Mycologia, 96(6), 2004, pp. 1294–1305.
q 2004 by The Mycological Society of America, Lawrence, KS 66044-8897
Cephalosporium maydis is a distinct species in the GaeumannomycesHarpophora species complex
Amgad A. Saleh1
John F. Leslie2
1962, 1963), is one of the most important fungal diseases in Egypt. This disease also has been reported
from India (Payak et al 1970, Ward and Bateman
1999) and Hungary (Pecsi and Nemeth 1998). C.
maydis reproduces asexually, and no perfect state has
been identified. Saleh et al (2003) and Zeller et al
(2000) showed that the pathogen is clonal in Egypt
and that the Egyptian population contains four lineages, three of which are widely distributed throughout the country.
C. maydis originally was described based on growth
characters and the morphology of hyphae, conidia
and conidiophores. Domsch and Gams (1972) suggested that the conidial state of C. maydis was a Phialophora (the anamorph of Gaeumannomyces Arx &
D. Olivier) and that spore production in C. maydis
was typical of that genus (Ward and Bateman 1999).
Most members of the genus Cephalosporium were
transferred to the genus Acremonium, a genus of hyaline hyphomycetes with aculeate (spine-like) phialides unrelated to either Phialophora or Harpophora,
when Gams (1971) reintroduced Acremonium. Gams
(2000) introduced Harpophora as a new genus (contains anamorphs of Gaeumannomyces and Magnaporthe) that is distinct from Phialophora. Harpophora spp.
are characterized by fast-growing, thin colonies with
sickle-shape conidia. Older hyphae are heavily pigmented, younger hyphae are nearly hyaline and
phialides are intermediate in pigmentation relative
to the older and younger hyphae. When he introduced Harpophora, Gams (2000) also introduced the
new combination Harpophora maydis (Samra, Sabet
and Hingorani) Gams as a replacement for Cephalosporium maydis.
Ward and Bateman (1999) used RFLP hybridization and portions of the rDNA repeat sequence to
associate C. maydis with the Gaeumannomyces species
complex, but their results were insufficient to determine whether C. maydis was a distinct taxon at the
species level and whether C. maydis should be reclassified. The distinguishing morphological characters
available for C. maydis are limited, and applying
them to identify the species is not easy, so we used
DNA sequence-based approaches to help differentiate this species, as has been done with numerous other fungal taxa.
Our objectives in this study were: (i) to determine
Department of Plant Pathology, Throckmorton Plant
Sciences Center, Kansas State University, Manhattan,
Kansas 66506-5502
Abstract: Cephalosporium maydis is an important
plant pathogen whose phylogenetic position relative
to other fungi has not been established clearly. We
compared strains of C. maydis, strains from several
other plant-pathogenic Cephalosporium spp. and several possible relatives within the GaeumannomycesHarpophora species complex, to which C. maydis has
been suggested to belong based on previous preliminary DNA sequence analyses. DNA sequences of the
nuclear genes encoding the rDNA ITS region, b-tubulin, histone H3, and MAT-2 support the hypothesis
that C. maydis is a distinct taxon within the Gaeumannomyces-Harpophora species complex. Based on
amplified fragment length polymorphism (AFLP)
profiles, C. maydis also is distinct from the other tested species of Cephalosporium, Phialophora sensu lato
and members of Gaeumannomyces-Harpophora species complex, which supports its classification as Harpophora maydis. Oligonucleotide primers for H. maydis were developed that can be used in a PCR diagnostic protocol to rapidly and reliably detect and
identify this pathogen. These diagnostic PCR primers
will aid the detection of H. maydis in diseased maize
because this fungus can be difficult to detect and isolate, and the movement of authentic cultures may be
limited by quarantine restrictions.
Key words: AFLP, b-tubulin, Corn, Harpophora
maydis, Histone H3, Phialophora sensu lato, maize,
mating type, rDNA ITS
INTRODUCTION
Late wilt of maize, caused by the fungus Cephalosporium maydis Samra, Sabet & Hingorani (Samra et al
Accepted for publication June 28, 2004.
1 Permanent address: Agricultural Genetic Engineering Research
Institute, Agricultural Research Center, 9 Gamaa Street, Giza,
Egypt.
2 Corresponding author. Department of Plant Pathology, 4002
Throckmorton Plant Sciences Center, Kansas State University, Manhattan, Kansas 66506-5502. Phone: 785-532-1363. Fax: 785-5322414. E-mail: jfl@plantpath.ksu.edu
1294
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1295
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TABLE I. Total number of amplified bands and polymorphic bands in the four AFLP profiles evaluated (EAA/MCA, EAA/
MCC, EAA/MCG, and EAA/MGA)
Species
Number
of strains
C. maydis
C. gramineum
A. strictum
G. cylindrosporus
G. graminis var. avenae
G. graminis var. tritici
G. graminis var. graminisa
Group 1
Group 2
17
6
10
15
8
16
9
4
2
Number of
bands in the four
AFLP profiles
Number of
polymorphic bands
(within species)
68
118
129
103
63
61
25
29
93
92
42
49
70
71
32
44
a
Based on the AFLP data, G. graminis var. graminis strains divided into five groups. Group 1 contained four strains; group
2 contained two strains; and groups 3, 4, and 5 (not shown) each contained only a single strain.
the relatedness of C. maydis to strains representing
the Gaeumannomyces-Harpophora species complex
and other Cephalosporium species, (ii) to assess the
integrity of the species examined and their genetic
relationships and (iii) to develop a rapid PCR method to detect C. maydis. Our working hypothesis is
that C. maydis is a distinct species in the Gaeumannomyces-Harpophora species complex and that new
molecular diagnostics are needed to rapidly and reliably identify this species.
