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New chromosome counts and genome size estimates for 28 species of Taraxacum sect. Taraxacum
expand article infoPetra Macháčková, Ľuboš Majeský, Michal Hroneš, Eva Hřibová§, Bohumil Trávníček, Radim J. Vašut
‡ Palacký University in Olomouc, Olomouc, Czech Republic
§ Centre of the Region Haná for Biotechnological and Agricultural Research, Olomouc, Czech Republic
Open Access

Abstract

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.

Keywords

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

Introduction

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.

Material and methods

Plant Material

We studied a total of 28 Taraxacum species (25 formally described and three still undescribed, referred to by their working names) belonging to Taraxacum sect. Taraxacum (Table 1). Plants and achenes of the investigated species were collected in natural habitats of several localities of Central Europe in the period 2014–2016. A detailed description of the localities, date, and collectors of samples is provided in Table 1. The studied plant material was documented by herbarium specimens and is deposited in the herbarium of the Department of Botany, Palacký University in Olomouc, Czech Republic (OL). All studied species are apomictic (agamospermous); thus, maternal plants and offspring plants (grown from seeds) are taxonomically (genetically) identical.

List of species used in this study, with sampling details. Country codes according to ISO 3166-1 alpha-2 (AT = Austria; CZ = Czechia, DE = Germany, HU = Hungary, IT = Italy, SK = Slovakia); Collectors: BT = Bohumil Trávníček; RJV = Radim Jan Vašut.

