Chromosome number report of three Carex sect. Mitratae taxa (Cyperaceae) in Korea
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Abstract
We report meiotic chromosome numbers of three taxa in Carex sect. Mitratae in Korea: Carex breviculmis R. Br. (n = 32II, 33II, 34II), C. polyschoena H. Lév. & Vaniot (n = 37II, 38II), and C. sabynensis Less. ex Kunth (n = 27II). Section Mitratae is one of the most species-rich Asian groups in Carex, comprising approximately 45–80 taxa. Twenty-seven of these occur in Korea, and they are some of the most challenging taxa to identify due to their obscure and inconspicuous diagnostic characters. Including the counts reported here, half of the native Korean sect. Mitratae chromosome numbers have been documented. Their haploid chromosome numbers range from n = 10 to n = 40, and many exhibit variations in the numbers counted within a taxon. These variations, along with the overall significant variation in sect. Mitratae, suggest that dynamic chromosome activity may be related to the high species diversity of Carex.
Carex sect. Mitratae Kükenthal (Cyperaceae) comprises 45–80 taxa, occurring in Asia, Australia, Europe, and New Zealand with high species diversity in East Asia (Tang et al., 2010; Hoshino et al., 2011). The section is characterized by a single, terminal spike inflorescence, tri- or di-stigmas, and membranous perigynia enclosing trigonous achenes with annulate or beak-like features at their apices (Tang et al., 2010; Nam, 2017). In Korea, there are about 180 Carex taxa in 43 sections, and sect. Mitratae is the largest one with 27 taxa, including one endemic, C. sabynensis Less. ex Kunth var. leiosperma Ohwi (Park et al., 2016; Nam, 2017). They grow sunny, wet places in forests and roadsides and bloom in early spring (Hoshino et al., 2011; Park et al., 2016).
Chromosome numbers dramatically vary in Carex, ranging from n = 6 to n = 66 (Tanaka, 1949; Roalson, 2008). Chromosomes in Cyperaceae are holocentric and lack localized centromeres. Holocentric chromosomes have spindle fibers attached along the entire length of the chromosome arms and viable fragments can be increased (fission, agmatoploidy) and/or decreased (fusion, symploidy) without DNA duplication/deletion events (Malheiros-Gardé and Gardé, 1950; Luceño, 1994; Luceño and Guerra, 1996; Hipp et al., 2013). Due to these features, continuous chromosome number variation in Carex can result from either agmatoploidy/symploidy or aneuploidy (Luceño and Guerra, 1996; Hipp et al., 2009; Chung et al., 2011). Chromosome evolution plays an important role in the species richness of Carex, which is the most diverse flowing plant genus in temperate zones with more than 5,000 taxa worldwide (Hipp et al., 2009; Global Carex Group, 2015).
Many members of sect. Mitratae have variation in chromosome numbers within taxa, and univalent, trivalent, and/or quadrivalent chromosomes are observed in some species such as Carex caryophyllea Latourr. and C. umbrosa Host subsp. huetiana (Boiss.) Soó (Luceño, 1993; Chung et al., 2016). Recently, meiotic chromosome numbers from Korean populations in the section have been reported: C. breviculmis R. Br. (n = 33II) (Chung et al., 2017, 2018), C. fernaldiana H. Lév. & Vaniot (n = 33II) (Chung and Im, 2019), C. polyschoena H. Lév. & Vaniot (n = 26II, 36II, 37II) (Chung et al., 2016, 2018), C. sabynensis Less. ex Kunth (n = 27II, 28II, 38II) (Chung et al., 2016, 2017, 2018), and C. tristachya Thunb. (n = 21II) (Chung et al., 2016, 2017). C. polyschoena and C. sabyenesis, common early-spring bloomers, exhibit high variation in chromosome number.
In the present study, we report additional chromosome numbers of three common sect. Mitratae taxa in Korea, Carex breviculmis, C. polyschoena, and C. sabynensis. These are the most common and widely distributed Mitratae species. Chromosome numbers reported for Korean taxa in the section are also documented.
Materials and Methods
Immature male spikes of Carex breviculmis, C. polyschoena, and C. sabynensis were fixed for meiotic chromosome number observation, following the methods of Rothrock and Reznicek (1996) and Chung et al. (2016). Spikes (terminal spikes in Carex sect. Mitratae) with immature anthers were preserved in a mixture of methanol, chloroform, and propionic acid (6:3:2) and then transferred to 70% ethanol. Fixed anthers were squashed in 1% acetic-orcein and observed at 1,000× magnification (Nikon Eclipse 50i, Nikon, Tokyo, Japan). More than two meiotic division cells per individual were observed and photographed. Voucher specimens with mature perigynia were collected and identified following Hoshino et al. (2011) and Park et al. (2016). The vouchers were deposited at Chonnam National University herbarium (CNU, department of division of biological science).
Results and Discussion
Meiotic chromosome numbers of three taxa in Carex sect. Mitratae were observed (Table 1). Both C. breviculmis and C. polyschoena exhibited variation in chromosome numbers within the taxon and/or individual. Four individuals of C. breviculmis exhibited continuous variation in chromosome numbers, from n = 32II to n = 34II. Two C. polyschoena individuals also had variation with n = 37II and 38II. C. sabynensis had the meiotic chromosome number of 27II. Their chromosomes were very small less than 2 μm long, and constricted centromeres were not visible (Fig. 1).
