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Department of Biology, University of Iceland, Grensásvegi 12, Reykjavík 108, Iceland
Received for publication February 15, 2000. Accepted for publication June 15, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Leymus • Psathyrostachys • FISH • RFLP • 18S–26S ribosomal genes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). These are Leymus arenarius (L.) Hochst. (octoploid, 2n = 8x = 56, northern European), L. mollis (Trin.) Pilger (tetraploid, 2n = 4x = 28, northern American/Pacific) and L. racemosus (Lam.) Tzvelev (tetraploid, 2n = 4x = 28, southeastern European and central Asian). They were also treated together under section Psammelymus of the genus Elymus L. (Bowden, 1957
), and elsewhere they were recognized as subspecies of Elymus arenarius, i.e., subsp. arenarius, mollis, and racemosus/giganteus (for example, Hultén and Fries, 1986
). These rhizomatous perennial Leymus (lymegrass) species are distinguished from species in other sections by their thick culms, awnless and hirsute lemmas, and lanceolate to linear-lanceolate glumes with 3–5 evident nerves. Characteristics that distinguish these three species lie mainly in the spikes and glumes (Melderis, 1980
; Barkworth and Atkins, 1984
). Leymus arenarius and L. mollis have spikelets in pairs at each node of the rachis and large glume size (and seed size) compared to that of L. racemosus, which has spikelets in groups of 2–7. Between L. arenarius and L. mollis, the former often has glaucous culms and leaves, papery glumes, and glaucous lemmas, whereas the latter tends to be more green, and its glumes and rachis densely pubescent. Overall these three species are very similar morphologically, leading to an assumption that they are closely related, taxonomically and phylogenetically.
A phylogenetic approach was adopted in Triticeae systematics (Löve, 1982, 1984
; Dewey, 1984
), and the classification is no longer based only on morphology and geographical distribution but also on cytogenetics. Löve (1982)
defined a genus as a group of species containing the same genome or a particular combination of a few genomes. Leymus was therefore recognized as a genomically distinct genus consisting of two different genomes JN (ENs, Dewey, 1984
; Löve, 1984
), where J (E) is a genome of Thinopyrum Löve and Ns is from Psathyrostachys Nevski. This genomic constitution is no longer held true, as molecular studies have rejected the involvement of the J (E) genome in Leymus (Zhang and Dvorak, 1991
; Wang and Jensen, 1994
), leading to a consensus that Xm or a genome of unknown origin be used in its place (Wang et al., 1994
).
All species of Leymus, 
30 in total, are considered to be phylogenetically related and share the same genome components. The relationships between species have therefore been interpreted by studying genome homology/homoeology based on meiotic pairing in various interspecific and intergeneric hybrids. However, studies involving species of the section Leymus, i.e., L. arenarius, L. mollis, and L. racemosus, have been limited and inconclusive. Wang and Hsiao (1984)
, for example, examined meiosis of L. mollis and L. arenarius hybrids and proposed that the tetraploid L. mollis formed half of the octoploid L. arenarius genomes, although it was not possible to identify whether the 14 bivalents formed were autosyndetic (intraspecific) or interspecific. Other authors observed high frequency of autosyndetic pairing in the L. arenarius genomes in a natural hybrid between L. arenarius and rye Secale cereale (Heneen, 1963
), in L. arenarius monoploids (Ahokas, 1997
), and in F1 hybrids between L. arenarius and wheat Triticum aestivum (Anamthawat-Jónsson and Bödvarsdóttir, 1998
). A recent molecular study (Ørgaard and Heslop-Harrison, 1994a
) has questioned the simplicity of Leymus phylogenetics and suggested that Asiatic and northern American Leymus species are more distantly related than previously thought.
The aim of the present study was therefore to investigate relationships among the three geographically distinct Leymus species described above, using fluorescence in situ hybridization (FISH) and restriction fragment length polymorphism (RFLP) analysis of the major ribosomal genes (18S–5.8S–26S rDNA). As reference, the RFLP analysis also included two other species of Leymus and four species of Psathyrostachys, the most closely related genus to Leymus. Our interest in the section Leymus has come from increasing use of Leymus genetic resources in ecological and breeding activities (Anamthawat-Jónsson et al., 1997, 1999
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The 9-kb fragment of wheat ribosomal genes (18S–5.8S–26S rDNA) in the plasmid clone pTa71 (Gerlach and Bedbrook, 1979
) was isolated and purified using Geneclean kit (Bio101, La Jolla, California, USA) before labeling specifically for the following chromosomal FISH or Southern RFLP.
