The species-rich and widespread genus Taraxacum F. H. Wiggers, 1780 (Asteraceae subfamily Cichorioideae) is one of the most taxonomically complex plant genera in the world, mainly due to its combination of different sexual and asexual reproduction strategies. Polyploidy is usually confined to apomictic microspecies, varying from 3x to 6x (rarely 10x). In this study, we focused on Taraxacum sect. Taraxacum (= T. sect. Ruderalia; T. officinale group), i.e., the largest group within the genus. We counted chromosome numbers and measured the DNA content for species sampled in Central Europe, mainly in Czechia. The chromosome number of the 28 species (T. aberrans Hagendijk, Soest & Zevenbergen, 1974, T. atroviride Štěpánek & Trávníček, 2008, T. atrox Kirschner & Štěpánek, 1997, T. baeckiiforme Sahlin, 1971, T. chrysophaenum Railonsala, 1957, T. coartatum G.E. Haglund, 1942, T. corynodes G.E. Haglund, 1943, T. crassum H. Øllgaard & Trávníček, 2003, T. deltoidifrons H. Øllgaard, 2003, T. diastematicum Marklund, 1940, T. gesticulans H. Øllgaard, 1978, T. glossodon Sonck & H. Øllgaard, 1999, T. guttigestans H. Øllgaard in Kirschner & Štěpánek, 1992, T. huelphersianum G.E. Haglund, 1935, T. ingens Palmgren, 1910, T. jugiferum H. Øllgaard, 2003, T. laticordatum Marklund, 1938, T. lojoense H. Lindberg, 1944 (= T. debrayi Hagendijk, Soest & Zevenbergen, 1972, T. lippertianum Sahlin, 1979), T. lucidifrons Trávníček, ineditus, T. obtusifrons Marklund, 1938, T. ochrochlorum G.E. Haglund, 1942, T. ohlsenii G.E. Haglund, 1936, T. perdubium Trávníček, ineditus, T. praestabile Railonsala, 1962, T. sepulcrilobum Trávníček, ineditus, T. sertatum Kirschner, H. Øllgaard & Štěpánek, 1997, T. subhuelphersianum M.P. Christiansen, 1971, T. valens Marklund, 1938) is 2n = 3x = 24. The DNA content ranged from 2C = 2.60 pg (T. atrox) to 2C = 2.86 pg (T. perdubium), with an average value of 2C = 2.72 pg. Chromosome numbers are reported for the first time for 26 species (all but T. diastematicum and T. obtusifrons), and genome size estimates for 26 species are now published for the first time.

Asteraceae , chromosome number, flow cytometry, karyology, Taraxacum officinale

Taraxacum F. H. Wiggers, 1780 (Asteraceae subfamily Cichorioideae) is a species-rich genus of common and widespread perennial grassland herbs growing from the subtropics to subarctic (arctic/alpine) regions across the world. Rough estimates suggest the genus contains approximately 2,800 species in approximately 60 sections (Kirschner et al. 2015), with the higher diversity in the mountains of Eurasia (Ge et al. 2011); a total of 1,900 species in 35 sections are listed for Europe (Kirschner et al. 2007). The complexity of Taraxacum taxonomy is caused by its combination of different reproduction strategies, including sexual reproduction (mainly outcrossing, less frequently selfing) and apomixis (meiotic diplospory; Richards 1973, Asker and Jerling 1992, Kirschner and Štěpánek 1994, Kirschner et al. 1994, Majeský et al. 2017). The vast majority of Taraxacum taxa are apomictic polyploid microspecies, only a few species are sexual diploids. The phenomenon of apomixis itself (i.e. clonal reproduction by seeds) attracts the attention of plant systematists as well as plant breeders for its possible application in crop breeding.

