Unidirectional mating-type switching is underpinned by a conserved MAT1 locus architecture

Unidirectional mating-type switching is a form of homothallic reproduction known only in a small number of filamentous ascomycetes. Their ascospores can give rise to either self-sterile isolates that require compatible partners for subsequent sexual reproduction, or self-fertile individuals capable of completing this process in isolation. The limited studies previously conducted in these fungi suggest that the differences in mating specificity are determined by the architecture of the MAT1 locus. In self-fertile isolates that have not undergone unidirectional mating-type switching, the locus contains both MAT1-1 and MAT1-2 mating-type genes, typical of primary homothallism. In the self-sterile isolates produced after a switching event, the MAT1-2 genes are lacking from the locus, likely due to a recombination-mediated deletion of the MAT1-2 gene information. To determine whether these arrangements of the MAT1 locus support unidirectional mating-type switching in the Ceratocystidaceae, the largest known fungal assemblage capable of this reproduction strategy, a combination of genetic and genomic approaches were used. The MAT1 locus was annotated in representative species of Ceratocystis, Endoconidiophora, and Davidsoniella. In all cases, MAT1-2 genes interrupted the MAT1-1-1 gene in self-fertile isolates. The MAT1-2 genes were flanked by two copies of a direct repeat that accurately predicted the boundaries of the deletion event that would yield the MAT1 locus of self-sterile isolates. Although the relative position of the MAT1-2 gene region differed among species, it always disrupted the MAT1-1-1 gene and/or its expression in the self-fertile MAT1 locus. Following switching, this gene and/or its expression was restored in the self-sterile arrangement of the locus. This mirrors what has been reported in other species capable of unidirectional mating-type switching, providing the strongest support for a conserved MAT1 locus structure that is associated with this process. This study contributes to our understanding of the evolution of unidirectional mating-type switching.

Phenolic degradation by catechol dioxygenases is associated with pathogenic fungi with a necrotrophic lifestyle in the Ceratocystidaceae

Fungal species of the Ceratocystidaceae grow on their host plants using a variety of different lifestyles, from saprophytic to highly pathogenic. Although many genomes of fungi in the Ceratocystidaceae are publicly available, it is not known how the genes that encode catechol dioxygenases (CDOs), enzymes involved in the degradation of phenolic plant defense compounds, differ among members of the Ceratocystidaceae. The aim of this study was therefore to identify and characterize the genes encoding CDOs in the genomes of Ceratocystidaceae representatives. We found that genes encoding CDOs are more abundant in pathogenic necrotrophic species of the Ceratocystidaceae and less abundant in saprophytic species. The loss of the CDO genes and the associated 3-oxoadipate catabolic pathway appears to have occurred in a lineage-specific manner. Taken together, this study revealed a positive association between CDO gene copy number and fungal lifestyle in Ceratocystidaceae representatives.

A New Real-Time PCR Assay for Detecting Fungi in Genus Ceratocystis

The genus Ceratocystis contains several significant plant pathogens, causing wilt and canker disease on a wide range of plant species. There are >40 known species of Ceratocystis, some of which are becoming increasingly important in agricultural or natural ecosystems. The diagnostic procedures for most Ceratocystis species rely on time-consuming and labor-intensive culturing approaches. To provide more time-efficient and sensitive molecular diagnostic tools for Ceratocystis, a generic TaqMan real-time PCR assay was developed using the ITS gene. This novel two-probe TaqMan assay amplified DNA from all tested Ceratocystis species. Some nonspecific amplification of a few species from closely related genera was observed under certain conditions; however, these false-positive detections could be ruled out using the additional PCR primers developed for further sequence-based identification of the detected species. The assay was found to be highly sensitive, as it detected 0.2 pg/μl of Ceratocystis DNA in water as well as in host DNA matrix. Further validation with artificially inoculated fig stem tissue demonstrated that the assay was also able to effectively detect the pathogen in infected asymptomatic stem tissue. This newly developed real-time PCR assay has practical applications in biosecurity, conservation, and agriculture; it will enable the detection of Ceratocystis species directly from plant material to facilitate more sensitive screening of imported plant germplasm, and allow rapid tracking of pathogens in the case of disease outbreaks.

