Research
Phylogenetic relationships in Elymus (Poaceae: Triticeae)
based on the nuclear ribosomal internal transcribed spacer
and chloroplast trnL-F sequences
Blackwell Publishing Ltd
Quanlan Liu1, Song Ge2, Haibao Tang1, Xianglin Zhang3, Genfeng Zhu3 and Bao-Rong Lu1
1
The Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, Fudan University,
Shanghai 200433, China;2 Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093,
China;3 Chinese National Human Genome Center at Shanghai, Shanghai 201203, China
Summary
Author for correspondence:
Bao-Rong Lu
Tel./Fax: +86 21 65643668
Email: brlu@fudan.edu.cn
Received: 22 October 2005
Accepted: 9 December 2005
• To estimate the phylogenetic relationship of polyploid Elymus in Triticeae, nuclear
ribosomal internal transcribed spacer (ITS) and chloroplast trnL-F sequences of 45
Elymus accessions containing various genomes were analysed with those of
five Pseudoroegneria (St), two Hordeum (H), three Agropyron (P) and two
Australopyrum (W) accessions.
• The ITS sequences revealed a close phylogenetic relationship between the
polyploid Elymus and species from the other genera. The ITS and trnL-F trees
indicated considerable differentiation of the StY genome species.
• The trnL-F sequences revealed an especially close relationship of Pseudoroegneria
to all Elymus species included. Both the ITS and trnL-F trees suggested multiple
origins and recurrent hybridization of Elymus species.
• The results suggested that: the St, H, P, and W genomes in polyploid Elymus were
donated by Pseudoroegneria, Hordeum, Agropyron and Australopyrum, respectively,
and the St and Y genomes may have originated from the same ancestor; Pseudoroegneria was the maternal donor of the polyploid Elymus; and some Elymus species
showed multiple origin and experienced recurrent hybridization.
Key words: Elymus, nuclear ribosomal internal transcribed spacer (ITS), chloroplast
trnL-F, phylogeny, genomic differentiation, hybridization, polyploidization.
New Phytologist (2006) 170: 411–420
© The Authors (2006). Journal compilation © New Phytologist (2006)
doi: 10.1111/j.1469-8137.2006.01665.x
Introduction
Polyploidy, resulting from either duplication of a single
but complete genome (autopolyploidy) or from combination
of two or more differentiated genomes (allopolyploidy), is
a prominent mode of speciation (Stebbins, 1971; Masterson,
1994; Soltis & Soltis, 2000). About 70% of angiosperms are
of polyploid origin (Masterson, 1994; Soltis & Soltis, 2000;
Wendel, 2000), which has significantly enriched diversity of
plants. A better understanding of the processes of polyploidization and rapid diversification of the descendants of a single
polyploidization event is therefore of widespread evolutionary
interest (Wendel, 2000; Soltis et al., 2003). Recent studies
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using genetic markers in many genera suggest that recurrent
origins for polyploid species are the rule rather than the
exception (Soltis & Soltis, 2000), and that genetic diversity
within recent polyploids is adequate to support rapid adaptive
evolution (Doyle et al., 2003; Soltis et al., 2003).
The wheat tribe (Poaceae: Triticeae), an important gene
pool for genetic improvement of cereal crops (Dewey, 1984;
Lu, 1993, 1994), includes many autopolyploid and allopolyploid taxa. Data from extensive cytogenetic analyses have been
used to illustrate systematic relationships of the tribe and to
clarify the ancestry of many polyploid species. One complex
group of polyploids within Triticeae is the genus Elymus
that, following the taxonomic delimitation by Löve (1984)
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412 Research
based essentially on genomic constitutions, includes approx.
150 perennial species distributed in a wide range of ecological
habitats over the temperate and subtropic regions. Elymus
has its origin through a typical allopolyploidy process (Dewey,
1984; Löve, 1984). Cytological studies suggest that five basic
genomes, namely, the St, Y, H, P and W in various combinations constitute Elymus species (Lu, 1994). The St genome is
a fundamental genome that exists in all Elymus species and
is donated by the genus Pseudoroegneria (Dewey, 1967). The
H, P and W genomes are derived from the genera Hordeum,
Agropyron and Australopyrum of Triticeae, respectively (Dewey,
1971; Jensen, 1990; Torabinejad & Mueller, 1993). However,
the donor of the Y genome that is present in the majority
of the Asiatic Elymus species has not yet been identified,
although extensive investigations have been carried out (Lu,
1993, 1994).
