Telomere Variability in the Monocotyledonous Plant Order Asparagales E
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Received 27 February 2003 Accepted 8 May 2003 Published online 28 July 2003 Telomere variability in the monocotyledonous plant order Asparagales E. Sy´korova´ 1,2,4, K. Y. Lim1, Z. Kunicka´ 4, M. W. Chase3, M. D. Bennett3, J. Fajkus2,4 and A. R. Leitch1* 1School of Biological Sciences, Queen Mary University of London, London E1 4NS, UK 2Institute of Biophysics, Czech Academy of Sciences, Brno, Czech Republic 3Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond TW9 3AB, UK 4Department of Functional Genomics and Proteomics, Faculty of Sciences, Masaryk University, Brno, Czech Republic A group of monocotyledonous plants within the order Asparagales, forming a distinct clade in phylogenetic analyses, was reported previously to lack the ‘typical’ Arabidopsis-type telomere (TTTAGGG)n. This stimulated us to determine what has replaced these sequences. Using slot-blot and fluorescent in situ hybridization (FISH) to species within this clade, our results indicate the following. 1. The typical Arabidopsis-type telomeric sequence has been partly or fully replaced by the human-type telomeric sequence (TTAGGG)n. Species in Allium lack the human-type variant. 2. In most cases the human variant occurs along with a lower abundance of two or more variants of the minisatellite sequences (of seven types evaluated), usually these being the consensus telomeric sequence of Arabidopsis, Bombyx (TTAGG)n and Tetrahymena (TTGGGG)n. FISH shows that the variants can occur mixed together at the telomere. 3. Telomerases generate products with a 6 base pair periodicity and when sequenced they reveal predomi- nantly a reiterated human-type motif. These motifs probably form the ‘true telomere’ but the error rate of motif synthesis is higher compared with ‘typical’ plant telomerases. The data indicate that the Aspara- gales clade is unified by a mutation resulting in a switch from synthesis of Arabidopsis-like telomeres to a low-fidelity synthesis of human-like telomeres. Keywords: plant; human; Tetrahymena; telomere; evolution; TRAP assay 1. INTRODUCTION mediated chromosome elongation system and elongate their chromosomes using recombination. In these organ- Telomere sequences are highly conserved at the ends of isms, the structure of the chromosome ends and their chromosomes across eukaryotes. Indeed, only point dependence on recombination proteins point towards mutations of the general, telomeric, minisatellite, oligonu- unequal recombination (gene conversion) occurring cleotide motif (T A G ) distinguish the telomeres of large m n o between telomeres (Lundblad & Blackburn 1993; McEa- groups of protozoa, algae, higher plants and animals. The chern & Blackburn 1996; Teng & Zakian 1999). Such conservation of telomere motifs results from their syn- alternative lengthening of telomere (ALT) mechanisms thesis by telomerase, a nucleoprotein enzyme complex have also been shown to occur in rare cases of human that adds oligonucleotide units to the telomere by reverse telomerase-negative tumours and in vitro immortalized transcription. This system is considered to be evol- cell-lines (Bryan et al. 1997). In relation to ALT, telomeric utionarily ancient, as shown by homology between cata- loops observed in mammals (see Griffith et al. 1999), lytic (protein) subunits of telomerases in different protozoa (see Murti & Prescott 1999; Munoz-Jordan et organisms, as well as by their similarity to viral reverse al. 2001) and recently in plants (J. D. Griffith, personal transcriptases. communication) could well serve, not only as the protec- The first exceptions noted to this ‘telomerase totality’ tive telomere capping structures, but also as substrates for were Drosophila and other insects. Drosophila species use a rolling circle gene conversion. strategy involving retrotransposition of HeT-A and TART In plants, the absence of Arabidopsis-type telomere sequences to chromosome ends (Biessmann et al. 1990; (TTTAGGG) was first observed in Alliaceae species Casacuberta & Pardue 2003), whereas in Chironomus n (notably Allium) and it was suggested that the chromo- species and Anopheles gambiae, telomeric DNA is formed somes may be terminated by satellite repeats, ribosomal by blocks of satellite repeat sequences that are probably DNA (rDNA) repeats or mobile elements (Fuchs et al. extended by gene conversion (see Biessmann et al. 2002 1995; Pich et al. 1996; Pich & Schubert 1998). The list for review). These sequences are more typical of subtel- of plants ‘lacking’ typical plant telomeres was then omeric regions in organisms with telomeres maintained by extended to include Aloe species (family Asphodelaceae; telomerase. The second group of exceptions were found in Adams et al. 2000) in which the