MATERIALS AND METHODS
Fungal strains.—We examined 44 strains from the Acremonium-Cephalosporium species complex and 48 strains from
Gaeumannomyces-Phialophora species complex (TABLE I).
The strains of C. maydis represent the four clonal lineages
found in Egypt (Saleh et al 2003). The other species of
Cephalosporium examined include: (i) Acremonium diospyri
(Crandall) W. Gams (syn., Cephalosporium diospyri Crandall), the causal agent of American persimmon wilt (Halls
1990); (ii) Cephalosporium gramineum Nisikado & Ikata in
Nisikado et al (Hymenula cerealis Ellis & Everh.), the causal
agent of Cephalosporium stripe of winter wheat (Bockus
and Claassen 1985); and (iii) Acremonium strictum W. Gams
(syn., Cephalosporium acremonium Auct. non Corda), the
causal agent of stalk rot and black bundle diseases of maize
or Acremonium wilt of sorghum (Bandyopadhyay et al
1987, Hanlin et al 1978). Three varieties of Gaeumannomyces graminis (anamorph Harpophora spp.) (Sacc.) Arx &
Olivier were examined: (i) G. graminis var. tritici J. Walker
(GGT), the causal agent of take-all of wheat and barley; (ii)
G. graminis var. avenae (E.M. Turner) Dennis (GGA), the
causal agent of take-all disease of oats and which also can
infect barley and cause take-all patch disease of bentgrass
(Dernoeden and O’Neill 1983, Couch 1995); and (iii) G.
graminis var. graminis (GGG), which has a wider host range
than either GGT or GGA, is a weak pathogen of wheat (Bryan et al 1995) and causes Bermudagrass decline (Elliott
1991), take-all root rot of St. Augustine grass (Elliott et al
1993, Wilkinson and Pedersen 1993), crown sheath rot of
rice (Walker 1981) and root rot of centipede grass (Wilkinson 1994). We also examined Gaeumannomyces cylindrosporus D. Hornby, D. Slope, R. Gutteridge & Sivanesan [anamorph Harpophora graminicola (Deacon) W. Gams], associated with root discoloration of Poa pratensis (5 Kentucky
bluegrass) ( Jackson and Landschoot 1986). Two field
strains of Fusarium verticillioides (Sacc.) Nirenberg, causal
agent of stalk and root rot of maize (Leslie et al 1990) were
used as outgroup.
DNA isolation.—Fungal cultures were grown in complete
medium (CM) broth (Correll et al 1987) and incubated on
an orbital shaker (150 rpm) at least 3 d at room temperature (25–28 C). The mycelia were harvested, ground to a
powder with liquid nitrogen in a mortar and pestle and
stored at 270 C until DNA was extracted. Fungal DNA was
isolated by a CTAB method (Murray and Thompson 1980)
as modified by Kerényi et al (1999).
AFLP reactions and data analysis.—AFLP reactions (Vos et
al 1995) were performed as described by Saleh et al (2003)
in a PTC-2000 Thermal Cycler (MJ Research Inc., Watertown, Massachusetts). The AFLP primers used in this study
were EcoRI primer (59-AGACTGCGTACCAATTC-39) followed by two base pairs (AA), abbreviated as EAA, and the
MseI primer (59-GATGAGTCCTGAGTAA-39) followed by
two base pairs (CA, CC, CG, or GA), abbreviated as MCA,
MCC, MCG, and MGA.
AFLP fingerprints were scored manually as ‘‘1’’ for the
presence of a band and ‘‘0’’ for the absence of a band,
assuming that bands with the same molecular size in different individuals were homologous. The Unweighted Pair
Grouping by Mathematical Averages (UPGMA) subroutine
of PAUP* 4.0b10 (Swofford 2000) was used to construct
phylograms (phenograms) and to estimate the genetic similarity among fungal strains of each species.
Conversion of AFLP markers into diagnostic PCR markers for
C. maydis.—AFLP bands that differentiate the lineages of
1296
TABLE II.