One important group of Elymus includes tetraploids
with the StY genomes. About 30 StY genome Elymus species
are found restrictedly in the temperate Asia, where more than
half of the known Elymus species originated (Salomon &
Lu, 1992; Lu, 1994). A large data set from cytological analyses
of artificial hybrids among the StY genome Elymus species
clearly indicates that the degree of chromosome pairing in
the hybrids gradually decreases with increase in geographical
distance from the locality of their parental species (Lu &
Salomon, 1992; Lu, 1993). This means that the StY genomes
in tetraploid Elymus species have been modified to a large extent
and have relatively high genetic diversity. This phenomenon
has not been found in the StH genomes, which have relatively
high homology among different tetraploid Elymus species
distributed in Asia (Lu et al., 1992). Knowledge of the molecular
phylogeny of the StY genome Elymus will provide a better
understanding of their genetic differentiation.
Molecular phylogenetic studies have successfully revealed
the origins and evolutionary history of polyploids in plants,
clarified the nature of different polyploids, and identified their
parental lineages and the hybridization events involved in their
formation (Soltis & Soltis, 1993; Wendel, 2000; Soltis et al., 2003).
Comparative phylogenies between nuclear and chloroplast/
mitochondrial sequences have become a powerful tool to
identify the mode of polyploidization in particular groups
(Ge et al., 1999; Mason-Gamer, 2001; Popp & Oxelman, 2001;
Mason-Gamer, 2004; Rauscher et al., 2004). Among the available
nuclear sequences, internal transcribed spacer (ITS) sequences
have been used successfully in studying phylogenetic and genomic
relationships of plants at lower taxonomic levels (Baldwin et al.,
1995; Hsiao et al., 1995; Wendel et al., 1995; Zhang et al., 2002;
Hao et al., 2004). The chloroplast DNA (cpDNA) sequences,
particularly the noncoding regions such as the intron of trnL
(UAA) and the intergenic spacer of trnL (UAA)–trnF (GAA)
are also valuable source of markers for identifying the maternal
donors of polyploids with additional capacity to reveal phylogenetic relationships of related species (Sang et al., 1997;
Mason-Gamer et al., 2002; Xu & Ban, 2004).
New Phytologist (2006) 170: 411–420
In this study, we sequenced and analysed the nuclear
ribosomal ITS and chloroplast trnL-F fragments for 30 Elymus
polyploids and their putative diploid donors to explore the
origin and relationships of the polyploid Elymus species. The
objectives of this study were (1) to reveal relationships of the
St, Y, P, W and H genomes of Elymus in relation to their putative
diploid ancestors; (2) to elucidate the phylogenetic relationships of the StY genome Asiatic tetraploids; and (3) to determine
the maternal genomic donor of Elymus polyploids.
Materials and Methods
Plant materials
A total of 57 Triticeae accessions were used in this study, including 45 Elymus accessions with different genomic combinations
(i.e. the StY, StStY, StH, StPY, StWY and StHY genomes),
five species of the related genus Pseudoroegneria (St and St1St2
genomes), three species of Agropyron (P genome), two species
of Hordeum (H genome) and two species of Australopyrum (W
genome). Bromus catharticus was used as the outgroup based
on previous phylogenetic studies of Poaceae (Hsiao et al., 1995;
Gaut, 2002). All seed materials were collected from the field
or provided by Dr B. Salomon of the Swedish University
of Agricultural Sciences, Sweden, and Drs H. Y. Zhou and
H. Q. Zhang of Sichuan Agriculture University, China. All the
accessions sequenced in this study, with their scientific names,
geographic origins and GenBank accession numbers are listed
in Table 1. The voucher specimens of this study are deposited
in the Swedish University of Agricultural Sciences, Sweden
and Sichuan Agriculture University, China.
DNA extraction and purification
Seeds were germinated and grown in a growth chamber with 15 h
in light and 9 h in dark at 25°C. Leaf samples collected from each
accession at the seedling stage were ground in liquid nitrogen
in a 1.5-ml microfuge tube, and DNA was extracted and purified
using a slight modification of the cetyltrimethylammonium
bromide (CTAB) procedure outlined in Doyle and Doyle (1990).
ITS amplification, cloning, and sequencing
The internal transcribed spacer (ITS) region of the nuclear
ribosomal DNA was amplified by polymerase chain reaction
(PCR) using the primers of ITS4 and ITS5 (Hsiao et al., 1995).
The PCR amplification of ITS DNA was carried out in a total
reaction volume of 25 µl containing 1× reaction buffer, 1.5
mM MgCl2, 0.5 µM of each primer, 200 µM of each dNTP
(TaKaRa Inc., Dalian, Liaoning, China), 0.5 units of ExTaq
Polymerase (TaKaRa Inc.), with an addition of 8% dimethyl
sulfoxide (DMSO) and water to the final volume. The thermocycling profile consisted of an initial denaturation step at 94°C
for 3 min, followed by 35 cycles of 0.5 min at 94°C, 1 min at
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Research
Table 1 Species of Elymus and other closely related genera used in this study
GenBank Accession No.
