Recent advances in the population biology and management of maize foliar fungal pathogens Exserohilum turcicum, Cercospora zeina and Bipolaris maydis in Africa

Maize is the most widely cultivated and major security crop in sub-Saharan Africa. Three foliar diseases threaten maize production on the continent, namely northern leaf blight, gray leaf spot, and southern corn leaf blight. These are caused by the fungi Exserohilum turcicum, Cercospora zeina, and Bipolaris maydis, respectively. Yield losses of more than 10% can occur if these pathogens are diagnosed inaccurately or managed ineffectively. Here, we review recent advances in understanding the population biology and management of the three pathogens, which are present in Africa and thrive under similar environmental conditions during a single growing season. To effectively manage these pathogens, there is an increasing adoption of breeding for resistance at the small-scale level combined with cultural practices. Fungicide usage in African cropping systems is limited due to high costs and avoidance of chemical control. Currently, there is limited knowledge available on the population biology and genetics of these pathogens in Africa. The evolutionary potential of these pathogens to overcome host resistance has not been fully established. There is a need to conduct large-scale sampling of isolates to study their diversity and trace their migration patterns across the continent.

Mating-Type Genes Control Sexual Reproduction, Conidial Germination, and Virulence in Cochliobolus lunatus

Cochliobolus lunatus (anamorph: Curvularia lunata) is a major pathogenic fungus that causes the Curvularia leaf spot of maize. ClMAT1-1-1 and ClMAT1-2-1, the C. lunatus orthologs of C. heterostrophus ChMAT1-1-1 and ChMAT1-2-1, were investigated in the present study to uncover their functions in C. lunatus. Southern blot analysis showed that these mating-type MAT genes exist in the C. lunatus genome as a single copy. ClMAT1-1-1 and ClMAT1-2-1 were knocked out and complemented to generate ΔClmat1-1-1 and ΔClmat1-2-1 and ΔClmat1-1-1-C and ΔClmat1-2-1-C, respectively. The mutant strains had defective sexual development and failed to produce pseudothecia. There were no significant differences in growth rate or conidia production between the mutant and wild-type strains. However, the aerial mycelia and mycelial dry weight of ΔClmat1-1-1 and ΔClmat1-2-1 were lower than those of wild type, suggesting that MAT genes affect asexual development. ClMAT genes were involved in the responses to cell wall integrity and osmotic adaptation. ΔClmat1-2-1 had a lower conidial germination rate than the wild-type strain CX-3. The virulence of ΔClmat1-2-1 and ΔClmat1-1-1 was also reduced compared with the wild-type. Complementary strains could restore all the phenotypes.

Self-fertility in Chromocrea spinulosa is a consequence of direct repeat-mediated loss of MAT1-2, subsequent imbalance of nuclei differing in mating type, and recognition between unlike nuclei in a common cytoplasm

The filamentous fungus Chromocrea spinulosa (Trichoderma spinulosum) exhibits both self-fertile (homothallic) and self-sterile (heterothallic) sexual reproductive behavior. Self-fertile strains produce progeny cohorts that are 50% homothallic, 50% heterothallic. Heterothallic progeny can mate only with homothallic strains, and progeny also segregate 50% homothallic, 50% heterothallic. Sequencing of the mating type (MAT) region of homothallic and heterothallic strains revealed that both carry an intact MAT1-1 locus with three MAT1-1 genes (MAT1-1-1, MAT1-1-2, MAT1-1-3), as previously described for the Sordariomycete group of filamentous fungi. Homothallic strains, however, have a second version of MAT with the MAT1-2 locus genetically linked to MAT1-1. In this version, the MAT1-1-1 open reading frame is split into a large and small fragment and the truncated ends are bordered by 115bp direct repeats (DR). The MAT1-2-1 gene and additional sequences are inserted between the repeats. To understand the mechanism whereby C. spinulosa can exhibit both homothallic and heterothallic behavior, we utilized molecular manipulation to delete one of the DRs from a homothallic strain and insert MAT1-2 into a heterothallic strain. Mating assays indicated that: i) the DRs are key to homothallic behavior, ii) looping out of MAT1-2-1 via intra-molecular homologous recombination between the DRs in self-fertile strains results in two nuclear types in an individual (one carrying both MAT1-1 and MAT1-2 and one carrying MAT1-1 only), iii) self-fertility is achieved by inter-nuclear recognition between these two nuclear types before meiosis, iv) the two types of nuclei are in unequal proportion, v) having both an intact MAT1-1-1 and MAT1-2-1 gene in a single nucleus is not sufficient for self-fertility, and vi) the large truncated MAT1-1-1 fragment is expressed. Comparisons with MAT regions of Trichoderma reesei and Trichoderma virens suggest that several crossovers between misaligned parental MAT chromosomes may have led to the MAT architecture of homothallic C. spinulosa.