Name
ITS3
ITS4
ITS5
H3-1a
H3-1b
T1
T2
T21
ChHMG1
ChHMG2
NcHMG1
NcHMG2
CMaflp11
CMaflp12
a
MYCOLOGIA
PCR primers for nuclear gene fragments sequenced in this study
Nucleotide sequencea
59-GCATCGATGAAGAACGCAGC-39
59-TCCTCCGCTTATTGATATGC-39
59-GGAAGTAAAGTCGTAACAAGG-39
59-ACTAAGCAGACCGCCCGCAGG-39
59-GCGGGCGAGCTGGATGTCCTT-39
59-AACATGCGTGAGATTGTAAGT-39
59-TAGTGACCCTTGGCCCAGTTG-39
59-GGTTTGCCAGAAAGCAGCACC-39
59-AAGGCNCCNCGYCCNATGAAC-39
59-CTNGGNGTGTAYTTGTAATTNGG-39
59-CCYCGYCCYCCYAAYGCNTAYAT-39
59-CGNGGRTTRTARCGRTARTNRGG-39
59-TTTCCTGCGGTGCCAA-39
59-TAATGCGGTTAGCCACTC-39
Gene
Reference
rDNA-ITS
White et al (1990)
Histone H3
Steenkamp et al (1999)
b-tubulin
O’Donnell and Cigelnik
(1997)
MAT-2
Arie et al (1997)
Unknown
This study
Abbreviations: Y 5 C or T; N 5 A, T, C, or G.
C. maydis were cut from the polyacrylamide gels and transferred to 1.5 mL microfuge tubes containing 8 ml of H2O.
These tubes were incubated at 37 C for 1 h (or overnight
at 4 C), and the resulting DNA suspension was used as template DNA for PCR reactions. PCR was performed in a total
volume of 20 ml in the presence of 27 ng of each primer
(EAA and MXX) and 200 mM deoxynuleoside triphosphates (New England Biolabs, Beverly, Massachusetts) in 13
NH4 buffer (Bioline USA Inc., Springfield, New Jersey), 1.5
mM MgCl2, and 0.2 U of Biolaset DNA polymerase (Bioline). The PCR cycling program used to re-amplify these
bands was the same as that used for the original AFLP amplification reactions (Saleh et al 2003). Two ml of the PCR
reaction products were separated in 1.5% agarose in 13
TAE buffer (40 mM Tris-acetate and 1 mM EDTA, pH 8.0).
PCR products were purified with the Wizard DNA Clean
Up kit (Promega, Madison, Wisconsin) to remove unincorporated nucleotides, proteins and other impurities. The
DNA concentration of the final purified PCR products was
determined with an ethidium bromide method (Sambrook
et al 1989) by estimating DNA concentrations relative to
HindIII-digested phage l DNA of known concentration.
The purified DNA products were sequenced with ABI
Prismt BigDyet Terminator Ready Cycle Sequencing Kits
(Applied Biosystems, Foster City, California). Sequencing
reactions were run on an ABI Prismt 3700 DNA Analyzer
at the Kansas State University DNA sequencing facility.
Specific PCR primers were designed based on the sequences of the AFLP fragments. PCR reactions, to test the
utility of the specific primers, were performed in a total
volume of 25 mL containing 0.5 mM of each primer, 200
mM deoxynucleoside triphosphates, 13 NH4 buffer, 2.5 mM
MgCl2, 1 U of Biolaset DNA polymerase, and 25 ng of fungal genomic DNA. The PCR program was one cycle of 94
C for 3 min, followed by 35 cycles of 94 C for 30 s, annealing
temperature (depending on the melting temperature of the
primers) for 30 s, and 72 C for 1 min, followed by a final
extension at 72 C for 5 min. PCR products were separated
on 1.2% agarose gels to assess amplification and reaction
specificity.
PCR amplification and DNA sequencing.—We amplified portions of four nuclear genes (TABLES II and III). Amplification reactions for each locus were performed in a total volume of 10 mL containing 0.25 mM of each primer and 200
mM deoxynucleoside triphosphates, 13 NH4 buffer, 2.5 mM
MgCl2, 0.2 U of Biolaset DNA polymerase, and 10 ng of
fungal genomic DNA. Cycling conditions for primer pairs
ITS3 1 ITS4; and ITS4 1 ITS5 (White et al 1990) were 94
C for 3 min and then 94 C for 1 min, 52 C for 1 min and
72 C for 1 min (35 cycles), followed by a 5 min extension
at 72 C. Cycling conditions for primer pairs T1 1 T2; and
T1 1 T21 (O’Donnnell and Cigelnik 1997) were 94 C for
3 min and then 94 C for 1 min, 63 C for 1 min, and 72 C
for 1 min (35 cycles), followed by a 5 min extension at 72
C. Cycling conditions for primers H3-1a and H3-1b (Steenkamp et al 1999) were 94 C for 3 min and then 94 C for 1
min, 68 C for 1 min, and 72 C for 1 min (35 cycles), followed by a 5 min extension at 72 C. Cycling conditions for
degenerate primers NcHMG1 and NcHMG2, and ChHMG1
and ChHMG2 (Arie et al 1997) were 94 C for 2 min and
then 94 C for 1 min, 55 C for 1 min, and 72 C for 1 min
(30 cycles), followed by a 5 min extension at 72 C. For
diagnostic purposes, the cycling conditions for primers
CMaflp11 and CMaflp 12 were 94 C for 3 min and then 94
C for 1 min, 67 C for 30 s, and 72 C for 45 s (35 cycles),
followed by a 5 min extension at 72 C. After PCR, amplified
DNA was quantified on agarose gels, purified with the Wizard PCR purification system (Promega), and sequenced as
described above.