No.
Species
Accession No.
Pseudoroegneria (Nevski) A. Löve
1
P. spicata
PI547161
2
P. libanotica
PI228389
3
P. strigosa
PI499637
4
P. alashanica
Z2006
5
P. elytrigioides
Z2005
Hordeum L.
6
H. bogdanii
PI531761
7
H. brevisubulatum
Y1604
Agropyron Gaertner
8
A. cristatum
H10154
9
A. cristatum
H10066
10
A. mongolicum
–
Australopyrum (Tsvelev) A. Löve
11
A. retrofractum
Crane 86146
12
A. pectinatum
–
Elymus L.
13
E. antiquus
H7087
14
E. antiquus
H3400
15
E. anthosachnoides
Y2236
16
E. barbicallus
H3267
Genome
Origin
ITS
trnL-F
St
St
St
St1St2
St1St2
Oregon, USA
Iran
Urumqi, Xinjiang, China
Yinchuan, Ninxia, China
Changdu, Tibet, China
AY740793
AY740794
AY740795
AY740796, AY740797
AY740798, AY740799
AF519159*
AY730567
AF519155*
AY73069
AY730568
H
H
China
Fuyun, Xinjiang, China
AY740876
AY740877
AY740789
AY740790
P
P
P
Altai, Xinjiang, China
Altai, Xinjiang, China
–
AY740890
AY740891
L36482*
AY740791
AY740792
AF519117*
W
W
–
–
–
L36483*, L36484*
AF519118*
–
StY
StY
StY
StY
Lixian, Sichuan, China
Sichuan, China
Yajiang, Sichuan, China
China, seeds from
D. R. Dewey, 1988, D 2509
China, seeds from
D. R. Dewey, 1988, D 3512
Yajiang, Sichuan, China
Hongyuan, Sichuan, China
Batang, Sichuan, China
Zogang, Tibet, China
Beijing, China
Zhaojie, Sichuan, China
Wenchuan, Sichuan, China
Lingtong, Shaanxi, China
Maowen, Sichuan, China
Li xian, Sichuan, China
Changdu, Tibet, China
Wenchuan, Sichuan, China
Wuhan, Hubei, China
Luhuo, Sichuan, China
Gongbogyamda, Tibet, China
Alma-Ata, Medeo, Kazakstan
Xinyuan, Xinjiang, China
NWFP, Pakistan
Habahe, Xinjiang, China
NWFP, Pakistan
Jeminay, Xinjiang, China
Altai, Xinjiang, China
Northern Iran
NWFP, Pakistan
Gissar, Tadzhikistan
Gissar, Tadzhikistan
Chatkal, Uzbekistan
Gissar, Tadzhikistan
Mandi, Pakistan
Hazara, Pakistan
Babusar, Gilgit, Pakistan
Naran village, Hazara, Pakistan
Dilidjan, Armenia
Crimea, Ukraine
AY740814, AY740815
AY740818, AY740819
AY740820, AY740821
AY740824, AY740825
–
AY730581
AY740770
–
AY740822, AY740823
AY730580
AY740826, AY740827
AY740872, AY740873
AY740870, AY740871
AY740874, AY740875
AY740830, AY740831
AY740834, AY740835
AY740836, AY740837
AY740828, AY740829
AY740856, AY740857
AY740854, AY740855
AY740846, AY740847
AY740848, AY740849
AY740816, AY740817
AY740862, AY740863
AY740864, AY740865
AY740899, AY740900
AY740812, AY740813
AY740832, AY740833
AY740838, AY740839
AY740840, AY740841
AY740844, AY740845
AY740842, AY740843
AY740804, AY740805
AY740806, AY740807
AY740850, AY740851
AY740852, AY740853
AY740860, AY740861
AY740858, AY740859
AY740800, AY740801
AY740802, AY740803
AY740866, AY740867
AY740868, AY740869
AY740808, AY740809
AY740810, AY740811
AY740771
–
AY740772
–
AY740773
–
AY730574
AY730572
–
AY730585
AY730582
–
AY730583
–
–
–
AY730584
AY740774
AY740775
–
AY740777
AY740776
AY740778
AY740779
AY730573
–
AY740780
AY730571
AY730576
AY730575
AY730578
AY730579
AY730577
AY730570
17
E. barbicallus
H3268
StY
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
E. brevipes
E. burchan-buddae
E. burchan-buddae
E. burchan-buddae
E. ciliaris
E. dolichatherus
E. dolichatherus
E. grandis
E. nakaii
E. nakaii
E. pendulinus
E. pendulinus
E. shandongensis
E. tibeticus
E. tibeticus
E. abolinii
E. abolinii
E. canaliculatus
E. fedtschenkoi
E. fedtschenkoi
E. glaberrimus
E. gmelinii
E. longearistatus
E. longearistatus
E. macrochaetus
E. macrochaetus
E. nevskii
E. nevskii
E. semicostatus
E. semicostatus
E. validus
E. validus
E. caucasicus
E. panormitanus
Y2245
Y3049
Y2219
Y2207
H7000
H8024
Y1411
H3879
H7386
H7371
H8986
Y1412
H3202
H8927
H8366
H3306
H8491
H4116
H7510
H4114a
Y2042
H1033
H3261
H4114b
H10303
H10208
H3305
H10213
H3288
H4101
H4078
H4100
H3207
H4152
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
StY
© The Authors (2006). Journal compilation © New Phytologist (2006) www.newphytologist.org
New Phytologist (2006) 170: 411–420
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Table 1 continued
GenBank Accession No.
