And Human-Type Telomere Repeats in Plants Using Fluorescence in Situ Hybridisation

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Survey of Arabidopsis- and Human-type Telomere Repeats in Plants Using Fluorescence in situ Hybridisation

Fukashi Shibata† and Masahiro Hizume*

Biological Institute, Faculty of Education, Ehime University, Matsuyama 790–8577, Japan

Received March 19, 2011, accepted June 1, 2011

Summary The nucleotide sequences of telomere repeats have been identiﬁed in many eukaryotes since Blackburn and Gall (1978), and these sequences are speciﬁc to the genus or higher taxonomic group. Wide vision telomere sequences are variable across eukaryotes. In plants, Arabidopsis-type

telomere repeats (TTTAGGG)n are dominant, although human-type telomere repeats (TTAGGG)n have been reported in a few taxa. Recently, the evolution of these telomere repeats in plants has be- come the focus of many studies. In this report, the Arabidopsis-type telomere repeat and human-type telomere repeat were surveyed in 87 species of gymnosperms and angiosperms. In 1 gymnosperm species and 62 angiosperm species, ﬂuorescence in situ hybridisation signals of both Arabidopsis- type and human-type telomere repeats were detected. Three gymnosperm species and 12 angiosperm species showed only signals of Arabidopsis-type telomere repeats. In 5 angiosperm species, only human-type telomere repeat signals were detected. The co-localisation of Arabidopsis- and human- type telomere repeats on chromosome ends in a wide range of plant species is a novel discovery that may elucidate the evolution of telomere repeats in plants.

Key words Arabidopsis-type telomere repeats, FISH, Human-type telomere repeats, Plant chromosome.

Eukaryotes possess numerous linear chromosomes, and the structure at the end of each chromosome, the telomere, is crucial in maintaining chromosome ends (Pryde et al. 1997, McKnight and Shippen 2004). Telomeres are constructed with telomere repeats, as well as speciﬁc proteins (Cech 2004), and are important in controlling chromosome positions in the nucleus (Naito et al. 1998, Riha et al. 2001, Bechard et al. 2009). Telomere repeats have been reported in many taxa. The human-type telomere repeat (TTAGGG)n is common in vertebrates (Meyne et al. 1989), and it has also been reported in some protozoa and fungi (Blackburn and Gall, 1978, Blackburn and Challoner,

1984, Schechtman, 1990). In plants, the telomere repeat (TTTAGGG)n was isolated in Arabidopsis (Richards and Ausubel 1988, Richards et al. 1992), and this Arabidopsis-type repeat has been reported in many angiosperms and gymnosperms (Ganal et al. 1991, Broun et al. 1992, Cox et al. 1993, Biessmann and Mason 1994, Fuchs et al. 1995, Wellinger and Sen 1997, Hizume et al. 1998, 2000, 2002). Allium cepa and A. fistulosum possess unique terminal repetitive sequences and/or rDNA sequences in place of the typical telomere sequence (Pich et al. 1996, Pich and Schubert 1998), and the absence of Arabidopsis-type telomere repeats were reported in other Asparagales (Adams et al. 2000, 2001). Weiss and Scherthan (2002) reported human-type telomere repeats instead of Arabidopsis- type telomere repeats in Aloe chromosomes, while Sykorova et al. (2003) described the coexistence of Arabidopsis-type and human-type telomere repeats on chromosomes in some plant species. In this study, we investigated the distribution of human-type and Arabidopsis-type telomere repeats using ﬂuorescence in situ hybridisation (FISH) on the chromosomes of 87 plant species.