Phylogenetic analysis.—DNA sequences were edited and
aligned with the Clustal W algorithm (Thompson et al
1994) as implemented in the program BioEdit (http://
www.mbio.ncsu.edu/BioEdit/bioedit.html). Final alignments were optimized visually. Intron/exon junctions in the
b-tubulin and histone H3 sequences were identified by
SALEH
TABLE III.
AND
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GAEUMANNOMYCES
GenBank accession numbers and TreeBank alignment numbers for sequences used in this study
Species
(KSU/ATCC)a
b-tubulin
rDNA ITS
Histone H3
MAT-2
C. maydis
diagnostic
AY435435
AY435436
AY435437
AY435438
AY428788
AY428787
AY428786
AY428785
AY435458
AY435459
AY435460
AY435461
NA
NA
AY435482
NA
AY435487
AY435488
AY435489
AY435490
C. gramineum
KSU 14436/MYA-3360
KSU 14437/MYA-3361
NA
NA
AY428791
AY428792
AY435462
NA
NA
NA
NA
NA
A. diospyri
KSU 14438/MYA-3362
AY435439
AY428793
AY435463
NA
NA
A. strictum
KSU 5144/MYA-3363
KSU 5147/MYA-3384
AY435440
AY435441
AY428789
AY428790
AY435464
AY435465
NA
NA
NA
NA
F. verticillioides
KSU 4773
KSU 5146
AY435443
AY435442
AY428795
AY428794
AY435467
AY435466
NA
NA
NA
NA
G. cylindrosporus
KSU 14447/MYA-3385
KSU 14448/MYA-3386
KSU 14913
KSU 14914
KSU 14916
KSU 14919
AY435445
AY435444
AY435446
AY435447
AY435448
AY435449
AY428772
AY428771
AY428773
AY428774
AY428775
AY428776
AY435469
AY435468
AY435470
AY435471
AY435472
AY435473
NA
AY435483
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
G. g. avenae
KSU 14439/MYA-3367
KSU 14440/MYA-3368
AY435450
AY435451
AY428777
AY428778
AY435474
AY435474
AY435484
NA
NA
NA
G. g. tritici
KSU 14441/MYA-3369
KSU 14449/MYA-3370
AY435452
AY435453
AY428783
AY428774
AY435476
AY435477
AY435485
NA
NA
NA
G. g. graminis
KSU 14445/MYA-3373
KSU 14442/MYA-3371
KSU 14443/MYA-3372
KSU 14446/MYA-3374
Alignment
AY435454
AY435455
AY435456
AY435457
SN1643
AY428779
AY428780
AY428781
AY428782
SN1646
AY435478
AY435479
AY435480
AY435481
SN1647
AY435486
NA
NA
NA
SN1649
NA
NA
NA
NA
C. maydis
KSU 10793/MYA-3357
KSU 10800/MYA-3358
KSU 14435/MYA-3359
KSU 10792/MYA-3356
(Lineage I)
(Linage II)
(Lineage III)
(Lineage IV)
NA—No amplification
a Strain numbers from the collections at the Department of Plant Pathology, Kansas State University (Manhattan, Kansas),
and the American Type Culture Collection (Manasas, Virginia).
aligning these sequences with the known F. verticillioides sequences of b-tubulin (GenBank accession number U34413)
and histone H3 (GenBank accession nnumber AF150859)
genes (O’Donnnell and Cigelnik 1997, Steenkamp et al
1999). Phylogenetic analyses of aligned DNA sequences
were performed with PAUP* version 4.0b10 (Swofford
2000). The heuristic search option was used to infer maximum parsimony trees. Clade stability was assessed by 1000
bootstrap replications (Felsenstein 1985, Hillis and Bull
1993) calculated from PAUP trees. Other measures, including tree length, consistency index (CI) and retention index
(RI), were calculated with PAUP* 4.0b10 (Swofford 2000).
Phylogenies were inferred from each of the three genes
individually and then for the combined data for the three
genes.
RESULTS
Comparison of AFLPs from C. maydis and related fungal species.—We used four AFLP primer pairs to assess
the relatedness of C. maydis to the other fungal spe-
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MYCOLOGIA
TABLE IV. Pairwise average genetic similarity within and between strains of species entities used in this study based on
AFLPs. Number of polymorphic bands used to generate these values was 411
Average of genetic similarity
CMa (27)b
CG (6)
AS (10)
AD (1)
GC (15)
GGA (8)
GGG (9)
GGT (16)
FV (1)
CM
CG
AS
AD
GC
GGA
GGG
GGT
FV
0.90
0.04
0.01
0.0
0.01
0.03
0.02
0.04
0.0
0.94
0.11
0.0
0.01
0.04
0.01
0.0
0.01
0.69
0.01
0.02
0.02
0.05
0.01
0.06
—c
0.04
0.06
0.08
0.18
0.0
0.60
0.03
0.02
0.0
0.07
0.74
0.01
0.18
0.0
0.15
0.02
0.02
0.85
0.0
—c
a
The letters in the strain name indicate the species name: (AD) A. diospyri, (AS) A. strictum, (CG) C. gramineum, (CM)
C. maydis, (GC) G. cylindrosporus, (GGA) G. graminis var. avenae, (GGG) G. graminis var. graminis, (GGT) G. graminis var.
tritici, and (FV) F. verticillioides.
b
Number of strains.
c Not calculated since only a single strain was analyzed.
cies. Each species had a distinctive AFLP profile. All
strains from the same species shared . 11% of the
bands (i.e., monomorphic bands, presumed to be
species specific), while strains from different species
had no bands common to all isolates (TABLE I). The
three varieties of G. graminis each had distinct AFLP
profiles that were no more similar to one another
than they were to other species examined. GGG
strains were the most diverse and produced five different AFLP patterns that shared almost no bands,
even with each other. Overall, the highest percentage
of shared bands (75%) occurred within C. gramineum (TABLE I).