Taxon Country Locality; GPS; Date; Collector
T. aberrans Hagendijk, Soest & Zevenbergen, 1974 AT Upper Austria, Obernberg am Inn town, lawn in the street of Therese-Riggle-Strasse; 48°19'14"N; 13°19'52"E; 10.05.2015; BT
T. atroviride Štěpánek & Trávníček, 2008 AT Altaussee village (near Bad Aussee town), lawns and roadsides in the ski resort NNW from the village (valley of Augstbach brook); 47°39'42"N; 13°44'38"E; 08.05.2014; BT
T. atrox Kirschner & Štěpánek, 1997 IT Cave del Predil settlement (S from Tarvisio town), lawns at the road no SP76 (at lake of Lago di Predil); 46°25'11"N; 13°33'42"E; 16.05.2015; BT
T. baeckiiforme Sahlin, 1971 HU Felsöcsatár village (W from the Szombathely town), grassy roadsides at the road towards Vaskeresztes village; 250 m a.s.l.; 47°12'20"N; 16°26'51"E; 26.04.2015; BT
T. chrysophaenum Railonsala, 1957 CZ Bartošovice village (near Nový Jičín town), lawns in park in central part of the village; 49°40'15"N, 18°02'59"E; 23.04.2014; BT
T. coartatum G. E. Haglund, 1942 CZ Lubná village (near Polička town), grassy places at brook in E part of the village; 480 m a.s.l.; 49°46'26"N, 16°13'57"E; 17.05.2016; BT & RJV
T. corynodes G. E. Haglund, 1943 CZ Hanušovice town, lawns at the railway station; 50°04'18"N, 16°55'52"E; 19.05.2015; BT
T. crassum H. Øllgaard & Trávníček, 2003 CZ Nové Město na Moravě town, grassy places at brook in the town, ca 0.6 km ESE from railway station of “Nové Město na Moravě-zastávka”; 600 m a.s.l.; 49°33'45"N, 16°04'04"E; 17.05.2016; BT & RJV
T. deltoidifrons H. Øllgaard, 2003 CZ Jimramov town, grassy places in the park of Bludník in N part of the town; 500 m a.s.l.; 49°38'19"N, 16°13'25"E; 17.05.2016; BT & RJV
T. diastematicum Marklund, 1940 CZ Svratka village, meadows and grassy places at NW margin of the settlement of Česká Cikánka; 630 m a.s.l.; 49°42'35"N, 16°03'01"E; 17.05.2016; BT & RJV
T. gesticulans H. Øllgaard, 1978 CZ Hanušovice town, lawns at the railway station; 50°04'18"N, 16°55'52"E; 19.05.2015; BT
T. glossodon Sonck & H. Øllgaard, 1999 CZ Studnice village (N from Nové Město na Moravě town), meadow at road near the Paseky settlement ca 1 km NNW from the village; 780 m a.s.l.; 49°36'51"N, 16°05'17"E; 17.05.2016; BT & RJV
T. guttigestans H. Øllgaard in Kirschner & Štěpánek, 1992 CZ Nové Město na Moravě town, grassy places at brook in the town, ca 0.6 km ESE from railway station of “Nové Město na Moravě-zastávka”; 600 m a.s.l.; 49°33'45"N, 16°04'04"E; 17.05.2016; BT & RJV
T. huelphersianum G. E. Haglund, 1935 CZ Pekařov settlement (near Hanušovice town), lawns and meadows in the settlement; 50°04'41"N, 17°01'31"E; 19.05.2015; BT
T. ingens Palmgren, 1910 CZ Svratka village, meadows and grassy places at NW margin of the settlement of Česká Cikánka; 630 m a.s.l.; 49°42'35"N, 16°03'01"E; 17.05.2016; BT & RJV
T. jugiferum H. Øllgaard, 2003 CZ Jedlí village (NW from Zábřeh town), lawns and roadsides in central part of the village; 49°55'54"N, 16°47'45"E; 19.05.2015; BT
T. laticordatum Marklund, 1938 CZ C Moravia, Hlinsko pod Hostýnem village, roadside at road towards Prusinovice village; 49°22'34"N; 17°36'47.8"E; 20.05.2016; BT
T. lojoense H. Lindberg, 1944 † CZ Úterý village (near Konstantinovy Lázně town), lawns at the brook on the eastern village margin; 510 m a.s.l.; 49°56'24"N, 13°00'21"E; 25.04.2014; BT
T. lucidifrons Trávníček, ineditus CZ Kunín village (near Nový Jičín town), lawns in chateau park; 49°38'39"N, 17°59'18"E; 23.04.2014; BT
T. obtusifrons Marklund, 1938 CZ Lubná village (near Polička town), grassy places at brook in E part of the village; 480 m a.s.l.; 49°46'26"N, 16°13'57"E; 17.05.2016; BT & RJV
T. ochrochlorum G. E. Haglund, 1942 CZ Svratka village, meadows and grassy places at NW margin of the settlement of Česká Cikánka; 630 m a.s.l.; 49°42'35"N, 16°03'01"E; 17.05.2016; BT & RJV
T. ohlsenii G. E. Haglund, 1936 DE Schönwald village (near Hof town), wet meadow and adjacent roadsides at the road (no. 15) towards Rehau village; 550 m a.s.l.; 50°13'37"N, 12°04'57"E; 27.04.2014; BT
T. perdubium Trávníček, ineditus CZ Záhlinice village (near Hulín town), wet meadow 1.3 km SSW from the railway station; 190 m a.s.l.; 49°16'52"N, 17°28'58"E; 20.04.2016; BT
T. praestabile Railonsala, 1962 IT Sella Nevea settlement (SW from Tarvisio town), lawns near hotel of Canin, road no. SP76; 46°23'19"N, 13°28'25"E; 16.05.2015; BT
T. sepulcrilobum Trávníček, ineditus CZ Záhlinice village (near Hulín town), wet meadow 1.3 km SSW from the railway station; 190 m a.s.l.; 49°16'52"N, 17°28'58"E; 20.04.2016; BT
T. sertatum Kirschner, H. Øllgaard & Štěpánek, 1997 CZ Svratka village, meadows and grassy places at NW margin of the settlement of Česká Cikánka; 630 m a.s.l.; 49°42'35"N, 16°03'01"E; 17.05.2016; BT & RJV
T. subhuelphersianum M. P. Christiansen, 1971 SK Spišské Podhradie village (near Levoča town), lawn at road not far from Sivá brada travertine spring; 49°00'28"N, 20°43'26"E; 01.05.2014; BT
T. valens Marklund, 1938 HU Szombathely town, lawns in the Szent István park (at the street of Jókai Mór); 225 m a.s.l.; 47°13'45"N, 16°36'15"E; 26.04.2015; BT

For karyological analyses, achenes were sown in Petri dishes containing 1% agar solution and germinated at room temperature. Fresh young leaves for flow cytometric analyses were collected from juvenile plants cultivated in a greenhouse at the Department of Botany, Faculty of Science, Palacký University in Olomouc.

