Gene genealogies reveal global phylogeographic structure and reproductive isolation among lineages of Fusarium graminearum, the fungus causing wheat scab

Strategies for managing Fusarium head blight and deoxynivalenol accumulation in wheat

Many mycotoxigenic fungi infect plant hosts and cause disease in the field. Therefore, control of field infection by these fungi is a critical step in managing mycotoxin accumulation in the harvested product. Fusarium graminearum, also known as Gibberella zeae, is the causal agent of Fusarium head blight (FHB), or scab, in cereals and is also the primary agent responsible for contamination of grain with deoxynivalenol (DON). Research efforts worldwide are devoted to the development of strategies to control field infection of wheat and barley by this pathogen. Strategies include the use of fungicides and biological control agents to protect flowering heads from infection. There is extensive effort in breeding for host resistance to infection and spread of the pathogen within the heads. Scientists are also seeking exogenous traits to introduce into cereals to enhance resistance. Cultural practices are also being examined, primarily as measures to reduce pathogen survival and inoculum production in crop residues. The successes and limitations of these strategies in the management of Fusarium head blight and deoxynivalenol are discussed.

Epidemiology of Fusarium head blight on small-grain cereals

Fusarium head blight (FHB) is one of the most serious diseases affecting wheat and barley worldwide. It is caused by Fusarium graminearum along with F. culmorum, F. avenaceum and other related fungi. These fungi also produce several mycotoxins. Though the disease results in reduced seed quality and yield, the toxins which may accompany the disease are often a more serious problem. Pathogen inoculum is usually very abundant, however production and dispersal of inoculum are weather-sensitive processes. An abundance of colonized substrate (i.e. maize or cereal debris) in a region contributes to airborne inoculum throughout the area. Local residues beneath the cereal crop (i.e. from previous crop) may have a less obvious effect, particularly in regions where long-distance dispersal is likely due to wind conditions. The host is most susceptible to infection at anthesis and shortly thereafter. A warm, moist environment characterized by frequent precipitation or heavy dew is highly favorable to fungal growth, infection and development of disease in head tissues. As the fungus grows, it produces mycotoxins which are water-soluble and may be translocated between tissues or leeched from source tissues. Important epidemiological issues have arisen recently and include an apparent shift in prevalence of Fusarium species on infected heads in Europe toward F. graminearum; and the presence of multiple chemotypes and aggressiveness variants within a species in a region.

Real-time PCR Quantification and Mycotoxin Production of Fusarium graminearum in Wheat Inoculated with Isolates Collected from Potato, Sugar Beet, and Wheat

ABSTRACT Fusarium graminearum causes Fusarium head blight (FHB) in small grains worldwide. Although primarily a pathogen of cereals, it also can infect noncereal crops such as potato and sugar beet in the United States. We used a real-time polymerase chain reaction (PCR) method based on intergenic sequences specific to the trichodiene synthase gene (Tri5) from F. graminearum. TaqMan probe and primers were designed and used to estimate DNA content of the pathogen (FgDNA) in the susceptible wheat cv. Grandin after inoculation with the 21 isolates of F. graminearum collected from potato, sugar beet, and wheat. The presence of nine mycotoxins was analyzed in the inoculated wheat heads by gas chromatography and mass spectrometry. All isolates contained the Tri5 gene and were virulent to cv. Grandin. Isolates of F. graminearum differed significantly in virulence (expressed as disease severity), FgDNA content, and mycotoxin accumulation. Potato isolates showed greater variability in producing different mycotoxins than sugar beet and wheat isolates. Correlation analysis showed a significant (P < 0.001) positive relationship between FgDNA content and FHB severity or deoxynivalenol (DON) production. Moreover, a significant (P < 0.001) positive correlation between FHB severity and DON content was observed. Our findings revealed that F. graminearum causing potato dry rot and sugar beet decay could be potential sources of inoculum for FHB epidemics in wheat. Real-time PCR assay provides sensitive and accurate quantification of F. graminearum in wheat and can be useful for monitoring the colonization of wheat grains by F. graminearum in controlled environments, and evaluating wheat germplasms for resistance to FHB.

Determination of the trichothecene mycotoxin chemotypes and associated geographical distribution and phylogenetic species of the Fusarium graminearum clade from China

A large number of isolates from the Fusarium graminearum clade representing all regions in China with a known history of Fusarium head blight (FHB) epidemics in wheat were assayed using PCR to ascertain their trichothecene mycotoxin chemotypes and associated phylogenetic species and geographical distribution. Of the 299 isolates assayed, 231 are from F. asiaticum species lineage 6, which produce deoxynivalenol and 3-acetyldeoxynivalenol (3-AcDON); deoxynivalenol and 15-acetyldeoxynivalenol (15-AcDON); and nivalenol and 4-acetylnivalenol (NIV) mycotoxins, with 3-AcDON being the predominant chemotype. Ninety-five percent of this species originated from the warmer regions where the annual average temperatures were above 15 degrees C, based on the climate data of 30 y during 1970-1999. However, 68 isolates within F. graminearum species lineage 7 consisted only of 15-AcDON producers, 59% of which were from the cooler regions where the annual average temperatures were 15 degrees C or lower. Identification of a new subpopulation of 15-AcDON producers revealed a molecular distinction between F. graminearum and F. asiaticum that produce 15-AcDON. An 11-bp repeat is present in F. graminearum within their Tri7 gene sequences but is absent in F. asiaticum, which could be directly used for differentiating the two phylogenetic species of the F. graminearum clade.

Speciation in Pyricularia inferred from multilocus phylogenetic analysis

Pyricularia isolates from various host plants were subjected to a multilocus phylogenetic analysis based on rDNA-ITS, actin, beta-tubulin, and calmodulin loci. A combined gene tree resolved seven groups with 100% BS support, suggesting that they are monophyletic groups supported concordantly by all four loci. By incorporating biological and morphological species criteria, each of the seven groups was considered to be a current species. However, phylogenetic relationships among these species were unresolved in the single-gene trees and in the combined tree. Furthermore, the transition from concordance to conflict occurred more than once in the combined gene tree. They were interpreted by assuming that Pyricularia has evolved through repeated species radiation. The transition point other than the current species limit was considered to be the limit of the former species.

Accelerated nrDNA evolution and profound AT bias in the medicinal fungus Cordyceps sinensis

Cordyceps sinensis is a reputed medicinal fungus growing parasitically on buried larvae of ghost moths in Asian high-altitude grassland ecosystems. We have analysed the intraspecific ITS nrDNA (ITS1, 5.8S gene, ITS2) variation among 71 sequences of C. sinensis available in EMBL/Genbank. The ITS sequences, submitted to Bayesian ML analyses, were distributed into five groups, referred to as A-E. Nine of the sequences (groups D and E) grouped with distantly related hypocrealean/clavicipitalean taxa and are interpreted as sequences erroneously accessioned under wrong taxon names. The remaining 62 sequences constituted three highly supported clades (groups A-C), that may represent cryptic (phylogenetic) species currently ascribed to C. sinensis. A remarkably high sequence divergence occurred in the 5.8S gene between the three groups. Sequences of groups B and C showed accelerated substitution rates and high AT nucleotide bias. We hypothesize that the accelerated evolution and AT bias have been caused by a shift in life historical attributes or ecology. We also suggest that the recorded differences in medicinal effects among C. sinensis populations may be attributed to the existence of genetically differentiated chemotypes in this morphotaxon.