Pairwise average genetic similarities within and between species were analyzed on the basis of AFLP
profiles, generated from two primer pairs (EAA/
MCC and EAA/MCG), for the 93 strains used in this
study (TABLE IV). Four hundred eleven polymorphic
bands were generated by these two primer pairs. The
highest average similarity was seen within C. gramineum (0.94), while the lowest was within GGG (0.15).
The average genetic similarity between strains from
different species generally was very low (TABLE IV).
The highest average genetic similarity was between
GGA and GGT and between GGT and AD (0.18).
The uniqueness of the clade for each species was supported with a bootstrap value $88% (FIG. 1). The
distinctness of the four lineages within C. maydis was
supported with bootstrap values $69%. The clade
that included strains from C. maydis and Gaeumannomyces-Harpophora species complex was supported
with a bootstrap value of 89%. The strains of GGT
grouped into two clusters, one contained Kansas
strains while the other contained a strain
(KSU14449) from Oregon. When the strains of GGA
and GGT were analyzed together, the KSU14449
strain formed a separate clade in the GGA cluster.
The strains of GGG divided into five groups, two of
which were supported with bootstrap values of 94 and
88%, while the other three groups were each represented by a single isolate. The first group contained
four strains isolated from Bermudagrass from Florida, and the second contained two strains, one from
Florida and the other from Missouri. The third,
fourth and fifth groups each contained only one
strain, isolated from rice, soybean and St. Augustine
grass, respectively.
Comparison of DNA sequences from C. maydis and related fungal species.—AFLP data were used to select
representative strains from each species from which
portions of four unrelated nuclear genes (rDNA, btubulin, histone H3 and MAT-2) were amplified and
sequenced (TABLES III and V). The exon sequences
of these genes were alignable across all species. Alignments of the intron sequences were not obvious for
the b-tubulin and histone H3 genes. However, ITS1
and ITS2 were alignable across all the species. Thus,
intron sequences of the b-tubulin and the histone H3
genes were excluded from the DNA alignments, and
the presence/absence of introns was used as a fifth
character state in the alignments.
rDNA ITS.—Amplification of the rDNA-ITS regions
yielded fragments ranging in length from 528 to 576
bp (TABLES III and V), with a total of 25 variable sites
throughout the exon across all isolates. No nucleotide variation in the rDNA exons was detected within
a species. The total number of variable nucleotide
sites across the entire region was 295, with 271 of
these sites being phylogenetically informative. A
SALEH
AND
LESLIE: GENETIC
RELATIONSHIPS IN
neighbor-joining analysis based on rDNA ITS sequences identified two major clades. C. gramineum
was used as outgroup for this analysis because it was
the most distant taxon. The first clade received 100%
bootstrap support and contained strains identified as
C. maydis and Gaeumannomyces spp. The second
clade, which received 99% bootstrap support, contained strains belonging to two other species in the
Acremonium-Cephalosporium complex and F. verticillioides. The clade containing the GGA strains and isolate KSU14449 received 99% bootstrap support. Each
fungal species clade had 100% bootstrap support.
The maximum parsimony analysis produced 18
equally parsimonious trees (tree length 5 520 steps,
CI 5 0.81, and RI 5 0.91) that differed from one
another only in the branching order within the clade
containing the G. graminis strains. These most parsimonious trees were similar in topology to that of
the neighbor-joining trees.
b-tubulin.—Amplification of the b-tubulin region
yielded fragments ranging from 564 to 819 bp in
length (TABLES III and V). The exon sequences of
the b-tubulin gene were alignable across all the species, with 71 variable sites in the exon, 61 of which
were phylogenetically informative. The b-tubulin region from C. gramineum strains did not amplify with
the T1, T2, and T21 primers under the tested PCR
reaction conditions. A neighbor-joining analysis
based on partial b-tubulin gene sequences gave almost exactly the same results as those obtained with
the rDNA sequences. C. maydis again was located in
the Gaeumannomyces-Harpophora species complex
with 94% bootstrap support. The bootstrap support
of the clade containing the remaining strains in the
Acremonium-Cephalosporium complex and F. verticillioides was relatively low (58%). The maximum parsimony analysis resulted in 12 most parsimonious
trees (tree length 5 114 steps, CI 5 0.71 and RI 5
0.88) that differed from one another only in the
branching order within the clade containing the G.
graminis strains. These most parsimonious trees were
similar in topology to the neighbor-joining tree produced from the b-tubulin sequences.
