Fungal defences against animal antagonists – lectins & more

Pathogenic fungi-induced susceptibility is mitigated by mutual Lactobacillus plantarum in the Drosophila melanogaster model

Background

Since animals frequently encounter a variety of harmful fungi in nature, their ability to develop sophisticated anti-fungal strategies allows them to flourish across the globe. Extensive studies have highlighted the significant involvement of indigenous microbial communities in human health. However, the daunting diversity of mammalian microbiota and host genetic complexity are major obstacles to our understanding of these intricate links between microbiota components, host immune genotype, and disease phenotype. In this study, we sought to develop a bacterium-fungus-Drosophila model to systematically evaluate the anti-fungal effects of commensal bacteria.

Results

We isolated the pathogenic fungal strain, Diaporthe FY, which was detrimental to the survival and development of Drosophila upon infection. Using Drosophila as a model system, Drosophila-associated Lactobacillus plantarum functioned as a probiotic, and protected the flies from mortality induced by Diaporthe FY. Our results show that L. plantarum hindered the growth of Diaporthe FY in vitro, and decreased the mortality rate of Diaporthe FY-infected flies in vivo, consequently mitigating the toxicity of Diaporthe FY to the hosts. Additionally, the presence of L. plantarum overrode the avoidance of oviposition on Diaporthe FY-associated substrates.

Conclusions

Diaporthe FY was identified as a potential Drosophila pathogen. Commensal L. plantarum mitigated the susceptibility of Drosophila to pathogenic fungi, providing insight into the natural interplay between commensal and pathogenic microbial communities that contribute to animal health and pathogenesis.

Fungal genotype determines survival of Drosophila melanogaster when competing with Aspergillus nidulans

Fungi produce an astonishing variety of secondary metabolites, some of which belong to the most toxic compounds in the living world. Several fungal metabolites have anti-insecticidal properties which may yield advantages to the fungus in competition with insects for exploitation of environmental resources. Using the Drosophila melanogaster/Aspergillus nidulans ecological model system to assess secondary metabolite mutant genotypes, we find a major role for the veA allele in insect/fungal confrontations that exceeds the influence of other factors such as LaeA. VeA along with LaeA is a member of a transcriptional complex governing secondary metabolism in A. nidulans. However, historically a mutant veA allele, veA1 reduced in secondary metabolite output, has been used in many studies of this model organism. To test the significance of this allele in our system, Aspergillus nidulans veA wild type, veA1, ΔveA and ΔlaeA were evaluated in confrontation assays to analyze egg laying activity, and the survival rate of larvae. The veA1 genetic background led to a significant increase of larval survival. Adult flies were observed almost exclusively on veA1, ΔveA or ΔlaeA genetic backgrounds, suggesting a role for the velvet complex in insect/fungal interactions. This effect was most profound using the veA1 mutant. Hence, larval survival in confrontations is highly affected by the fungal genotype.

The genome and development-dependent transcriptomes of Pyronema confluens: a window into fungal evolution

Fungi are a large group of eukaryotes found in nearly all ecosystems. More than 250 fungal genomes have already been sequenced, greatly improving our understanding of fungal evolution, physiology, and development. However, for the Pezizomycetes, an early-diverging lineage of filamentous ascomycetes, there is so far only one genome available, namely that of the black truffle, Tuber melanosporum, a mycorrhizal species with unusual subterranean fruiting bodies. To help close the sequence gap among basal filamentous ascomycetes, and to allow conclusions about the evolution of fungal development, we sequenced the genome and assayed transcriptomes during development of Pyronema confluens, a saprobic Pezizomycete with a typical apothecium as fruiting body. With a size of 50 Mb and ∼13,400 protein-coding genes, the genome is more characteristic of higher filamentous ascomycetes than the large, repeat-rich truffle genome; however, some typical features are different in the P. confluens lineage, e.g. the genomic environment of the mating type genes that is conserved in higher filamentous ascomycetes, but only partly conserved in P. confluens. On the other hand, P. confluens has a full complement of fungal photoreceptors, and expression studies indicate that light perception might be similar to distantly related ascomycetes and, thus, represent a basic feature of filamentous ascomycetes. Analysis of spliced RNA-seq sequence reads allowed the detection of natural antisense transcripts for 281 genes. The P. confluens genome contains an unusually high number of predicted orphan genes, many of which are upregulated during sexual development, consistent with the idea of rapid evolution of sex-associated genes. Comparative transcriptomics identified the transcription factor gene pro44 that is upregulated during development in P. confluens and the Sordariomycete Sordaria macrospora. The P. confluens pro44 gene (PCON_06721) was used to complement the S. macrospora pro44 deletion mutant, showing functional conservation of this developmental regulator.

Fungi are a morphologically and physiologically diverse group of organisms with huge impacts on nearly all ecosystems. In recent years, genomes of many fungal species have been sequenced and have greatly improved our understanding of fungal biology. Ascomycetes are the largest fungal group with the highest number of sequenced genomes; however, for the Pezizales, an early-diverging lineage of filamentous ascomycetes, only one genome has been sequence to date, namely that of the black truffle. While truffles are among the most valuable edible fungi, they have a specialized life style as plant symbionts producing belowground fruiting bodies; thus it is difficult to draw conclusions about basal ascomycetes from one truffle genome alone. Therefore, we have sequenced the genome and several transcriptomes of the basal ascomycete Pyronema confluens, which has a saprobic life style typical of many ascomycetes. Comparisons with other fungal genomes showed that P. confluens has two conserved mating type genes, but that the genomic environment of the mating type genes is different from that of higher ascomycetes. We also found that a high number of orphan genes, i.e. genes without homologs in other fungi, are upregulated during sexual development. This is consistent with rapid evolution of sex-associated genes.

Molecular analysis of fungal gene expression upon interkingdom competition with insects

Fungi produce an astonishing variety of secondary metabolites, some of which belong to the most toxic compounds in the living world. However, the benefits of secondary metabolites for fungi are often obscure. Here we describe a method that will aid in the analysis of fungal gene expression upon interkingdom competition with insects. It allows for isolation of fungal RNA from surface-grown mycelium after confrontation with insect larvae, and subsequent molecular analysis.