Karyology

For chromosome counts, we used mitotically active root tip meristems of dandelion seedlings. Seedlings of the investigated species with 1–2 cm long roots were collected in the morning. To obtain the desired metaphase index, the roots were pre-treated in a 2 mM solution of 8-hydroxyquinoline for two hours at room temperature and an additional two hours at 4 °C in the dark. Then, the material was fixed in Carnoy’s fixative (a mixture (3:1, v/v) of absolute ethanol and acetic acid) and stored in a refrigerator (4 °C) until further processing (Hasterok and Maluszynska 2000). For slide preparation, a combination of protocols in Hasterok and Maluszynska (2000) and van Baarlen et al. (2000) was used with the following changes for the investigated species of dandelions. Fixed root tips were washed in citrate buffer (0.01 M, pH 4.8) for 5 min and then enzymatically digested in a mixture of 0.1% cellulose Onozuka RS (Trichoderma Persoon, 1794; Sigma), 0.1% pectolyase (Aspergillus japonicus Saito, 1906; Sigma) and 0.1% cytohelicase (Helix pomatia Linnaeus, 1758; Sigma) in the citrate buffer for 90 min at 37–40 °C. To remove trace amounts of the enzymatic mixture, the root tips were then gently washed in citrate buffer for 5 min. Only the mitotically active meristematic tissue of a root tip was cut off under a stereoscopic microscope, transferred into a drop of 50% acetic acid on a slide and covered by a coverslip. After heating the preparation to 42 °C for 1–2 min, cells were spread between a glass slide and coverslip in a drop of 50% acetic acid. The coverslip was mechanically removed by a razor blade after deep freezing in liquid nitrogen, and the slide was air dried. To increase the contrast of metaphase chromosomes for counting, the preparations were stained with DAPI (4',6-Diamidine-2'-phenylindole dihydrochloride; Vectashield Mounting Medium with DAPI, Vector Laboratories). For each species, at least ten metaphases were analysed to determine the chromosome number. Well-spread metaphase images were captured using Olympus BX 60 and Axio Imager Z.2 Zeiss fluorescence microscopes, both equipped with a CCD camera and ISIS software (Metasystems, Altlussheim, Germany).

Genome size estimation

The absolute genome size (2C-value; Doležel et al. 2007) of the fresh plant samples was quantified using a BD Accuri C6 flow cytometer (BD Biosciences, San Jose) equipped with a blue laser (488 nm, 20 mW, BD Accuri; BD Biosciences, San Jose). Sample preparation followed the standard protocol using LB01 isolation buffer supplemented with PVP (polyvinylpyrrolidone, 10 g/500 ml of buffer) to suppress interference of phenolic compounds with DNA staining (Doležel and Bartoš 2005, Doležel et al. 2007). Approximately 0.2 cm2 of the plant tissue between secondary veins was chopped in 500 μl of LB01 buffer together with a similar amount of tissue of an internal standard. Due to peak overlap in some accessions, Solanum lycopersicum Linnaeus, 1753 ‘Stupické polní rané’ (2C = 1.96 pg; Doležel et al. 2007) served as the primary reference standard, and Glycine max (Linnaeus, 1753) Merrill, 1917 ‘Polanka’ (2C = 2.33 pg, re-calculated against a primary standard) served as the secondary standard. The suspension was filtered through a 42 μm nylon mesh, supplemented with 20 μl of RNase A type II-A (with a final concentration of 50 μg/ml) and incubated at room temperature for approximately 10 min. The sample was then stained with 20 μl of propidium iodide (PI; final concentration of 50 μg/ml) and incubated with occasional shaking for approximately 5 min at room temperature. A flow-through fraction was then run on the flow cytometer, and the relative fluorescence intensity of at least 5,000 particles was recorded. Each sample was analysed at least three times. If the range of variation in the three measurements exceeded the 2% threshold, then the outlying value was discarded, and the sample was re-analysed. Only G0/G1 peaks with coefficients of variation < 4% were accepted. The 2C-value was calculated by multiplying the 2C-value of the standard with the sample/standard fluorescence ratio. Monoploid genome size (1Cx-value) was calculated by dividing the 2C-value by the inferred chromosome number.

