Tuesday March 30


Parallel session 2: Fungal-Host Biology



Role of a mucin-like membrane protein in signalling and pathogenicity of Fusarium oxysporum

Elena Perez-Nadales, Antonio Di Pietro

Department of Genetics, University of Cordoba



The soilborne fungus Fusarium oxysporum causes vascular wilt in a wide range of plant species by penetrating roots, invading the cortex and colonizing the vascular tissue. Fmk1, a mitogen activated protein kinase (MAPK) orthologous to S. cerevisiae Fus3 and Kss1, is essential for plant infection. The signalling components upstream of the Fmk1 cascade are currently unknown. In yeast, the membrane mucin Msb2 functions at the head of the filamentous growth MAPK cascade. We identified a gene from F. oxysporum whose predicted product has sequence homology with yeast Msb2 and shows a similar domain structure, including an N-terminal signal sequence, a predicted serine-threonine rich mucin region, a transmembrane domain and a short cytoplasmic tail. Western analysis using an HA-tagged Msb2 version showed that F. oxysporum Msb2 is an integral membrane protein which is expressed during vegetative growth and tomato root infection. Deletion mutants lacking msb2 showed reduced phosphorylation levels of Fmk1, suggesting that Msb2 may function upstream of this MAPK. In contrast to Dfmk1 strains, Dmsb2 single and Dfmk1/Dmsb2 double mutants exhibited enhanced sensitivity to the cell wall-targeting compounds Congo Red and Calcofluor White, suggesting that Msb2 also signals in an Fmk1-independent pathway functioning in the cell wall stress response. The Dmsb2 strains showed delayed invasive growth across cellophane membranes and significantly reduced virulence on tomato plants. Our results suggest that Msb2 is a mucin-like membrane protein that contributes to invasive growth and virulence of F. oxysporum by signalling partly via the Fmk1 MAPK cascade.  





The evolution and maintenance of pathogen specialization in the fungus-growing ant symbiosis

Nicole Gerardo

Emory University, Atlanta, USA



For approximately 50 million years, fungus-growing ants have been cultivating fungi for food. Over the evolutionary history of this ancient agricultural association, the ants have diversified into more than 200 species that are divided into five distinct phylogenetic and ecological groups, each with their own favored fungal crops. The antsí fungal crops are plagued by microfungal pathogens in the genus Escovopsis. To combat these pathogens, the ants engage in a mutualism with antibiotic-producing actinobacteria. Bioassays interacting strains of the ants' cultivated fungi with strains of the pathogen demonstrate that pathogen strains are coevolved and specialized, which would likely prevent rampant switching between hosts. This specialization is driven by the ability of the host fungi to inhibit some pathogen strains but not others, and the ability of the pathogens to recognize and grow towards chemical signals of some hosts but not others. The bacteria, however, exhibit less evolutionary specialization in their inhibition patterns towards the pathogen. Elucidation of this intricate system of symbiotic coevolution will be facilitated by ongoing genome sequencing of the ants and their microbial associates





High symbiont relatedness stabilizes mutualistic cooperation in fungus-growing termites

Duur K. Aanen
Wageningen University



It is unclear how mutualistic relationships can be stable when partners disperse freely and have the possibility of forming associations with many alternative genotypes. Theory predicts that high symbiont relatedness should resolve this problem, but the mechanisms to enforce this have rarely been studied. In this presentation, I describe experiments addressing this question for the mutualistic symbiosis between fungus-growing termites and Termitomyces. Our experiments show how colonies succeed in propagating only a single heterokaryon of their Termitomyces symbiont, despite initiating cultures from genetically variable sexual spores from the habitat at the start of a colony. High inoculation density of asexual heterokaryotic spores in the substrate followed by successful fusion among clonally related mycelia enhances the efficiency of asexual spore production in proportion to strain frequency. This positive reinforcement results in an exclusive lifetime association of each host colony with a single fungal symbiont and hinders the evolution of cheating. Our findings explain why vertical symbiont transmission in fungus-growing termites is rare and evolutionarily derived.


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