Taxonomy, phylogeny, and distribution of Puccinia graminis, the black stem rust: New insights based on rDNA sequence data

Abstract

Puccinia graminis (Uredinales) is an economically important and common host-alternating rust species on Berberidaceae/Poaceae (subfamilies Pooideae and Panicoideae) that has been spread globally by human activities from an unknown center of origin. To evaluate the taxonomic implications, phylogenetic relationships, and distribution/spread of this complex species, we sequenced and cladistically analyzed the ITS1, 5.8S, and ITS2 regions from herbarium specimens on various host plants from Iran (17), Europe (1), and North America (4). The ITS region plus the 5.8S gene ranged from 686 to 701 bp, including the flanking partial sequences of the 18S and 28S rDNA. Our phylogenetic analysis included 54 bp of the 18S sequence, the entire ITS1 + 5.8S + ITS2, and 58 bp of the 28S sequence. A second analysis used only the last 42 bp of ITS1, and all the 5.8S and ITS2, to incorporate data from additional sequences downloaded from GenBank. In addition to variation in sequence length, there was variation in sequence content. The analysis does not support classical morphology-based taxonomic concepts of the P. graminis complex. Also, host range, host taxonomy, and geographic origin provide minor information on taxonomic relationships. Puccinia graminis is most probably monophyletic. Coevolutionary aspects can hardly be discussed because of lack of sequence data from alternate host specimens. The occurrence of unrelated fungal taxa on the same host species suggests that, besides coevolution with the host, host jumps and hybridization may have played an important role in the evolution of P. graminis. From rDNA data we conclude that the pathogen was introduced to North America at least twice independently. For a new taxonomic concept, we think the complex has to be split into at least two species. New morphological features and further features other than sequence data, however, must be checked for taxonomic value first and, if necessary, be considered.

Full text

Mycoscience (2005) 46:241–247 © The Mycological Society of Japan and Springer-Verlag Tokyo 2005
DOI 10.1007/s10267-005-0244-x
FULL PAPER
Mehrdad Abbasi · Stephen B. Goodwin · Markus Scholler
Taxonomy, phylogeny, and distribution of
Puccinia graminis
, the black stem
rust: new insights based on rDNA sequence data
Received: January 24, 2005 / Accepted: April 25, 2005
Abstract Puccinia graminis (Uredinales) is an economi-
cally important and common host-alternating rust species
on Berberidaceae/Poaceae (subfamilies Pooideae and
Panicoideae) that has been spread globally by human activi-
ties from an unknown center of origin. To evaluate the
taxonomic implications, phylogenetic relationships, and dis-
tribution/spread of this complex species, we sequenced and
cladistically analyzed the ITS1, 5.8S, and ITS2 regions from
herbarium specimens on various host plants from Iran (17),
Europe (1), and North America (4). The ITS region plus the
5.8S gene ranged from 686 to 701bp, including the flanking
partial sequences of the 18S and 28S rDNA. Our phyloge-
netic analysis included 54bp of the 18S sequence, the entire
ITS1 + 5.8S + ITS2, and 58bp of the 28S sequence. A second
analysis used only the last 42bp of ITS1, and all the 5.8S and
ITS2, to incorporate data from additional sequences down-
loaded from GenBank. In addition to variation in sequence
length, there was variation in sequence content. The analy-
sis does not support classical morphology-based taxonomic
concepts of the P. graminis complex. Also, host range, host
taxonomy, and geographic origin provide minor informa-
tion on taxonomic relationships. Puccinia graminis is most
probably monophyletic. Coevolutionary aspects can hardly
be discussed because of lack of sequence data from alter-
nate host specimens. The occurrence of unrelated fungal
taxa on the same host species suggests that, besides coevo-
lution with the host, host jumps and hybridization may have
played an important role in the evolution of P. graminis.
