We produced DNA data of thirteen Svalbard plant species and estimated the genome sizes of eight species out of the thirteen. The genome sizes estimated by flow cytometry and by k-mer analysis of genome sequences were similar in Salix brachista: 400 Mb and 421 Mb, respectively (Chen et al. 2019). Because there is no information on the genome data of the eight Svalbard species, to check the accuracy of our genome size, we listed the range of genome sizes using flow cytometry for the Family the plant belonged to (Table 1; Pellicer and Leitch 2019). Draft genome assembly has never been reported for these Svalbard plants, and Betula nana of the United Kingdom is the only plant with genome sequencing data (Wang et al. 2013). As the other species had no genome data, we listed the genome sizes of individual plants in the same genus on the basis of genome sequencing data (Table 1; Sun et al. 2022).
Cochlearia groenlandica is a diploid plant with 14 chromosomes, common in Svalbard and Greenland (Luka et al., 2022). The genome size of C. groenlandica estimated in this study was 180 Mb, which was smaller than those of Cochlearia species ranged 196–735 Mb estimated by flow cytometry (Peer et al. 2003; Kochjarova et al. 2006; Lysak et al. 2009). Cochlearia plants with a large DNA content have more chromosome numbers: C. tatrae (2n = 42; 1C = 491 Mb), C. borzaeana (2n = 48; 1C = 683 Mb), C. danica (2n = 42; 1C = 686 Mb), and C. officinalis (2n = 32; 1C = 735 Mb) (Kochjarova et al. 2006; Lysak et al. 2009). The genome size of C. groenlandica was similar to 196 Mb of C. pyrenaica with 12 chromosomes.
Dryas octopetala is a diploid plant with 18 chromosomes, a common prostrate shrub forming a dense mat in the Arctic. The Dryas genus comprises of three species: D. drummondii, D. integrifolia and D. octopetala. Among them, D. drummondii contains root nodules while the others don’t (Billault-Penneteau et al. 2019). The genome size of D. drummondii was reported to be 253 Mb, which was obtained through whole genome sequencing (Griesmann et al. 2018). The genome size of D. octopetala, 204 Mb, was smaller than that of D. drummondii. The genome size of D. octopetala was estimated as 567 Mb from the fluoresce of nuclei (Dickson et al. 1992), which seemed to be collected from North America. It is not clear whether this difference in genome size is due to differences in experimental methods or whether the plants are taxonomically different. Further study such as a comparative analysis of the genomes of Svalbard plants and North American plants is therefore needed for clarification.
Eriophorum scheuchzeri ssp. arcticum is a diploid plant with 58 chromosomes, a circumpolar species living in the High Arctic. Its genome size, 306 Mb, is relatively small in the Cyperaceae (Table 1). The genome sizes of E. angustifolium and E. vaginatum were 587 Mb and 489 Mb, respectively (Grime and Mowforth 1982), and their chromosome number was both 58 (Love and Love 1981). The genome size of E. scheuchzeri ssp. arcticum was smaller than that of Eriophorum plants with the same chromosome number.
Oxyria digyna is a diploid plant with 14 chromosomes, a common forb in the Arctic and high mountains. The genus Oxyria has only three species, and the genome size has not been reported on any of them. The genome sizes of the closest genus, Rumex species, ranged from 470–6,110 Mb, which is larger than 352 Mb of O. digyna.
Betula nana ssp. nana is a diploid plant with 28 chromosomes, a cold-adapting circumpolar species. The triploid Betula plant with 42 chromosomes was reported, which was a hybrid of B. nana and B. pubescens (Anamthawat-Jónsson et al. 2010). The genome size of B. nana ssp. nana analyzed in this study was 689 Mb, which is similar to the DNA content of the triploid, 670.8 Mb (Anamthawat-Jónsson et al. 2010). The plastid DNA of B. nana living in Svalbard shared its characteristics with B. pubescens (Eidesen et al. 2015). Therefore, the genome size estimated in this study seemed to be that of a triploid hybrid plant. Further study is required to clarify the ploidy level, morphology, and chromosome number of Svalbard B. nana ssp. Nana.
Silene is a giant genus with nearly 900 species. The DNA content of six species out of 900 was analyzed using flow cytometry and they were more than 900 Mb: S. ciliate 924 Mb; S. vulgaris 1,100 Mb; S. pendula 1,149 Mb; S. rupestris 1,663 Mb; S. latifolia female 2,802 Mb; S. latifolia male 2,861 Mb; S. chalcedonica 3,223 Mb (Siroký et al. 2001). The genome size of S. uralensis ssp. arctica, 894 Mb, estimated in this study was smaller than those of Silene species. We could not estimate the genome size of S. acaulis, and it is probably because the genome size is over 1 Gb.