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, using combined denaturation temperatures of 88°–89°C for 10 min in a modified thermocycler (Cambio, Cambridge, UK). For use with L. arenarius and L. mollis chromosomes, the rDNA probe from pTa71 was labeled with digoxygenin-11-dUTP (Boehringer, Manheim, Germany) using the random-primed labeling method, the in situ hybridization was detected with green-fluorescing anti-digoxygenin-FITC (Boehringer) and counterstained with red-fluorescing propidium iodide (Sigma). For use with L. racemosus chromosomes, the rDNA probe was labeled directly with red-fluorescing rhodamine-4-dUTP (Amersham-Pharmacia, Denmark) by nick translation, and after in situ hybridization the chromosomes were counterstained with blue-fluorescing DAPI (4, 6-diamidino-2-phenylindole, Sigma). The chromosomes were examined in Leitz epifluorescence microscope with 1000x magnification using appropriate filter sets, A for DAPI, I3 for FITC and propidium iodide, and N2 for rhodamine. The results were recorded by photography using Fuji print films. The chromosomal mapping of ribosomal genes was analyzed by making an idiogram of rDNA loci, based on at least ten metaphase cells each species. Chromosomes showing major rDNA loci were also those having secondary constriction at the nucleolar organizer region (NOR) and a satellite that could be seen clearly in standard metaphase chromosome preparations. They were also called satellite (SAT) chromosomes.
rDNA-RFLP
For Southern analysis, total genomic DNAs of all the species listed in Table 1 were digested with restriction enzymes BamHI, EcoRI, and DraI separately, size-fractionated by gel electrophoresis in 0.6–0.7% agarose, and transferred to Hybond N+ nylon membrane (Amersham). The blot was then hybridized with the purified rDNA probe from the clone pTa71 according to a standard protocol of ECL chemiluminescence (Amersham). Hybridization and washing stringencies were 78 (based on 0.5 mol/L NaCl) and 86%, respectively, and exposure time on film was from 5 to 45 min. Twenty RFLP fragments from BamHI, EcoRI, and DraI digestions were obtained (Table 2) and they were scored from each plant sample as present (1) and absent (0). These scores were used to calculate genetic distance according to Nei and Li (1979)
, from which a UPGMA cluster phenogram was constructed using the NTSYS-pc version 2.02 (Applied Biostatistics, New York, New York, USA).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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These results are consistent with data reported from other studies. Heneen (1963)
described SAT-chromosomes of L. arenarius, whose map is identical to our map of major loci. The ribosomal probe pTa71 was also used to detect rDNA loci in L. arenarius (Ørgaard and Heslop-Harrison, 1994b
), but their results included four major sites, probably the two pairs of major loci a1 and a2. Their four weaker sites would therefore be the pair a3 and two minor sites. According to the C-banded karyotype of L. racemosus (Qi et al., 1997
), the loci a1/r1 appear to be on chromosome E. Recent mapping using the ribosomal probe pTa71 revealed better resolution, whereby the three pairs of rDNA loci detected are in good agreement with our results, i.e., loci r1, r2, and r3 in the present study appear to be on chromosomes B, A, and C (Kishii et al., 1999
). No report on rDNA loci of L. mollis by others has been found. None of our Leymus major loci, a1/r1, m1, a2/r2, m2, corresponds to those detected in any of the following Psathyrostachys species: P. juncea, P. lanuginosa, and P. fragilis (William and Mujeeb-Kazi, 1992
; Linde-Laursen and Baden, 1994a, b
). Molecular in situ mapping of the ribosomal genes will certainly provide a more accurate comparison of rDNA loci of Leymus and Psathyrostachys.
Restriction fragment length polymorphism (RFLP) of the ribosomal genes
The analysis based on BamHI, EcoRI, and DraI restriction fragments (Table 2, Fig. 5) revealed genetic relationships among five species of Leymus (L. arenarius, L. mollis, L. racemosus, L. karelinii, and L. angustus) and four species of Psathyrostachys (P. fragilis, P. huashanica, P. juncea, and P. lanuginosa). The rDNA-RFLP data confirmed a close relationship between northern European L. arenarius and central Eurasian L. racemosus and their distance to the American L. mollis. Furthermore, the analysis revealed a closer relationship between the two genera Leymus and Psathyrostachys than that among species within a genus.