From rDNA data we conclude that the pathogen was intro-
M. Abbasi
Plant Pests and Diseases Research Institute, Tehran, Iran
S.B. Goodwin
Crop Production and Pest Control Research Unit,
USDA-Agricultural Research Service; Department of Botany and
Plant Pathology, Purdue University, West Lafayette, IN, USA
M. Scholler (*)
Staatliches Museum für Naturkunde, Abteilung Botanik, D-76133
Karlsruhe, Germany
Tel. +49-721-175-2810; Fax +49-721-175-2884
e-mail: scholler@naturkundeka-bw.de
duced to North America at least twice independently. For a
new taxonomic concept, we think the complex has to be
split into at least two species. New morphological features
and further features other than sequence data, however,
must be checked for taxonomic value first and, if necessary,
be considered.
Key words Coevolution · Collective species · Species
concept · Taxonomy
Introduction
The black stem rust, Puccinia graminis Pers. (Uredinales),
is a common heteroecious species with plant species of
Berberidaceae as aecial hosts and members of the Poaceae
as telial hosts. Cummins (1971) lists hosts of 77 genera of
Poaceae (primarily in subfamily Pooideae but also a few in
the Panicoideae) containing species that harbor P. graminis.
In addition, more than 70 species of Berberis and some
of Mahonia are listed as aecial hosts (Gäumann 1959;
Cummins 1971; Anikster and Wahl 1979). Puccinia
graminis is a complex species consisting of numerous bio-
logically specialized formae speciales, and has been divided
into infraspecific taxa that differ mainly in urediniospore
length. Urban’s (1967) morphological species concept is
generally acknowledged as definitive. For example, Abbasi
et al. (2002) classified specimens according to urediniospore
length and also found differences in the number of germ
pores. Urban further divides P. graminis into the two sub-
species P. graminis subsp. graminis and P. graminis subsp.
graminicola Z. Urb. The first subspecies infects mainly cul-
tivated cereals and related hosts, whereas the latter occurs
primarily on wild grasses. Urban also divides the type sub-
species into two varieties, namely P. graminis subsp.
graminis var. graminis (mainly on Triticum, Aegilops,
Elymus) and P. graminis subsp. graminis var. stakmannii
A.L. Guyot et al. (mainly on Avena, Hordeum, Secale).
Because of overlapping sizes of urdiniospores and subse-
quent problems in delimiting the infraspecific taxa, recent

242
authors have considered additional characters and tech-
niques such as germling morphology (Swertz 1994), isozyme
banding patterns (Burdon and Marshall 1981; Swertz 1994),
and DNA sequence data (Zambino and Szabo 1993). Those
studies, however, included only a small part of the full range
of genetic variation present within the species so were not
sufficient to propose new taxonomic concepts.
Zambino and Szabo (1993) analyzed the internal tran-
scribed spacer (ITS) region of the ribosomal DNA and
concluded that the P. graminis is monophyletic, but that
study was not comprehensive enough (only 13 specimens,
mainly from the United States, were sequenced) to draw
further conclusions on the phylogeny and (co-)evolution of
this pathogen. According to most authors (Anikster and
Wahl 1979; Leppik 1961, 1970; Savile and Urban 1982;
Urban and Markova 1983, 1984), Asia or North Africa
(Central Asia, the Middle East, Near East, Ethiopia,
Azerbaijan, or Iran depending on the author) is the origin
from which the pathogen has been introduced to other con-
tinents by man. This assumption is based mainly on the fact
that these regions have floras with a high diversity of poten-
tial telial and aecial hosts (i.e., grasses in the Pooideae and
members of the Berberidaceae, respectively). In the follow-
ing, we provide a phylogenetic study based on rDNA se-
quence data of 28 specimens (6 of which were downloaded
from GenBank) on 19 different host species to obtain more
information about phylogeny, evolutionary aspects, and
spread of P. graminis. Furthermore, it should help to pro-
vide better arguments for a future taxonomic revision of the
complex species.