Results

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.

Nuclear DNA content of studied Taraxacum sect. Taraxacum species (Lyc = Solanum lycopersicon ‘Stupické polní rané’; Gly = Glycine max ‘Polanka’; n.a. = not analysed, N = number of plants analysed; 1Cx = monoploid genome size, 2C = DNA amount/ploidy level).

Species 2C DNA amount [pg] (mean ± s.d.) N Ploidy 1Cx [pg] Standard
T. aberrans 2.71 ± 0.010 3 3x 0.90 Lyc
T. atroviride 2.70 ± 0.020 2 3x 0.90 Lyc
T. atrox 2.60 ± 0.002 2 3x 0.87 Lyc
T. baeckiiforme 2.62 ± 0 1 3x 0.87 Lyc
T. chrysophaenum n.a. n.a. 3x n.a. n.a.
T. coartatum 2.72 ± 0.070 2 3x 0.91 Lyc
T. corynodes 2.67 ± 0.001 2 3x 0.89 Lyc
T. crassum 2.62 ± 0.020 2 3x 0.87 Lyc
T. deltoidifrons 2.69 ± 0.007 3 3x 0.90 Lyc
T. diastematicum 2.67 ± 0 1 3x 0.89 Lyc
T. gesticulans 2.83 ± 0.040 2 3x 0.94 Lyc
T. glossodon 2.77 ± 0.010 2 3x 0.92 Lyc
T. guttigestans 2.74 ± 0.004 2 3x 0.91 Lyc
T. huelphersianum 2.79 ± 0.006 2 3x 0.93 Lyc
T. ingens 2.68 ± 0.013 3 3x 0.89 Gly + Lyc
T. jugiferum 2.71 ± 0.001 2 3x 0.90 Lyc
T. laticordatum 2.84 ± 0.008 2 3x 0.95 Lyc
T. lojoense 2.62 ± 0.020 4 3x 0.87 Lyc
T. lucidifrons 2.81 ± 0 1 3x 0.94 Lyc
T. obtusifrons 2.75 ± 0.03 2 3x 0.92 Lyc
T. ochrochlorum 2.67 ± 0 1 3x 0.95 Gly
T. ohlsenii 2.63 ± 0 1 3x 0.88 Lyc
T. perdubium 2.86 ± 0 1 3x 0.95 Lyc
T. praestabile 2.73 ± 0.050 3 3x 0.91 Lyc
T. sepulcrilobum 2.72 ± 0 1 3x 0.91 Lyc
T. sertatum 2.69 ± 0.010 2 3x 0.90 Lyc
T. subhuelphersianum n.a. n.a. 3x n.a. n.a.
T. valens 2.70 ± 0 1 3x 0.90 Lyc

Discussion

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 of T. officinale group in literature record. Values with asterisk (*) indicate re-calculated values according to conversion rate of 1 pg ~ 9.78×108 bp (Doležel et al. 2003).

Literature 2C [pg] 2C [Gbp]
Bennett et al. 1982 2.55 2.49*
Záveský et al. 2005 1.74–2.70 1.70–2.64*
Vidic et al. 2009 2.56* 2.50
Temsch et al. 2010 2.51 2.45*
Bainard et al. 2011 2.67 2.61*
Iaffaldano et al. 2017 1.65–3.09* (2.45–2.76*) 1.61–3.02 (2.40–2.70)
this study 2.60–2.86 2.54–2.80*

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.

Acknowledgements

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).