Pronounced ecological separation between two closely related lineages of the polyporous fungus Gloeoporus taxicola

Gloeoporus taxicola is a saprotrophic polypore that produces annual fruit bodies, in Scandinavia exclusively on coniferous wood. In this study, we demonstrate the existence of two ecotypes of G. taxicola in Norway; a coastal lowland form fruiting on dead logs and dying branches of standing Pinus sylvestris, and a highland form largely fruiting on trunks of Picea abies in moist, old-growth forests. Thus, a distinct ecological separation has happened between the two forms both concerning substrate and environmental requirements, leading to a rather distinct difference in distribution. Genetically, there seems to be a more stepwise transition between the two forms, as revealed by sequence analyses of two DNA regions. Genetic variation was significantly higher in the highland form compared with the lowland form, which could be due to disparate immigration histories in Scandinavia. Different evolutionary scenarios may explain the observed pattern, including a former allopatric distribution in isolated glacial refugia, succeeded by immigration into the same region. The partition of the genetic variation among the two forms suggests a recent or ongoing speciation event and possibly some ongoing introgression. Our findings have implications on the conservations status of the taxon, as their susceptibility to modern forestry seems to be different.

Multilocus phylogenetic analyses within Blumeria graminis, a powdery mildew fungus of cereals

Blumeria graminis, a powdery mildew fungus, is an important plant pathogen that causes serious damage to a variety of cereal crops. In spite of the importance of the pathogen, information on phylogenetic structure within B. graminis is scarce. In this study we conducted phylogenetic analyses of B. graminis based on the DNA sequences of four different DNA regions (ITS, 28S rDNA, chitin synthase 1, and beta-tubulin). The analyses revealed that the protein-coding regions have higher amounts of phylogenetic signals than rDNA regions and are useful for phylogenetic analyses of B. graminis. The present phylogenetic analyses revealed nine distinct groups in the B. graminis isolates used in this study, a result which was commonly supported by all trees constructed from the four DNA regions. Isolates from a single host genus belonged to a single group except for isolates from Lolium and Bromus, in which the isolates were split into two and three groups, respectively. Isolates from Agropyron, Secale and Triticum formed a distinct clade (Triticum clade) with identical or similar DNA sequences. The Hordeum clade was a sister of the Triticum clade, and Poa and Avena clades were distantly related to the Triticum and Hordeum clades. This phylogenetic relationship of B. graminis is well concordant with the level of reproductive isolation between formae speciales and also with phylogeny inferred from a cytological study. Shimodaira-Hasegawa and Templeton tests using sequences of four different DNA regions significantly rejected the tree topology of plants. Therefore, possibility of co-speciation between B. graminis and its host plants was obscure in this study.

Eukaryotic microbes, species recognition and the geographic limits of species: examples from the kingdom Fungi

The claim that eukaryotic micro-organisms have global geographic ranges, constituting a significant departure from the situation with macro-organisms, has been supported by studies of morphological species from protistan kingdoms. Here, we examine this claim by reviewing examples from another kingdom of eukaryotic microbes, the Fungi. We show that inferred geographic range of a fungal species depends upon the method of species recognition. While some fungal species defined by morphology show global geographic ranges, when fungal species are defined by phylogenetic species recognition they are typically shown to harbour several to many endemic species. We advance two non-exclusive reasons to explain the perceived difference between the size of geographic ranges of microscopic and macroscopic eukaryotic species when morphological methods of species recognition are used. These reasons are that microbial organisms generally have fewer morphological characters, and that the rate of morphological change will be slower for organisms with less elaborate development and fewer cells. Both of these reasons result in fewer discriminatory morphological differences between recently diverged lineages. The rate of genetic change, moreover, is similar for both large and small organisms, which helps to explain why phylogenetic species of large and small organisms show a more similar distribution of geographic ranges. As a consequence of the different rates in fungi of genetic and morphological changes, genetic isolation precedes a recognizable morphological change. The final step in speciation, reproductive isolation, also follows genetic isolation and may precede morphological change.

Suitability of methods for species recognition in the Phialocephala fortinii-Acephala applanata species complex using DNA analysis

Sequence data of two coding and three non-coding loci were used to study the taxonomic identity within and relatedness among seven previously defined cryptic species (CSP) of Phialocephala fortinii and Acephala applanata using two approaches of species recognition. Identification of taxonomic groups corresponding to CSP was ambiguous in some cases when applying solely the genealogical concordance phylogenetic species recognition (GCPSR) concept. The definition of groups corresponding to CSP using GCPSR was complicated due to shared sequence haplotypes between CSP, unresolved CSP for several loci, and possible introgression. GCPSR in conjunction with a population genetic approach improved resolution significantly and the CSP status could be confirmed for all seven CSP of P. fortinii s.l. The most critical step in both analyses was the definition of groups. The combination of several classes of markers differing in resolution helped to define species boundaries.

Combinatorially selected defense peptides protect plant roots from pathogen infection

Agricultural productivity and sustainability are continually challenged by emerging and indigenous pathogens. Currently, many pathogens can be combatted only with biocides or environmentally dangerous fumigants. Here, we report a rapid and pathogen-specific strategy to reduce infection by organisms that target plant roots. Combinatorially selected defense peptides, previously shown to effect premature encystment of Phytophthora capsici zoospores, were fused to maize cytokinin oxidase/dehydrogenase as a display scaffold. When expressed in tomato roots, the peptide-scaffold constructs were secreted and accumulated to sufficient concentrations in the rhizosphere to induce zoospore encystment and thereby deter taxis to the root surface. Pathogen infection was significantly inhibited in roots expressing bioactive peptides fused to the maize cytokinin oxidase/dehydrogenase scaffold. This peptide-delivery technology is broadly applicable for rapid development of plant defense attributes against plant pathogens.

Functional characterization of acetylglutamate synthase and phosphoribosylamine-glycine ligase genes in Gibberella zeae

Gibberella zeae (anamorph, Fusarium graminearum) is an important pathogen of cereal crops found in many regions of the world. In this study, we have characterized two auxotrophic strains, designated S4B1279 and S4B3008, which were discovered from a collection of insertional mutants of G. zeae generated by restriction enzyme-mediated integration (REMI). Both mutant strains exhibited pleiotropic phenotypic changes that include reduction of mycelial growth and virulence and abolished sexual reproduction. Molecular analysis of the REMI mutants has shown that the auxotrophy of S4B1279 is due to a mutation of the ARG2 gene encoding an acetylglutamate synthase, and the auxotrophy of S4B3008 is due to a mutation of the ADE5 gene encoding a phosphoribosylamine-glycine ligase. Subsequent gene disruption and complementation studies have confirmed the functions for ARG2 and ADE5, respectively, in G. zeae. Our study has demonstrated the feasibility of using the REMI technique in studying G. zeae virulence mechanisms, in addition to providing two new selectable markers allowing genetic transformation of the fungus.

