Research Article |
Corresponding author: Z. Wilhelm de Beer ( wilhelm.debeer@fabi.up.ac.za ) Academic editor: Thorsten Lumbsch
© 2017 Z. Wilhelm de Beer, Seonju Marincowitz, Tuan A. Duong, Michael J. Wingfield.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
de Beer ZW, Marincowitz S, Duong TA, Wingfield MJ (2017) Bretziella, a new genus to accommodate the oak wilt fungus, Ceratocystis fagacearum (Microascales, Ascomycota). MycoKeys 27: 1-19. https://doi.org/10.3897/mycokeys.27.20657
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Recent reclassification of the Ceratocystidaceae (Microascales) based on multi-gene phylogenetic inference has shown that the oak wilt fungus Ceratocystis fagacearum does not reside in any of the four genera in which it has previously been treated. In this study, we resolve typification problems for the fungus, confirm the synonymy of Chalara quercina (the first name applied to the fungus) and Endoconidiophora fagacearum (the name applied when the sexual state was discovered). Furthermore, the generic placement of the species was determined based on DNA sequences from authenticated isolates. The original specimens studied in both protologues and living isolates from the same host trees and geographical area were examined and shown to represent the same species. A lectotype was designated for Chalara quercina and Endoconidiophora fagacearum and an epitype linked to a living ex-epitype isolate was designated. Phylogenetic analyses confirmed that the species resides in a well-supported monophyletic lineage in the Ceratocystidaceae, distinct from all other genera in the family. The new genus Bretziella is described to accommodate the oak wilt fungus.
Quercus , Ceratocystidaceae , Microascales , heterothallic
Oak wilt is a serious disease of many Quercus spp. in the Midwestern and Eastern United States, as well as Texas (
Oak wilt is caused by a fungus in the genus Ceratocystis, which is widely known as Ceratocystis fagacearum (
Generic boundaries within the Ceratocystidaceae were recently reconsidered based on DNA sequence data for three gene regions in 70 species (
The fourth unresolved lineage in the study of
During the course of the six decades following the
The aim of this study was firstly to consider the appropriate generic placement of the oak wilt fungus in the Ceratocystidaceae based on phylogenetic analyses of the three gene regions used by
Herbarium specimens labelled as Chalara quercina from the study of
Culture numbers‡ | Host | Locality | GenBank accession number | ||
---|---|---|---|---|---|
60S | LSU | MCM7 | |||
CMW 2039 = CBS 130770 | Quercus sp. | Minnesota | =KM495518 | =KM495341 | =KM495430 |
CMW 2656EP = CBS 138363 | Quercus rubra | Iowa | KM495518 § | KM495341 | KM495430 |
CMW 2658 | Quercus sp. | Iowa | =KM495518 | =KM495341 | =KM495430 |
CMW 38759 = CBS 129241 | Quercus sp. | Iowa | =KM495518 | =KM495341 | =KM495430 |
Three gene regions, the nuclear ribosomal DNA large subunit (LSU), the 60S ribosomal protein RPL10 (60S), and mini-chromosome maintenance complex component 7 (MCM7), were amplified and sequenced for all four living isolates. These gene regions were the same as those selected and used by
Representative species of the dominant genera in the Ceratocystidaceae were included in the phylogenetic analyses. The recently described Meredithiella was not included because appropriate sequence data were not available for this taxon. Species of Knoxdaviesia and Graphium were included as outgroups. Datasets for each of the three gene regions were compiled and aligned separately with the online version of MAFFT v. 7 (
Morphological characters of sexual and asexual structures taken from the herbarium specimens and living isolates were compared with each other and with the original descriptions (
Microscopic structures taken from herbarium specimens were mounted and studied in 10 % KOH, and those from living cultures were mounted in water, later replaced with 85 % lactic acid in which they were then studied. Up to 50 measurements were made for each characteristic structure where possible. Microscopic structures were studied with a Nikon SMZ18 stereoscope and a Nikon Eclipse Ni compound microscope. Images were captured using a Nikon DS-Ri2 camera. Measurements were made using the Nikon Imaging Software (NIS) Elements (v. 4.3).
DNA sequences obtained for the LSU, 60S, and MCM7 regions of the four living isolates were used for phylogenetic analyses. These sequences, as well as the ITS and TEF1α sequences for CBS 138363 = CMW 2656 (ex-epitype, see below), have been deposited in the RefSeq Targeted Loci (RTL) database in NCBI GenBank (
A total of 39 isolates representing 35 species were included in the phylogenetic analyses. Alignment of the 60S dataset resulted in ambiguously aligned regions and long gaps that were a result of the inconsistency in the presence/absence of introns and highly variable intron sequences. Gap-containing positions from the 60S dataset were thus excluded from further analyses. After removing all gap positions, the 60S dataset consisted of 314 characters with 105 variable characters. The LSU dataset consisted of 875 characters with 173 variable characters. The MCM7 dataset consisted of 628 characters with 321 variable characters. The ML and BI analyses of the concatenated dataset of all three gene regions resulted in trees with almost identical topology. Monophyletic clades representing all genera included in the analyses could be identified and these clades were strongly supported in both ML and BI analyses.