References

  • Asker SE, Jerling L (1992) Apomixis in Plants. CRC Press, Boca Raton, Ann Arbor, London, Tokyo, 298 pp.
  • Bainard JD, Husband BC, Baldwin SJ, Fazekas A, Gregory TR, Newmaster SG, Kron P (2011) The effects of rapid desiccation on estimates of plant genome size. Chromosome Research 19: 825–842. https://doi.org/10.1007/s10577-011-9232-5
  • Cardona MA, Contandriopoulos J (1983) IOPB chromosome number reports LXXIX. Taxon 32(2): 323–324.
  • Chrtek Jr J, Zahradníček J, Krak K, Fehrer J (2009) Genome size in Hieracium subgenus Hieracium (Asteraceae) is strongly correlated with major phylogenetic groups. Annals of Botany 104: 161–178. https://doi.org/10.1093/aob/mcp107
  • den Nijs JCM, Kirschner J, Štěpánek J, van der Hulst A (1990) Distribution of diploid plants of Taraxacum sect. Ruderalia in East-Central Europe, with special reference to Czechoslovakia. Plant Systematics and Evolution 170: 71–84. https://doi.org/10.1007/BF00937850
  • den Nijs JCM, Sterk AA (1984) Cytogeography of Taraxacum section Taraxacum and section Alpestria in France and adjacent parts of Italy and Switzerland, including some taxonomic remarks. Acta Botanica Neerlandica 33(1): 1–24. https://doi.org/10.1111/j.1438-8677.1984.tb01769.x
  • den Nijs JCM, van der Hulst A (1988) Cytogeography of Taraxacum section Erythrosperma: Diploid sexuals in SE and SW Europe. Botanische Jahrbücher für Systematik 110: 83–93.
  • Doležel J, Bartoš J, Voglmayr H, Greilhuber J (2003) Nuclear DNA content and genome size of trout and human. Cytometry Part A 51A: 127–128. https://doi.org/10.1002/cyto.a.10013
  • Doležel J, Greilhuber J, Lucretti S, Meister A, Lysák MA, Nardi L, Obermayer R (1998) Plant Genome Size Estimation by Flow Cytometry: Inter-laboratory Comparison. Annals of Botany 82(suppl. 1): 17–26. https://doi.org/10.1093/oxfordjournals.aob.a010312
  • Ge XJ, Kirschner J, Štěpánek J (2011) 71. Taraxacum Wiggers FH, Prim Fl Holsat 56. 1780. In: Wu ZY, Raven PH, Hong DY (Eds) Flora of China (Vol. 20–21). Science Press (Beijing) and Missouri Botanical Garden Press, St. Louis, 270–325. http://www.efloras.org [Accessed 11. April 2018]
  • Greilhuber J, Doležel J, Lysák M, Bennett MD (2005) The origin, evolution, and proposed stabilisation of the terms ‘genome size’ and ‘C-value’ to describe nuclear DNA contents. Annals of Botany 95: 255–260. https://doi.org/10.1093/aob/mci019
  • Hasterok R, Maluszynska J (2000) Cytogenetic markers of Brassica napus L. chromosomes. Journal of Applied Genetics 41(1): 1–10.
  • Iaffaldano BJ, Zhang YX, Cardina J, Cornish K (2017) Genome size variation among common dandelion accessions informs their mode of reproduction and suggests the absence of sexual diploids in North America. Plant Systematics and Evolution 303(6): 719–725. https://doi.org/10.1007/s00606-017-1402-2
  • Kirschner J, Štěpánek J (1984) Taraxacum (Spectabilia) nordstedtii Dahlst. in Central Europe (Studies in Taraxacum). Folia Geobotanica et Phytotaxonomica 19(3): 287–297. https://doi.org/10.1007/BF02853093
  • Kirschner J, Štěpánek J (1994) Clonality as a part of the evolution process in Taraxacum. Folia Geobotanica et Phytotaxonomica 29: 265–275. https://doi.org/10.1007/BF02803800
  • Kirschner J, Štěpánek J (1998b) A Monograph of Taraxacum sect. Palustria. Průhonice, 281 pp.