Multilocus sequence analysis of Fusarium pseudograminearum reveals a single phylogenetic species

Fusarium pseudograminearum causes crown rot of wheat in Australia and most other wheat growing regions, but its evolutionary history is largely unknown. We demonstrate for the first time that F. pseudograminearum is a single phylogenetic species without consistent lineage development across genes. Isolates of F. pseudograminearum, F. graminearum sensu lato, and F. cerealis, were collected from four countries and four single copy, nuclear genes were partially sequenced, aligned with previously published sequences of these and related species, and analysed by maximum parsimony and Bayesian inference. Evolutionary divergence varied between genes, with high phylogenetic incongruence occurring between the gene genealogies. The absence of geographic differentiation between isolates indicates that the introduction of new fungal strains to a region has the potential to introduce new pathogenic and toxigenic genes into the native population through sexual recombination.

Hybridization among cryptic species of the cellar fungus Coniophora puteana (Basidiomycota)

In this study we have analysed the genetic variation and phylogeography in a global sample of the cellar fungus Coniophora puteana, which is an important destroyer of wooden materials indoor. Multilocus genealogies of three DNA regions (beta tubulin, nrDNA ITS and translation elongation factor 1alpha) revealed the occurrence of three cryptic species (PS1-3) in the morphotaxon C. puteana. One of the lineages (PS3) is apparently restricted to North America while the other two (PS1-2) have wider distributions on multiple continents. Interspecific hybridization has happened between two of the lineages (PS1 and PS3) in North America. In three dikaryotic isolates, two highly divergent beta tubulin alleles coexisted, one derived from PS1 and one from PS3. Furthermore, one isolate included a recombinant ITS sequence, where ITS1 resembled the ITS1 version of PS3 while ITS2 was identical to a frequent PS1 ITS2 version. This pattern must be due to hybridization succeeded by intralocus recombination in ITS. The results further indicated that introgression has happened between subgroups appearing in PS1. We hypothesize that the observed reticulate evolution is due to previous allopatric separation followed by more recent reoccurrence in sympatry, where barriers to gene flow have not yet evolved. A complex phylogeographical structure is observed in the morphotaxon C. puteana caused by (i) cryptic speciation; (ii) the interplay between natural migration and distribution patterns and probably more recent human mediated dispersal events; and (iii) hybridization and introgression.

Nonribosomal peptide synthetase (NPS) genes in Fusarium graminearum, F. culmorum and F. pseudograminearium and identification of NPS2 as the producer of ferricrocin

Fungi have the potential to produce a wide range of secondary metabolites including polyketides and small peptides produced by nonribosomal peptide synthetases (NPS). Fusarium graminearum is a mycotoxin producing pathogen of cereals and knowledge of the infection process is essential for the development of disease control. Bioinformatics provide a means to identify genes encoding NPSs, the products of which may act as fungal virulence factors. The F. graminearum genome sequence was analysed and similarity searches and application of prediction server service identified 15 putative NPS genes. NPS1 and NPS2, were found to be related to genes involved in NPS hydroxamate siderophore biosynthesis and chemical analysis of a F. graminearum NPS2 deletion mutant showed that this gene encodes the NPS responsible for the biosynthesis of ferricrocin. The expression of the NPS genes was analysed in Fusarium culmorum. NPS1 and NPS19 differed from the remainder of the genes, as they were only expressed during infection of barley roots and not under the different culture conditions tested. Strains of F. graminearum, F. culmorum and Fusarium pseudograminearum were examined for the presence and expression of the 15 identified NPS genes. With the exception of NPS18, that is absent in F. pseudograminearum, all the NPS genes are represented in the diffferent species. Lack of transcripts from some genes and the presence of frameshift and stop codons in four of the NPS genes in the sequenced F. graminearum strain suggest that some are pseudogenes.

Genetic Diversity of Gibberella zeae Isolates from Manitoba

Gibberella zeae (anamorph Fusarium graminearum) causes Fusarium head blight, one of the most important diseases of cereals in the Canadian prairies for the last decade. In 2002, 60 isolates of G. zeae were collected and single spored from naturally infected spikes of wheat from Carman and Winnipeg in Manitoba. These isolates were compared using vegetative compatibility analysis and polymerase chain reaction (PCR)-based sequence related amplified polymorphisms (SRAP). Sixteen vegetative compatibility groups (VCG) were found among the 50 isolates tested. Five VCGs were found in the two locations, five in Carman and six in Winnipeg. Eight SRAP primer pairs amplified 90 polymorphic DNA fragments from 60 isolates and identified 59 distinct haplotypes. Among seven pairs of isolates, each pair from a distinct spike, four had isolates with different VCGs and six comprised different SRAP haplotypes. Principal component analysis and UPGMA separated the dataset into two main groups, each with isolates from both locations. The analysis of molecular variance also revealed that 75 and 20% of the variance was associated with differences among individual isolates and varieties sampled, respectively. Geographic location was not a significant source of variation at P = 0.05 and accounted for only 4% of total variance. A low correlation between VCG and SRAP marker data was detected. This study showed that, although genetic diversity is high among G. zeae isolates, Carman and Winnipeg collections have a similar genetic makeup and are likely part of the same population. The significant proportion of variance accounted by the variety compared with the geographic origin of isolates suggests that seedborne inoculum might have contributed to the genetic diversity within the G. zeae collection under study.

Genetic Relationships Between Cercospora kikuchii Populations from South America and Japan

ABSTRACT A collection 160 isolates of Cercospora kikuchii was made from South America and 245 from Japan. DNA fingerprint patterns were analyzed based on amplified fragment length polymorphism among the sample isolates, dividing the isolates into seven lineages (I to VII). Partial nucleotide sequence analyses of the beta-tubulin gene supported this division into seven lineages. Lineages I and III commonly existed in South America and Japan. In all, 136 of the 160 isolates from South America and 223 of the 245 isolates from Japan belonged to lineage I, indicating that lineage I was the major lineage in each area; 5 isolates from South America and 8 isolates from Japan belonged to lineage III. Lineages II (12 isolates) and IV (2 isolates) were specific to Japan and lineages V (3 isolates), VI (1 isolate), and VII (15 isolates) specifically existed in South America. These results suggest that the population genetic structure of C. kikuchii was different between South America and Japan, but the dominance of lineage I was common between the two areas.