No.
Species
Accession No.
Genome
Origin
ITS
trnL-F
52
E. tschimganicus
H3302
StStY
Xinjiang, China
AY740783
53
54
E. caninus
E. himalayanus
PI564910
H4134
StH
StHY
Russian Federation
Astor valley, Gilgit, Pakistan
55
E. rectisetus
H3152
StWY
Lake Lyndon, Australia
56
E. melantherus
ZY3146
StPY
Sichuan, China
57
E. rigidulus
ZY3113
StPY
Xiahe, Gansu, China
AY740878, AY740879,
AY740880
AY740897, AY740898
AY740881, AY740882,
AY740883
AY740893, AY740894
AY740896, AY740786
AY740884, AY740885,
AY740886
AY740887, AY740888,
AY740889
S20004
–
Kunming, Yunnan, China
Bromus L.
58
B. catharticus
AF521898*
AY740781
AY740782
AY740895
AY740785
AY740784
AY829228
Numbers before species’ names serve as identifiers of specific accessions that correspond to the numbers in Figs 1,2, and 3. GenBank accessions
with an asterisk (*) represent previously published sequences from the GenBank (http://www.ncbi.nlm.nih.gov).
55°C, 1 min at 72°C and final extension step of 10 min at
72°C. PCR reactions from the polyploid Elymus were run in
triplicates in different thermocyclers and the PCR products
were combined in an attempt to offset the potential effects
of PCR drifts (Wagner et al., 1994). The PCR products were
purified using a gel extraction kit (TaKaRa Biotechnology
(Dalian) Co., Ltd, Dalian, China) and linked into a pMD-T
vector according to the manufacturer’s instruction (TaKaRa
Biotechnology (Dalian) Co., Ltd). Transformation, plating and
isolation of plasmids were performed as described in Sambrook
et al. (1989). Purified plasmid DNAs were digested with EcoRI
and HindIII. For each of the Elymus species, 10–15 cloned
PCR products were sequenced to include all the possible
ITS sequences from the donor species, using ABI BigDye
terminators according to the product instructions, and run on
an ABI 3730 sequencer.
CpDNA amplification and sequencing
The chloroplast tRNA genes trnT, trnL-5′, trnL-3′ and trnF,
along with their intervening noncoding regions, were amplified
using the primers a and f, or c and f of Taberlet et al. (1991).
Amplification of the cpDNA was performed in a total reaction
volume of 25 µl with the same components as described in
the ITS amplification, except that DMSO was not added. The
PCRs were performed as for the ITS nuclear ribosomal DNA,
except the annealing and extension times were 1.5 min each.
The PCR products were cleaned as described in the previous
section, the primers b, d, e and f of Taberlet et al. (1991) were
used to sequence both strands of the PCR fragments to
unambiguously identify all sites, and the sequencing reactions
were as described earlier.
New Phytologist (2006) 170: 411–420
Phylogenetic analysis
The ITS and trnL-F sequences were aligned with CLUSTAL X
(Thompson et al., 1999) and refined manually. The boundaries of the ITS region (ITS1–5.8S–ITS2) and trnL-F (trnL
intron–trnL 3′exon–intergenic spacer–trnF 5′exon) were determined according to Hsiao et al. (1995) and Ogihara et al.
(2002), respectively. Gaps were coded as binary characters by
their presence/absence, and were used for the phylogenetic
analyses. The basic sequence statistics, including nucleotide
frequencies, transition/transversion (ns : nv) ratio and variability
in different regions of the sequences were computed by MEGA
3 (Kumar et al., 2004).
The aligned sequences were used as the input data for PAUP
4.0 (Swofford, 1998), MEGA 3 and the PHYLIP software package
(Felsenstein, 1995). Parsimony analyses were performed by
heuristic search with tree bisection-reconnection (TBR) branch
swapping, MULPARS option, ACCTRAN optimization, and
100 random addition replicates. Topological robustness was
assessed by bootstrap analysis with 500 replicates using simple
taxon addition. The aligned sequences were also analyzed with
the Neighbor-Joining Program of the PHYLIP package, and
carried out with 1000 bootstrap replicates.