Fungi employ one of two mating tactics for sexual reproduction: self-sterile/heterothallic species can mate only with a genetically distinct partner while self-fertile/homothallic species do not require a partner. In ascomycetes, sexual reproduction is controlled by master regulators encoded by the mating-type (MAT) locus. The architecture of MAT differs in heterothallic versus homothallic species; heterothallics carry one of two forms (MAT1-1 or MAT1-2) per nucleus, whereas most homothallics carry both MAT forms in a single nucleus. There are intriguing exceptions. For example, the yeast models, Saccharomyces cerevisiae, and Schizosaccharomyces pombe undergo reversible MAT switching, not demonstrated in filamentous fungi. Here, we describe the mating mechanism in Chromocrea spinulosa (Trichoderma spinulosum), a filamentous ascomycete that exhibits both homothallic and heterothallic behavior. Self-fertile strains produce progeny cohorts that are 50% homothallic, 50% heterothallic. Self-sterile strains can mate only with homothallic strains, and when this occurs, homothallic and heterothallic progeny are also produced in a 1:1 ratio. By MAT sequencing and manipulation, we discovered unique MAT architecture and determined that self-fertility is achieved by deletion of MAT1-2 from most homothallic nuclei and subsequent inter-nuclear recognition between the resulting two, unevenly present, nuclear types in a common cytoplasm.

Maintenance of sex-related genes and the co-occurrence of both mating types in Verticillium dahliae

Verticillium dahliae is a cosmopolitan, soilborne fungus that causes a significant wilt disease on a wide variety of plant hosts including economically important crops, ornamentals, and timber species. Clonal expansion through asexual reproduction plays a vital role in recurring plant epidemics caused by this pathogen. The recent discovery of recombination between clonal lineages and preliminary investigations of the meiotic gene inventory of V. dahliae suggest that cryptic sex appears to be rare in this species. Here we expanded on previous findings on the sexual nature of V. dahliae. Only 1% of isolates in a global collection of 1120 phytopathogenic V. dahliae isolates contained the MAT1-1 idiomorph, whereas 99% contained MAT1-2. Nine unique multilocus microsatellite types comprised isolates of both mating types, eight of which were collected from the same substrate at the same time. Orthologs of 88 previously characterized sex-related genes from fungal model systems in the Ascoymycota were identified in the genome of V. dahliae, out of 93 genes investigated. Results of RT-PCR experiments using both mating types revealed that 10 arbitrarily chosen sex-related genes, including MAT1-1-1 and MAT1-2-1, were constitutively expressed in V. dahliae cultures grown under laboratory conditions. Ratios of non-synonymous (amino-acid altering) to synonymous (silent) substitutions in V. dahliae MAT1-1-1 and MAT1-2-1 sequences were indistinguishable from the ratios observed in the MAT genes of sexual fungi in the Pezizomycotina. Patterns consistent with strong purifying selection were also observed in 18 other arbitrarily chosen V. dahliae sex-related genes, relative to the patterns in orthologs from fungi with known sexual stages. This study builds upon recent findings from other laboratories and mounts further evidence for an ancestral or cryptic sexual stage in V. dahliae.