Histone H3.—The amplification of the histone region
yielded fragments ranging from 398 to 525 bp in
length (TABLES III and V). The histone H3 gene sequence was the most variable of those examined. As
with the b-tubulin gene, the histone H3 exon sequences were alignable but not the introns. The total
number of variable nucleotide sites in the exon regions was 82, with 63 being phylogenetically informative sites. Again C. gramineum was the most genetically distant of the tested species and was used as
outgroup. Branching orders observed in the neigh-
GAEUMANNOMYCES
1299
bor-joining and maximum parsimony trees were not
as strongly supported by the bootstrap analysis as they
were for the other loci examined, although the clade
containing strains of Gaeumannomyces sp. and C.
maydis received 69% bootstrap support.
Mating type.—The NcHMG degenerate primers amplified DNA fragments ranging from 214 to 216 bp
in length from one strain each of C. maydis, G. cylindrosporus and three varieties of G. graminis. No amplification was detected when the ChHMG primers
were used. The GGA and GGT fragment sequences
were identical. The pairwise comparisons of nucleotide sequence similarity for this region between C.
maydis and G. cylindrosporus, GGA/GGT and GGG
were 75, 79 and 76%, respectively. When the MAT-2
DNA sequences were translated into amino acid sequences, they aligned very well with MAT-2 sequences (Treebase accession number SN1649) from other
Ascomycetes in the GenBank database (e.g., Magnaporthe grisea accession number BAC65094). Based on
these alignments the MAT-2 HMG box of C. maydis
and its allied species have a novel intron that has not
yet been reported from any of the other Ascomycete
MAT-2 sequences available in GenBank.
Combined analysis of rDNA ITS, b-tubulin, and histone
H3.—The maximum parsimony phylogeny inferred
from the combined datasets (806 steps) is shorter
than the sum of the tree lengths for each of the individual datasets (840 steps). These data sets consequently can be combined (Farris et al 1994). Again
C. maydis was in the same clade (received 100% bootstrap support) as the Gaeumannomyces-Harpophora
strains (FIG. 2). The clade containing strains from
GGA and GGT received 86% bootstrap support. Maximum parsimony analysis of the combined datasets
produced two most parsimonious trees (tree length
5 806 steps, CI 5 0.77 and RI 5 0.88) and differed
from one another in the branching order within the
clade containing G. graminis strains (FIG. 2). The topology of the trees from the maximum parsimony
and neighbor-joining analyses were the same.
Diagnostic PCR primers for C. maydis.—Of the 25
AFLP bands polymorphic between C. maydis lineages, 11 were cut from the polyacrylamide gel, purified
and amplified. Six of these bands were amplified and
sequenced directly. PCR with the primer pair
CMaflp11 1 CMaflp12:), derived from AFLP fragment EAA/MCG-5, amplifies a 300-bp sequence
unique to C. maydis (TABLE III, FIG. 3). Amplification
at different annealing temperatures (55, 58, 60, 65
and 67 C) with numerous strains of C. maydis (Saleh
et al 2003), as well as with all the C. maydis strains
used in this study, produced the same results. The
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MYCOLOGIA
FIG. 1. UPGMA tree based on AFLP fingerprints generated from two primer pairs (EAA/MCC and EAA/MCG) for 93
fungal strains used in this study. Percent values on the branches of the tree generated with 1000 bootstrap replicates.
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GAEUMANNOMYCES
TABLE V. Representative fungal strains used in the phylogenetic studies and the size in bp of the PCR fragments generated,
for the rDNA-ITS, b-tubulin, and histone H3 coding regions
Species
G. cylindrosporus
G. graminis var. avenae
G. graminis var. graminis
G. graminis var. tritici
C. maydis lineages I and III
C. maydis lineage II
C. maydis lineage IV
A. stricum
A. diospyri
F. verticillioides
C. gramineum
Strain numbera
rDNA-ITS
b-tubulin
Histone H3
KSU 14448/MYA-3366
KSU 14447/MYA-3365
KSU 14913, KSU 14914
KSU 14916
KSU 14919
KSU 14439/MYA-3367
KSU 14440/MYA-3368
KSU 14445/MYA-3373
KSU 14442/MYA-3371
KSU 14443/MYA-3372
KSU 14446/MYA-3374
KSU 14441/MYA-3369
KSU 14449/MYA-3370
KSU 14435/MYA-3359,
KSU 10793/MYA-3357
KSU 10800/MYA-3358
KSU 10792/MYA-3356
KSU 5144/MYA-3363,
KSU 5147/MYA-3364
KSU 14438/MYA-3362
KSU 5146, KSU4773
KSU 14436/MYA-3360,
KSU 14437/MYA-3361
Range
576
576
576
576
576
541
549
538
539
537
537
540
543
556
819
815
817
816
817
779
779
771
776
784
776
779
778
759
525
525
517
527
523
479
479
470
473
460
478
479
479
488
557
555
560
759
579
644
488
488
398
363
528
561
591
564
NA
446
479
523
528–576
564–819
398–525
a
Strain numbers are those from the Department of Plant Pathology, Kansas State University and the American Type Culture
Collection.