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. The EcoRI profiles consisted of three classes of fragments: 11–9.5 kb, 7–6.6 kb, and 4.4–3 kb. The DraI profiles consisted of only one band class, 11–9.5 kb. One repeat unit of the ribosomal genes in this Leymus–Psathyrostachys group varied between 9.5 and 11 kb in length. Leymus mollis was unique for having 9.5-kb genes, while all other species had 11-kb or 10-kb genes or both. Smaller fragments add up to be the whole genes in most cases, apparently due to point mutations in the repeat units. No RFLP variation was found among individuals of Icelandic L. arenarius, whereas variation in Alaskan L. mollis was limited within RFLP classes, which in themselves were species specific (Anamthawat-Jónsson et al., 1999
). Therefore the different copies within species may reflect allelic variation in polyploids (all Leymus) or multiple loci in diploid species like Psathyrostachys. Some of these species even have more than one rDNA locus on each chromosome, for example, two pairs of P. stoloniformis (diploid) chromosomes showed rDNA signals on both arms of the chromosomes, possibly due to chromosomal translocation (Ørgaard and Heslop-Harrison, 1994b
). The phenogram constructed from rDNA-RFLP fragments in Table 2 showed two major clusters, one of them consisted of L. mollis, P. lanuginosa, and P. juncea, while the other included all other species studied here (Fig. 5). The rDNA phenogram undoubtedly confirmed the chromosomal mapping results of a close relationship between L. arenarius and L. racemosus. This RFLP similarity is obviously based on the genes in the major loci of these two species (Fig. 4). According to the rDNA-RFLP, these two European Leymus species are more related to Eurasian species of Leymus than to the American L. mollis. Most interestingly, the genus Psathyrostachys is also divided, leading to a larger rDNA variation within a genus than between the two genera Leymus and Psathyrostachys.
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DISCUSSION |
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According to the ribosomal gene mapping by FISH, the half of the octoploid L. arenarius genomes, which contain the major rDNA loci, are likely to have derived from the tetraploid genomes of L. racemosus, almost without change. The other half, in which reside the minor rDNA loci, are more likely to have come from another species rather than being a duplicate of the L. racemosus genomes. The sites of the minor loci are different from any of the sites detected here. Whereas the rDNA copy number can change in a short time, probably due to reduced constraint as the genes are in ample excess of that required to sustain ribosomal synthesis (Leitch and Heslop-Harrison, 1993
), the structural changes of chromosomes resulting in site variation would be rare or occurring at a slower rate. It is known that the organization of genes within plant genomes, i.e., the gene order or linkage arrangement, has been conserved over a longer evolutionary period than previously thought (Gale and Devos, 1998
). The ribosomal gene mapping, either by in situ hybridization, C-banding, or other means, reflects similarly good synteny across evolutionarily related Triticeae species and hence is useful for tracing genome/species origin in polyploids and hybrids (Jiang and Gill, 1994
; Linde-Laursen and Baden, 1994b
; Anamthawat-Jónsson et al., 1997
). Based on in situ mapping of the ribosomal genes and the rDNA-RFLP analysis, it is clear that the northern European L. arenarius is closely related to the central Eurasian L. racemosus, and they both have different genome origins from that of the northern American L. mollis.
The close relationship between L. arenarius and L. racemosus is not surprising because of their geographic proximity. Molecular studies of chloroplast genomes as well as other genomes show that northerly and southerly plant migrations reflect climatic oscillations in the Quaternary in Europe (Comes and Kadereit, 1998
; Taberlet et al., 1998
). Leymus arenarius might have originated from L. racemosus, via interspecific hybridization as proposed here and/or polyploidy, and might have moved northward as the ice retreated, and isolation thereafter led to speciation. A similar process has been documented, for example, in Saxifraga species (Tollefsrud et al., 1998
). Leymus arenarius populations in Iceland have been examined, and their low level of molecular variation has suggested postglacial arrival of the species from Europe in contrast to the diverse and much older Alaskan L. mollis (Anamthawat-Jónsson et al., 1999
). The long-standing belief in a close genetic relationship between northern European L. arenarius and northern American/Asian L. mollis is not supported by our data.