Salix species are creeping shrubs that live in the Arctic and the alpine. The estimated genome size of S. polaris was 383 Mb, which is similar to that of two Salix species based on the whole genome sequence data: S. brachista 420 Mb, S. dunnii 376 Mb, S. matsudana 656 Mb, S. suchowensis 425 Mb, and S. viminalis 360 Mb (Chen et al. 2019; Almeida et al. 2020; Wei et al. 2020; Zhang et al. 2020; He et al. 2021). These Salix species were all diploid plants with 38 chromosomes, on the contrary, S. polaris is a hexaploid species with 114 chromosomes (Table 1). Due to the difference in the ploidy level, the C-value of hexaploid S. polaris was 1,148 Mb.
The four Svalbard plants — C. groenlandica, D. octopetala, E. scheuchzeri ssp. arcticum, and O. digyna — had small genome sizes (< 500 Mb), which could be good candidates to assemble high-quality, reference-level genomes using current long-read sequencing technologies.
Unfortunately, we failed to estimate the genome size for five out of the thirteen species, which suggests that these five Svalbard plants may have a much larger genome size than 1.0 Gb. Indeed, of these five species, S. oppositifolia was known to have a genome size of over 1.4 Gb (Loureiro et al. 2013). The sequencing data in this study was just 32 Gb, as such it may not be enough to estimate such a large genome. The genome sizes of relative plants of Polemonium boreale and Silene acaulis are much larger than 1 Gb (Table 1). Bistorta vivipara and Papaver dahlianum are polyploidy plants. To analyze the genomes of these plants, sufficiently large sequence data will be required.
The genome size of plants is proportional to the amount of noncoding DNA (Barakat et al. 1997), and the noncoding DNA is mainly composed of repetitive DNA, such as transposable elements and telomeric repeats (Kubis et al.1998). We have analyzed telomeric-repeat motifs from the genome sequencing data of the thirteen Svalbard plants. We discovered a novel telomeric-repeat motif TTCAGGG in Papaver dahlianum, which again emphasizes the importance of investigating poorly studied species in the polar regions.
The DNA amount in a nucleus has been considered to be closely related to nuclear and cell sizes, and negatively correlated with cell division rate (Bennett and Leitch 2005). Bennett (1972) suggested that DNA amount is positively correlated with minimal generation time (MGT), which is the minimum time required to produce the first mature seed since germination. The seed weight of legume species and the seed dry mass of Allium species showed a positive correlation with the DNA amount (Bennett and Leitch 2005). As plant growth is affected by environmental factors such as temperature and moisture, MGT changes according to the environment; in the Arctic tundra, with low temperature and short growing season, the MGT becomes long. If the MGT of a plant is longer than one year, the plant should be perennial. In fact, most Arctic tundra plants are perennial.
The genome size of Svalbard plants analyzed in this study was smaller than that of plants belonging to the same genus or Family. Small amounts of DNA in polar plants have also been observed in Antarctic and sub-Antarctic plants (Bennett et al. 1982). It is suggested that there is a maximum DNA amount per diploid genome that allows plants to survive in extreme environments with low temperatures and a short growing season (Bennett et al. 1982; Knight et al. 2005). Plants with higher DNA amounts than the maximum DNA amount are supposed to be gradually excluded, and the DNA amount may determine the latitudinal limits where the plants can be distributed (Bennett et al. 1982). The plants under global and local conservation concerns in United Nations Environment Programme World Conservation and Monitoring Centre (UNEP-WCMC) Species Database (http://quin.unep-wcmc.org/isdb/taxonomy/) posed larger genome sizes than no concerned plants (Vinogradov 2003). In the Arctic where temperature increases faster than anywhere else on Earth, and moisture condition changes dramatically in permafrost, genome size could be one of the prioritized criteria to be considered to conserve endangered tundra plants.
The findings in this study provide a quantitative guideline for whole-genome sequencing analysis of Arctic plants. They also show the potential of Arctic plants to be a new source of telomere diversity. As we used only thirteen species among the multitude of Arctic plants, it is anticipated that we could have elucidated much more unprecedented genetic and genomic variation if we study more arctic plants. It will deepen our understanding of Arctic plants and their genome evolution.