Results
Variation in ITS and trnL-F sequences
The ITS sequences in this study included three regions: (1) 43
nucleotides of the 18S rRNA gene; (2) the complete sequences
of ITS1, 5.8S rRNA gene and ITS2; and (3) 58 nucleotides
of the 26S rRNA. Sequences of the 18S rRNA and the 26S
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Research
rRNA genes showed no variation for all accessions included in
this study. The length of sequences ranged from 212 to 221 bp
in the ITS1 region, and from 216 to 218 bp in the ITS2
region. The 5.8S rRNA gene was 164 bp long and completely
identical for all the cloned sequences of the 58 accessions. The
average of G + C content was 61.2%. Sequence alignment
necessitated gaps of one to eight bases in length. Of 223 variable
sites, 115 were parsimoniously informative.
The trnL-F fragment sequenced in this study included four
regions: (1) the partial trnL intron; (2) the trnL 3′ exon; (3) the
trnL–trnF intergenic spacer; and (4) the partial trnF exon with
40 bp. The length of the sequenced chloroplast trnL-F varied
from 859 to 882 bp in all accessions. The average of G + C
content was 30.2%. Of the 210 variable sites, 107 were
parsimoniously informative, including the polymorphisms
introduced by insertions/deletions.
Phylogenetic analysis of the nuclear ITS sequences
We first constructed the phylogeny of all the diploid species
based on their ITS sequences in order to reveal relationships
of the putative genomic donors of Elymus. Three equally most
parsimonious trees were obtained with the tree length of 147
steps, a consistency index (CI) of 0.857 and a retention index
(RI) of 0.764. As shown in the strict consensus tree (Fig. 1), species
with the same genomes formed highly supported monophylies,
indicating that the four genomes have been well differentiated
and corresponded to different morphologically recognized genera.
The H genome (Hordeum) was at the basal position, followed
Fig. 1 The strict consensus tree of three most parsimonious (MP)
trees inferred from the ITS sequences of the diploid species (Tree
length = 147, consistency index (CI) = 0.857, retention index
(RI) = 0.764). The topologies obtained by Neighbor-Joining method
(NJ) are the same except for some nodes having different bootstrap
values. Numbers above and below the branches indicate bootstrap
values > 50% by MP and NJ analyses, respectively. Numbers after the
species names refer to accession numbers as indicated in Table 1.
Capital letters in parentheses following the species names indicate the
genome type of the species. The genome type (St, P, W or H) of a
monophyletic group is given to the right.
by the St genome (Pseudoroegneria) that was sister to the other
two related P and W genomes (Agropyron and Australopyrum).
To further analyse genomic relationships and the origin
of the polyploid Elymus, ITS sequences of all the polyploid
species (Elymus polyploids with the StH, StY, StStY, StHY,
StPY and StWY genomes and Pseudoroegneria tetraploids
with the St1St2 genomes) were included in the phylogenetic
analysis, together with those of diploids containing the St,
H, P and W genomes. To ensure obtaining all possible types
of ITS sequences of a polyploid, 10–15 clones from each of the
selected Elymus species were sequenced. In the case of multiple
identical sequences resulting from cloned PCR products of
one accession, only one sequence was included in the data set.
Consequently, 107 unique sequences were obtained and used
for phylogenetic analyses.
Maximum parsimony analysis resulted in 440 equally most
parsimonious trees. Each of the trees was 431 steps with a CI
of 0.620 and a RI of 0.819. In one of the most parsimonious
tree (Fig. 2), all the homeologous ITS sequences from polyploid
accessions grouped with those of the diploid parental clades
expected from cytological studies. Four major clades with high
bootstrap support (83–100%) were found, which correspond
to the four genomic types (H, P, W and St). The first clade
consisted of the Hordeum and Elymus species with the StH
and StHY genomes (100% bootstrap support). The second
clade included the Australopyrum and StWY genome Elymus
species (96% bootstrap support), while the third clade included
the Agropyron and StPY genome Elymus species (95% bootstrap
support). The fourth (largest) clade (83% bootstrap support)
comprised the Pseudoroegneria species (St and St1St2 genomes)
and all polyploid Elymus with the StStY, StY, StH, StHY, StPY,
and StWY genomes. Three subclades (A, B, and C subclades)
were recognized in this clade. It is worth mentioning that no
obvious Y-genome specific clade was detected in the phylogenetic
tree. Neighbor-Joining analysis generated a similar topology
with minor variation in bootstrap values.