Genetic Structure of Atmospheric Populations of Gibberella zeae

ABSTRACT Gibberella zeae, causal agent of Fusarium head blight (FHB) of wheat and barley and Gibberella ear rot (GER) of corn, may be transported over long distances in the atmosphere. Epidemics of FHB and GER may be initiated by regional atmospheric sources of inoculum of G. zeae; however, little is known about the origin of inoculum for these epidemics. We tested the hypothesis that atmospheric populations of G. zeae are genetically diverse by determining the genetic structure of New York atmospheric populations (NYAPs) of G. zeae, and comparing them with populations of G. zeae collected from seven different states in the northern United States. Viable, airborne spores of G. zeae were collected in rotational (lacking any apparent within-field inoculum sources of G. zeae) wheat and corn fields in Aurora, NY in May through August over 3 years (2002 to 2004). We evaluated 23 amplified fragment length polymorphism (AFLP) loci in 780 isolates of G. zeae. Normalized genotypic diversity was high (ranging from 0.91 to 1.0) in NYAPs of G. zeae, and nearly all of the isolates in each of the populations represented unique AFLP haplotypes. Pairwise calculations of Nei's unbiased genetic identity were uniformly high (>0.99) for all of the possible NYAP comparisons. Although the NYAPs were genotypically diverse, they were genetically similar and potentially part of a large, interbreeding population of G. zeae in North America. Estimates of the fixation index (G(ST)) and the effective migration rate (Nm) for the NYAPs indicated significant genetic exchange among populations. Relatively low levels of linkage disequilibrium in the NYAPs suggest that outcrossing is common and that the populations are not a result of a recent bottleneck or invasion. When NYAPs were compared with those collected across the United States, the observed genetic identities between the populations ranged from 0.92 to 0.99. However, there was a significant negative correlation (R = -0.59, P < 0.001) between genetic identity and geographic distance, suggesting that some genetic isolation may occur on a continental scale. The contribution of long-distance transport of G. zeae to regional epidemics of FHB and GER remains unclear, but the diverse atmospheric populations of G. zeae suggest that inoculum may originate from multiple locations over large geographic distances. Practically, the long-distance transport of G. zeae suggests that management of inoculum sources on a local scale, unless performed over extensive production areas, will not be completely effective for the management of FHB and GER.

A highly efficient transient protoplast system for analyzing defence gene expression and protein-protein interactions in rice

SUMMARY The transient assay system based on mesophyll or cultured cell-derived protoplasts has been exploited in several plant species and has become a powerful tool for rapid gene functional analysis and biochemical manipulations. However, the system has not been widely used in rice owing to the difficulties in large-scale isolation of viable rice protoplasts from leaves or suspension-cultured cells. Here, we describe a significantly improved method to isolate a large number of protoplasts from stem and sheath tissues of both young and mature plants. High-level coexpression of multiple constructs and efficient suppression of exogenous and endogenous genes were observed in the stem- and sheath-derived protoplasts. A transient green fluorescent protein and luciferase-based reporter system for defence-related genes expression analysis has been established, which is useful for screening and characterizing genes involved in rice defence signalling pathways. Furthermore, a protoplast-based bimolecular fluorescence complementation (BiFC) system for the detection of protein-protein interactions in living rice cells was developed. The YFP complementation of two split-YFP halves mediated by homodimerization of the GUS and SPIN1, a cell-death related protein, was observed in transfected protoplasts. In combination with genetic, genomic and proteomic approaches, the established versatile protoplast transient assay system will facilitate large-scale functional analysis of defence-related genes in rice.

Genomic analysis of host-pathogen interaction between Fusarium graminearum and wheat during early stages of disease development

Fusarium graminearum strains responsible for causing the plant disease Fusarium head blight vary greatly in their ability to cause disease and produce mycotoxins on wheat. With the goal of understanding fungal gene expression related to pathogenicity, three cDNA libraries were created by suppression subtractive hybridization using wheat heads inoculated with a highly aggressive strain and either water or a less aggressive strain of this pathogen. Eighty-four fungal genes expressed during initial disease development were identified. The probable functions of 49 of these genes could be inferred by bioinformatic analysis. Thirty-five ESTs had no known homologues in current databases and were not identified by ab initio gene prediction methods. These ESTs from infected wheat heads probably represent F. graminearum genes that previously were not annotated. Four genes represented in one of these libraries were selected for targeted gene replacement, leading to the characterization of a two-component response regulator homologue involved in pathogenicity of the fungus. The mutants for this gene showed reduced sporulation and delayed spread of Fusarium head blight on wheat.

A northward colonisation of the Andes by the potato cyst nematode during geological times suggests multiple host-shifts from wild to cultivated potatoes

The cyst nematode Globodera pallida is a major pest of potato in South America where this specialist parasite is native. To investigate its phylogeography, we have genotyped individuals from 42 Peruvian populations using mitochondrial and nuclear molecular markers. A clear south-to-north phylogeographical pattern was revealed with five well-supported clades. The clade containing the southern populations is genetically more diverse and forms the most basal branch. The large divergence among cytochrome b haplotypes suggests that they diverged before human domestication of potato. As the nematodes studied have been sampled on cultivated potato, multiple host-shifts from wild to cultivated potatoes must have occurred independently in each clade. We hypothesise that this south-to-north pattern took place during the uplift of the Andes beginning 20 My ago and following the same direction. To our knowledge, this is the first study of a plant parasite sampled on cultivated plants revealing an ancient phylogeographical pattern.

The PKS4 gene of Fusarium graminearum is essential for zearalenone production

Zearalenones are produced by several Fusarium species and can cause reproductive problems in animals. Some aurofusarin mutants of Fusarium pseudograminearum produce elevated levels of zearalenone (ZON), one of the estrogenic mycotoxins comprising the zearalenones. An analysis of transcripts from polyketide synthase genes identified in the Fusarium graminearum database was carried out for these mutants. PKS4 was the only gene with an enoyl reductase domain that had a higher level of transcription in the aurofusarin mutants than in the wild type. An Agrobacterium tumefaciens-mediated transformation protocol was used to replace the central part of the PKS4 gene with a hygB resistance gene through double homologous recombination in an F. graminearum strain producing a high level of ZON. PCR and Southern analysis of transformants were used to identify isolates with single insertional replacements of PKS4. High-performance liquid chromatography analysis showed that the PKS4 replacement mutant did not produce ZON. Thus, PKS4 encodes an enzyme required for the production of ZON in F. graminearum. Barley root infection studies revealed no alteration in the pathogenicity of the PKS4 mutant compared to the pathogenicity of the wild type. The expression of PKS13, which is located in the same cluster as PKS4, decreased dramatically in the mutant, while transcription of PKS4 was unchanged. This differential expression may indicate that ZON or its derivatives do not regulate expression of PKS4 and that the PKS4-encoded protein or its product stimulates expression of PKS13. Furthermore, both the lack of aurofusarin and ZON influenced the expression of other polyketide synthases, demonstrating that one polyketide can influence the expression of others.

Phylogenetic Analyses of Phytopathogenic Isolates of Verticillium spp

ABSTRACT To better understand the genetic relationships between Verticillium dahliae isolates from lettuce and other phytopathogenic Verticillium spp. isolates from various hosts and geographic locations, the complete intergenic spacer (IGS) region of the nuclear ribosomal RNA gene (rDNA) and the beta-tubulin gene were amplified and sequenced. The sequences of the complete IGS region and the beta-tubulin gene were used alone and in combination to infer genetic relationships among different isolates of Verticillium with the maximum-likelihood distance method. Phylogenetic analyses set sequences into four distinct groups comprising isolates of V. albo-atrum, V. tricorpus, and V. dahliae from cruciferous and noncruciferous hosts. Within the four Verticillium groups, isolates of V. dahliae from cruciferous hosts displayed the closest affinity to V. dahliae from noncruciferous hosts. Isolates of V. dahliae from noncruciferous hosts could be further divided into four subgroups based on sequence similarities within the IGS region. Cross-pathogenicity tests demonstrated that most Verticillium isolates were as virulent on other hosts as on their hosts of origin. A phenogram based on the cross pathogenicity of individual isolates resembled those derived from the IGS and beta-tubulin sequence comparisons. On the basis of the data presented, the potential origin of some isolates of V. dahliae pathogenic on lettuce is proposed.