Mating type protein Mat1-2 from asexual Aspergillus fumigatus drives sexual reproduction in fertile Aspergillus nidulans

The lack of an experimentally amenable sexual genetic system in Aspergillus fumigatus is a major limitation in the study of the organism's pathogenesis. A recent comparative genome analysis revealed evidence for potential sexuality in A. fumigatus. Homologs of mating type genes as well as other genes of the "sexual machinery" have been identified in anamorphic A. fumigatus. The mat1-2 gene encodes a homolog of MatA, an HMG box mating transcriptional factor (Mat(HMG)) that regulates sexual development in fertile Aspergillus nidulans. In this study, the functionalities of A. fumigatus mat1-2 and the Mat1-2 protein were determined by interspecies gene exchange between sterile A. fumigatus and fertile A. nidulans. Ectopically integrated A. fumigatus mat1-2 (driven by its own promoter) was not functional in a sterile A. nidulans Delta matA strain, and no sexual development was observed. In contrast, the A. fumigatus mat1-2 open reading frame driven by the A. nidulans matA promoter and integrated by homologous gene replacement at the matA locus was functional and conferred full fertility. This is the first report showing that cross species mating type gene exchange between closely related Ascomycetes did not function in sexual development. This is also the first report demonstrating that a Mat(HMG) protein from an asexual species is fully functional, with viable ascospore differentiation, in a fertile homothallic species. The expression of mat1-2 was assessed in A. fumigatus and A. nidulans. Our data suggest that mat1-2 may not be properly regulated to allow sexuality in A. fumigatus. This study provides new insights about A. fumigatus asexuality and also suggests the possibility for the development of an experimentally amenable sexual cycle.

Isolation and characterization of the mating-type locus of the barley pathogen Pyrenophora teres and frequencies of mating-type idiomorphs within and among fungal populations collected from barley landraces

Pyrenophora teres f. sp. teres mating-type genes (MAT-1: 1190 bp; MAT-2: 1055 bp) have been identified. Their predicted proteins, measuring 379 and 333 amino acids, respectively, are similar to those of other Pleosporales, such as Pleospora sp., Cochliobolus sp., Alternaria alternata, Leptosphaeria maculans, and Phaeosphaeria nodorum. The structure of the MAT locus is discussed in comparison with those of other fungi. A mating-type PCR assay has also been developed; with this assay we have analyzed 150 isolates that were collected from 6 Sardinian barley landrace populations. Of these, 68 were P. teres f. sp. teres (net form; NF) and 82 were P. teres f. sp. maculata (spot form; SF). Within each mating type, the NF and SF amplification products were of the same length and were highly similar in sequence. The 2 mating types were present in both the NF and the SF populations at the field level, indicating that they have all maintained the potential for sexual reproduction. Despite the 2 forms being sympatric in 5 fields, no intermediate isolates were detected with amplified fragment length polymorphism (AFLP) analysis. These results suggest that the 2 forms are genetically isolated under the field conditions. In all of the samples of P. teres, the ratio of the 2 mating types was consistently in accord with the 1:1 null hypothesis. This ratio is expected when segregation distortion and clonal selection among mating types are absent or asexual reproduction is rare. Overall, sexual reproduction appears to be the major process that equalizes the frequencies of the 2 mating types within populations.

Reproduction inAspergillus fumigatus: sexuality in a supposedly asexual species?

Aspergillus fumigatus has long been considered to reproduce only by asexual means. However, accumulating evidence suggest that a sexual stage for A. fumigatus may yet be identified. We describe results from published and ongoing studies involving population genetic analyses, genome analysis, studies of mating-type gene presence and distribution, expression of sex-related genes, and taxonomic work which support the assertion that A. fumigatus has the potential to reproduce by sexual means. The consequences of sexual reproduction for the population biology and disease management of the species are discussed. The possible mechanisms of evolution of asexuality are then considered. It is proposed that asexual species may arise in one step by mutation or loss of a key gene(s), and/or there may be a 'slow decline' in sexual fertility within the species as a whole. Thus, it is argued that species should not be considered simply as sexual or asexual, but rather as individual isolates being present on a continuum of sexual fertility, with the implications for understanding sexuality/asexuality in A. fumigatus discussed.