For clarity, we named the four clades using genomic
symbols of the diploid species, i.e. the H, W, P, and St clades,
respectively (Fig. 2). The phylogenetic tree showed that all Elymus
polyploids, except for those with the St and Y genomes,
had two distinct types of ITS sequences with one forming
a clade with its respective putative diploid donors and the other
grouped with the St genome clade. By contrast, all Elymus
species with the Y and St genomes (StY and StStY) were retained
in the St clade. In addition, a well-supported branch (97%
bootstrap) was found within the St clade, including the St1St2
genome Pseudoroegneria and the StY genome Elymus tetraploids
that shared an 8-bp deletion in their ITS sequences (Fig. 2).
Phylogenetic analysis of the chloroplast trnL-F
sequences
The chloroplast trnL-F sequences of all Elymus polyploids
and their putative diploid donor species were included for
© The Authors (2006). Journal compilation © New Phytologist (2006) www.newphytologist.org
New Phytologist (2006) 170: 411–420
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New Phytologist (2006) 170: 411–420
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Research
phylogenetic analysis. Maximum parsimony analysis resulted
in 316 equally most parsimonious trees with 307 steps, a consistency index of 0.785, and a retention index of 0.816. It was
evident from the phylogenetic tree (Fig. 3) that the diploid
species grouped into four distinct clades corresponding
to the St, P, W and H genomes, respectively. The tetraploid
Pseudoroegneria (St1St2) and all Elymus species formed a large
but highly supported clade (100%) together with the diploid
Pseudoroegneria species, suggesting a close relationship between
Pseudoroegneria and Elymus species in terms of their chloroplast
genomes. This clade was named as St clade because all diploid
and polyploid accessions contained the St genome. The diploid
Hordeum species (H clade) was the earliest divergent lineage,
followed by the diploid Agropyron (P clade) and Australopyrum
(W clade) species that were sisters to the St clade. NeighborJoining analysis produced a similar topology with only very
minor differences in bootstrap supports (Fig. 3).
Discussion
Phylogenetic relationships of Elymus and its proposed
diploid ancestors
The genus Elymus consists of polyploids that are widely distributed over different continents and includes a large number
of endemic species. Only a few molecular studies addressing
phylogenetic relationships of the StH and StHY genome Elymus
species are reported (Mason-Gamer, 2001; Mason-Gamer
et al., 2002; McMillan & Sun, 2004; Xu & Ban, 2004). Little
is known about phylogeny of the Asiatic StY genome Elymus
species at molecular level. Analyses of ITS sequences collected
from a wide range of polyploid Elymus species and their
related genera will provide opportunities for understanding
their phylogenetic relationships, ancestral donors and polyploidization events in the speciation processes.
In the diploid and polyploid ITS trees, four of the five genomes
presented in Elymus formed distinct clades, the exception
being the Y genome. Of the four clades, the H genome clade
was basal, with the W, P, and St clades being successively more
distantly related. There was no obvious Y genome clade, and
all the StY species were placed in the St genome clade. These
results indicate that ITS sequences of all the genomes derived
from the diploid ancestors have remained clearly differentiated
in the polyploid Elymus. This can be reflected by the fact
that all the allopolyploid species (except for those with the Y
genome) contained two distinct types of ITS sequences, with
one type in the St clade and the others in different genomic
clades (H, P or W clade), respectively. This strongly suggests
that ITS sequences in different Elymus species showed a clear
linkage with those in their diploid ancestors. This is illustrated
by the fact that the StH, StHY, StPY and StWY genome
Elymus species were simultaneously clustered in both of
their ancestral groups, indicating that two distinct types of
ITS sequences exist in these polyploid Elymus. Obviously,
the homogenization of ITS sequences is not significant in
the polyploid Elymus. This provides strong evidence that the
polyploid Elymus species are derived from polyploidization
through hybridization between different ancestral genera,
as indicated by cytological analyses (Dewey, 1984; Lu, 1994).
For example, the StH genome Elymus species are derived from
the hybridization between Pseudoroegneria (St) and Hordeum
(H). Although the rDNA sequences are considered to undergo
a rapid homogenization through concerted evolution, growing
evidence shows that ITS polymorphism or incomplete homogenization is the rule rather than the exception (Buckler et al.,
1997; Hershkovitz et al., 1999). Directional and bidirectional
interlocus concerted evolution following allopolyploid speciation have been documented (Wendel et al., 1995; Fulnecek
et al., 2002), but allopolyploid species often maintain both
parental sequences of the ITS region. The phenomenon of
incomplete homogenization is useful for understanding the
origin of hybrids and genomic constructions in polyploidy
species (Ainouche & Bayer, 1997; Hershkovitz et al., 1999;
Popp & Oxelman, 2001; Yonemori et al., 2002; Koch et al.,
2003). It is evident that the homogenization of ITS sequences
in the allopolyploid Elymus is not completed, which probably
suggests the recent origin of these polyploids because sufficient
time is needed to allow ITS sequences to be homogenized
(Ainouche & Bayer, 1997).