Two different polyketide synthase genes are required for synthesis of zearalenone in Gibberella zeae

Zearalenone (ZEA) is a polyketide mycotoxin produced by some species of Gibberella/Fusarium and causes hyperestrogenic syndrome in animals. ZEA occurs naturally in cereals infected by Gibberella zeae in temperate regions and threatens animal health. In this study, we report on a set of genes that participate in the biosynthesis of ZEA in G. zeae. Focusing on the non-reducing polyketide synthase (PKS) genes of the G. zeae genome, we demonstrated that PKS13 is required for ZEA production. Subsequent analyses revealed that a continuous, 50 kb segment of DNA carrying PKS13 consisted of three additional open reading frames that were coexpressed as a cluster during the condition for ZEA biosynthesis. These genes, in addition to PKS13, were essential for the ZEA biosynthesis. They include another PKS gene (PKS4) encoding a fungal reducing PKS; zearalenone biosynthesis gene 1 (ZEB1), which shows a high similarity to putative isoamyl alcohol oxidase genes; and ZEB2 whose deduced product carries a conserved, basic-region leucine zipper domain. ZEB1 is responsible for the chemical conversion of beta-zearalenonol (beta-ZOL) to ZEA in the biosynthetic pathway, and ZEB2 controls transcription of the cluster members. Transcription of these genes was strongly influenced by different culture conditions such as nutrient starvations and ambient pH. Furthermore, the same set of genes regulated by ZEB2 was dramatically repressed in the transgenic G. zeae strain with the deletion of PKS13 or PKS4 but not in the ZEB1 deletion strain, suggesting that ZEA or beta-ZOL may be involved in transcriptional activation of the gene cluster required for ZEA biosynthesis in G. zeae. This is the first published report on the molecular characterization of genes required for ZEA biosynthesis.

Variability and characterization of mycotoxin-producing Fusarium spp isolates by PCR-RFLP analysis of the IGS-rDNA region

In the present report, a total of 75 Fusarium spp isolates (35 of the Gibberella fujikuroi species complex, 26 of F. oxysporum, 7 of F. graminearum, 5 of F. culmorum, 1 of F. cerealis, and 1 of F. poae) from different hosts were characterized morphologically, physiologically and genetically. Morphological characterization was performed according to macroscopic and microscopic aspects. Physiological characterization was based on their ability to produce fumonisin B1 (FB1), fumonisin B2 (FB2), zearalenone (ZEA) and type B trichothecenes (deoxynivalenol, nivalenol and 3-acetyldeoxynivalenol). FB1, FB2, and ZEA were determined by liquid chromatography and trichothecenes by gas chromatography. Molecular characterization of isolates was carried out using an optimized and simple method for isolation of DNA from filamentous fungi and polymerase chain reaction-restriction fragment length polymorphisms (PCR-RFLP) of the intergenic spacer region (IGS) of the rDNA. The results indicated that G. fujikuroi complex isolates can be divided into low and high fumonisin producers. The haplotypes obtained with HhaI, EcoRI, AluI, PstI and XhoI enzymes provided very characteristic groupings of G. fujikuroi isolates as a function of host type and fumonisin producing capacity. F. graminearum, F. culmorum and F. cerealis isolates were high ZEA and type B trichothecene producers, while F. oxysporum and the G. fujikuroi complex isolates did not show this ability. The haplotypes obtained with CfoI, AluI, HapII, XhoI, EcoRI and PstI enzymes permitted to discern these five Fusarium species and G. fujikuroi complex isolates but the restriction patterns of the IGS region did not show any relationship with the geographic origin of isolates.

Multiple gene genealogies and AFLPs suggest cryptic speciation and long-distance dispersal in the basidiomycete Serpula himantioides (Boletales)

Serpula himantioides (Boletales, Basidiomycota) produces thin resupinate basidiocarps on dead coniferous wood worldwide and causes damage in buildings as well. In this study, we present evidence for the existence of at least three phylogenetically defined cryptic species (referred to as Sib I-III) within the morphospecies S. himantioides, a conclusion based on analyses of sequence data from four DNA regions and amplified fragment length polymorphisms (AFLPS). A low degree of shared sequence polymorphisms was observed among the three lineages indicating a long-lasting separation. The AFLPs revealed two additional subgroups within Sib III. Results from mating studies were consistent with the molecular data. In Sib III, no correspondence between genetic and geographical distance was observed among isolates worldwide, presumably reflecting recent dispersal events. Our results indicate that at least two of the lineages (Sib II and Sib III) have wide sympatric distributions. A population genetic analysis of Sib III isolates, scoring sequence polymorphisms as codominant SNP markers, indicates that panmictic conditions exist in the Sib III group. This study supports the view that cryptic speciation is a common phenomenon in basidiomycete fungi and that phylogenetic species recognition can be a powerful inference to detect cryptic species. Furthermore, this study shows that AFLP data are a valuable supplement to DNA sequence data in that they may detect a finer level of genetic variation.

Low genetic variation and no detectable population structure in aspergillus fumigatus compared to closely related Neosartorya species

Aspergillus fumigatus is an anamorphic euascomycete mold with a ubiquitous presence worldwide. Despite intensive work to understand its success as a pathogen infecting immunosuppressed patients, the population dynamics and recent evolutionary history of A. fumigatus remain understudied. We examined patterns of genetic variation at three intergenic loci for 70 natural isolates from Europe, North America, South America, Asia, Africa, and Australia. The same loci were used to analyze within-population genetic variation for 33 isolates obtained from five geographic locations. Neither data set detected evidence of population differentiation or found any association between the genetic and geographic distances among these isolates. No evidence for genetic differentiation within the two A. fumigatus mating types was detected. The genetic diversity of A. fumigatus, contrasted with that of its close teleomorphic relatives, Neosartorya fischeri and Neosartorya spinosa, is remarkably low.

Development of a Fusarium graminearum Affymetrix GeneChip for profiling fungal gene expression in vitro and in planta

Recently the genome sequences of several filamentous fungi have become available, providing the opportunity for large-scale functional analysis including genome-wide expression analysis. We report the design and validation of the first Affymetrix GeneChip microarray based on the entire genome of a filamentous fungus, the ascomycetous plant pathogen Fusarium graminearum. To maximize the likelihood of representing all putative genes (approximately 14,000) on the array, two distinct sets of automatically predicted gene calls were used and integrated into the online F. graminearum Genome DataBase. From these gene sets, a subset of calls was manually annotated and a non-redundant extract of all calls together with additional EST sequences and controls were submitted for GeneChip design. Experiments were conducted to test the performance of the F. graminearum GeneChip. Hybridization experiments using genomic DNA demonstrated the usefulness of the array for experimentation with F. graminearum and at least four additional pathogenic Fusarium species. Differential transcript accumulation was detected using the F. graminearum GeneChip with treatments derived from the fungus grown in culture under three nutritional regimes and in comparison with fungal growth in infected barley. The ability to detect fungal genes in planta is surprisingly sensitive even without efforts to enrich for fungal transcripts. The Plant Expression Database (PLEXdb, http://www.plexdb.org) will be used as a public repository for raw and normalized expression data from the F. graminearum GeneChip. The F. graminearum GeneChip will help to accelerate exploration of the pathogen-host pathways that may involve interactions between pathogenicity genes in the fungus and disease response in the plant.