  • Kirschner J, Štepánek J, Greuter W (2007-onwards) Taraxacum. In: Greuter W, Raab-Straube E. von (Eds) Compositae, Euro+Med Plantbase – the information resource for Euro-Mediterranean plant diversity. http://www.emplantbase.org/home.html [Accessed 11. April 2018]
  • Kirschner J, Štěpánek J, Tichý M, Krahulcová A, Kirschnerová L, Pellar L (1994) Variation in Taraxacum bessarabicum and allied taxa of the section Piesis (Compositae): Allozyme diversity, karyotype and breeding behaviour. Folia Geobotanica et Phytotaxonomica 29: 61–83. https://doi.org/10.1007/BF02807777
  • Kirschner J, Záveská-Drábková L, Štěpánek J, Uhlemann I (2015) Towards a better understanding of the Taraxacum evolution (Compositae-Cichorieae) on the basis of nrDNA of sexually reproducing species. Plant Systematics and Evolution 301: 1135–1156. https://doi.org/10.1007/s00606-014-1139-0
  • Kirschner J, Štěpánek J (2011) Typification of Leontodon Taraxacum L. (≡ Taraxacum officinale F.H.Wigg.) and the generic name Taraxacum: A review and a new typification proposal. Taxon 60: 216–220.
  • Kula A, Grabowska-Joachimiak A, Kasjaniuk M, Legutko J, Marciniuk P, Musiał K (2013) Chromosome numbers in 10 Taraxacum species from Poland. Acta Biologica Cracoviensia series Botanica 55(2): 153–157. https://doi.org/10.2478/abcsb-2013-0030
  • Leitch IJ, Soltis DE, Soltis PS, Bennett MD (2005) Evolution of DNA amounts across land plants (Embryophyta). Annals of Botany 95: 207–217. https://doi.org/10.1093/aob/mci014
  • Majeský Ľ, Krahulec F, Vašut RJ (2017) How apomictic taxa are treated in current taxonomy: A review. Taxon 66: 1017–1040. https://doi.org/10.12705/665.3
  • Majeský Ľ, Vašut RJ, Kitner M, Trávníček B (2012) The pattern of genetic variability in apomictic clones of Taraxacum officinale indicates the alternation of asexual and sexual histories of apomicts. PLoS One 7(8): e41868. https://doi.org/10.1371/journal.pone.0041868
  • Małecka J (1967) Cyto-embryological studies in Taraxacum scanicum Dt. Acta Biologica Cracoviensia Series Botanica 10: 195–208.
  • Małecka J (1969) Further cyto-taxonomic studies in the genus Taraxacum section Erythrosperma Dt. I. Acta Biologica Cracoviensia Series Botanica 12: 57–72.
  • Małecka J (1972) Further cyto-taxonomic studies in the genus Taraxacum section Palustria Dahlstedt. Acta Biologica Cracoviensia Series Botanica 15: 113–126.
  • Małecka J (1973) Problems of the mode of reproduction in microspecies of Taraxacum section Palustria Dahlstedt. Acta Biologica Cracoviensia Series Botanica 16: 37–84.
  • Małecka J (1978) Further cyto-taxonomic studies in the genus Taraxacum section Palustria Dahlstedt, Part III. Acta Biologica Cracoviensia Series Botanica 21: 109–115.
  • Marciniuk J, Rerak J, Grabowska-Joachimiak A, Jastrząb I, Musiał K, Joachimiak AJ (2010) Chromosome numbers and stomatal cell length in Taraxacum sect. Palustria from Poland. Acta Biologica Cracoviensia Series Botanica 52(1): 117–121. https://doi.org/10.2478/v10182-010-0015-7
  • Marciniuk P, Musiał K, Joachimiak AJ, Marciniuk J, Oklejewicz K, Wolanin M (2012) Taraxacum zajacii (Asteraceae), a new species from Poland. Annales Botanici Fennici 49(5): 387–390. https://doi.org/10.5735/085.049.0611