NA—no data, did not amplify.
clearest results were obtained with an annealing temperature of 67 C and 1.5 mM MgCl2 concentration,
because only the unique C. maydis band was amplified (FIG. 3). These primers and conditions did not
result in the amplification of similar sized DNA fragments from F. verticillioides or from any of the other
tested members of either the Gaeumannomyces-Harpophora or the Acremonium-Cephalosporium species
complexes. When the DNA sequence of the PCR
product unique to C. maydis was searched in GenBank, there was no significant match with any other
DNA or protein sequences in the database.
DISCUSSION
The primary objective of our study was to determine
the genetic relatedness of C. maydis to several morphologically similar fungi and to confirm its identity
as a distinct taxon. AFLPs have been used to group
strains into species in Fusarium (Marasas et al 2001,
Zeller et al 2003), with strains in biologically distinct
species sharing no more than 40% of the bands in a
profile. AFLPs usually are most useful for studying
genetic diversity at or below species level and often
provide little, if any, useful information about genetic
relatedness between taxa above species level other
than that they are different (e.g., Rehmany et al
2000, Marasas et al 2001, Zeller et al 2003). C. maydis
shared no AFLP bands with any of the other species
examined, suggesting that the C. maydis strains belong to a single distinct species. The GGG strains had
the lowest within-group average genetic similarity
(0.15) and produced very different AFLP profiles,
suggesting that this variety of G. graminis probably
contains several cryptic species. When the strains
from GGA and GGT were analyzed together, the
KSU14449 strain was related more closely to the GGA
isolates than it was to the other GGT isolates. Isolates
of G. graminis identified as GGT on the basis of ascospore length—isolates capable of infecting oats
and isolates that are genetically close to GGA—have
been reported from Australia (Bryan et al 1995,
1999). Bryan et al (1999) suggested that these strains
of G. graminis represent either a third cereal-attacking variety or are intervarietal hybrids between strains
of GGA and GGT. Our limited data support the hybrid hypothesis in which outcrosses can occur between strains of GGA and GGT. C. gramineum, A.
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MYCOLOGIA
FIG. 2. One of the most parsimonious trees inferred from sequences of rDNA-ITS, b-tubulin and histone H3. The percent
bootstrap values obtained from 1000 replications are indicated above the branches. Tree length is 806 steps, CI 5 0.77 and
RI 5 0.88. C. gramineum served as outgroup.
strictum (C. acremonium) and G. cylindrosporus had
no bands in common with any of the other species
examined and should all be considered distinct species.
The analysis of three sequenced genes is consistent
with the conclusions of the AFLP study. The similarity
of the b-tubulin, rDNA ITS and histone H3 sequences was almost 100% among the strains representing
the four lineages of C. maydis. The only difference
in the nucleotide sequences of the three genes between the four lineages occurred at two single nucleotide insertions in the ITS-1 region of the rDNA se-
quence. The lineage II strains had both these inserted nucleotides, whereas lineage IV strains had neither. Strains of lineages I and III had one or the other
of these nucleotide insertions. Based on DNA sequences already deposited at GenBank (Ward and
Bateman 1999), the Egyptian strains previously included in GenBank belong to lineage II and the Indian strain represented there belongs to lineage IV.
Moreover, C. maydis rDNA-ITS sequences were not
the same as those from G. graminis var. maydis Yao
et al (GenBank accession number AY120939), which
causes take-all disease of maize (Yao et al 1993).
SALEH
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FIG. 3. PCR amplification products obtained with the
cmaflp11 and cmaflp12 primers to differentiate between H.
maydis strains from other related fungi. Lanes: M, 100 bp
DNA Ladder marker (Invitrogen, Carlsbad, California); C,
negative control that contains everything in the reaction
mix except DNA; 1–8, H. maydis (includes representatives
of all four lineages); 9, C. gramineum; 10, A. strictum; 11,
A. diospyri; 12, G. cylindrosporus; 13, G. graminis var. avenae;
14, G. graminis var. graminis; 15, G. graminis var. tritici; and
16, F. verticillioides.
Although C. maydis is a vascular wilt pathogen, and
hence unlike the other pathogens in the Gaeumannomyces-Harpophora species complex, which are rootinfecting pathogens, it is clearly a soilborne disease.
The AFLP profiles and DNA sequences reported
here clearly place C. maydis in the GaeumannomycesHarpophora species complex, a conclusion consistent
with previous studies (Domsch and Gams 1972, Gams
2000, Walker 1981, Ward and Bateman 1999). Thus,
Cephalosporium maydis Samra, Sabet & Hingorani (in
Phytopathology 53:404–405, 1963) is recognized as
Harpophora maydis (Samra, Sabet & Hingorani)
Gams (in Studies in Mycology 45:192), because its
morphological and cultural characters resemble
those of Harpophora (Domsch and Gams 1972, Gams
2000, Walker 1981), and the molecular characters
place it firmly within the Gaeumannomyces-Harpophora species complex.