Importance of the life cycle in sympatric host race formation and speciation of pathogens

ABSTRACT Numerous morphological species of pathogenic fungi have been shown to actually encompass several genetically isolated lineages, often specialized on different hosts and, thus, constituting host races or sibling species. In this article, we explore theoretically the importance of some aspects of the life cycle on the conditions of sympatric divergence of host races, particularly in fungal plant pathogens. Because the life cycles classically modeled by theoreticians of sympatric speciation correspond to those of free-living animals, sympatric divergence of host races requires the evolution of active assortative mating or of active host preference if mating takes place on the hosts. With some particular life cycles with restricted dispersal between selection on the host and mating, we show that divergence can occur in sympatry and lead to host race formation, or even speciation, by a mere process of specialization, with strong divergent adaptive selection. Neither active assortative mating nor active habitat choice is required in these cases, and this may explain why the phylo-genetic species concept seems more appropriate than the biological species concept in these organisms.

Identification of the down-regulated genes in a mat1-2-deleted strain of Gibberella zeae, using cDNA subtraction and microarray analysis

Gibberella zeae (anamorph: Fusarium graminearum), a self-fertile ascomycete, is an important pathogen of cereal crops. Here, we have focused on the genes specifically controlled by the mating type (MAT) locus, a master regulator of sexual developmental process in G. zeae. To identify these genes, we employed suppression subtractive hybridization between a G. zeae wild-type strain Z03643 and the isogenic self-sterile mat1-2 strain T43deltaM2-2. Both reverse Northern and cDNA microarray analyses using 291 subtractive unigenes confirmed that 58.8% (171 genes) were significantly down-regulated in T43deltaM2-2. Among these, 98 could be either manually or automatically annotated based on known functions of their possible homologs. Northern blot analysis revealed that all of the genes examined were differentially regulated by MAT1-2 during sexual development. This study is the first report on the set of genes that are transcriptionally altered by the deletion of MAT1-2 during sexual reproduction in G. zeae.

GIP2, a putative transcription factor that regulates the aurofusarin biosynthetic gene cluster in Gibberella zeae

Gibberella zeae (anamorph: Fusarium graminearum) is an important pathogen of maize, wheat, and rice. Colonies of G. zeae produce yellow-to-tan mycelia with the white-to-carmine red margins. In this study, we focused on nine putative open reading frames (ORFs) closely linked to PKS12 and GIP1, which are required for aurofusarin biosynthesis in G. zeae. Among them is an ORF designated GIP2 (for Gibberella zeae pigment gene 2), which encodes a putative protein of 398 amino acids that carries a Zn(II)2Cys6 binuclear cluster DNA-binding domain commonly found in transcription factors of yeasts and filamentous fungi. Targeted gene deletion and complementation analyses confirmed that GIP2 is required for aurofusarin biosynthesis. Expression of GIP2 in carrot medium correlated with aurofusarin production by G. zeae and was restricted to vegetative mycelia. Inactivation of the 10 contiguous genes in the DeltaGIP2 strain delineates an aurofusarin biosynthetic gene cluster. Overexpression of GIP2 in both the DeltaGIP2 and the wild-type strains increases aurofusarin production and reduces mycelial growth. Thus, GIP2 is a putative positive regulator of the aurofusarin biosynthetic gene cluster, and aurofusarin production is negatively correlated with vegetative growth by G. zeae.

Pathogenicity and In Planta Mycotoxin Accumulation Among Members of the Fusarium graminearum Species Complex on Wheat and Rice

ABSTRACT Fusarium head blight (FHB), or scab, is a destructive disease of small grains caused by members of the Fusarium graminearum species complex, comprised of at least nine distinct, cryptic species. Members of this complex are known to produce mycotoxins including the trichothecenes deoxynivalenol (DON) along with its acetylated derivatives and nivalenol (NIV). In this study, 31 strains, belonging to eight species of this complex and originating from diverse hosts or substrates, were tested for differences in aggressiveness and mycotoxin production. Large variation among strains, both in terms of their aggressiveness and the ability to produce trichothecenes on a susceptible cultivar of wheat was found; variation appears to be a strain-specific rather than species-specific characteristic. While pathogenicity was not influenced by the type of mycotoxin produced, a significant correlation was observed between the amount of the dominant trichothecene (DON and its acetylated forms or NIV) produced by each strain and its level of aggressiveness on wheat. Some isolates also were tested for their ability to infect rice cv. M201, commonly grown in the United States. While tested strains were capable of infecting rice under greenhouse conditions and causing significant amount of disease, no trichothecenes could be detected from the infected rice florets.

Leptographium longiclavatum sp. nov., a new species associated with the mountain pine beetle, Dendroctonus ponderosae

The mountain pine beetle, Dendroctonus ponderosae, and its fungal associates are devastating the lodgepole pine forests in British Columbia, Canada. During our fungal survey, an unknown Leptographium species has been consistently isolated from both D. ponderosae and infested lodgepole pine (Pinus contorta var. latifolia). This Leptographium species has similar morphology with the Leptographium anamorph of Ophiostoma clavigerum whose association with the D. ponderosae is well known. However, thorough morphological comparisons showed that this fungus is distinct from all the other Leptographium species described in the literature, and especially from O. clavigerum. Comparison of DNA sequences of multiple loci and the profiles by the PCR-RFLP marker also confirmed that this Leptographium species represents an undescribed taxon. Based on its distinct morphological, physiological characteristics and phylogenetic position, we describe it as L. longiclavatum sp. nov.

Relationships Between Weather Conditions, Agronomic Practices, and Fermentation Characteristics with Deoxynivalenol Content in Fresh and Ensiled Maize

The deoxynivalenol (DON) content of maize silage was determined in samples collected at harvest and after ensiling in 2001 and 2002 from 30 to 40 Pennsylvania dairies. Information on cultural practices, hybrid maturity, planting, and harvest date was collected from each site. Site-specific weather data and a corn development model were used to estimate hybrid development at each site. Correlation analysis was used to assess the relationship between weather data, hybrid development, cultural practices and preharvest DON. Fermentation characteristics (moisture, pH, and so on) of ensiled samples were measured to study their relationship to postharvest DON contamination. No significant difference (P ≤ 0.05) was noted between the numbers of samples containing DON in 2001 and 2002, although concentration was higher in 2002 samples. A positive correlation was observed between DON concentration of harvest samples and daily average temperature, minimum temperature, and growing degree day during tasselling, silking, and milk stages. A negative correlation was observed between daily average precipitation at blister stage and DON concentration in harvest samples. Samples from no-till or minimum-till locations had higher DON concentrations than moldboard or mixed-till locations. Harvest samples had higher DON concentration than ensiled samples, suggesting that some physical, chemical, or microbiological changes, resulting from ensiling, may reduce DON in storage.