Materials and methods
The samples used for DNA sequencing were 17 herbarium
specimens collected by M. Abbasi in Iran during the past
15 years (voucher specimens are deposited in IRAN and
further duplicates in PUR), 1 specimen from Germany,
and 4 from the USA (Table 1). DNA was purified from
dried herbarium material by the extraction protocol of
Taylor and Swann (1994) and by the grinding method. For
the latter method, spores (teliospores, urediniospores, or
aeciospores) were scraped from the herbarium specimens
and suspended in 50ml low ethylenediaminetetraacetic acid
(EDTA) TE (0.089 M Tris base, 0.045 M boric acid, 0.05mM
EDTA) + 1% 2-mercaptoethanol buffer in 1.5-ml plastic
tubes and ground with a mini-pestle mounted in an electric
drill. The complete ITS region (3¢-end of the 18S rRNA
gene, ITS1, 5.8S rRNA gene, ITS2, and 5¢-end of the 28S
rRNA gene) of each specimen was amplified with primers
ITS4 and ITS5 of White et al. (1990). Amplification was as
described by Zambino and Szabo (1993) with the following
cycling parameters: 40 cycles of 94°C for 30s, 50°C for
1min, 72°C for 2min, and a final extension of 10min at
72°C. The size and quantity of amplification products were
Table 1. Specimens of Puccinia graminis included in the sequence analysis
Reference Host Locality Length of Herbarium Infraspecific GenBank
number the ITS number classification accession
region (bp) (Urban 1967; number
Abbasi et al. 2002)
3Berberis sp. Iran, Dizin 695 IRAN 11459F nd AY874145
5Triticum aestivum Iran, Shahrud 696 IRAN 9803F gg AY874142
11 Elymus hispidus Iran, Alburz, Dizin 701 IRAN 11082F gg AY874148
12 Elymus libanoticus Iran, Alburz, Dizin 700 IRAN 11083F gs AY874135
13 Elymus elongatiformis Iran, Alburz, Dizin 695 IRAN 11084F gs AY874136
14 Elymus elongatiformis Iran, Alburz, Khor 701 IRAN 11089F gg AY874134
20 Triticum aestivum Iran, Shavur 697 IRAN 10839F gg AY874146
26 Aegilops crassa Iran, Bakhtaran 701 IRAN 6646F gg AY874138
29 Avena sativa Iran, Gorgan 695 IRAN 6832F gg AY874137
31 Poa trivialis Iran, Javaherdeh 694 IRAN 11086F gr AY874147
33 Taeniathrum crinitum Iran, Golestan 696 IRAN 9261F gs AY874144
National Park
34 Boissiera squarrosa Iran, Golestan 695 IRAN 9260F gs AY874143
National Park
35 Eremopyrum distans Iran, Golestan 699 IRAN 9258F gs AY874140
National Park
39 Hordeum spontaneum Iran, Golestan 697 IRAN 11087F gs AY874133
National Park
40 Leucopoa sclerophylla Iran, Mount Saluk 695 IRAN 10838F gs AY874141
96 Bromus tomentosus Iran, Kelardasht 695 IRAN 11088F gs AY874139
105 Avena ludoviciana Iran, Amarlu 686 IRAN 11453F gs AY874149
SC Secale cereale Germany, Mecklenburg-Vorpommern 701 PUR N1125 gs AY874151
CDL Triticum aestivum USA 697 — nd AY114289
PGI Triticum aestivum USA, Arkansas 698 PUR 89472 gg AY874153
PGII Triticum aestivum USA, Indiana 697 PUR gg AY874154
PGIII Poa pratensis USA, Indiana 694 PUR N1189 gr AY874155
nd, not determined; ITS, internal transcribed spacer; gg, P. graminis subsp. graminis var. graminis; gs, P. graminis subsp. graminis var. stakmanii;
gr, P. graminis subsp. graminicola

243
verified on 1% agarose gels. DNA bands of sufficient quan-
tity and of the expected size were excised from the gels, and
the DNA was purified with the GeneClean spin kit (BIO
101, Vista, CA, USA) according to the manufacturer’s in-
structions. Purified products were quantified with a Hoefer
DyNAQuant 2000 fluorometer (Hoefer, San Francisco,
CA, USA) and cloned with the TA cloning kit (Invitrogen,
Carlsbad, CA, USA). The presence of inserts was
confirmed by digestion with EcoRI and agarose electro-
phoresis. Plasmid DNA was prepared with the Wizard
miniprep kit (Promega, Madison, WI, USA), and DNA