Identity and conservation of mating type genes in geographically diverse isolates of Phaeosphaeria nodorum

Mating type idiomorphs (MAT1-1 and MAT1-2) were identified from the heterothallic loculoascomycete Phaeosphaeria nodorum (wheat biotype) using DNA from a pair of isolates from Poland and Georgia, USA that are known to mate. MAT predicted proteins of P. nodorum are similar in sequence and in phylogenetic relationship to those described for other loculoascomycetes such as Cochliobolus spp., Alternaria alternata, and Didymella zeae-maydis. The organization of the MAT locus of the P. nodorum differs from these species in that its idiomorph begins within an adjacent upstream conserved ORF of unknown function. MAT-specific primers were used to identify isolates of both mating types in field populations, demonstrating that an absence of either mating type is not the reason that the teleomorph has not been found in New York. Portions of MAT1-1 and MAT1-2 were sequenced from geographically diverse isolates, including those from regions where the teleomorph has been reported. MAT was highly conserved and no significant differences in sequence were found.

Distribution of mating type alleles in the wheat pathogen Mycosphaerella graminicola over spatial scales from lesions to continents

A total of 2035 Mycosphaerella graminicola strains collected from 16 geographic locations on four continents were assayed for the mating type locus. RFLP fingerprints were used to identify clones in each population. At the smallest spatial scale analyzed, both mating types were found among fungal strains sampled from different lesions of the same leaf as well as from different pycnidia in the same lesion. At larger spatial scales, the two mating types were found at equal frequencies across spatial scales ranging from several square meters to several thousand square kilometers. Though the absolute frequencies of the two mating types sometimes varied for different sampling units within the same spatial scale in the hierarchy (plots within a field, fields within a country, or different continents of the world), none of the differences were statistically significant from the null hypothesis of equal frequencies for the two mating types. The evolutionary forces likely to maintain the even distribution of the two mating types in this pathogen were discussed.

A multiplex PCR test for determination of mating type applied to the plant pathogens Tapesia yallundae and Tapesia acuformis

A multiplex PCR test for determining mating type of the pathogens Tapesia yallundae and Tapesia acuformis is described. The test involves three primers: a "common" primer annealing to DNA sequence conserved in the flanking region of both mating-type idiomorphs and two specific primers annealing to sequence in either the MAT-1 or the MAT-2 idiomorphs. Locating the specific primers in different positions relative to the common primer yielded PCR products of 812 or 418 bp from MAT-1 and MAT-2 isolates, respectively. The test was used successfully to determine the mating type of 118 isolates of T. yallundae and T. acuformis from Europe, North America, and New Zealand. Isolates of both mating types were found on all continents for both species despite the rarely observed occurrence of sexual reproduction of T. acuformis. The multiplex test design should be applicable to other ascomycete species, of use in studies of MAT distribution and facilitating sexual crossing by identifying compatible isolates.

Cell and nuclear recognition mechanisms mediated by mating type in filamentous ascomycetes

Sexual development in filamentous ascomycetes requires mating-type genes to mediate recognition of compatible cell and nuclear types. Characterization of mating-type genes from various fungi shows that they primarily encode transcriptional regulators. Recent studies on mating-type-specific pheromones and internuclear recognition have shed light on how mating-type genes specify mating and nuclear identity in filamentous ascomycetes.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

A G protein alpha subunit from Cochliobolus heterostrophus involved in mating and appressorium formation

A Galpha subunit-encoding gene (CGA1) was cloned from Cochliobolus heterostrophus, a heterothallic foliar pathogen of corn. The deduced amino acid sequence showed similarity to Galpha proteins from other filamentous fungi and suggested that CGA1 is a member of the Galphai class. cga1 mutants had reduced ability to form appressoria on glass surfaces and on corn leaves; mutants nevertheless caused lesions on corn plants like those of wild type. cga1 mutants were female sterile; sexual development was completely abolished when the mutant allele was homozygous in a cross. Ascospores produced in crosses heterozygous at Cga1 were all wild type. The signal transduction pathway represented by CGA1 appears to be involved in developmental pathways leading to either appressorium formation or mating; in sexual development CGA1 is required for both fertility and ascospore viability.