Chromosome complement of the fungal plant pathogen Fusarium graminearum based on genetic and physical mapping and cytological observations

A genetic map of the filamentous fungus Fusarium graminearum (teleomorph: Gibberella zeae) was constructed to both validate and augment the draft whole-genome sequence assembly of strain PH-1. A mapping population was created from a cross between mutants of the sequenced strain (PH-1, NRRL 31084, originally isolated from Michigan) and a field strain from Minnesota (00-676, NRRL 34097). A total of 111 ascospore progeny were analyzed for segregation at 235 loci. Genetic markers consisted of sequence-tagged sites, primarily detected as dCAPS or CAPS (n = 131) and VNTRs (n = 31), in addition to AFLPs (n = 66) and 7 other markers. While most markers exhibited Mendelian inheritance, segregation distortion was observed for 25 predominantly clustered markers. A linkage map was generated using the Kosambi mapping function, using a LOD threshold value of 3.5. Nine linkage groups were detected, covering 1234 cM and anchoring 99.83% of the draft sequence assembly. The nine linkage groups and the 22 anchored scaffolds from the sequence assembly could be assembled into four chromosomes, leaving only five smaller scaffolds (59,630 bp total) of the nuclear DNA unanchored. A chromosome number of four was confirmed by cytological karyotyping. Further analysis of the genetic map data identified variation in recombination rate in different genomic regions that often spanned several hundred kilobases.

Characterization of Fusarium spp. isolates by PCR-RFLP analysis of the intergenic spacer region of the rRNA gene (rDNA)

In the present study, 44 Fusarium spp. isolates (5 Fusarium culmorum, 7 Fusarium graminearum, 1 Fusarium cerealis, 1 Fusarium poae, 26 Fusarium oxysporum, and 4 Gibberella fujikuroi species complex) were characterized morphologically, physiologically and genetically. All except one (Dutch Collection: CBS 620.72) were isolated from different hosts grown in various Spanish localizations. Morphological characterization was made according to macroscopic and microscopic aspects. Physiological characterization was based on their ability to produce zearalenone (ZEA) and type B trichothecenes (deoxynivalenol, nivalenol and 3-acetyldeoxynivalenol). ZEA was determined by liquid chromatography and trichothecenes by gas chromatography. Confirmation was carried out by liquid chromatography-ion trap-mass spectrometry (ZEA) or gas chromatography-mass spectrometry (trichothecenes). Molecular characterization of isolates was performed using an optimized, simple and low-cost method for isolation of DNA from filamentous fungi and polymerase chain reaction-restriction fragment length polymorphisms (PCR-RFLP) of the intergenic spacer region (IGS) of the rRNA gene (rDNA). The results indicate that F. graminearum, F. culmorum and F. cerealis isolates were high ZEA and type B trichothecene producers, the F. poae isolate produced very low level of nivalenol while F. oxysporum and the G. fujikuroi complex isolates did not show this ability. Restriction patterns of the IGS region did not show any relationship with the host, geographic origin of the isolate and mycotoxin-producing capacity. However, the haplotypes obtained with six restriction enzymes (CfoI, AluI, HapII, XhoI, EcoRI and PstI) permitted to discern the six assayed Fusarium species. Therefore, this is a rapid and suitable methodology that allows closely related strains to group and to estimate the genetic relationships between the groups.

PCR detection assays for the trichothecene-producing species Fusarium graminearum, Fusarium culmorum, Fusarium poae, Fusarium equiseti and Fusarium sporotrichioides

Contamination of small-grain cereals with the fungal species Fusarium graminearum, F. culmorum, F. poae, F. sporotrichioides and F. equiseti is an important source of trichothecenes, Zearalenone and other mycotoxins which cause serious diseases in human and animals. Additionally, these species contribute to Fusarium Head Blight, a disease which produces important losses in cereal yield. Early detection and control of these Fusarium species is crucial to prevent toxins entering the food chain and a useful tool in disease management practices. We describe the development of specific PCR assays to F. graminearum, F. culmorum, F. poae, F. sporotrichioides and F. equiseti using DNA from pure fungal cultures as well as from naturally infected wheat seeds, using in this case a rapid and easy protocol for DNA isolation. The specific primers were designed on the basis of IGS sequences (Intergenic Spacer of rDNA), a multicopy region in the genome that permits to enhance the sensitivity of the assay in comparison with PCR assays based on single-copy sequences.

Fusarium graminearum, F. cortaderiae and F. pseudograminearum in New Zealand: molecular phylogenetic analysis, mycotoxin chemotypes and co-existence of species

Fusarium graminearum and F. pseudograminearum are important plant pathogens in New Zealand and around the world. Headblight and crown rot diseases of cereals caused by these species are responsible for large economic losses due to reduction in seed quality and contamination of grain with tricothecene mycotoxins. In the current study we have used two different molecular phylogenetic approaches, AFLPs and gene genealogies, to gain insight into the evolutionary relationships between F. graminearum, and F. pseudograminearum in New Zealand. The worldwide genetic diversity of F. graminearum clade is represented by at least eight biogeographically distinct species (previously designated as lineages of F. graminearum). Our analysis demonstrated that this clade is represented by F. graminearum (= F. graminearum Lineage 7) and F. cortaderiae (= F. graminearum Lineage 8) in New Zealand. Through our analysis we also confirm the presence of F. pseudograminearum in New Zealand as a first record for this organism. Information on species is necessary for preventing the inadvertent intercontinental introduction of genetically unique foreign pathogens associated with world trade. The ability to place species information into a worldwide context enabled postulation that the New Zealand representatives of F. graminearum clade originated from at least two regions, and probably on at least two hosts. Correlation of species descriptions with biogeographical and host information revealed evidence for co-localisation of F. graminearum clade species with potential for genetic outcrossing in the field. Mycotoxin analysis showed F. graminearum (= lineage 7) isolates produce either nivalenol (NIV) or deoxnivalenol (DON). In contrast, F. cortaderiae isolates produced only NIV. These findings support earlier observations that mycotoxin production in the F. graminearum clade is not species specific, but suggest maintenance of chemotype diversity through speciation may have been restricted to a subset of species.

Putative polyketide synthase and laccase genes for biosynthesis of aurofusarin in Gibberella zeae

Mycelia of Gibberella zeae (anamorph, Fusarium graminearum), an important pathogen of cereal crops, are yellow to tan with white to carmine red margins. We isolated genes encoding the following two proteins that are required for aurofusarin biosynthesis from G. zeae: a type I polyketide synthase (PKS) and a putative laccase. Screening of insertional mutants of G. zeae, which were generated by using a restriction enzyme-mediated integration procedure, resulted in the isolation of mutant S4B3076, which is a pigment mutant. In a sexual cross of the mutant with a strain with normal pigmentation, the pigment mutation was linked to the inserted vector. The vector insertion site in S4B3076 was a HindIII site 38 bp upstream from an open reading frame (ORF) on contig 1.116 in the F. graminearum genome database. The ORF, designated Gip1 (for Gibberella zeae pigment mutation 1), encodes a putative laccase. A 30-kb region surrounding the insertion site and Gip1 contains 10 additional ORFs, including a putative ORF identified as PKS12 whose product exhibits about 40% amino acid identity to the products of type I fungal PKS genes, which are involved in pigment biosynthesis. Targeted gene deletion and complementation analyses confirmed that both Gip1 and PKS12 are required for aurofusarin production in G. zeae. This information is the first information concerning the biosynthesis of these pigments by G. zeae and could help in studies of their toxicity in domesticated animals.