Carex sect. Mitratae taxa investigated with voucher specimens and chromosome numbers.
Photomicrographs of Carex meiotic chromosomes. A–E. C. breviculmis. A, B. n = 34II, Chung 7013. C. n = 34II, Chung 7017. D. n = 33II, Chung 7029. E. n = 32II, Chung 7035. F–H. C. polyschoena. F. n = 37II, Chung 7019. G. n = 37II, Chung 7032. H. n = 38II, Chung 7032. I. C. sabynensis, n = 27II, Chung 7009. Scale bars = 10 μm.
Carex breviculmis R. Br. (n = 32II, 33II, 34II) (Fig. 1A–E)
One count of n = 32II, one count of n = 33II, and two counts of n = 34II were observed in C. breviculmis (Fig. 1A–E). The counts made in the present work were identical to those in Tanaka (1939), Hoshino (1981), Ohkawa and Yokota (1998), de Lange and Murray (2002), and Chung et al. (2017, 2018). Following the taxonomic treatment of C. breviculmis (Park et al., 2016; Nam, 2017), previous chromosome counts for C. leucochlora were included in the species (Table 1). All the individuals examined were collected in sunny and open habitats. The species commonly occurs throughout the country. By the long awns on pistillate scales, pubescent perigynia, and terminal, staminate spikes without peduncles, C. breviculmis is distinguished from morphologically similar species such as C. mitrata var. aristata Ohwi (Park et al., 2016; Nam, 2017).
Carex polyschoena H. Lév. & Vaniot (n = 37II, 38 II) (Fig. 1F–H)
From two individuals two different meiotic chromosome numbers were observed, n = 37II and the first report of n = 38II in the species (Fig. 1F–H). This new count expands the meiotic chromosome number variation range to n = 26II, 36 II, 37 II, 38 II (Chung et al., 2016, 2018). The species occurs in Japan, Korea, and China (Park et al., 2016). Although the species very common in Korea growing through the county, in Japan only a few populations are found only in Tsushima Island, Nagasaki Prefecture, where the holotype of species was collected (Hoshino et al., 2011). Tang et al. (2010) and Govaerts (2020) treated C. polyschoena as a synonym of C. pisiformis Boott, but Hoshino et al. (2011) recognized them as two independent species and considered C. pisiformis as an endemic species in Japan. In addition, Hoshino (1981) reported chromosome number of 2n = 68 for C. pisiformis. More recently, two taxa are distinguished by rhizome, pistillate inflorescence shape, and perigynium beak characters (Nam, 2017). Species delimitation of the species C. polyschoena and C. pisiformis should be reexamined covering entire distribution areas of the species.
Carex sabynensis Less. ex Kunth (n = 27II) (Fig. 1I)
Multiple cells from one individual of C. sabynensis constantly had a meiotic chromosome number of n = 27II (Fig. 1I) as in Chung et al. (2016, 2018), which was also observed from Korean populations. Counts from other Korean individuals show variation with numbers of n = 27II, 28II, 38II (Table 1). The species occurs broadly in East Asia, and variation in chromosome number is evident throughout its range (Hoshino et al., 2011). The species was considered as a subspecies of C. umbrosa Host – C. umbrosa subsp. sabynensis (Less. ex Kunth) Kük Govaerts (2020). However, Nam (2017) treated C. sabynensis as an independent species based on distinct pistillate inflorescence and perigynium characters.
Chromosome number variation in sect. Mitratae
The chromosome numbers of half of native Korean sect. Mitratae taxa have been documented (Table 2). The haploid numbers range from n = 10 (C. blepharicarpa Franch.) to n = 40 (C. stenostachys Franch. & Sav.), and most taxa exhibit variation in their chromosome numbers. Among the fourteen taxa, only three have consistent chromosome numbers: C. nervata Franch. & Sav. (2n = 76), C. toyoshimae Tuyama (2n = 62), and C. tristachya Thunb. (2n = 42). In contrast, C. multifolia Ohwi exhibits the broadest range of variation (2n = 30, 60, 64–66, 70). Because chromosome variation in Carex provides important information on taxonomy as well as genetic diversity, it is encouraged to investigate additional individuals of every taxon (Hoshino et al., 1993; Hip et al., 2010).
Chromosome numbers reported in Carex sect. Mitratae in Korea.
Positive correlations between chromosome number and genetic diversity and geographic distance have been found (Luceño and Castroviejo, 1991; Hipp et al., 2010). Varying chromosome numbers might reflect genetic diversity within and/or among individuals in a taxon but also might result from incongruent hypotheses on species delimitations among researchers. Previous taxonomic studies were limited to certain geographic areas, such as China (Tang et al., 2010) and Japan (Hoshino et al., 2011). Comprehensive systemic research targeting the section’s entire geographic range are needed. In addition, laboratory and/or biological errors might present. For instance, somatic metaphase chromosome number of C. blepharicarpa Franch. reported by Lee and Kim (2008) is 2n = 20, but many chromosomes are overlapping each other in the image and seem to be considered as mono-centromere chromosomes by the authors. It could be 2n = about 40. All the voucher specimens and raw chromosome data should be available to researchers, so that critical cytological data of Carex, taxonomically challenging taxa, are well-documented for further research on phylogeny and evolution.
Acknowledgements
We thank Amy Buthod (Robert Bebb Herbarium, University of Oklahoma) for comments on an earlier version of the manuscript.
Notes
Conflict of Interest
The authors declare that there are no conflicts of interest.