Relationship between Leymus and Psathyrostachys
The ribosomal gene RFLP results show that (1) Leymus is related to and could have been derived from Psathyrostachys or they have common origins, and (2) particular species of Leymus and Psathyrostachys are more related to one another than to other species of these two genera. It is known that Leymus and Psathyrostachys are genetically and genomically closely related. Meiotic pairing data obtained from interspecific and intergeneric crosses have been used as evidence that Psathyrostachys genome (Ns) forms half of Leymus (NsXm), whereas the other half are thought to be of unknown origin (Dewey, 1984
; Wang and Hsiao, 1984
; Wang and Jensen, 1994
; Wang et al., 1994
). Molecular studies based on repetitive DNA sequences have led to a different conclusion, that all of the Leymus genomes have probably come from Psathyrostachys, i.e., making Leymus autopolyploids (or segmental alloploids) having Ns1Ns2 genomes (Zhang and Dvorak, 1991
; Dvorak and Zhang, 1992
). Other molecular and cytogenetic studies (Ørgaard and Heslop-Harrison, 1994a, b
) cannot define genomic relationships between Psathyrostachys and Leymus. What is the source of such a discrepancy?
Our present study suggests that the choices of species in crosses could have influenced the interpretation of meiotic pairing data. Most meiotic studies have assumed that Psathyrostachys is monophyletic and Leymus is allopolyploid. Any bivalent pairing in intergeneric hybrids is therefore considered to be between the Ns of Leymus and the Ns of Psathyrostachys, whereas univalents are left to the Xm genome of Leymus, and Leymus autosyndetic pairing is unlikely because the species behave normally like diploids. Some studies (e.g., Dewey, 1972
; Sun, Wu, and Liu, 1995
) analyzed hybrids between Asiatic Leymus (L. racemosus and L. multicaulis) and P. juncea, the species that are quite distant according to the present rDNA-RFLP results. In such cases, the univalents could well be the Ns of P. juncea and the bivalents autosyndetic Leymus. Molecular cytogenetic analysis of meiotic chromosomes in wheat x Leymus amphihaploid hybrids shows that almost all metaphase ring bivalents are formed by intraspecific pairing, e.g., Leymus to Leymus chromosomes, whereas interspecific (Leymus to wheat) bivalents exist in a much lower frequency (Anamthawat-Jónsson and Bödvarsdóttir, 1998
). In addition, Leymus multivalents are observed in meiosis of the backcross derivatives that contain a duplicate set of the Leymus genomes (Anamthawat-Jónsson, 1999
). In addition, high chromosome pairing has also been observed in L. arenarius monoploids (Ahokas, 1997
). Evidently, autosyndetic pairing is prevalent in these Leymus genomes. Bivalents and multivalents were indeed common in the L. racemosus x P. juncea hybrid (Dewey, 1972
) but as usual the species origin of the multivalents was not known since a standard chromosome staining cannot reveal species or genome origin of chromosomes. In situ hybridization, using genomic or species-specific probes, is therefore an essential method to verify meiotic data and genome designation of a genus.
Large genetic diversity within the genera Leymus and Psathyrostachys has been observed by various means. The ribosomal RFLP studies suggested a large genome differentiation in northern temperate Leymus and Chinese Psathyrostachys (Ørgaard and Heslop-Harrison, 1994a
) and within South American Leymus (Dubcovsky, Schlatter, and Echaide, 1997
). Morphological data, sequenced chloroplast genes, and nuclear rDNA-ITS, as well as RAPD markers, revealed a similar pattern of diversity in the genus Psathyrostachys (Baden, 1991
; Hsiao et al., 1994
; Seberg, Petersen, and Baden, 1994
; Wei and Wang, 1995
). In particular, all the data indicate that P. fragilis is closely related to P. huashanica, but they are diverged from P. juncea and P. lanuginosa. Even before this, Nevski (1934)
placed them in different sections, P. fragilis in Camtolepis and the latter group (P. juncea and P. lanuginosa) in Eupsathyrostachys. This division is also apparent in our rDNA-RFLP study. Many more species from both genera would have to be analyzed, but so far there tends to be larger diversity within the genera Leymus and Psathyrostachys than between them. It seems obvious that the taxonomy of this group should be reexamined.
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FOOTNOTES |
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2 Author for correspondence (Tel: +354 525 4620, FAX: +354 525 4069, e-mail: kesara{at}hi.is)
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LITERATURE CITED |
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