Recent and Ongoing Horizontal Transfer of Mitochondrial Introns Between Two Fungal Tree Pathogens

Two recently introduced fungal plant pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) are responsible for Rapid ‘ōhi‘a Death (ROD) in Hawai‘i. Despite being sexually incompatible, the two pathogens often co-occur in diseased ‘ōhi‘a sapwood, where genetic interaction is possible. We sequenced and annotated 33 mitochondrial genomes of the two pathogens and related species, and investigated 35 total Ceratocystis mitogenomes. Ten mtDNA regions [one group I intron, seven group II introns, and two autonomous homing endonuclease (HE) genes] were heterogeneously present in C. lukuohia mitogenomes, which were otherwise identical. Molecular surveys with specific primers showed that the 10 regions had uneven geographic distribution amongst populations of C. lukuohia. Conversely, identical orthologs of each region were present in every studied isolate of C. huliohia regardless of geographical origin. Close relatives of C. lukuohia lacked or, rarely, had few and dissimilar orthologs of the 10 regions, whereas most relatives of C. huliohia had identical or nearly identical orthologs. Each region included or worked in tandem with HE genes or reverse transcriptase/maturases that could facilitate interspecific horizontal transfers from intron-minus to intron-plus alleles. These results suggest that the 10 regions originated in C. huliohia and are actively moving to populations of C. lukuohia, perhaps through transient cytoplasmic contact of hyphal tips (anastomosis) in the wound surface of ‘ōhi‘a trees. Such contact would allow for the transfer of mitochondria followed by mitochondrial fusion or cytoplasmic exchange of intron intermediaries, which suggests that further genomic interaction may also exist between the two pathogens.

Characterization of the Ergosterol Biosynthesis Pathway in Ceratocystidaceae

Terpenes represent the biggest group of natural compounds on earth. This large class of organic hydrocarbons is distributed among all cellular organisms, including fungi. The different classes of terpenes produced by fungi are mono, sesqui, di- and triterpenes, although triterpene ergosterol is the main sterol identified in cell membranes of these organisms. The availability of genomic data from members in the Ceratocystidaceae enabled the detection and characterization of the genes encoding the enzymes in the mevalonate and ergosterol biosynthetic pathways. Using a bioinformatics approach, fungal orthologs of sterol biosynthesis genes in nine different species of the Ceratocystidaceae were identified. Ergosterol and some of the intermediates in the pathway were also detected in seven species (Ceratocystis manginecans, C. adiposa, Huntiella moniliformis, Thielaviopsis punctulata, Bretziella fagacearum, Endoconidiophora polonica and Davidsoniella virescens), using gas chromatography-mass spectrometry analysis. The average ergosterol content differed among different genera of Ceratocystidaceae. We also identified all possible terpene related genes and possible biosynthetic clusters in the genomes used in this study. We found a highly conserved terpene biosynthesis gene cluster containing some genes encoding ergosterol biosynthesis enzymes in the analysed genomes. An additional possible terpene gene cluster was also identified in all of the Ceratocystidaceae. We also evaluated the sensitivity of the Ceratocystidaceae to a triazole fungicide that inhibits ergosterol synthesis. The results showed that different members of this family behave differently when exposed to different concentrations of triazole tebuconazole.

Epitypification of Ceratocystis fimbriata

Ceratocystis accommodates many important pathogens of agricultural crops and woody plants. Ceratocystis fimbriata, the type species of the genus is based on a type that is unsuitable for a precise application and interpretation of the species. This is because no culture or DNA data exist for the type specimen. The aim of this study was to select a reference specimen that can serve to stabilize the name of this important fungus. We selected a strain, CBS 114723, isolated from sweet potato in North Carolina, USA, in 1998 for this purpose. The strain was selected based on the availability of a living culture in a public depository. A draft genome sequence is also available for this strain. Its morphological characteristics were studied and compared with the existing and unsuitable type specimen as well as with the original descriptions of C. fimbriata. The selected strain fits the existing concept of the species fully and we have consequently designated it as an epitype to serve as a reference specimen for C. fimbriata.