The four C. fagacearum isolates included in this study formed a well-supported monophyletic clade (Figure
Bayesian phylogram derived from the analyses of the concatenated dataset (60S, LSU, MCM7). Maximum likelihood bootstrap values (≥ 70 %, 1000 replicates) and Bayesian posterior probabilities values (≥ 0.95) are indicated at nodes. “-” indicated no phylogenetic support or the support values are below 70% for ML and 0.95 for BI.
The herbarium specimen of Chalara quercina (BPI 595712) from study of
Description: Conidiophores cylindrical tapering towards the apex, single, upright, straight or slightly curved, occasionally branched, pale to dark brown, becoming paler to the apex, 3–9 septate, up to 140 µm long including conidiogenous cells, 3–5 µm wide at the base. Conidiogenous cells cylindrical, tapering towards the apex, slightly pigmented to hyaline, 20–32 µm long, 2.5–3.5 µm wide at the base, 2–3 µm wide near the apex. Conidia endogenous, hyaline, rectangular shaped, 4–8.5 × 2–3 µm, produced in chains. Aleurioconidia not observed.
The herbarium specimen of Endoconidiophora fagacearum (FP 97476) from the study of
Description: Ostiolar hyphae observed in a single ascomatal neck hyaline, divergent. Ascospores recovered from broken ascoma hyaline, ellipsoidal, occasionally curved, 4.5–9.5 × 2–3.5 µm, embedded in gelatinous sheath. Conidiophores cylindrical tapering towards the apex, single, upright, straight or slightly curved, occasionally branched, pale to dark brown, becoming paler towards the apex, 2–6 septate, up to 100 µm long including conidiogenous cells, 3.5–5 µm wide at the base (these measurements reflect a limited number of intact conidiophores due to the brittle condition of the specimen). Conidiogenous cells cylindrical, tapering towards the apex, slightly pigmented to hyaline, 19–35 µm long, 2.5–3.5 µm at the base, 2–3.5 µm near the apex. Conidia endogeneous, hyaline, rectangular-shaped, 3–6.5 × 2–3 µm, produced in chains. Aleurioconidia not observed.
Laboratory crosses between the living isolates (Table
Description: On 2 % YMA with oak sticks mycelia fluffy, pale to dark grey. Spore-bearing structures hidden in mycelial mat. Conidiophores cylindrical, tapering towards the apex, single, upright, straight or slightly curved, occasionally branched or reduced to conidiogenous cells, pale to dark brown, becoming paler towards the apex, 3–9 septate, up to 155 µm long including conidiogenous cells, 3–5 µm wide at the base, often constricted at septum. Conidiogenous cells cylindrical tapering towards the apex, slightly pigmented to hyaline, 25–35 µm long, 2.5–4.5 µm wide at the base, 2.5–3.5 µm wide near the apex. Conidia endogenous, rectangular shaped, hyaline, 3.5–9 × 1.5–3.5 (avg. 5.9 × 2.5 µm), produced in chains. Aleurioconidia not observed.
Features of the conidiophores were almost identical between the two herbarium specimens and the living isolates (Figure
Culture characteristics of the fresh isolates were similar to those of the Bretz specimen (FP 97476), forming fluffy, thick mycelial mats containing the sexual structures (Figure
Only a few broken ascomata were removed from the Bretz specimen (FP 97476) for this study. The shape of the ascomata was similar to those described by
Morphological features of herbarium specimens and a living isolate of the oak wilt fungus. A, E, F, K Chalara quercina (BPI 595712, Lectotype) B, C, D, G, H, L Endoconidiophora fagacearum (FP 97476, Lectotype) I, J, M Living isolate treated as Ceratocystis fagacearum (CMW 2656 = CBS 138363, ex-epitype) A, B Dried cultures (arrow in B indicates the piece where ascomata were found) C, D Ascospores with sheaths (arrows) E–J Conidiophores K–M Conidia. Scale bars: C–J = 20 µm, K–M = 10 µm.
Morphological comparisons with herbarium specimens representing Chalara quercina and Endoconidiophora fagacearum, confirmed that the four living isolates included in this study represented the same taxon. Unresolved typification and nomenclatural issues relating to this taxon are considered below. Phylogenetic analyses including DNA sequences showed that the four isolates grouped in a well-supported clade in the Ceratocystidaceae (Figure
Named after Theodore W. Bretz who first discovered and described the sexual state of the type species of this genus (
The genus is distinguished from all other genera of the Ceratocystidaceae based on the mycelial mats that it forms on infected oak trees. These mats form pressure cushions or pads that push the bark away from the underlying sapwood. This causes cracks in the bark, exposing the mats to fungal-feeding arthropod vectors, primarily nitidulid beetles.
