (1960), Bailey (1999),         Hollingsworth and Bailey (2000b) 44 Japan
  Kim and Park (2000) 44, 66, 88 Korea
  Lee (1972) 88 Korea
 Adventive distribution area
  Bailey (2003) 88 Hungary, USA, Canada, Australia, New Zealand
  Bailey and Stace (1992) 88 Great Britain
  Graham and Wood (1965) 44, approx. 88 USA
  Hollingsworth and Bailey (2000b) 88 Great Britain
  Májovský et al. (1974), Murín (1974), Váchová         and Feráková (1986), Májovský et al. (1987) 44 Slovakia
  Mandák et al., this study 88 Czech Republic
  Wcisło (1977), Pogan and Wcisło (1983) 88 Poland
  Zhukova [1967, in Bolkhovskikh et al. (1969)] approx. 60 Russia
Reynoutria japonica Houtt. var. compacta Moldenke
 Native distribution area
  Bailey (2003) 44 Japan
 Adventive distribution area
  Bailey and Conolly (1985), Bailey and Stace (1992),         Hollingsworth and Bailey (2000b), Bailey (2003) 44 Great Britain
Bailey (2003), Mandák et al., this study 44 Czech Republic
Reynoutria sachalinensis (F. Schmidt) Nakai
 Native distribution area
  Bailey (2003) 102, 103 Korea
  Bailey and Stace (1992), Bailey (2003) 44 Japan
  Kim and Park (2000) 132 Korea
  Kim and Park (2000) 44 Japan
  Lee (1972) 102 Korea
  Sokolovskaya [1960, 1965 in         Bolkhovskikh et al. (1969)] 44 Russia (Sakhalin)
 Adventive distribution area
  Bailey (2003) 44, 66 USA
  Bailey and Stace (1992) 44 Great Britain
  Májovský and Váchová (1986), Májovský et al. (1987) 44 Slovakia
  Mandák et al., this study 44, 66, 88 Czech Republic
  Menshikova [1964, in Bolkhovskikh et al. (1969)] ca. 66 Russia
  Wcisło (1977), Pogan and Wcisło (1983) 44 Poland
Reynoutria × bohemica Chrtek et Chrtková
 Native distribution area
  Bailey (2003) 66 Japan
 Adventive distribution area
  Bailey and Stace (1992), Bailey et al. (1996), Bailey (2003) 44, 66, 88 Great Britain
  Bailey (2003) 88 France, Germany
  Bailey (2003) 66 USA, Australia, New Zealand
  Mandák et al., this study 44, 66, 88 Czech Republic
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The dwarf alpine form R. japonica var. compacta has been repeatedly reported to be tetraploid (Table 1) which is in accordance with our study. Nevertheless, chromosome counts published to date are only from the adventive distribution area where R. japonica var. compacta is distinguished as a morphologically distinct type. This distinction certainly results from the bottleneck effect; in the native distribution area, there is a high morphological variability creating a continuum between R. japonica var. japonica and R. japonica var. compacta (Shiosaka and Shibata, 1993). For this reason, most authors working in the Far East do not recognize this variety (Lee, 1972; Kim and Park, 2000; but see Ohwi, 1965). On the other hand, Yonekura and Ohashi (1997) recognized, in addition to var. japonica, two more varieties of R. japonica, namely var. hachidyoensis (Honda) Yonekura et H. Ohashi and var. uzenesis (Honda) Yonekura et H. Ohashi. Both are endemic to Japan and have never been introduced into Europe.

Whereas R. japonica is cytologically uniform in Europe (Table 1), the situation with R. sachalinensis and R. ×bohemica is much more complicated. Our study yielded new ploidy levels (Figs 3 and 4). Reynoutria sachalinensis is predominantly tetraploid, but we also found hexaploids and octoploids (Fig. 3). There are two possible methods by which this could arise: (1) by generative reproduction via unreduced gametes or (2) by somatic mutations (i.e. autopolyploidization leading to an octoploid cytotype). The former method is limited by rarely occurring sexual reproduction in Reynoutria taxa in the territory studied. Hence, occasional generative reproduction fixed by clonality over time could store ‘aberrant’ ploidy levels for a long time. Massive clonal growth could function in the same way in the case of somatic mutation without the necessity for generative reproduction. However, since the frequency of somatic mutations in R. sachalinensis is not known, the latter explanation of ‘aberrant’ ploidy levels only remains a speculation. An alternative explanation is the introduction of these ‘aberrant’ ploidy levels directly from the native distribution areas, Japan, Sakhalin or Ullung‐do (an island between Japan and Korea). The ability of hexaploid and octoploid R. sachalinensis to hybridize with other Reynoutria taxa has never been studied, i.e. there are no data concerning the fertility of pollen grains, seed viability or the ability of seedlings to survive. On the other hand, hybridization between and within particular Reynoutria taxa is common (Bailey and Stace, 1992). Hence, we can expect further hybridization with other Reynoutria taxa and the subsequent rise of new hybrid combinations with unknown properties.