  • Mártonfiová L (2015) Hybridization in natural mixed populations of sexual diploid and apomictic triploid dandelions (Taraxacum sect. Taraxacum): Why are the diploid sexuals not forced out? Folia Geobotanica 50(4): 339–348. https://doi.org/10.1007/s12224-015-9231-y
  • Mártonfiová L, Majeský Ľ, Mártonfi P (2007) Polyploid progeny from crosses between diploid sexuals and tetraploid apomictic pollen donors in Taraxacum sect. Ruderalia. Acta Biologica Cracoviensia Series Botanica 49(1): 47–54.
  • Mártonfiová L, Mártonfi P, Šuvada R (2010) Breeding behavior and its possible consequences for gene flow in Taraxacum sect. Erythrosperma (H. Lindb.) Dahlst. Plant Species Biology 25(2): 93–102. https://doi.org/10.1111/j.1442-1984.2010.00270.x
  • Meirmans PG, Calame FG, Bretagnolle F, Felber F, den Nijs JHC (1999) Anthropogenic disturbance and habitat differentiation between sexual diploid and apomictic triploid Taraxacum sect. Ruderalia. Folia Geobotanica 34(4): 451–469. https://doi.org/10.1007/BF02914922
  • Mogie M, Richards AJ (1983) Satellite chromosomes, systematics and phylogeny in Taraxacum (Asteraceae). Plant Systematics and Evolution 141(3–4): 219–229. https://doi.org/10.1007/BF00989003
  • Noirot M, Barre P, Louarn J, Duperray C, Hamon S (2000) Nucleus-cytosol interactions – A source of stoichiometric error in flow cytometric estimation of nuclear DNA content in plants. Annals of Botany 86: 309–316. https://doi.org/10.1006/anbo.2000.1187
  • Oberprieler C, Vogt R (1993) Chromosome numbers of North African Phanerogams II. Willdenowia 23(1/2): 211–238.
  • Rice A, Glick L, Abadi S, Einhorn M, Kopelman NM, Salman-Minkov A, Mayzel J, Chay O, Mayrose I (2015) The Chromosome Counts Database (CCDB) – a community resource of plant chromosome numbers. New Phytologist 206(1): 19–26. https://doi.org/10.1111/nph.13191
  • Richards AJ (1969) Chromosome Number Reports XXIII. Taxon 18(5): 560–562.
  • Sahlin CI (1984) Zur Taraxacum-Flora Bayerns. Berichte der Bayerischen Botanischen Gesellschaft 55: 49–57.
  • Salih RHM, Majeský Ľ, Schwarzacher T, Gornall R, Heslop-Harrison P (2017) Complete chloroplast genomes from apomictic Taraxacum (Asteraceae): Identity and variation between three microspecies. PLoS One 12(2): e0168008. https://doi.org/10.1371/journal.pone.0168008
  • Sato K, Yamazaki T, Iwatsubo Y (2014) Chromosome Diversity of Japanese Taraxacum officinale Weber ex F. H. Wigg. s.l. (Common Dandelion; Asteraceae). Cytologia 79(3): 395–408. https://doi.org/10.1508/cytologia.79.395
  • Schmid M, Vašut RJ, Oosterveld P (2004) Taraxacum prunicolor spec. nova, a new species of the Taraxacum scanicum group (sect.Erythrosperma). Feddes Repertorium 115: 220–229. https://doi.org/10.1002/fedr.200311038
  • Siljak-Yakovlev S, Pustahija F, Šolić EM, Bogunić F, Muratović E, Bašić N, Catrice O, Brown SC (2010) Towards a Genome Size and Chromosome Number Database of Balkan Flora: C-Values in 343 Taxa with Novel Values for 242. Advanced Science Letters 3(2): 190–213. https://doi.org/10.1166/asl.2010.1115
  • Suda J, Krahulcová A, Trávníček P, Rosenbaumová R, Peckert T, Krahulec F (2007) Genome size variation and species relationships in Hieracium subgenus Pilosella (Asteraceae) as inferred by flow cytometry. Annals of Botany 100: 1323–1335. https://doi.org/10.1093/aob/mcm218
