Carex L. is the most species-rich genus in the temperate zones, with over 2,000 species worldwide, and is characterized by unisexual flowers on spikes and enclosed bracts of fruits called perigynia (Global Carex Group, 2015). In Korea, the genus is also the most diverse among vascular plants, comprising about 180 species across 42 sections (Park et al., 2016). Of these, Carex section Mitratae Kük. is the most diverse, with approximately 21 taxa, while Carex section Siderostictae Franch. ex Ohwi, with 4 taxa, is phylogenetically significant as the basal group in Carex (Yano et al., 2014; Park et al., 2016). The high species diversity within the genus has been attributed to the presence of non-localized centromere chromosomes, known as holocentric chromosomes (Roalson, 2008; Hipp et al., 2009).

Holocentric chromosomes lack localized centromeres during cell divisions, which makes chromosomes capable of separating and/or fusing, thereby promoting rapid speciation (Hipp et al., 2013). Due to this chromosome feature, Carex displays a wide range of variation in chromosome numbers from n = 6 to n = 66, with continuous variation between n = 6 and n = 48 (Roalson, 2008). Chromosome number variations are more significant in species-rich and/or recently diverging lineages such as Carex sect. Ovales Kunth and Carex sect. Mitratae than in basal and/or smaller lineages such as Carex section Siderostictae (Global Carex Group, 2021). In basal lineages, polyploidization is prominent, while continuous chromosome number variation is common in recently derived lineages (Hipp et al., 2007; Yano et al., 2014).

In Korea, ongoing efforts have been made to document key cytogenetic data for Carex, though only 31 of approximately 180 Carex taxa in the Korean flora have been studied for their chromosome numbers (Chung et al., 2023; Masaki et al., 2024). Nine taxa in Carex sect. Mitratae, the most common and species-rich section, show continuous variation in chromosome numbers within species and/or individuals (Masaki et al., 2024). Conversely, C. siderosticta, a member of the basal lineage Carex sect. Siderosticta, exhibits polyploidy (Chung, 2024). Recently, a triploid population was identified in Korea, adding another ploidy level to the species: diploidy (2n = 12, Japan and Korea), triploidy (2n = 18, Korea), and tetraploidy (2n = 24, Japan, China, Russia) (Tanaka, 1939, 1940; Hoshino and Tanaka, 1977; Hoshino, 1981; Starodubtsev, 1989; Tang and Xiang, 1989; Hoshino et al., 1993; Probatova et al., 1998; Probatova, 2000; Chung et al., 2013, 2017; Yano et al., 2014; Chung, 2024).

South Korea has a total of 3,390 islands, with 93% located in the Yellow Sea and along the country’s southwest coast (Ministry of the Interior and Safety, 2025; Ministry of Oceans and Fisheries, 2025). Most of these islands are continental in origin, formed by isolation from the mainland due to sea level rises after the last glaciation period during the Quaternary (Chung and Oh, 2018). Despite the large number of islands in the west and southwest of the Korean Peninsula and the clear importance of cytogenetic diversity in Carex, cytological studies of Carex have never been conducted there. Islands offer great opportunities to hypothesize and test such cytogenetic and evolutionary phenomena.

Cytological investigations on Carex have been carried out to determine chromosome number variation in island populations. As an initial report, we present meiotic chromosome numbers of two species in Carex sect. Mitratae (Carex breviculmis R. Br. and C. tristachya Thunb.) and somatic chromosome numbers of two species in Carex sect. Siderostictae (C. ciliatomarginata Nakai and C. siderosticta Hance) from insular populations in the Yellow Sea and southwestern coast of Korea.