Reynoutria ×bohemica is a hybridogenous taxon that either originated in the adventive distribution area or was directly introduced from the native distribution area. Hybridization events of alien plants followed by the spread of the hybrid have been documented (Vilà et al., 2000). Such events can lead to the rapid evolution of new taxa and expansion of their range. Eventually, such hybrid taxa may interfere with human objectives and become weedy or invasive (Abbott, 1992) as is the case with R. ×bohemica (B. Mandák and P. Pyšek, unpubl. res.).

However, the pattern of cytological variation in R. ×bohemica follows that found in R. sachalinesis but with different underlying mechanisms (Bailey et al., 1995). Reynoutria ×bohemica is predominantly hexaploid (Fig. 3). The clones with different ploidy levels, i.e. tetraploids and octoploids, have different origins. The tetraploid R. ×bohemica is probably a hybrid between R. sachalinensis and R. japonica var. compacta (Fig. 4). Bailey and Stace (1992) pointed out that this hybrid is very difficult to distinguish on morphological grounds from hexaploid R. ×bohemica, and chromosome numbers are often the only means of identifying these clones. The origin of octoploid R. ×bohemica plants is inevitably more speculative (Bailey, 1999). They are probably the product of fusion of an unreduced (hexaploid plant of R. ×bohemica) and a reduced (tetraploid plant of R. sachalinensis) gamete. Another possibility is autopolyploidization of the tetraploid R. ×bohemica cytotype (Fig. 4).

Bailey and Stace (1992) showed that hexaploid R. ×bohemica has extremely irregular meiosis, with large numbers of univalents and multivalents up to quadrivalents. This fact is reflected in low pollen fertility. In contrast, both tetraploid and octoploid R. ×bohemica have much more regular meiosis than the hexaploid. This agrees with our finding that the pollen grains of hexaploid R. ×bohemica showed lower germination than the octoploid clones (Koukolíková 2001). We can partly confirm this result that regularity of meiosis in Reynoutria hybrids is determined mainly by the ploidy level of these taxa.

The most common ploidy levels of particular Reynoutria taxa are scattered over the whole of the Czech Republic (Fig. 1). Most of the habitats occupied by Reynoutria taxa are in human settlements and the occurrence of the taxa is therefore associated with cultivation and subsequent accidental spreading by man, water or soil movements connected with building activities. Compared with the other Reynoutria taxa, the presence of octoploid R. ×bohemica and R. sachalinensis is more associated with gardens (Fig. 5). Hence, there is a possibility that these ‘aberrant’ ploidy levels were introduced directly from native distribution areas or evolved in gardens and then spread locally.

Although a direct genotype analysis is needed to confirm this conclusion, the high cytological variability of R. ×bohemica indicates that the evolution of new taxa is in progress within the complex studied. This fact, in combination with a high invasive potential within the group, is alarming. Selection of new, more invasive clones at the cytological level brings about the possibility that new cytotypes with a greater ability to spread and survive could emerge in central Europe.

SUPPLEMENTARY INFORMATION

Supplementary data contain a list of Reynoutria localities with ploidy levels and geographical coordinates in the Czech Republic.

ACKNOWLEDGEMENTS

We thank Ivan Ostrý, Irena Koukolíková and Jan Pergl for technical assistance. We are also grateful to John P. Bailey for his help with various Reynoutria problems and for reading the manuscript. The work was supported by grant no. A6005805/1998 from the Grant Agency of the Academy of Sciences of the Czech Republic, and grant no. AV0Z6005908 from the Academy of Sciences of the Czech Republic.

Supplementary Material

Supplementary data
supp_92_2_265__index.html (719B, html)
Content Snapshot
supp_92_2_265_v2_index.html (987B, html)

Received: 22 November 2002; Returned for revision: 12 March 2003; Accepted: 7 May 2003

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