Random Insertional Mutagenesis Identifies Genes Associated with Virulence in the Wheat Scab Fungus Fusarium graminearum

ABSTRACT Fusarium graminearum is an important pathogen of small grains and maize in many areas of the world. To better understand the molecular mechanisms of F. graminearum pathogenesis, we used the restriction enzyme-mediated integration (REMI) approach to generate random insertional mutants. Eleven pathogenicity mutants were identified by screening 6,500 hygromycin-resistant transformants. Genetic analyses indicated that the defects in plant infection were tagged by the transforming vector in six of these mutants. In mutant M8, the transforming plasmid was integrated 110-bp upstream from the start codon of the cystathionine betalyase gene (CBL1). Gene replacement mutants deleted for CBL1 and the methionine synthase gene MSY1 were also obtained. Both the cbl1 and msy1 deletion mutants were methionine auxotrophic and significantly reduced in virulence on corn silks and wheat heads. We also identified genes disrupted by the transforming DNA in three other REMI mutants exhibiting reduced virulence. In mutants M68, the transforming vectors were inserted in the NADH: ubiquinone oxidoreductase. The putative b-ZIP transcription factor gene and the transducin beta-subunit-like gene disrupted in mutants M7 and M75, respectively, had no known homologs in filamentous fungi and were likely to be novel fungal virulence factors.

Identification of a gene cluster responsible for the biosynthesis of aurofusarin in the Fusarium graminearum species complex

The red pigmentation of Fusarium graminearum and related species that cause stem and head blight of cereals is due to the deposition of aurofusarin in the cell walls. To determine the importance of this polyketide for fungal physiology and pathogenicity, aurofusarin deficient mutants were produced by random and targeted mutagenesis of F. pseudograminearum and F. graminearum. We show that a gene cluster, including the F. graminearum PKS12 gene, is responsible for the biosynthesis of aurofusarin. Three F. pseudograminearum aurofusarin deficient mutants were disrupted in a region upstream from a gene with sequence homology to the aflatoxin regulatory gene aflR. Comparative PCR analyses of the aurofusarin gene cluster in F. graminearum, F. culmorum, and F. pseudograminearum show conserved organization and expression analyses detected no PKS12 transcripts in any of the mutants. To confirm that PKS12 encodes the precursor for aurofusarin, targeted mutagenesis was carried out in F. graminearum. All disruptants showed an albino phenotype. The DeltaPKS12 mutants have higher growth rate and a 10-fold increase in conidia production compared to the wild type. Aurofusarin does not appear to aid in radiation protection and all the mutants are fully pathogenic on wheat and barley. HPLC analyses of aurofusarin deficient mutants confirm the absence of aurofusarin and show an increase in the level of the mycotoxin zearalenone.

Occurrence and distribution of Microdochium nivale and Fusarium species isolated from barley, durum and soft wheat grains in France from 2000 to 2002

Fusarium Head Blight of small grain cereal is a disease of growing concern in Europe. Along with Microdochium nivale, several species of Fusarium may be associated with the disease, including species that are potentially toxigenic. This paper describes the results of a large scale survey of the variety and frequency of different Fusarium species and M. nivale in France. A total of 749 soft wheat, durum wheat and barley samples were collected and analyzed from 2000 to 2002. The most frequent species isolated were F.graminearum, F. avenaceum and F. poae. The frequency of F. poae seems to have increased while M.nivale and F. culmorum appear less frequent than previously described in France. Other Fusarium species detected in decreasing prevalence were F. tricinctum, F. equiseti, F. acuminatum, F. sambucinum, F.sporotrichioides, F. moniliforme, F. heterosporum, F. subglutinans and F. oxysporum. All the most frequent pathogenic species and also the less pathogenic ones were frequently associated with individual fields. The implications of these associations for the protection of cereals crops and for contamination by mycotoxins are discussed.

Concordant evolution of trichothecene 3-O-acetyltransferase and an rDNA species phylogeny of trichothecene-producing and non-producing fusaria and other ascomycetous fungi

The cereal pathogen Fusarium graminearum species complex (e.g. Fusarium asiaticum, previously referred to as F. graminearum lineage 6) produces the mycotoxin trichothecene in infected grains. The fungus has a gene for self-defence, Tri101, which is responsible for 3-O-acetylation of the trichothecene skeleton in the biosynthetic pathway. Recently, trichothecene non-producers Fusarium oxysporum and Fusarium fujikuroi (teleomorph Gibberella fujikuroi) were shown to have both functional (Tri201) and non-functional (pseudo-Tri101) trichothecene 3-O-acetyltransferase genes in their genome. To gain insight into the evolution of the trichothecene genes in Gibberella species, the authors examined whether or not other (pseudo-)biosynthesis-related genes are found near Tri201. However, sequence analysis of a 12 kb region containing Tri201 did not result in identification of additional trichothecene (pseudo-)genes in F. oxysporum. In a further attempt to find other trichothecene (pseudo-)genes from the non-producer, the authors examined whether or not the non-trichothecene genes flanking the ends of the core trichothecene gene cluster (i.e. the Tri5 cluster) comprise a region of synteny in Gibberella species. However, it was not possible to isolate trichothecene (pseudo-)genes from F. oxysporum (in addition to the previously identified pseudo-Tri101), because synteny was not observed for this region in F. asiaticum and F. oxysporum. In contrast to this unsuccessful identification of additional trichothecene (pseudo-)genes in the non-producer, a functional trichothecene 3-O-acetyltransferase gene could be identified in fusaria other than Gibberella: Fusarium decemcellulare and Fusarium solani; and in an ascomycete from a different fungal genus, Magnaporthe grisea. Together with the recent functional identification of Saccharomyces cerevisiae ScAYT1, these results are suggestive of a different evolutionary origin for the trichothecene 3-O-acetyltransferase gene from other biosynthesis pathway genes. The phylogeny of the 3-O-acetyltransferase was mostly concordant with the rDNA species phylogeny of these ascomycetous fungi.

Characterization of California Isolates of Fusarium oxysporum f. sp. vasinfectum

Thirty isolates of Fusarium oxysporum f. sp. vasinfectum from California, Australia, China, and the American Type Culture Collection were characterized by partial sequences of translational elongation factor (EF-1α), phosphate permase (PHO), and beta-tubulin (BT) genes, restriction digests of the intergenic spacer (IGS) region of nuclear rDNA, and pathogenicity tests. Based on phylogenetic analysis of combined sequences of EF-1α, PHO, and BT genes, California isolates represented four lineages. Lineage I contained race 3, lineage II contained races 1, 2, and 6, lineage III contained race 8, and lineage IV contained race 4. The Australian isolates formed a strongly supported independent clade. There were nine haplotypes based on restriction digests of the IGS region. In greenhouse pathogenicity tests with California isolates, those from the race 4 lineage were highly aggressive on certain Pima cotton (Gossypium barbadense) cultivars and less aggressive on Upland cotton (Gossypium hirsutum) cultivars. All isolates belonging to the other lineages caused relatively mild symptoms on both Pima and Upland cultivars. This is the first report of the occurrence of races 3, 4, and 8 in California.