By contrast, all the tetraploid StY genome Elymus species had
only one genomic type of ITS sequences since all these species
were included in the St clade (Fig. 2). This was also true of the
tetraploid Pseudoroegneria species. However, the hexaploids
that combined the St and Y genomes with one of the three
differentiated genomes (H, P or W genome) had two genomic
types of ITS sequences. There are two possible explanations
for this phenomenon. One of the explanations is that the
St and Y genomes may have the same origin, because the St
genome clade in this study did not show any St- and Y-genomeassociated subclades. This explanation is supported by the
Fig. 2 One of the 440 most parsimonious (MP) trees inferred from the internal transcribed spacer (ITS) sequences of all the accessions used in
this study (Tree length = 431, consistency index (CI) = 0.620, retention index (RI) = 0.819). The topologies obtained by Neighbor-Joining
method (NJ) are the same except for some nodes having different bootstrap values. Numbers above and below the branches indicate bootstrap
values > 50% by MP and NJ analyses, respectively. Branch lengths are proportional to the number of nucleotide substitutions; the scale bar at
the upper-left corner indicates one substitution. Numbers after species names refer to the accession numbers shown in Table 1. Capital letters
in parentheses indicate the genome type of the species. Names in boldface indicate species of Pseudoroegneria and other diploid genera. The
arrow indicates the 8-bp deletion shared by species in the subclade. *, polyploid species with the ‘H’ genome; **, polyploid species with the
‘W’ genome; ***, polyploid species with the ‘P’ genome. The genome type (St, P, W or H) of a monophyletic group is given to the right.
© The Authors (2006). Journal compilation © New Phytologist (2006) www.newphytologist.org
New Phytologist (2006) 170: 411–420
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418 Research
Fig. 3 One of 316 most parsimonious (MP)
trees inferred from the trnL-F sequences of
all the accessions used in this study (Tree
length = 307, consistency index
(CI) = 0.785, retention index (RI) = 0.816).
The topologies obtained by NeighborJoining method (NJ) are the same except for
some nodes having different bootstrap
values. Numbers above and below the
branches indicate bootstrap values greater
than 50% by MP and NJ analyses,
respectively. Branch lengths are proportional
to the number of nucleotide substitutions,
and the scale bar at the upper-left corner
indicates one substitution. Capital letters in
parentheses indicate the genome type of the
species. Names in boldface indicate species
of Pseudoroegneria and other diploid
genera. *, polyploid species with the ‘H’
genome; **, polyploid species with the
‘W’ genome; ***, polyploid species with the
‘P’ genome. The genome type (St, P, W or H)
of a monophyletic group is given to the right.
relatively close affinities between the St and Y genomes reported
by Lu et al. (1992), and Lu & Bothmer (1991) based on
cytological investigations, although a close affinity between
the St and P genomes are also reported (Wang et al., 1985).
In addition, no diploid Y genome species has been found so
far despite great efforts world-wide, further supporting the this
explanation. It is possible that homogenization of ITS sequences
has occurred between the St and Y genomes in polyploid Elymus
species. However, since there is no (or extremely low) chromosome pairing between the St and Y genomes under normal
conditions (the situation is the same between the St and H,
P or W genomes) the likelihood of homogenization of the ITS
sequences between the St and Y genomes seems to be low in
the polyploid Elymus species. Previous studies also indicate that
homogenization of ITS sequences in allopolyploids rarely results
in the formation of a single genomic type of sequences (Wendel
et al., 1995; Rauscher et al., 2004). Therefore, we prefer the
explanation that the St and Y genomes may have the same origin.
Differentiation of the StY genome Elymus tetraploids
Previous cytological investigations suggest that the StY genomes
in Elymus tetraploids are considerably differentiated in relation
New Phytologist (2006) 170: 411–420
to their geographical distribution (Lu & Salomon, 1992;
Lu, 1993). The analysis of ITS sequences based on nearly all
the StY genome Elymus species covering a wide range of distribution also demonstrated genomic differentiation of these species.
It is obvious that a subclade including two Pseudoroegneria
(St1St2) tetraploids and eight Elymus (StY) species are supported
with high bootstrap support (97%) and are differentiated from
other StY species (Fig. 2). This provides an evidence of genomic
differentiation of the StY genome species from molecular data.
Genomic differentiation among species through polyploidy
or at the same ploidy level usually results in great diversity
of polyploid species, as suggested by Soltis & Soltis (2000).