Genome comparisons suggest an association between Ceratocystis host adaptations and effector clusters in unique transposable element families

Ceratocystis fimbriata is a host specific fungal pathogen of sweet potato (Ipomoea batatas). The closely related species, C. manginecans, is an important pathogen of trees (e.g. Acacia mangium and Mangifera indica) but has never been isolated from tuber crops. The genetic factors that determine the host range and host specificity of these species have not been determined. The aim of this study was to compare the genomes of C. fimbriata and C. manginecans in order to identify species-specific genetic differences that could be associated with host specificity. This included whole-genome alignments as well as comparisons of gene content and transposable elements (TEs). The genomes of the two species were found to be very similar, sharing similar catalogues of CAZymes, peptidases and lipases. However, the genomes of the two species also varied, harbouring species-specific genes (e.g. small secreted effectors, nutrient processing proteins and stress response proteins). A portion of the TEs identified (17%) had a unique distribution in each species. Transposable elements appeared to have played a prominent role in the divergence of the two species because they were strongly associated with chromosomal translocations and inversions as well as with unique genomic regions containing species-specific genes. Two large effector clusters, with unique TEs in each species, were identified. These effectors displayed non-synonymous mutations and deletions, conserved within a species, and could serve as mutational hot-spots for the development of host specificity in the two species.

Distribution and Evolution of Nonribosomal Peptide Synthetase Gene Clusters in the Ceratocystidaceae

In filamentous fungi, genes in secondary metabolite biosynthetic pathways are generally clustered. In the case of those pathways involved in nonribosomal peptide production, a nonribosomal peptide synthetase (NRPS) gene is commonly found as a main element of the cluster. Large multifunctional enzymes are encoded by members of this gene family that produce a broad spectrum of bioactive compounds. In this research, we applied genome-based identification of nonribosomal peptide biosynthetic gene clusters in the family Ceratocystidaceae. For this purpose, we used the whole genome sequences of species from the genera Ceratocystis,Davidsoniella,Thielaviopsis, Endoconidiophora,Bretziella, Huntiella, and Ambrosiella. To identify and characterize the clusters, different bioinformatics and phylogenetic approaches, as well as PCR-based methods were used. In all genomes studied, two highly conserved NRPS genes (one monomodular and one multimodular) were identified and their potential products were predicted to be siderophores. Expression analysis of two Huntiella species (H. moniliformis and H. omanensis) confirmed the accuracy of the annotations and proved that the genes in both clusters are expressed. Furthermore, a phylogenetic analysis showed that both NRPS genes of the Ceratocystidaceae formed distinct and well supported clades in their respective phylograms, where they grouped with other known NRPSs involved in siderophore production. Overall, these findings improve our understanding of the diversity and evolution of NRPS biosynthetic pathways in the family Ceratocystidaceae.

Taxonomic revision and multi-locus phylogeny of the North American clade of Ceratocystis

The North American clade (NAC) of Ceratocystis includes pathogenic species that infect a wide range of woody hosts. Previous phylogenetic analyses have suggested that this clade includes cryptic species and a paraphyletic C. variospora. In this study, we used morphological data and phylogenetic analyses to characterize NAC taxa, including Ceratocystis isolates causing a serious disease of almond trees in California. Phylogenetic analyses based on six gene regions supported two new species of Ceratocystis. Ceratocystis destructans is introduced as the species causing severe damage to almond trees in California, and it has also been isolated from wounds on Populus and Quercus in Iowa. It is morphologically similar to C. tiliae, a pathogen on Tilia and the most recently characterized species in the NAC. Ceratocystis betulina collected from Betula platyphylla in Japan is also newly described and is the sister taxon to C. variospora. Our six-locus phylogenetic analyses and morphological characterization resolved several cryptic species in the NAC.