† Present address: Institute of Plant Science and Resources, Okayama University, Kurashiki 710–0046, Japan * Corresponding author, e-mail: [email protected] 354 F. Shibata and M. Hizume Cytologia 76(3)

Materials and methods

Plant materials and chromosome preparation Plants of 87 species in 81 genera purchased from commercial sources or collected in ﬁelds were used in this study, and planted in pots or seeded in petri dishes. Species names are listed in Table 1. The taxonomic treatment of order and family in Table 1 followed the Angiosperm Phylogeny Group (2009). Chromosome preparations of gymnosperm species were prepared as described by Hizume et al. (1999). For angiosperm plants, the root tips were collected for chromosomal analysis. The root tips were treated with 0.05% colchicine at 20°C for 2–4 h, ﬁxed in a chilled mixture of ethanol–acetic acid (3 : 1) overnight, and then stored in a freezer. Fixed root tips were macerated in an enzyme solution containing 2% Cellulase Onozuka RS (Yakult) and 0.5% Pectolyase Y-23 (Seishin) in 10 mM sodium citrate buffer (pH 4.5) at 37°C for 30–60 min. The meristematic cells were isolated and crushed in 45% acetic acid under a coverslip on a glass slide. The coverslips were removed by the dry-ice method followed by air-drying overnight.

FISH procedures The Arabidopsis-type telomere sequence repeats and human-type telomere sequence repeats were amplified by PCR using (TTTAGGG)5 and (CCCTAAA)5 primers for Arabidopsis-type and (TTAGGG)5 and (CCCTAA)5 primers for human-type in the absence of template DNA (Ijodo et al. 1991, Cox et al. 1993). The amplified telomere repeats were labelled with biotin using the Biotin– Nick Translation Mix (Roche). The labelled DNA probe was dissolved in 50% formamide and 10% dextran sulfate in 2×SSC and used as a FISH probe solution. We used the FISH procedures described by Hizume et al. (1999). The chromosomal DNA on glass slides was denatured by immers- ing them into 70% formamide in 2×SSC at 76°C for 60 s. The hybridized probes on the chromosomes were detected using Alexa Fluor® 488-labelled streptavidin (Invitrogen). The chromosomes were counterstained with 0.1 μg/ml 4.6-diamidino-2-phenylindole (DAPI). Epifluorescence micro- scope images of FISH chromosomes were recorded using a chilled charge-coupled device camera (Sensys 1400, Photometrics) and analysed using IPLab (Scanalytics).

Results

After FISH with probes for the Arabidopsis-type telomere repeat, 78 species classified in all of the 26 orders that were examined revealed FISH signals at the chromosome ends (Fig. 1). In gymnosperms, all 4 species classified in 3 orders showed signals of Arabidopsis-type telomere repeats. In angiosperms, 9 species classified in the order Asparagales showed no Arabidopsis-type telomere signals (Table 1). When probed with human-type telomere repeats, 68 species classified in 21 orders showed signals at each chromosome end (Fig. 2). In gymnosperms, only Cycas revoluta displayed human-type telomere signals. In angiosperms, 16 species in 12 orders (Asparagales, Asterales, Caryophyllales, Commelinales, Geraniales, Lamiales, Malvales, Ranunculales, Rosales, Saxifragales, Solanales, and Zingiberales) exhibited no human-type telomere signal (Table 1). 4 Allium species showed no signal in FISH with probes for both types of telomere repeats (Table 1). In 42 species, clear differences in signal intensity were observed between the 2 probes (Table 1). 12 species in the order Asparagales displayed stronger signals when probed with human-type telomere repeats. 8 species (Cycas revoluta, Pinus densiflora, Hyacinthus orientalis, Matricaria chamomilla, Tanacetum ptarmiciflorum, Tricyrtis hirta, Ranunculus muricatus, and Solanum tuberosum) 2011 FISH Survey of Telomere Repeats in Plants 355

Table 1. FISH results of 87 species probed with Arabidopsis-type (TTTAGGG)n or human-type (TTAGGG)n telomere repeats. d: signals were detected at chromosome ends. nd: no signal. d/i: signals were detected at chromosome end and interstitial or proximal regions

FISH Results

Order Family Genus Species Arabidopsis-type Human-type Inequality (TTTAGGG)n (TTAGGG)n