concentration was estimated with a fluorometer. DNA
samples were prepared for sequencing with the Thermo-
Sequenase fluorescent labeled primer cycle sequencing kit
(Amersham Pharmacia Biotech) and sequenced on an
ALFexpress automated DNA sequencer (Amersham
Pharmacia Biotech) as described by Goodwin and Zismann
(2001). Each clone was sequenced in both directions
with the M13 reverse and M13-40 primers. For the majority
of specimens, more than one clone was sequenced to mini-
mize errors caused by polymerase chain reaction (PCR)
amplification.
DNA sequences were aligned with the profile mode of
Clustal X 1.81 (Thompson et al. 1997) with default settings
as described elsewhere (Goodwin et al. 2001), and were
edited manually when necessary. Following alignment, ge-
netic distances among all isolates were calculated and
neighbor-joining trees were prepared with the Draw N-J
Tree option of Clustal X. This option uses Kimura’s two-
parameter method for estimating evolutionary distances
(Kimura 1980) and the neighbor-joining algorithm of Saitou
and Nei (1987). Bootstrap analyses (1000 replications)
were performed on the resulting trees with the Bootstrap
N-J Tree option of Clustal X, and the final trees were
visualized and printed with Njplot (Perrière and Gouy
1996).
Two analyses were performed. The first was on the com-
plete ITS region of the 22 herbarium specimens listed in
Table 1 plus single representatives of three related species
as outgroups. In the second analysis, the ITS database was
augmented with six sequences representing different
formae speciales of P. graminis that were downloaded from
GenBank (Table 2). Because sequences downloaded from
GenBank included only the last 42 bases at the 3¢-end of
ITS1, the second analysis used only the alignable data com-
mon to all specimens (i.e., the last 42bp of ITS1 plus the
complete 5.8S and ITS2 sequences). For both analyses, se-
quences of P. striiformis (GenBank AY874152) and P.
recondita (GenBankAY880845) from Triticum aestivum
and of P. hordei (GenBankAY874150) from Hordeum
vulgare were used as outgroups. All outgroup specimens
were new sequences obtained as described above from
herbarium material collected in Iran.
Morphological analysis of the specimens and their iden-
tification to subspecies and variety according to the criteria
of Urban (1967) were published previously (Abbasi et al.
2002). Infraspecific classification could not be determined
for sequences downloaded from GenBank because the
original specimens were not available for examination.
Infraspecific classification also could not be determined
for the specimen from Berberis sp. because it included only
the aecial state and, therefore, did not include the
urediniospores required for morphological analysis (see
Table 1).
Results
Nucleotide sequence analysis of the ITS region of P.
graminis revealed a high level of molecular variation for
both sequence length and content. The boundaries of the
internal transcribed spacers ITS1 and ITS2 were deter-
mined by comparison with several published sequences in
the rust fungi. The complete amplified region ranged from
686 to 701bp, including flanking partial sequences of the
18S (54bp) and 28S (58bp) rDNA. Therefore, the length of
the ITS region itself ranged from 574 to 589bp. As ex-
pected, sequences of the 5.8S gene were highly conserved
among the 22 specimens, whereas those for ITS1 and ITS2
exhibited polymorphisms due to base substitutions, inser-
tions, or deletions of up to 36 nucleotides. Both ITS1 and
ITS2 contained phylogenetically informative sites. Two in-
formative sites were also found near the 5¢-end of the 18S
sequence.