The basic chromosome number in Taraxacum is x = 8, and it is constant across all the sections. The diploid number (2n = 2x = 16) is confined to only sexually reproducing species, and sexual species are nearly all diploids, with only a few exceptions of sexual tetraploids known in section Piesis (Kirschner and Štěpánek 1994, 1998a, Trávníček et al. 2013). In contrast, apomictic species are never diploids but always polyploids (Majeský et al. 2017), having one of the genes involved in regulation of apomixis (DIPLOSPOROUS) located on the NOR chromosome (Vašut et al. 2014). Most of the known chromosome numbers for apomictic Taraxacum species are at a triploid level (2n = 3x = 24), especially those of the widespread European sections Taraxacum sect. Taraxacum (Mártonfiová 2006, Kula et al. 2013), T. sect. Erythrosperma (Małecka 1967, 1969, Vašut 2003, Schmid et al. 2004, Vašut et al. 2005, Uhlemann 2007, 2010, Vašut and Majeský 2015, Wolanin and Musiał 2017), T. sect. Palustria (Małecka 1972, 1973, 1978, Kirschner and Štěpánek 1998b, Marciniuk et al. 2010) and T. sect. Hamata (Mogie and Richards 1983, Øllgaard 1983). However, tetraploids (2n = 4x = 32) also occur quite frequently in some sections, such as the European dandelions in sections T. sect. Palustria (e.g., T. vindobonense Soest, 1965, T. brandenburgicum Hudziok, 1969 and T. portentosum Kirschner & Štěpánek, 1998), T. sect. Erythrosperma (e.g., T. tortilobum Florström, 1914, T. fulvum Raunkiaer, 1906 and T. bifurcatum Hagendijk et al., ineditus), T. sect. Naevosa (e.g., T. euryphyllum (Dahlstedt, 1911) M. P. Christiansen, 1940 and T. naevosum Dahlstedt, 1900), T. sect. Scariosa and T. sect. Celtica (T. unguilobum Dahlstedt, 1912 and T. fulvicarpum Dahlstedt, 1927). Higher ploidy levels are uncommon in Taraxacum, while natural pentaploids (2n = 5x = 40; e.g., in the European species T. skalinskanum Małecka & Soest, 1972 and T. zajacii J. Marciniuk et P. Marciniuk, 2012 and 6 other species of section Palustria, T. faeroense Dahlstedt in H. H. Johnston, 1926 of T. sect. Spectabilia, T. caledonicum A. J. Richards, 1972 of section Celtica and T. albidum Dahlstedt, 1907 of section Mongolica from Japan), hexaploids (2n = 6x = 48 for T. ranunculus Kirschner & Štěpánek, 1998 of section Palustria and T. nordstedtii Dahlstedt, 1911 of section Celtica), and aberrant heptaploid (2n = 7x = 56) or decaploid (2n = 10x = 80) mutants of natural species have been documented (Richards 1969, Małecka 1973, Mogie and Richards 1983, Kirschner and Štěpánek 1984, 1998b, Sato et al. 2011, Marciniuk et al. 2012). The geographic distribution of diploids and polyploids in Europe is more or less separated, with polyploids mainly distributed in the colder regions of mountains in the north and with diploid sexuals distributed in warmer regions of the south, which results in the phenomenon of geographic parthenogenesis (den Nijs et al. 1990, den Nijs and van der Hulst 1988, Uhlemann 2001, Verduijn et al. 2004a).

Genome size estimation (plant genome C-value) (Greilhuber et al. 2005) is a rapid cytogenetic method that helps provide a better understanding of the evolutionary relationships among studied taxa. The method itself has methodological limitations (multiple factors can affect the measurements; the method does not provide any information on repetitive sequences involved; etc.); however, patterns of genome size estimates in species groups provide additional information on possible pathways of evolution (Soltis et al. 2003, Leitch et al. 2005, Šmarda et al. 2012). Although flow cytometry was widely used in Taraxacum research for rapidly identifying the ploidy level in mixed apomictic-sexual populations (e.g., Meirmans et al. 1999, Verduijn et al. 2004a, 2004b, Mártonfiová 2006, 2015, Mártonfiová et al. 2007, 2010) or in taxonomic revisions (e.g., Vašut 2003), genome size estimates are very limited. Genome size (C-value) in Taraxacum varies (in known species) between 2C = 1.74 pg in diploid T. linearisquameum Soest, 1966 and 2C = 6.91 pg in tetraploid T. albidum (Záveský et al. 2005, Siljak-Yakovlev et al. 2010); European triploid apomicts have a value of 2C ≈ 2.4–2.76 pg (Bennett et al. 1982, Záveský et al. 2005, Bainard et al. 2011, Iaffaldano et al. 2017). Considerable variation (~1.2-fold difference) in DNA content, measured as the C-value, was observed in T. stenocephalum Boissier et Kotschy ex Boissier, 1875 (Trávníček et al. 2013) and in a sample of an unidentified species of the Taraxacum officinale group in North America (Iaffaldano et al. 2017).