New Ceratocystis species associated with rapid death of Metrosideros polymorpha in Hawai'i

The native ‘ōhi’a lehua (Metrosideros polymorpha) has cultural, biological and ecological significance to Hawai’i, but it is seriously threatened by a disease commonly referred to as rapid ‘ōhi’a death (ROD). Preliminary investigations showed that a Ceratocystis species similar to C. fimbriata s.lat. was the cause of the disease. In this study, we used a combination of the phylogenetic, morphological and biological species concepts, as well as pathogenicity tests and microsatellite analyses, to characterise isolates collected from diseased ‘ōhi’a trees across Hawai’i Island. Two distinct lineages, representing new species of Ceratocystis, were evident based on multigene phylogenetic analyses. These are described here as C. lukuohia and C. huliohia. Ceratocystis lukuohia forms part of the Latin American clade (LAC) and was most closely associated with isolates from Syngonium and Xanthosoma from the Caribbean and elsewhere, including Hawai’i, and C. platani, which is native to eastern USA. Ceratocystis huliohia resides in the Asian-Australian clade (AAC) and is most closely related to C. uchidae, C. changhui and C. cercfabiensis, which are thought to be native to Asia. Morphology and interfertility tests support the delineation of these two new species and pathogenicity tests show that both species are aggressive pathogens on seedlings of M. polymorpha. Characterisation of isolates using microsatellite markers suggest that both species are clonal and likely represent recently-introduced strains. Intensive research is underway to develop rapid screening protocols for early detection of the pathogens and management strategies in an attempt to prevent the spread of the pathogens to the other islands of Hawai’i, which are currently disease free.

Non-Mendelian segregation influences the infection biology and genetic structure of the African tree pathogen Ceratocystis albifundus

The African fungal tree pathogen, Ceratocystis albifundus, undergoes uni-directional mating type switching, giving rise to either self-fertile or self-sterile progeny. Self-sterile isolates lack the MAT1-2-1 gene and have reduced fitness such as slower growth and reduced pathogenicity, relative to self-fertile isolates. While it has been hypothesized that there is a 1:1 ratio of self-fertile to self-sterile ascospore progeny in relatives of C. albifundus, some studies have reported a significant bias in this ratio. This could be due to the fact that either fewer self-sterile ascospores are produced or that self-sterile ascospores have low viability. We quantified the percentage of self-sterile and self-fertile ascospores from ascospore masses in C. albifundus using real-time PCR. Primers were designed to distinguish between spores that contained the MAT1-2-1 gene and those where this gene had been deleted. A significant bias towards the self-fertile mating type was observed in all single ascospore masses taken from sexual structures produced in haploid-selfed cultures. The same result was observed from a disease outbreak situation in an intensively managed field of cultivated native trees, and this was coupled with very low population diversity in the pathogen. This was in contrast to the results obtained from ascospore masses taken from the crosses performed under laboratory conditions or ascomata on native trees in a non-disease situation, where either self-fertile or self-sterile ascospores were dominant. The results suggest that reproductive strategies play a significant role in the infection biology and genetic structure of C. albifundus populations.

IMA Genome-F 8: Draft genome of Cercospora zeina, Fusarium pininemorale, Hawksworthiomyces lignivorus, Huntiella decipiens and Ophiostoma ips

The genomes of Cercospora zeina, Fusarium pininemorale, Hawksworthiomyces lignivorus, Huntiella decipiens, and Ophiostoma ips are presented in this genome announcement. Three of these genomes are from plant pathogens and otherwise economically important fungal species. Fusarium pininemorale and H. decipiens are not known to cause significant disease but are closely related to species of economic importance. The genome sizes range from 25.99 Mb in the case of O. ips to 4.82 Mb for H. lignivorus. These genomes include the first reports of a genome from the genus Hawksworthiomyces. The availability of these genome data will allow the resolution of longstanding questions regarding the taxonomy of these species. In addition these genome sequences through comparative studies with closely related organisms will increase our understanding of how these species or close relatives cause disease.