Neighbor-joining analyses of the entire ITS1+5.8S+ITS2
region of the 22 specimens plus the three outgroup species
revealed that P. graminis as a whole is monophyletic (Fig.
1). However, the species was separated clearly into three
clades, each with bootstrap support of 96% or higher. Clade
1 contained specimens from a wide array of wild hosts that
were collected only in Iran (Fig. 1). This clade also con-
tained specimens from cultivated hosts (oats and wheat) as
well as the aecial-stage sample from Berberis sp., confirming
that Berberis is an alternate host for members of this clade.
Specimen 26 from Aegilops crassa was clearly distinct, but
Table 2. DNA sequencesa of Puccinia graminis downloaded from GenBank
GenBank
accession no. Host Country, state formae speciales
L08696 Avena sativa USA f. sp. avenae
L08698 Dactylis glomerata USA, Minnesota f. sp. dactylidis
L08699 Lolium perenne USA, Minnesota f. sp. lolii
L08701 Poa pratensis USA, Minnesota f. sp. poae
L08703 Secale cereale USA, New York f. sp. secalis
L08708 Triticum aestivum USA, Kansas f. sp. tritici
aIncludes the last 42bp of ITS1 and all the 5.85 and ITS2 region

244
also clustered with the other members of clade 1 in this
analysis (Fig. 1).
Clade 2 contained specimens from rye and wheat from
Germany and the USA, respectively, plus specimens from
Eremopyrum distans and two species of Elymus from Iran
(Fig. 1). Thus, clade 2 contained specimens from both culti-
vated and wild hosts spanning three continents. All the
hosts infected by members of this clade were in the tribe
Triticeae.
Clade 3 contained two specimens, one from wild Avena
in Iran and the other from Poa pratensis in the USA (Fig. 1).
Therefore, members of this clade also occurred on at least
two continents.
The same three clades were obtained when the analysis
was augmented with sequences from six formae speciales
obtained from the USA (Fig. 2). Two of the additional
specimens (formae speciales secalis and tritici) clustered
within clade 2, while the remaining four specimens clus-
tered within clade 3. Clade 1 remained composed solely of
specimens from Iran that were collected from four host
tribes. Bootstrap values for each clade in the second analy-
sis remained high and ranged from 79% for clade 2 to 100%
Fig. 1. Phylogram from neighbor-joining analysis of Puccinia graminis
DNA data including some cereal rusts as an outgroup. The topology
and bootstrap analysis were based on the entire internal transcribed
spacer (ITS)1, 5.8, and ITS2 regions. P. g., Puccinia graminis; P. s.,
Puccinia striiformis; P. h., Puccinia hordei; P. r., Puccinia recondita; gg,
P. graminis subsp. graminis var. graminis; gs, P. graminis subsp.
graminis var. stakmanii; gr, P. graminis subsp. graminicola; A, Avenae;
B, Bromeae; P, Poeae; T, Triticeae

245
for clade 3. The slightly lower bootstrap values for clades 1
and 2 compared to those in Fig. 1 probably occurred be-
cause polymorphic sites at the 3¢-end of the 18S gene and
the first half of ITS1 were excluded from the second analy-
sis. Tree topology was almost identical between the two
analyses except for the position of specimen number 26,
which clustered as a sister to clade 1 in the first analysis but
as a sister to clade 2 in the second analysis.
There was no correlation between the three clades iden-
tified by the ITS sequences and morphology-based infraspe-
cific taxa, as all three infraspecific taxa were distributed
widely among the three clades. For example, clade 1 con-
tained representatives of all three infraspecific taxa, clade 2
contained P. graminis subsp. graminis varieties graminis
and stakmanii, and clade 3 contained P. graminis subsp.
graminis and P. graminis subsp. graminicola.