Arabidopsis-type telomeric yeasts, which possess irregular sequences at the telomeres. sequence is apparently replaced by the human-type These yeasts have survived mutations in the telomerase- sequence (TTAGGG)n (Weiss & Scherthan 2002). The lack of Arabidopsis-type telomeric sequence has since been demonstrated in 16 other species from 12 families of * Author for correspondence ([email protected]). Asparagales (Adams et al. 2001). In phylogenetic analyses Proc. R. Soc. Lond. B (2003) 270, 1893–1904 1893 2003 The Royal Society DOI 10.1098/rspb.2003.2446 1894 E. Sy´korova´ and others Telomere variability in Asparagales telomere telomerase ants of the minisatellite repeat as found as the consensus analysis assay sequence in vertebrates (including humans) (TTAGGG)n, Agapanthaceae T +6 Tetrahymena (TTGGGG)n, Oxytricha (TTTTGGGG)n, + Amaryllidaceae T 6 Bombyx (TTAGG)n, Chlamydomonas (TTTTAGGG)n and Ascaris (TTAGGC) . These sequence motifs were Alliaceae T – n chosen for their sequence similarity to the Arabidopsis telo- meric sequence, and we presumed that the simplest hypo- + Agavaceae T 6 thetical event resulting in telomeric change to a different minisatellite repeat would be a mutation in the template Hyacinthaceae T +6 replacement of Aphyllanthaceae T region of telomerase. Arabidopsis- We demonstrate that in most species investigated by Themidaceae type telomere slot-blot hybridization the telomere consensus motif of sequence Asparagaceae T +6 Arabidopsis is replaced by that typical of humans or by a Ruscaceae T +6 mixture of the Arabidopsis-, human- and Tetrahymena-type Laxmanniaceae T motifs. To answer the question as to whether the human Asphodelaceae T +6 motif, detected in situ on chromosome ends of many Aspa- Xanthorrhoeaceae T ragales, is synthesized by telomerase and thus constitutes Hemerocallidaceae T +6 the ‘true’ telomere, the telomerase repeat amplification Xeronemataceae T Iridaceae T +6 protocol (TRAP) was performed, and the products Doryanthaceae P sequenced. The results were unambiguously positive in Tecophilaeaceae P sampled Asparagales species, except for Allium cepa,in Ixioliriaceae which no activity was detected. We also show that the lack Boryaceae of telomerase activity in A. cepa is not due to inhibition Asteliaceae T of the TRAP assay. Hypoxidaceae T Lanariaceae Blandfordiaceae T Orchidaceae P +7 2. MATERIAL AND METHODS Commelinales T (a) Plant material Zingiberales The sources of plants used to extract total genomic DNA for Liliales P slot-blot analyses are given in figures 2–4. Control genomic Pandanales DNA for these experiments were Silene latifolia (Caryophilaceae; Institute of Biophysics, Czech Republic), Nicotiana sylvestris (Solanaceae, Queen Mary University of London, UK), A. cepa Figure 1. Phylogenetic tree of Asparagales (Fay et al. 2000). cv. Ailsa Craig (Alliaceae; Royal Botanic Gardens, Kew, UK; Arrow shows the node at which the Arabidopsis-type telomere RBGK), Homo sapiens DNA (Cambio UK Ltd) and Bombyx sequences were substituted (considered as lost by Adams et al. 2001). T denotes that slot-blot analyses are presented mori, a gift of Frantisˇek Marec (Institute of Entomology, Czech ˇ here; P denotes additional families studied by Adams et al. Acad. Sci., Ceske´ Budeˇjovice, Czech Republic). (2001); ϩ indicates that the TRAP assay generated a For FISH experiments, vigorously growing root tips were product; the subscript 7 or 6 denotes the periodicity in base obtained from pot-grown plants of the following: Ruscus pairs of the TRAP product; Ϫ indicates no TRAP products aculeatus (Ruscaceae), Lycoris squamigera (Amaryllidaceae), and generated. Ornithogalum umbellatum (Hyacinthaceae; all grown at Royal Botanic Gardens, Kew); Ornithogalum virens (Hyacinthaceae; Chelsea Physic Gardens, London, UK); Bowiea volubilis (Fay et al. 2000), these species cluster in a clade within (Hyacinthaceae) and Iris versicolor × I. virginica (Iridaceae; Asparagales, thereby indicating that the Arabidopsis-type Queen Mary University of London). telomeric sequence was lost in a single evolutionary event For telomerase activity assays (TRAP assays), the following ca.80–90 Myr ago (figure 1). More recently, Sykorova et plants were used. al. (2003) showed that the Arabidopsis-type telomere was also absent in some Cestrum species (family Solanaceae), (i) Harvested seedlings of A. cepa cv. Stuttgarter Riesen although the minisatellite sequences were present at inter- (Alliaceae; Semo Smrzˇice, Czech Republic), Asparagus stitial locations. officinalis (Asparagaceae; commercial cultivar), Gennaria These data raise questions as to what sequences have chaplinii (RBGK 1986-5342; Amaryllidaceae), Galtonia come to replace the Arabidopsis-type consensus sequence candicans (RBGK 1969-19589; Hyacinthaceae), and how do the new sequences function as telomeres. Agapanthus umbellatus