Gymnosperms Cycads Cycadaceae Cycas revoluta d/i Fig. 2 A > d/i Fig. 3 A

Ginkgoaceae Ginkgoaceae Ginkgo biloba d Fig. 2 B nd

Pinales Pinaceae Abies alba d Fig. 2 C nd Pinales Pinaceae Pinus densiflora d/i Fig. 2 D nd

Angiosperms Alismatales Araceae Epipremnum aureum d Fig. 2 E > d Fig. 3 B Alismatales Hydrocharitaceae Egeria densa d Fig. 2 F d Fig. 3 C

Apiales Apiaceae Coriandrum sativum d Fig. 2 G > d Fig. 3 D

Arecales Arecaceae Chamaedorea elegans d Fig. 2 H > d Fig. 3 E

Asparagales Agapanthaceae Agapanthus africanus d Fig. 2 I < d Fig. 3 F Asparagales Agavaceae Chlorophytum comosum nd d Fig. 3 G Asparagales Alliaceae Allium cepa nd nd Asparagales Alliaceae Allium tuberosum nd nd Asparagales Alliaceae Allium sativum nd nd Asparagales Alliaceae Allium chinense nd nd Asparagales Alliaceae Tristagma uniflorum nd d Fig. 3 H Asparagales Amaryllidaceae Narcissus triandrus d Fig. 2 J < d Fig. 3 I Asparagales Asparagaceae Asparagus sprengeri d Fig. 2 K d Fig. 3 J Asparagales Asphodelaceae Bulbine frutescens d Fig. 2 L d Fig. 3 K Asparagales Hyacinthaceae Hyacinthoides hispaniaca d Fig. 2 M d Fig. 3 L Asparagales Hyacinthaceae Hyacinthus orientalis d/i Fig. 2 N < d/i Fig. 3 M Asparagales Hyacinthaceae Muscari neglectum d Fig. 2 O < d Fig. 3 N Asparagales Hyacinthaceae Ornithogalum virens d Fig. 2 P d Fig. 3 O Asparagales Hyacinthaceae Ornithogalum nutans d Fig. 2 Q < d Fig. 3 P Asparagales Hyacinthaceae Ornithogalum umbellatum d Fig. 2 R < d Fig. 3 Q Asparagales Hyacinthaceae Ornithogalum dubium d Fig. 2 S < d Fig. 3 R Asparagales Hyacinthaceae Puschkinia scilloides var. d Fig. 2 T < d Fig. 3 S libanotica Asparagales Hyacinthaceae Scilla scilliodes d Fig. 2 U < d Fig. 3 T Asparagales Iridaceae Babiana stricta nd d Fig. 3 U Asparagales Iridaceae Crocus chrysanthus d Fig. 2 V < d Fig. 3 V Asparagales Iridaceae Freesia refracta nd d Fig. 3 W Asparagales Iridaceae Iris sanguinea nd d Fig. 3 X Asparagales Ixioliriaceae Ixiolirion tataricum d Fig. 2 W > d Fig. 3 Y Asparagales Orchidaceae Bletilla striata d Fig. 2 X > d Fig. 3 Z Asparagales Ruscaceae Aspidistra elatior d Fig. 2 Y d Fig. 3 AA Asparagales Ruscaceae Ophiopogon japonicus d Fig. 2 Z < d Fig. 3 AB Asparagales Ruscaceae Reineckea carnea d Fig. 2 AA < d Fig. 3 AC

Asterales Asteraceae Chrysanthemums coronarium d Fig. 2 AB > d Fig. 3 AD Asterales Asteraceae Crossostephium artemisioides d Fig. 2 AC > d Fig. 3 AE Asterales Asteraceae Matricaria chamomilla d/i Fig. 2 AD d/i Fig. 3 AF Asterales Asteraceae Nipponanthemum nipponicum d Fig. 2 AE > d Fig. 3 AG Asterales Asteraceae Tanacetum ptarmiciflorum d/i Fig. 2 AF d/i Fig. 3 AH Asterales Campanulaceae Lobelia erinus d Fig. 2 AG d Fig. 3 AI Asterales Goodeniaceae Leschenaultia formosa d Fig. 2 AH > d Fig. 3 AJ Asterales Stylidiaceae Stylidium debile d Fig. 2 AI nd