Taraxacum sect. Taraxacum (formerly known as T. sect. Ruderalia; generally known as Taraxacum officinale group; see Kirschner and Štěpánek 2011) has a strongly prevailing triploid ploidy level of 2n = 3x = 24, by which it differs from other closely related sections (Erythrosperma, Palustria, and Celtica) with known ploidies of 3x and 4x or even higher. In this study, we aimed to count the chromosome number of 28 species for which knowledge was lacking and to detect the ploidy level for these species. Furthermore, we searched for variability in genome size among these species to determine whether we can detect variation in DNA content among species similar to that found in a sample of unidentified taxa of T. officinale group.

The chromosome number of all 28 studied species of Taraxacum sect. Taraxacum (T. aberrans, T. atroviride, T. atrox, T. baeckiiforme, T. chrysophaenum, T. coartatum, T. corynodes, T. crassum, T. deltoidifrons, T. diastematicum, T. gesticulans, T. glossodon, T. guttigestans, T. huelphersianum, T. ingens, T. jugiferum, T. laticordatum, T. lojoense, T. lucidifrons, T. obtusifrons, T. ochrochlorum, T. ohlsenii, T. perdubium, T. praestabile, T. sepulcrilobum, T. sertatum, T. subhuelphersianum, T. valens) was counted invariably as 2n = 3x = 24 (Figs 1, 2). With respect to the position of the centromere, the chromosomes of all studied species were predominantly sub-metacentric or metacentric. The chromosome sizes were relatively small (Figs 1, 2). The smallest chromosome size in this study was 1.02 µm (T. ochrochlorum), and the largest one was 4.94 µm (T. baeckiiforme).

Figure 1.

Mitotic metaphase chromosomes of studied triploid species (2n=3x=24) of Taraxacum sect. Taraxacum. A T. aberrans B T. atroviride C T. atrox D T. baeckiiforme E T. chrysophaenum F T. coartatum G T. corynodes H T. crassum I T. deltoidifrons J T. diastematicum K T. gesticulans L T. glossodon M T. guttigestans N T. huelphersianum O T. ingens. Scale Bar: 5µm.

Figure 2.

Mitotic metaphase chromosomes of studied triploid species (2n=3x=24) of Taraxacum sect. Taraxacum. A T. jugiferum B T. laticordatum C T. lojoense D T. lucidifrons E T. obtusifrons F T. ochrochlorum G T. ohlsenii H T. perdubium I T. praestabile J T. sepulcrilobum K T. sertatum L T. subhuelphersianum M T. valens. Scale Bar: 5µm.

The DNA content of the twenty-six studied Taraxacum species (two species, i.e., T. chrysophaenum and T. subhuelphersianum, were not analysed due to low-quality fresh material) ranged 1.08-fold from 2C = 2.60 pg in T. atrox to 2C = 2.86 pg in T. perdubium (Table 2). The average and median 2C-values for Taraxacum sect. Taraxacum (based on these 26 species) are 2.72 pg and 2.71 pg, respectively.

Chromosome number variation differs among sections of the genus Taraxacum and more frequently occurs in sections such as Palustria or Celtica, whereas in section Taraxacum (and also section Hamata), it is nearly unknown. In our study, we aimed to either find variation in ploidy or confirm the prevailing triploid level. Our findings confirmed previously published records of 2n = 3x = 24 for T. diastematicum and T. obtusifrons (Uhlemann 2001, Salih et al. 2017); the chromosome numbers for all other 26 species are new findings. The ploidy level measured by flow cytometry was previously documented for 11 species (T. atrox, T. baeckiiforme, T. corynodes, T. crassum, T. glossodon, T. guttigestans, T. ingens, T. laticordatum, T. ohlsenii, T. sertatum and T. valens; Trávníček et al. 2010); we now provide exact information on chromosome numbers and genome size estimations.