Discussion
Phylogeny
This analysis of diverse specimens from three continents
confirms the conclusion of Zambino and Szabo (1993) that
Fig. 2. Phylogram from neighbor-joining analysis of Puccinia graminis
DNA data including some cereal rusts as an outgroup. The topology
and bootstrap analysis were based on the last 42bp of the ITS1 plus the
entire 5.8S and ITS2 regions. P. g., Puccinia graminis; P. s., Puccinia
striiformis; P. h., Puccinia hordei; P. r., Puccinia recondita; gg, P.
graminis subsp. graminis var. graminis; gs, P. graminis subsp. graminis
var. stakmanii; gr, P. graminis subsp. graminicola; A, Avenae; B,
Bromeae; P, Poeae; T, Triticeae

246
P. graminis is monophyletic. A high bootstrap value (100%)
separates P. graminis clades from the three species of grass
rusts used as outgroups. The results also confirmed the util-
ity of herbarium specimens for phylogenetic analyses of rust
fungi. The complete ITS region was amplified without
difficulty from specimens up to 15 years old, and analysis of
older specimens may be possible if the region is amplified in
smaller, overlapping segments. This method could greatly
expand the range of specimens available for analysis to
include the type specimens on which the morphological
classifications were based.
Although P. graminis as a whole is monophyletic, it
clearly is a compound species with high genetic variability.
There are three different clades supported by high
bootstrap values. Differences between clades 1 and 2 speci-
mens on the one side and clade 3 on the other are particu-
larly high. So, considering clade 1 and 2 as a single clade
would be reasonable as well. The analyses provides some
interesting information on the phylogeny of the complex
species that is not typical for obligate plant parasitic fungi.
Host range and subspecific classification seem not to pro-
vide very much phylogenetically relevant information. In
contrast to many other obligate fungal plant parasites (see
Scholler 1998), related host plants within the P. graminis
complex do not indicate related fungi and vice versa. Triti-
cum aestivum (clades 1, 2) and Avena sativa (1, 3) are even
represented in two different clades (Figs. 1, 2). This result
indicates that there was no continuous coevolution and per-
manent binding to certain host plants; we assume that hy-
bridization (as suggested by Johnson 1949; Johnson et al.
1932; Green 1971) and jumps to different hosts have played
a major role in the evolution of the fungus. Host jumps may
have taken place from a telial host to new telial hosts via
urediniospores, from telial hosts to aecial hosts via
basidiospores, or from an aecial host to a new telial host
via aeciospores. The role of the aecial host in the phylogeny
of P. graminis remains unclear, as it is in other heteroecious
rust fungi. On one hand, P. graminis is facultatively hetero-
ecious (McAlpine 1906: 121), i.e., the fungus does not need
the aecial host for reproduction and survival, but on the
other hand, no sexual recombination is possible without
aecia and the aecial host. This finding indicates that
new hosts may have been “conquered” via aeciospores and
that the Berberis may have played an important role in
phylogenetic history of the rust. Consequently, more speci-
mens on the aecial host need to be studied. Furthermore, it
would be helpful to know more about the host range of a
certain strain before sequencing it. We do not know the host
range of all the strains from Iran, except for the host plant
species on which we found it. As already mentioned,
most of the host plants of P. graminis belong to subfamily
Pooideae, including all material we evaluated for this
study. From the phylogenetic point of view, it would be
interesting to sequence P. graminis of species on
Panicoideae (e.g., on Echinochloa spp. or Setaria spp.;
Cummins 1971) to answer the question whether P. graminis
is polyphyletic (on at least two different subfamilies) or
monophyletic, switching from Pooideae to Panicoideae or
vice versa.
The subspecific taxa delimited mainly using morphologi-
cal data as proposed by Urban (1967) and Abbasi et al.