Brassicales Brassicaceae Brassica rapa var. perviridis d Fig. 2 AJ d Fig. 3 AK Brassicales Brassicaceae Orychophragmus violaceus d Fig. 2 AK > d Fig. 3 AL Brassicales Limnanthaceae Limnanthes douglasii d Fig. 2 AL d Fig. 3 AM Brassicales Tropaeolum Tropaeolum majus d Fig. 2 AM > d Fig. 3 AN

Caryophyllales Amaranthaceae Spinacia oleracea d Fig.N 2 A d Fig. 3 AO Caryophyllales Nyctaginaceae Mirabilis jalapa d Fig. 2 AO nd Caryophyllales Plumbaginaceae Limonium sinuatum d Fig. 2 AP > d Fig. 3 AP 356 F. Shibata and M. Hizume Cytologia 76(3)

Table 1. (continued)

FISH Results

Order Family Genus Species Arabidopsis-type Human-type Inequality (TTTAGGG)n (TTAGGG)n

Commelinales Commelinaceae Tradescantia pallida d Fig. 2 AQ d Fig. 3 AQ Commelinales Pontederiaceae Eichhornia crassipes d Fig. 2 AR nd

Cucurbitales Cucurbitaceae Cucurbita moschata d Fig. 2 AS d Fig. 3 AR

Dioscoreales Dioscoreaceae Dioscorea japonica d Fig. 2 AT > d Fig. 3 AS

Dipsacales Caprifoliaceae Lonicera japonica d Fig. 2 AU > d Fig. 3 AT

Ericales Ebenaceae Diospyros kaki d Fig. 2 AV d Fig. 3 AU

Fabales Fabaceae Entada rheedii d Fig. 2 AW > d Fig. 3 AV Fabales Fabaceae Vicia faba d Fig. 2 AX > d Fig. 3 AW

Gentianales Apocynaceae Asclepias physocarpa d Fig. 2 AY > d Fig. 3 AX Gentianales Rubiaceae Coffea arabica d Fig. 2 AZ > d Fig. 3 AY

Geraniales Geraniaceae Geranium sanguineum d Fig. 2 BA nd

Lamiales Acanthaceae Thunbergia alata d Fig. 2 BB > d Fig. 3 AZ Lamiales Lamiaceae Mentha spicata d Fig. 2 BC nd Lamiales Lamiaceae Nepeta cataria d Fig. 2 BD nd Lamiales Oleaceae Fraxinus japonica d Fig. 2 BE > d Fig. 3 BA

Liliales Alstroemeriaceae Alstroemeria saturne d Fig. 2 BF d Fig. 3 BB Liliales Liliaceae Fritillaria assyrica d Fig. 2 BG d Fig. 3 BC Liliales Liliaceae Lilium longiflorum d Fig. 2 BH > d Fig. 3 BD Liliales Liliaceae Tricyrtis hirta d/i Fig. 2 BI d/i Fig. 3 BE

Malvales Malvaceae Corchorus olitorius d Fig. 2 BJ nd Malvales Malvaceae Hibiscus syriacus d Fig. 2 BK > d Fig. 3 BF

Chloranthales Chloranthaceae Chloranthus spicatus d Fig. 2 BL d Fig. 3 BG Chloranthales Chloranthaceae Chloranthus japonicus d Fig. 2 BM > d Fig. 3 BH Chloranthales Chloranthaceae Sarcandra glabra d Fig. 2 BN d Fig. 3 BI

Ranunculales Papaveraceae Papaver rhoeas d Fig. 2 BO > d Fig. 3 BJ Ranunculales Ranunculaceae Helleborus niger d Fig. 2 BP > d Fig. 3 BK Ranunculales Ranunculaceae Ranunculus muricatus d/i Fig. 2 BQ nd