Bretziella fagacearum (Bretz) Z.W.deBeer, Marinc., T.A.Duong & M.J.Wingf.
Ascomatal bases black, globose, with undifferentiated ornamental hyphae, often embedded in mycelial mat. Ascomatal necks elongated, black at base, lighter at apex. Ostiolar hyphae present. Asci dehiscent. Ascospores one-celled, hyaline, ellipsoidal, occasionally curved, embedded in hyaline sheath. Conidiophores arise laterally from vegetative hyphae, occasionally branched. Conidiogenous cells phialidic, cylindrical, pale to dark brown. Conidia unicellular, cylindrical with flattened ends, hyaline, borne in chains of varying length. Aleurioconidia not present.
The only known species in the genus causes vascular wilt on various oak species in North America.
Bas.: Endoconidiophora fagacearum Bretz, Phytopathology 42: 436. 1952; Ceratocystis fagacearum (Bretz) Hunt, Lloydia 19: 21. 1956. TYPES: USA. Dry culture resulting from a cross between two isolates, locations unknown, from Quercus sp., 26 Feb 1952, T.Bretz (Lectotype designated here: FP 97476, MycoBank typification number: MBT 378423). USA. Iowa, on Quercus rubra, 1991, S.Seegmueller (Epitype designated here: BPI 893238, MycoBank typification number: MBT 378424; ex-epitype culture CBS 138363 = CMW 2656). Representative sequences from epitype: 60S = KM495518, LSU = KM495341, MCM7 = KM495430, ITS = KU042044, TEF1α = KU042043. See Notes 1, 2 and 3 below.
= Chalara quercina Henry, Phytopathology 34: 633. 1944; Thielaviopsis quercina (Henry) A.E.Paulin, T.C.Harr. & McNew, Mycologia 94: 70. 2002. TYPE: USA. Dry culture, Wisconsin, Madison, on Quercus sp., Sept. 1943, B.Henry (Lectotype designated here: BPI 595712, MycoBank typification number: MBT 378425). See Note 4 below.
Line drawings of the oak wilt fungus. These illustrations are based on previously published line drawings and observations of the herbarium specimens (BPI 595712, FP 97476) in the present study. A Conidiophore and conidia in 10 % KOH (BPI 595712) B Ascomatal primordium re-drawn from
Based on the one fungus one name principles adopted in the Melbourne Code (
In the present study, we have shown that the oak wilt fungus does not belong in Ceratocystiss. str., Endoconidiophora, Thielaviopsis or any of the other genera currently accepted in the Ceratocystidaceae (
In the protologue of E. fagacearum,
The lectotypes designated here for Ch. quercina and E. fagacearum both consist of dried specimens for which DNA sequence data are not available. However, based on careful microscopic comparisons between these two specimens and a living isolate from Iowa (Figure
The oak wilt fungus is an economically important pathogen in the USA, with the potential to become a serious, alien invasive if it was ever introduced into other countries having oak forests. It is listed as a quarantine organism by the European and Mediterranean Plant Protection Organization (EPPO) and the European Union (EU) (http://www.q-bank.eu/). Making a change to the name of a species having this level of importance must clearly be done responsibly and with care (
In addition to phylogenetic data, the unusual biology of the oak wilt fungus supports the description of the new genus, Bretziella, to accommodate this species. After infection of healthy trees through wounds or root grafts, the fungus forms pressure pads under the bark that lead to cracks in the bark, exposing mats of mycelium and fruiting structures, attractive to fungus-feeding arthropods such as nitidulid beetles that then act as vectors of the fungus (
The choice of an epithet for the new species name in Bretziella was problematic. If we were to follow the Melbourne Code strictly, the unknown basionym of the asexual morph, Ch. quercina, would have priority over E. fagacearum, the basionym for C. fagacearum and the name that has been widely used. A formal proposal has thus been submitted to conserve the better known basionym against one that would be unfamiliar to most plant pathologists and mycologists. In this way, it is possible to ensure that even though the species has to be treated in a new genus, the epithet will remain familiar to those working with the fungus.
Subsequent to careful morphological comparisons, two lectotypes and an epitype have bene designated for the two basionyms, Chalara quercina and Endoconidium fagacearum. These procedures ensure that the basionyms are now permanently linked to specimens. Sequences obtained from the epitype have been deposited in the RefSeq Targeted Loci (RTL) database in NCBI GenBank to enable accurate and reliable identifications when BLAST searches are conducted (
We thank the curators at BPI (U.S. National Fungus Collections, Beltsville, Maryland) and FP (Forest Service at the Center for Forest Mycology Research, Madison, Wisconsin), for making specimens available for this study. We also acknowledge the financial support of members of the Tree Protection Cooperative Programme (TPCP), the NRF/DST Centre of Excellence in Tree Health Biotechnology (CTHB), and the University of Pretoria.