Ecology and population structure of a tree wound-infecting fungus in a native South African forest environment

Ceratocystis tsitsikammensis was first isolated from bark harvesting wounds on two indigenous tree species in the Afromontane forests of the Western Cape Province of South Africa. Inoculation studies indicated that it is a potential pathogen of native Rapanea melanophloeos trees. In this study, we investigated the distribution, ecology and biology of C. tsitsikammensis in the Garden Route National Park of South Africa. Isolates were obtained from wounds on R. melanophloeos, three non-native hosts as well as from nitidulid and staphylinid beetles visiting wounds on these trees. The genetic diversity and population biology of the fungus was examined using microsatellite markers. Its mating strategy was also determined by amplifying its mating type genes and the fungus was shown to be homothallic. Despite the homothallic nature of the fungus, high levels of random mating and absence of genetic structure was found in the investigated population, suggesting a strong effect of gene flow, probably linked to insect dispersal. The gene diversity of C. tsitsikammensis was similar to that of a related fungus, Ceratocystis albifundus, that is known to be native in Africa. This, together with the fact that C. tsitiskamensis is not known elsewhere, within or outside South Africa, suggests that it is native and endemic to the Cape Afromontane region.

Cornuvesica: A little known mycophilic genus with a unique biology and unexpected new species

Little is known about the biology of the monotypic genus Cornuvesica (Microascales), apart from that isolates are notoriously difficult to culture on artificial media. A recent collection of material resembling this genus from freshly made wounds on Gmelina arborea in Indonesia, provided an opportunity to reconsider all available material of Cornuvesica falcata, type species of the genus. In addition to morphological comparisons, multigene phylogenetic analyses were made using sequences of the SSU, ITS, LSU and TEF-1α genes. Our results showed that the holotype of Cor. falcata from pine in Canada differed from all other material previously considered to represent this species and also from the new Indonesian collections. The collections considered represented three additional species that we describe here as new. Three New Zealand isolates and an isolate from UK were respectively described as Cor. acuminata and Cor. crypta, while the Indonesian isolates were described as Cor. magnispora. Phylogenies based on the SSU and LSU data sets showed that Cornuvesica spp. do not belong in the Ceratocystidaceae as previously suggested, but represent a distinct lineage in the Microascales that has yet to be named. Results showed that culture filtrates from other fungi or ferric chloride markedly stimulated the growth of Cor. magnispora.

Redefining Ceratocystis and allied genera

The genus Ceratocystis was established in 1890 and accommodates many important fungi. These include serious plant pathogens, significant insect symbionts and agents of timber degradation that result in substantial economic losses. Virtually since its type was described from sweet potatoes, the taxonomy of Ceratocystis has been confused and vigorously debated. In recent years, particulary during the last two decades, it has become very obvious that this genus includes a wide diversity of very different fungi. These have been roughly lumped together due to their similar morphological structures that have clearly evolved through convergent evolution linked to an insect-associated ecology. As has been true for many other groups of fungi, the emergence of DNA-based sequence data and associated phylogenetic inferences, have made it possible to robustly support very distinct boundaries defined by morphological characters and ecological differences. In this study, DNA-sequence data for three carefully selected gene regions (60S, LSU, MCM7) were generated for 79 species residing in the aggregate genus Ceratocystis sensu lato and these data were subjected to rigorous phylogenetic analyses. The results made it possible to distinguish seven major groups for which generic names have been chosen and descriptions either provided or emended. The emended genera included Ceratocystis sensu stricto, Chalaropsis, Endoconidiophora, Thielaviopsis, and Ambrosiella, while two new genera, Davidsoniella and Huntiella, were described. In total, 30 new combinations have been made. This major revision of the generic boundaries in the Ceratocystidaceae will simplify future treatments and work with an important group of fungi including distantly related species illogically aggregated under a single name.