Rosales Moraceae Ficus umbellata d Fig. 2 BR > d Fig. L3 B Rosales Rosaceae Malus domestica d Fig. 2 BS nd

Sapindales Rutaceae Citrus hassaku d Fig. 2 BT > d Fig. 3 BM Sapindales Rutaceae Ruta graveolens d Fig. 2 BU > d Fig. 3 BN

Saxifragales Paeoniaceae Paeonia lactiflora d Fig. 2 BV nd

Solanales Convolvulaceae Evolvulus pilosus d Fig. 2 BW d Fig. 3 BO Solanales Solanaceae Capsicum annuum d Fig. 2 BX nd Solanales Solanaceae Solanum tuberosum d/i Fig. 2 BY > d/i Fig. 3 BP

Zingiberales Zingiberaceae Zingiber mioga d Fig. 2 BZ nd showed interstitial or proximal telomere signals (Figs. 1 and 2).

Discussion

The sequences of 2 telomere repeats, Arabidopsis- and human-type, used as probes in FISH are identical except for a difference of 1 nucleotide, but they exist as tandem repeats in the chromosome. Importantly, they do not cross-hybridise with each other under FISH conditions. In fact, some plants showed only 1 telomere repeat, but the majority of the species maintained both 2011 FISH Survey of Telomere Repeats in Plants 357

Fig. 1. FISH images probed with Arabidopsis-type telomere repeats. Species names are shown in Table 1. Bar=2 μm.

Arabidopsis-type and human-type telomere repeats at the chromosome ends. Generally, the FISH results of this study corroborated former reports (Pich and Schubert 1998, Adams et al. 2000, 2001, Sykorova et al. 2003), and any differences could be accounted for by the sensitivity of FISH detection. In fact, in our results, some species displayed very faint signals, and a po ssibility remains that Arabidopsis-type or human-type telomere repeats existed in the species that we reported as negative in this study. 358 F. Shibata and M. Hizume Cytologia 76(3)

Fig. 2. FISH images probed with human-type telomere repeats. Species names are shown in Table 1. Bar=2 μm.

Gymnosperms originated at an earlier time point than angiosperms, and one of the gymnosperm species, Cycas revoluta, showed both types of telomere repeat signals at chromosome ends. Chloranthales, which is the oldest and earliest diverged angiosperm used in this study, as well as Asterales and Apiales, which are believed to have branched recently in the angiosperms, also have both types of telomere repeats at chromosome ends. This suggests that both types of telomere repeats are preserved at chromosome ends, at least from the beginning of the spermatophytes, and is suggested by the fact that both repeats have a role at chromosome ends. Sykorova et al. (2003) reported that human-type telomeres switch from Arabidopsis-type telomeres in 3 genera of Solanaceae, in which other genera possess Arabidopsis-type telomeres. Except 2011 FISH Survey of Telomere Repeats in Plants 359 for the Asparagales species, the FISH signal intensity of Arabidopsis-type telomere repeats tended to be stronger than that of human-type telomere repeats in the species showing different signal intensity between the 2 probes. In plants, the Arabidopsis-type telomere repeat is dominant, and this might be inherited from a common ancestor of angiosperms and gymnosperms. Arabidopsis-type telomere repeats have also been reported in 5 species of pteridophytes and brytophytes (Suzuki 2004), indicating that Arabidopsis-type telomere repeats originated in land plants. The human-type telomere repeats have not been examined in these plants. In green algae, Chlamydomonas reinhardtii has Arabidopsis-type telomere repeats (Petracek et al. 1990) and Chlorella vulgaris has human-type telomere repeats (Higashiyama et al. 1995). These studies suggest that green algae may have both telomere sequences. The colocalisation of Arabidopsis-type and human-type telomere repeats at chromosome ends was also dominant in gymnosperms and angiosperms. The question remains regarding the origin of the co-localisation of these 2 telomere repeats, and this information may be gathered by expanding the target taxa of the FISH experiment.

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