Characterization of mat A-2, mat A-3 and deltamatA mating-type mutants of Neurospora crassa

The mating-type locus of Neurospora crassa regulates mating identity and entry into the sexual cycle. The mat A idiomorph encodes three genes, mat A-1, mat A-2, and mat A-3. Mutations in mat A-1 result in strains that have lost mating identity and vegetative incompatibility with mat a strains. A strain containing mutations in both mat A-2 and mat A-3 is able to mate, but forms few ascospores. In this study, we describe the isolation and characterization of a mutant deleted for mat (deltamatA), as well as mutants in either mat A-2 or mat A-3. The deltamatA strain is morphologically wild type during vegetative growth, but it is sterile and heterokaryon compatible with both mat A and mat a strains. The mat A-2 and mat A-3 mutants are also normal during vegetative growth, mate as a mat A strain, and produce abundant biparental asci in crosses with mat a, and are thus indistinguishable from a wild-type mat A strain. These data and the fact that the mat A-2 mat A-3 double mutant makes few asci with ascospores indicate that MAT A-2 and MAT A-3 are redundant and may function in the same pathway. Analysis of the expression of two genes (sdv-1 and sdv-4) in the various mat mutants suggests that the mat A polypeptides function in concert to regulate the expression of some sexual development genes.

Application of mating type gene technology to problems in fungal biology

In ascomycetes, the single mating type locus (MAT) controls sexual development. This locus is structurally unusual because the two alternate forms ("alleles") are completely dissimilar sequences, encoding different transcription factors, yet they occupy the same chromosomal position. Recently developed procedures allow efficient cloning of MAT genes from a wide array of filamentous ascomycetes, thereby providing MAT-based technology for application to several ongoing issues in fungal biology. This article first outlines the basic nature of MAT genes, then addresses the following topics: efficient cloning of MAT genes; the unusual molecular characteristics of these genes; phylogenetics using MAT; the issues of why some fungi are self-sterile, others self-fertile, and yet others asexual; the long-standing mystery of possible mating type switching in filamentous fungi; and finally the evolutionary origins of pathogenic capability.

An asexual fungus has the potential for sexual development

The availability of cloned genes that control sexual reproduction (mating type genes) in high fungi has allowed us to consider the causes of failure to mate in asexual fungi. We report here that the asexual fungus Bipolaris sacchari has a homolog of the MAT-2 gene of its sexual ascomycete relative Cochliobolus heterostrophus. The B. sacchari MAT-2 sequence is highly similar to that of C. heterostrophus MAT-2 and, in fact, functions in transgenic C. heterostrophus. Thus, the asexual nature of B. sacchari is not due to absence or mutation of MAT. When either of the C. heterostrophus MAT genes was transformed into B. sacchari, the recipient could neither self nor cross with other B. sacchari strains, in contrast to transgenic C. heterostrophus strains which can do both. Persistent asexuality of B. sacchari, in spite of the presence of complementary functional MAT genes, suggests that this fungus lacks genes other than MAT which are essential for mating. Notably, the transgenic B. sacchari strains were sometimes able to initiate, but not complete, sexual development in interspecific pairings with C. heterostrophus. Transcript analysis showed that the B. sacchari MAT-2 gene is expressed in transgenic C. heterostrophus and that the C. heterostrophus MAT genes are expressed in transgenic B. sacchari. No transcript of the native B. sacchari MAT-2 gene was detected under any growth condition tested.