(2002) are not natural groups and, consequently, do not
reflect the phylogeny of the rust. Although information on
the subspecific placement of eight specimens was not avail-
able, Figs. 1 and 2 provide sufficient evidence for a poly-
phyletic origin (all three taxa occur in at least two different
clades). Later, we discuss this again from the taxonomic
point of view.
Distribution and spread
We evaluated 28 specimens from three different countries
and continents, namely from Asia/Iran (17), from Europe/
Germany (1), and from North America/USA (9). As noted
earlier, P. graminis is introduced in Germany and the
United States but is a native of Iran. In Figs. 1 and 2, Iranian
specimens represent all specimens in clade 1 and are repre-
sented in clade 2 and 3 as well. USA specimens are in clade
2 and 3 and the European (German) specimen clusters in
clade 2. As mentioned, P. graminis is a species of Asiatic
origin and was introduced to other countries (Leppik 1961;
Anikster and Wahl 1979). The question whether Iran is the
center or one of the centers of origin for the black stem rust
(as it assumed for wheat and related species; see Vavilov
1992) cannot be answered yet. Variability of the Iranian
specimens is high, but we have no comparative data from
regions/countries where P. graminis is native as well. Gen-
erally, genetic diversity of a spreading species is always
highest in its geographic origin. In addition, the higher ge-
netic diversity in Iranian specimens may also be a conse-
quence of the number of species and of species on different
host plants studied. The non-Iranian specimens were all
from cereals or forage grasses. Definitely, P. graminis must
have been introduced to North America at least twice inde-
pendently because in this continent we found genetically
strongly different clade 2 and clade 3 specimens.
Taxonomy
Our study shows that P. graminis is a complex species.
Certain morphological characters proposed for subspecific
classification (Urban 1967; Abbasi et al. 2002) do not
represent natural groups and turned out to be polyphyletic.
Interestingly, host range or forma speciales provide no taxo-
nomic information, either. Therefore, a new taxonomic con-
cept is urgently required. There are, however, two major
questions and problems involved with a new concept
(including new or unknown scientific names) of such an
important pathogen.
The first question is whether the complex should be
reclassified just by using sequence data. In our opinion, it
should not. In general we think that especially morphologi-
cal and ontogenetical data should be used to characterize a
taxon, because these features tell us much more about a
species and its biology than a variable sequence of the
rDNA-ITS region as a small and nonfunctional part of
the genome. Particularly, in P. graminis, the “molecular

247
way” only would be hardly accepted because many
nontaxonomists and nonmicrobiologists, such as plant
pathologists, extension people, growers, farmers, etc., who
permanently deal with this fungus and who need a classifica-
tion based on features that can be traced within a short time
and without major technical effort. Therefore, taxonomists
should invest some time and resume looking for additional
morphological or ontogenetical features starting with those
specimens used for sequencing. Other simply determinable
and taxonomically valuable features such as the production
of taxon-specific chemicals (e.g., for carotene type and con-
tents in rust fungi; see Zwetko and Pfeifhofer 1991) also
could be tested for the P. graminis complex. Second, into
how many species (or subspecies) should the complex be
split? This question cannot be answered yet. As mentioned,
we cannot base a classification on sequence data only and
urgently need further non-molecular data. Based on the
present rDNA sequence data available, it seems reasonable
to split P. graminis into at least two species, the first species
consisting of taxa belonging to clade 1 and 2 and the second
species belonging to clade 3. However, even the sequence
data are still not sufficient to carry out major taxonomic
changes. Especially, specimens on Panicoideae and type
specimens should be integrated in the study.
Acknowledgments Studienstiftung Mykologie (Köln, Germany) sup-
ported M. Abbasi for a stay of 6 months at the Botany and Plant
Pathology Department, Purdue University (USA), to carry out mo-
lecular studies and to study grass rust specimens in the Arthur Her-
barium. Special thanks are due to Jessica Cavaletto (West Lafayette)
for technical help, and to Dr. Les Szabo (Saint Paul), who gave us
permission to use the sequence CDL on wheat.
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