A tetraploid chromosome number (2n = 4x = 32) was counted for only a few species of the 165 species of T. sect. Taraxacum with known chromosome numbers in the Chromosome Counts Database (CDDB, version 1.45; Rice et al. 2015). None of the records can be considered fully reliable due to frequent misidentifications of the Taraxacum microspecies (lack of identification by specialists). Den Nijs and Sterk (1984) published two chromosome counts, i.e., triploid (2n = 3x = 24) and tetraploid (2n = 4x = 32), for species named as T. lacistrum Sahlin, 1982, and collected in France; however, the tetraploid number is listed as a question mark, and this chromosome number must therefore be considered dubious (due to the apomictic behaviour of microspecies, it is implausible to have 2 different ploidy levels for the same species). The chromosome number for a species from the High Atlas, T. atlantis-majoris H. Lindberg, 1932 was counted as tetraploid, but the species identification is mentioned as “T. cf. atlantis-majoris”, and misidentification as other species (even from other sections, such as Piesis) cannot be excluded (Oberprieler and Vogt 1993). The tetraploid record for T. albertshoferi Sahlin, 1984 (Sahlin 1984) cannot be accepted without doubt either, because in the same paper, T. franconicum Sahlin, 1984 (which is now considered a synonym of T. plumbeum Dahlstedt, 1911) is also described with a tetraploid chromosome number, which was confirmed to be erroneous; the correct one is triploid (e.g., Vašut 2003). The tetraploid record for T. mediterraneum Soest, 1954 (Cardona and Contandriopoulos 1983; identified as T. balearicum Soest, 1961) does not refer how the taxon was determined. Rousi et al. (1985) published a tetraploid record for T. penicilliforme H. Lindberg, 1907 as a member of T. sect. Vulgaria (= T. sect. Taraxacum), but this species belongs to T. sect. Borea. Thus, the only somewhat reliable record of a tetraploid in Taraxacum sect. Taraxacum is for the alpine species T. venticola A. J. Richards, 1972 (Richards 1972).

Our list of species of T. sect. Taraxacum mainly includes typical members of the section, which differ slightly in their eco-geographic preferences. Some species have (in Central Europe) a preference for wet and sub-oceanic regions (such as T. corynodes, T. chrysophaenum, T. lucidifrons and T. ochrochlorum); on the other hand, some occupy more xerothermic regions (e.g., T. atrox, T. baeckiiforme, and T. lojoense). Some species resemble members of T. sect. Celtica (T. lucidifrons) or T. sect. Palustria (T. perdubium and T. sepulcrilobum). However, although the species in our study differ somewhat in ecology and geography, there is no variation in their ploidy levels. This is in agreement with previous studies in which only a triploid level was undoubtedly recorded for Nordic (“Atlantic”) and Pannonian or Mediterranean species.

Genome size estimates in Taraxacum sect. Taraxacum are very limited. Only a few papers dealt with its genome size (Bennett et al. 1982, Záveský et al. 2005, Bainard et al. 2011, Iaffaldano et al. 2017), but none of these papers studied known apomictic microspecies; only unknown species of the T. officinale group were measured. Generally, the genome size of the T. officinale group varies between 2C = 1.65 pg and 2C = 3.09 pg (Bennett et al. 1982, Záveský et al. 2005, Vidic et al. 2009, Temsch et al. 2010, Bainard et al. 2011, Iaffaldano et al. 2017; summarized in Table 3); values between 2C = 1.65–1.74 pg (Záveský et al. 2005, Iaffaldano et al. 2017) are equal to a diploid ploidy level (i.e., the species T. linearisquameum). The genome size of triploid apomicts apparently ranges from 2C = 2.45 pg to 2.76 (3.09) pg (see literature above). Our results are among the highest recorded values. The overall variation in recorded values is approximately 16 % (excluding the highest value of 2C = 3.09 pg, which may represent an aneuploid or tetraploid plant). Such variation can reflect real genome size variation among different species (individuals). Within a single species, Taraxacum stenocephalum (T. sect. Piesis), an ~1.2-fold difference in DNA content is documented (1.194-fold difference for DAPI and 1.219-fold difference for PI; Trávníček et al. 2013). Greater variation in DNA content can be attributed to the sexual reproduction of the species (in contrast to the apomictic reproduction of the species in our study). Even greater variation in DNA content was documented in Picris hieracioides Linnaeus, 1753 (Asteraceae, Cichorioideae, Cichorieae); in diploid sexual species, it ranged from 2C = 2.26 to 3.11 pg (1.37-fold difference; Slovák et al. 2009). In other genera of Asteraceae with the occurrence of apomictic taxa, such as Hieracium Linnaeus, 1753 and Pilosella Hill, 1756 DNA content variation is considerably larger than the known variation in Taraxacum sect. Taraxacum, i.e., 2.37-fold and 4.3-fold, respectively (Suda et al. 2007, Chrtek et al. 2009).