Meiotic and somatic chromosomes were observed using methods from Masaki et al. (2024) and Chung (2024), respectively. For the meiotic chromosome observation of Mitratae species, immature male spikes were treated in a mixture of methanol, chloroform, and propionic acid at room temperature, then stored in 70% ethanol. To observe somatic chromosomes in sect. Siderostictae, active root tips were sampled in 0.002 M 8-hydroxiquinoline and fixed in a mixture of ethanol and glacial acetic acid (3:1). Fixed pollen and root cells were stained with 1% acetic-orcein and squashed. Meiotic and somatic chromosomes observed at ×1,000 magnification were drawn, photographed, and analyzed. At least three cells per individual were examined to determine chromosome numbers and variations. An optical microscope (Nikon Eclipse 50i, Nikon, Tokyo, Japan) was used for observation. Species identification followed Hoshino et al. (2011) and Park et al. (2016), and voucher specimens were housed in the herbarium of the Honam National Institute of Biological Resources (HIBR).

Chromosomes of four Carex taxa were confirmed from twelve island populations in the west and southwest of South Korea (Table 1). Two Mitratae taxa, C. breviculmis and C. tristachya, had very small chromosomes, about 1 μm long, while two Siderostictae species, C. ciliatomarginata and C. siderosticta, displayed relatively large chromosomes with lengths ranging from 1.5 μm to 5.5 μm (Fig. 1). In the sect. Siderostictae, C. siderosticta showed two ploidy levels, diploidy and triploidy, and C. ciliatomarginata was only diploid.

Carex breviculmis R. Br. 청사초 (n = 34_II) (Fig. 1A) – Sect. Mitratae Kük.

The meiotic chromosomes of n = 34_II were observed in C. breviculmis collected from Sinan-gun, Jeollanam-do (Fig. 1A, Table 1). Their chromosomes were very small, less than 2 μm long, and constricted centromeres were not visible. The species is widespread throughout East Asia, and variation in chromosome numbers has been reported as 2n = 54, 58, 62, 64, 66, 68, 70, 72 (Hoshino et al., 2011) (Table 1). In Korea, it is one of the most common Carex species, growing across the country, with chromosome numbers varying as 2n = 54, 58, 62, 64, 66, 68, 72 (Park et al., 2016) (Table 1). The chromosome number of 2n = 68 has been reported from several populations from central to southern Korea, including Jejudo Island: Ansan-si (Gyeonggi), Goesan-gun (Chungbuk), Cheonan-si (Chungnam), Tongyeong-si (Gyeongnam), and Jeju-si (Jejudo Island) (Chung and Im, 2020; Chung and Chung, 2021; Masaki et al., 2024). Conversely, a meiotic chromosome number of n = 32 _II was observed in Saengildo Island, located south of the Korean Peninsula (Masaki et al., 2024).

Carex tristachya Thunb. 반들사초 (n = 21_II) (Fig. 1B) – Sect. Mitratae Kük.

The meiotic chromosome number for C. tristahya was observed as n = 21_II, and chromosome lengths ranged from about 1.5 μm to 2.7 μm (Fig. 1B). Previous counts from Korean populations were consistent in number, and only bivalents were reported (Chung et al., 2016, 2017). However, Japanese populations showed variable numbers with univalents, bivalents, trivalents, and tetravalents (2n = 38, 40, 41, 42, 61) (Tanaka, 1948). The species inhabits sunny grasslands or forest edges across China, Japan, Taiwan, and Korea (Park et al., 2016). In Korea, the species is common in southern areas and appears to be both morphologically and cytologically well structured.

Carex ciliatomarginata Nakai 털대사초 (2n = 12) (Fig. 1C–G) – Sect. Siderostictae Franch. ex Ohwi

The somatic chromosome number of 2n = 12 was observed in C. ciliatomarginata collected from five populations across four islands (Table 1). Chromosome lengths ranged from 1.5 μm to 4.5 μm, and constricted centromeres were indistinguishable (Fig. 1C–G). The species is characterized by pubescent, broad leaves (up to 2 cm wide), staminate terminal spikes, and pubescent perigynia; it grows in China, Japan, and Korea (Hoshino et al., 2011; Park et al., 2016). Diploids (2n = 2x = 12) have been reported in Japanese and Korean populations (Hoshino and Tanaka, 1977; Hoshino, 1981; Hoshino and Shimizu, 1986; Chung et al., 2013; Yano et al., 2014). Based on mitotic chromosomes from Japanese populations, a karyotype was proposed: 2n = 12 = 2L + 6M + 4S, ranging from 1.83 μm to 3.67 μm in length; no irregular division in meiotic chromosomes was observed (Hoshino, 1981; Hoshino and Shimizu, 1986).