  • Temsch EM, Temsch W, Ehrendorfer-Schratt L, Greilhuber J (2010) Heavy metal pollution, selection, and genome size: The species of the Žerjav study revisited with flow cytometry. Journal of Botany 2010: 596542. https://doi.org/10.1155/2010/596542
  • Trávníček B, Kirschner J, Štěpánek J, Vašut RJ (2010) 23. Taraxacum Wigg. In: Štěpánková J (Ed.) Flora of the Czech Republic (Vol. 8). Academia Praha, Praha, 23–269.
  • Trávníček P, Kirschner J, Chudáčková H, Rooks F, Štěpánek J (2013) Substantial Genome Size Variation in Taraxacum stenocephalum (Asteraceae, Lactuceae). Folia Geobotanica 48(2): 271–284. https://doi.org/10.1007/s12224-013-9151-7
  • Uhlemann I (2001) Distribution of reproductive systems and taxonomical concepts in the genus Taraxacum F. H. Wigg. (Asteraceae, Lactuceae) in Germany. Feddes Repertorium 112: 15–35. https://doi.org/10.1002/fedr.20011120105
  • van Baarlen P, van Dijk PJ, Hoekstra RF, de Jong JH (2000) Meiotic recombination in sexual diploid and apomictic triploid dandelions (Taraxacum officinale L.). Genome 43(5): 827–835. https://doi.org/10.1139/g00-047
  • van Dijk PJ (2003) Ecological and evolutionary opportunities of apomixis: insights from Taraxacum and Chondrilla. Philosophical Transactions of The Royal Society of London Series B-Biological Sciences 358(1434): 1113–1120. https://doi.org/10.1098/rstb.2003.1302
  • Vašut RJ (2003) Taraxacum sect. Erythrosperma in Moravia (Czech Republic): Taxonomic notes and the distribution of previously described species. Preslia 75(4): 311–338.
  • Vašut RJ, Štěpánek J, Kirschner J (2005) Two new apomictic Taraxacum microspecies of section Erythrosperma from Central Europe. Preslia 77: 197–210.
  • Vašut RJ, Vijverberg K, van Dijk PJ, de Jong H (2014) Fluorescent in situ hybridization shows DIPLOSPOROUS located on one of the NOR chromosomes in apomictic dandelions (Taraxacum) in the absence of a large hemizygous chromosomal region. Genome 57(11/12): 609–620. https://doi.org/10.1139/gen-2014-0143
  • Vašut RJ, Majeský Ľ (2015) Taraxacum pudicum, a new apomictic microspecies of T. section Erythrosperma (Asteraceae) from Central Europe. Phytotaxa 227(3): 243–252. https://doi.org/10.11646/phytotaxa.227.3.3
  • Verduijn MH, van Dijk PJ, van Damme JM (2004a) Distribution, phenology and demography of sympatric sexual and asexual dandelions (Taraxacum officinale s.l.): geographic parthenogenesis on a small scale. Biological Journal of the Linnean Society 82(2): 205–218. https://doi.org/10.1111/j.1095-8312.2004.00325.x
  • Verduijn MH, van Dijk PJ, van Damme JMM (2004b) The role of tetraploids in the sexual-asexual cycle in dandelions (Taraxacum). Heredity 93(4): 390–398. https://doi.org/10.1038/sj.hdy.6800515
  • Vidic T, Greilhuber J, Vilhar B, Dermastia M (2009) Selective significance of genome size in a plant community with heavy metal pollution. Ecological Applications 19(6): 1515–1521. https://doi.org/10.1890/08-1798.1
  • Wolanin MM, Musiał K (2017) Chromosome numbers in 11 species of Taraxacum section Erythrosperma Dt. from Poland. Acta Biologica Cracoviensia Series Botanica 59(2): 77–82. https://doi.org/10.1515/abcsb-2017-0008
  • Záveský L, Jarolímová V, Štěpánek J (2005) Nuclear DNA content variation within the genus Taraxacum (Asteraceae). Folia Geobotanica 40(1): 91–104. https://doi.org/10.1007/BF02803047