Genome size estimates vary in all taxa. Multiple factors can affect the measurement of genome size, e.g., differences in instrument settings among the instruments used (Doležel et al. 1998), using inadequate dye (DAPI vs. PI; Doležel et al. 1992), interactions between the dye and other molecules that lead to cytosolic effects (Noirot et al. 2000), and discrepancies in standardization (Doležel and Greilhuber 2010). Applying different laboratory procedures to the same species can lead to up to <10% variation; in the T. officinale group, different treatments led to a difference of up to 8.7% (Bainard et al. 2011). Therefore, at least part of the difference among published records can be attributed to a bias due to differences in laboratory procedures. We used a standardized procedure (buffers, tissue treatments, etc.) in our lab; therefore, the observed variation among the species used in this study likely reflects the real variation in DNA content.

Our study provided new data for 26 species of T. sect. Taraxacum, which confirmed no variation in chromosome number and ploidy level (2n = 3x = 24) and revealed only minor variation in DNA content that roughly equalled a possible methodological bias. The species sampled cover variation within the section: a sample of typical T. sect. Taraxacum species (most of the studied species) but also species that by morphology or ecology are intermediates of other sections, i.e., T. perdubium and T. sepulcrilobum, which are morphological and ecological intermediates between the studied section and T. sect. Palustria; or T. lucidifrons, which is morphologically similar to T. sect. Celtica or species resembling members of T. sect. Borea (T. ohlsenii, T. lojoense and T. atrox). Two species in our list are apolliniferous (T. atrox and T. subhuelphersianum). Such unusual homogeneity among species in T. sect. Taraxacum rather than great morphological (and ecological) variability might reflect a young evolutionary origin, which is likely in contrast to sections Palustria, Erythrosperma and others that may partly consist of evolutionarily older species (Wittzell 1999, Majeský et al. 2012, Kirschner et al. 2015). Although there is no evidence for the potential evolutionary scenario in European Taraxacum sections, we can speculate that the origin of apomictic species of T. sect. Taraxacum (T. officinale group) may be a result of “recent” hybridization between triploid apomicts and diploid sexuals in the sexual-asexual cycle in a mixed dandelion population, a phenomenon experimentally described in this group (Tas and van Dijk 1999, van Dijk 2003, van Dijk and Vijverberg 2005). In a mixed population (2x and 3x cytotypes; sexual and apomictic types), triploids are results of hybridization between triploid apomicts (diploid pollen) and diploid sexuals (haploid egg cell); however, a rare occurrence of tetraploidy (probably of temporary occurrence) can accelerate the formation of novel triploids (Verduijn et al. 2004b). These tetraploids probably occur in nature as a (rare) product of hybridization in mixed populations (probably discovered in the papers of Sato et al. 2014 or Iaffaldano et al. 2017; Ľ. Majeský, unpublished results) and function as a bridge in the formation of novel stable apomictic microspecies, but probably no such temporary tetraploid hybrids evolved in stable microspecies.

This study was supported by the Internal Grant Agency of Palacký University (IGA PrF-2018-001), the European Social Fund, Education for Competitiveness Operational Programme (CZ.1.07/2.2.00/28.0158) and by the National Program of Sustainability I (award LO1204).