Carex siderosticta Hance 대사초 (2n = 12, 18) (Fig. 1H–L) – Sect. Siderostictae Franch. ex Ohwi

Two levels of ploidy were observed in C. siderosticta collected from five populations across two islands, Eocheongdo and Daenanjido: 2n = 2x = 12 and 2n = 3x = 18 (Table 1). Chromosome lengths ranged from 2.0 μm to 5.5 μm, and constricted centromeres were not visible (Fig. 1H–L). The total chromosome lengths of diploids and a triploid measured 43.6 μm (average) and 62.7 μm, respectively. Compared to the total chromosome length of the previous triploid at 54.5 μm, DNA duplication is suggested (Chung, 2024). This species is the most widely distributed in its section, with three different ploidy levels reported: Japan (2x and 4x), China (4x), Russia (4x), and Korea (2x and 3x) (Table 1).

This study presents the second report of a triploid population of Carex siderosticta. The first triploid population was identified in mountainous areas of Gangwon-do province, central eastern Korea (Chung, 2024), while here, we discovered a triploid population on Eocheongdo Island, a small island (1.80 km²) in the Yellow Sea, located 72 km west of the Korean mainland and 300 km east of Jiaodong (tsiaotung) Peninsula, China (Ministry of Oceans and Fisheries, 2025). The Eocheongdo triploid population was relatively large, with well-developed rhizomes and typical morphological features consistent with those generally observed in the species. Observations of four populations from various sites (inside and seashore forests) on Daenanjido Island showed a somatic chromosome number of 2n = 12. In addition to ploidy variation, different karyotypes and chromosome lengths have been reported in C. siderosticta (Chung, 2024). Pollen grains from the triploid populations should be examined to determine whether they are fertile or sterile.

Carex sect. Mitratae is a species-rich group consisting of about 80 species found across Central, East, and Southeast Asia, Australia, Europe, and the Pacific islands (Dai et al., 2010; Hoshino et al., 2011). Dynamic chromosome activity has been proposed to explain the high species diversity within Carex, with cytotypes and/or phylogenetic types also reported (Hipp et al., 2009; Yano et al., 2010). In Korean populations, we have identified both cytologically diverse and structured species within the section (Masaki et al., 2024). Carex breviculmis shows two meiotic chromosome numbers, n = 32_II in Saengildo Island (south) and n = 34_II in Imjado Island (west).

Polyploidy is found almost exclusively in Carex sect. Siderostictae within Carex. Specifically, C. siderosticta exhibits three levels of polyploidy: diploidy, triploidy, and tetraploidy (Chung, 2024). Tetraploidy is hypothesized to result from autopolyploidization, while triploidy has been proposed through various scenarios such as fertilization of unreduced or reduced diploid gametes and the involvement of a triploid bridge (Yano et al., 2014; Chung, 2024). Furthermore, polyploid speciation on islands can be facilitated by potential congeneric colonists and changes in ploidy levels relative to overseas sister taxa (Meudt et al., 2021). In Rhynchospora Vahl (Cyperaceae), a genus with holocentric chromosomes, various polyploid mechanisms have been identified, including whole gene duplication, fissions, fusions, and diploidization (Mata-Sucre et al., 2024). Eocheongdo Island presents an excellent location to test hypotheses of polyploidization in C. siderosticta. Understanding polyploidization in the basal lineage of Carex is crucial for a better understanding of species diversity and chromosome evolution. Detailed studies of C. siderosticta populations on the island will offer insights into polyploidization not only in Carex but also in holocentric chromosome-organisms.