Reconsidering species boundaries in the Ceratocystis paradoxa complex, including a new species from oil palm and cacao in Cameroon

The Ceratocystis paradoxa complex accommodates a group of fungal pathogens that have become specialized to infect mostly monocotyledonous plants. Four species currently are recognized in this group, including C. paradoxa, which has a widespread distribution and broad host range. In this study, multigene phylogenetic analyses involving sequences of the ITS, β-tubulin and TEF-1α gene loci, in combination with phenotypic and mating studies, were used to characterize purported C. paradoxa isolates from Cameroon and to compare them with isolates from elsewhere, including protologs and type specimens of known species. We show that the C. paradoxa complex comprises substantially greater species diversity than previously recognized. One new species in this group is described from Cameroon as Ceratocystis cerberus, while C. paradoxa sensu stricto (s. str.) and four other species are redefined. Lectotypes are designated for C. ethacetica and Endoconidium fragrans (synonym of C. ethacetica), while epitypes are designated for C. paradoxa s. str., C. ethacetica and C. musarum. A neotype is designated for Catenularia echinata (synonym of C. ethacetica) and two species, previously treated in Thielaviopsis, are transferred to Ceratocystis.

Monilochaetes and allied genera of the Glomerellales, and a reconsideration of families in the Microascales

We examined the phylogenetic relationships of two species that mimic Chaetosphaeria in teleomorph and anamorph morphologies, Chaetosphaeriatulasneorum with a Cylindrotrichum anamorph and Australiasca queenslandica with a Dischloridium anamorph. Four data sets were analysed: a) the internal transcribed spacer region including ITS1, 5.8S rDNA and ITS2 (ITS), b) nc28S (ncLSU) rDNA, c) nc18S (ncSSU) rDNA, and d) a combined data set of ncLSU-ncSSU-RPB2 (ribosomal polymerase B2). The traditional placement of Ch. tulasneorum in the Microascales based on ncLSU sequences is unsupported and Australiasca does not belong to the Chaetosphaeriaceae. Both holomorph species are nested within the Glomerellales. A new genus, Reticulascus, is introduced for Ch. tulasneorum with associated Cylindrotrichum anamorph; another species of Reticulascus and its anamorph in Cylindrotrichum are described as new. The taxonomic structure of the Glomerellales is clarified and the name is validly published. As delimited here, it includes three families, the Glomerellaceae and the newly described Australiascaceae and Reticulascaceae. Based on ITS and ncLSU rDNA sequence analyses, we confirm the synonymy of the anamorph genera Dischloridium with Monilochaetes. Consequently Dischloridium laeënse, type species of the genus, and three related species are transferred to the older genus Monilochaetes. The teleomorph of D. laeënse is described in Australiasca as a new species. The Plectosphaerellaceae, to which the anamorph genus Stachylidium is added, is basal to the Glomerellales in the three-gene phylogeny. Stilbella annulata also belongs to this family and is newly combined in Acrostalagmus. Phylogenetic analyses based on ncLSU, ncSSU, and combined ncLSU-ncSSU-RPB2 sequences clarify family relationships within the Microascales. The family Ceratocystidaceae is validated as a strongly supported monophyletic group consisting of Ceratocystis, Cornuvesica, Thielaviopsis, and the type species of Ambrosiella. The new family Gondwanamycetaceae, a strongly supported sister clade to the Ceratocystidaceae, is introduced for the teleomorph genus Gondwanamyces and its Custingophora anamorphs. Four families are accepted in the Microascales, namely the Ceratocystidaceae, Gondwanamycetaceae, Halosphaeriaceae, and Microascaceae. Because of a suggested affinity of a Faurelina indica isolate to the Microascales, the phylogenetic position of the Chadefaudiellaceae is reevaluated. Based on the results from a separate ncLSU analysis of the Dothideomycetes, Faurelina is excluded from the Microascales and placed in the Pleosporales.