Gaeumannomyces-Harpophora complex.—Morphological characters used to delimit species of Gaeumannomyces include hyphopodia (structures of attachment and penetration produced by epiphytic hyphae on the host), perithecia, asci and ascospores
(Walker 1981). We included two species of Gaeumannomyces in this study, G. cylindrosporus and G. graminis. The three varieties of G. graminis cannot be
distinguished by their Harpophora anamorphs. GGA
and GGT have simple hyphopodia, whereas GGG has
lobed hyphopodia both on plants and in axenic culture. Nucleotide sequence variation in the exon regions of the rDNA, b-tubulin and histone H3 sequences from the G. cylindrosporus strains was very
low, while the nucleotide variation of similar sequences from the G. graminis strains were much more heterogeneous.
The G. graminis strains formed a strongly supported clade (FIG. 2) in which the GGA and GGT strains
were closer to each other than either set of strains
GAEUMANNOMYCES
1303
was to the strains representing GGG. Although the
GGG strains formed a monophyletic clade in our
study, differences in both nucleotide sequence and
AFLP profile were sufficient to prevent any conclusions as to the number of species into which this taxon eventually might be resolved. These results are
consistent with previous studies based on morphology and host range (Walker 1981, Bryan et al 1999),
RFLP hybridization and rDNA-ITS sequences (Bryan
et al 1995, Ward and Akrofi 1994, Fouly et al 1997,
Ward and Bateman 1999) and RAPD profiles (Fouly
et al 1996).
Both isolates of A. strictum (KSU 5144 and KSU
5147) were isolated from sorghum in Egypt. The
DNA sequences of the three genes we examined were
identical for these two strains. The rDNA ITS sequence of KSU 5144 was ;91% similarity to that of
the A. strictum type strain CBS 346.70 (GenBank accession number AY138845). The sequences from the
strains of A. strictum we examined were more similar
(.95%) to a strain of Nectria mauritiicola (NRRL
20420; GenBank accession number AJ557830) that
was identified morphologically as A. strictum by Novicki et al (2003). They suggested that A. strictum either is polyphyletic or a genetically diverse taxon, a
conclusion supported by our analysis of the two
strains described here.
C. gramineum is the causal agent of Cephalosporium stripe of winter wheat as well as many other graminaceous plants (Bockus 1992), and its generic position is known to be in need of correction (Bruehl
1963, Farr et al 1989). We could not amplify DNA
fragments from this fungus with the b-tubulin or
MAT-2 primers we used. For the rDNA-ITS region,
there were 25 polymorphic sites in the exon region
for the entire set of species examined. C. gramineum
differed from H. maydis at 14 of these sites, with 10
of these site differences unique to C. gramineum.
With respect to the histone H-3 region, the introns
within the C. gramineum sequence are positioned differently from those in any of the other species examined. Similarly, of the 82 polymorphic sites for the
entire set of species in the histone H3 exon, C. gramineum differed from H. maydis at 43 sites, 13 of
which were unique to C. gramineum. Thus, C. gramineum appears to be distantly related to both the
Acremonium-Cephalosporium and the Gaeumannomyces-Harpophora species complexes fungi. Indeed, F.
verticillioides is more closely related to the other
members of the Acremonium-Cephalosporium species
complex than is C. gramineum (FIG. 2). The closest
match with C. gramineum rDNA-ITS sequence in the
EMBL and GenBank databases was to Rhynchosporium secalis (96%) (Goodwin 2002). Thus, additional
1304
MYCOLOGIA
work is needed to determine the evolutionary position of this species.
PCR primers for identifying H. maydis.—PCR primers
designed on the basis of AFLP markers potentially
can be used for rapid identification and detection of
H. maydis. This diagnostic PCR-based method is
quick and easy to apply. Such primers are of particular importance because H. maydis is not widely distributed yet and both pure cultures and infected materials are subjected to plant quarantines and other
restrictions in movement. These primers can be synthesized locally and used for diagnosis even if authenticated cultures of the fungus are not available
for comparative analyses.
In conclusion, H. maydis belongs in the Gaeumannomyces-Harpophora species complex even though the
vascular wilt it causes is quite different from the root
diseases caused by other pathogenic species in this
species complex. The species-specific primers we developed can be used to rapidly identify this pathogen
when it is found in new locations. We found that C.
gramineum clearly falls outside either the Acremonium-Cephalosporium or Gaeumannomyces-Harpophora
species complexes and that the evolutionary position
and nomenclatural status of this species require further investigation and reconsideration. Within Gaeumannomyces graminis, the two varieties avenae and tritici could be the same species and GGG may need to
be divided into several species.
ACKNOWLEDGMENTS
This work was financially supported in part by the Kansas
Agricultural Experimentation Station, and ATUT collaborative research grant No. 58-314-7-057 through the U.S.
Agency for International Development (Cairo, Egypt). Mr.
Saleh was supported by a fellowship from the Institute of
International Education. We thank Bill Bockus for providing us with cultures of Cephalosporium gramineum and
Gaeumannomyces graminis var. tritici; Ned Tisserat for providing us with cultures of Acremonium diospyri, Gaeumannomyces cylindrosporus, Gaeumannomyces graminis var. avenae, Gaeumannomyces graminis var. graminis and Gaeumannomyces graminis var. tritici; Amy Beyer and Brook van Scoyoc for technical assistance; and Walter Gams and Kurt
Zeller for critically reading the manuscript. Contribution
04-179-J from the Kansas Agricultural Experimentation Station, Manhattan.
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