However, because of the weak resolution of the current ITS
data in the St clade, whether or not the differentiation of the
StY genome Elymus species is associated with their geographic
distribution, as suggested by previous cytological observations,
needs to be verified further with additional evidence.
Multiple origin is a very important evolutionary process
of polyploid species, which emphasizes ‘each polyploid species
forms over and over again from different parental genotypes
generating a diverse array of polyploid genotypes’ (Soltis &
Soltis, 1999). The ITS tree in this study also suggested a
multiple origin of some StY genome species resulting from
www.newphytologist.org © The Authors (2006). Journal compilation © New Phytologist (2006)
Research
recurrent hybridization, which can be shown by different
accessions of the same species, will appear at different clades
of a phylogenetic tree. For example, in this study, different
accessions of Elymus antiquus (−13 and −14) and Elymus
pendulinus (−28 and −29) were grouped in different subclades
of the St clade. This helps to explain the abundant genetic
diversity within an Elymus species. The recurrent hybridization
also promoted rapid adaptation of the Elymus species to
different ecological habitats, resulting in the formation of many
endemic genotypes and species.
It is worth pointing out that the two Pseudoroegneria and
eight StY genome Elymus species from the central and eastern
Asia consistently had an 8-bp deletion in their ITS sequences
and clustered distinctly into one subclade. It is most likely that
the 8-bp deletion had already occurred in the Pseudoroegneria
species before being passed on to some Elymus species during
the polyploidization process. If this assumption holds true,
the St genome in Elymus should have been derived from more
than one Pseudoroegneria species/populations through hybridization. In other words, Elymus species with the 8-bp deletion
in the ITS sequences may have evolved from hybridization of
the Pseudoroegneria ancestor with the 8-bp deletion, whereas
other Elymus species without the 8-bp deletion might originate
from hybridization of other Pseudoroegneria ancestors. This
could also explain the rich diversity and wide adaptation of
the StY genome Elymus species (Lu, 1994).
The maternal donor of Elymus species
The trnL-F gene tree represents a maternal genealogy of the
Elymus species, because the chloroplast genome is maternally
inherited in grasses (Ge et al., 1999; Mason-Gamer et al., 2002).
This offers an opportunity to identify the maternal parents
of the Elymus species. All polyploid species including the
Pseudoroegneria and Elymus species formed a highly supported
monophyletic group (100% and 92% bootstraps for MP
and NJ trees, respectively) on the maternal trnL-F tree. This
suggests that the Pseudoroegneria species (St genome) served
as the maternal donor during the polyploid speciation of the
Elymus species (tetraploids and hexaploids). This result, in
conjunction with the biparentally inherited ITS tree (Fig. 2),
implies that diploid Hordeum (H genome), Agropyron (P) and
Australopyrum (W) species were the paternal parents for the
Elymus species with the H, P and W genomes. This conclusion
is in good agreement with previous studies by McMillan & Sun
(2004) and Mason-Gamer et al. (2002) using restriction fragment
length polymorphism (RFLP) analysis of cpDNA and chloroplast
DNA sequence data, where Pseudoroegneria was suggested as
the chloroplast genome donor of the northern American StH
genome Elymus and two StY genome species. A recent study
based on analysis of partial trnL-F sequences of a few Elymus
species by Xu & Ban (2004) also presented similar results.
The phylogenetic tree based on trnL-F sequences of the
Elymus species also indicated a multiple origin of polyploids
in the evolutionary process of some Elymus species. For example,
different accessions of Elymus longearistatus were clustered in
the different subclades of the St clade. In addition, all Elymus
species used in this study were scattered in different subclades
in the trnL-F tree, which may also suggest different maternal
lineages of the polyploid genus. Conversely, in the ITS tree,
at least two hybridization events involving Elymus caninus
(StH) and Elymus himalayanus (StHY) occurred to form these
polyploids with the H genome species because they appeared
in different subclades of the St clade (E. caninus in the Asubclade and E. himalayanus in the B-subclade). In addition,
two hybridization events involving Elymus melantherus (StPY),
Elymus rigidulus (StPY), and Elymus rectisetus (StWY) can be
identified for generating the polyploid species with the P and
W genomes. Therefore, nuclear DNA and cpDNA sequences
showed that many Elymus species had a multiple origin and
experienced recurrent hybridization between species with
different genomes. This suggests that hybridization and
polyploidization were the major driving force in the diversity
and evolution of the genus Elymus.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Grant no. 30270092) and the Program
for Key International S & T Cooperation Project of China (Grant
no. 2001CB711103). Prof. M. E. Barkworth of the Utah State
University, USA, provided critical comments. Dr N. Clarke of
the Norwegian Forest Research Institute, Norway, kindly edited
English of the manuscript.
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