Science - February 16, 2011

Gene transfer makes fungi harmful

Pathogenic fungi do spontaneously what organizations such as Greenpeace dread: they exchange genes or even complete chromosomes with other fungi species. So says an article in FEMS Microbiology Reviews written by Wageningen geneticists Rahim Mehrabi, Pierre de Wit and Gert Kema.

Fusarium fungi in winter wheat.
For years, the fungus Pyrenophora tritici-repentis did not cause any harm to wheat. But at about the time of the Second World War, this fungus suddenly started to damage wheat plants in the same way that the pathogenic fungus Stagonospora nodorum did. When geneticists followed the genome sequences of both these fungi and compared them, they found what appeared to be an 'island' of Stagonospora-DNA in the genome of Pyrenophora. It seemed too that this little cluster of genes was present in moulds at about 1940, but not before. Therefore, it is very plausible that P. tritici-repentis became a pathogenic fungus after a DNA-exchange with S. nodorum.
Mobile chromosomes
Something similar happened to the fungus Fusarium oxysporum, a resilient pathogen in tomatoes. The Amsterdam research team of Martijn Rep has recently demonstrated in Nature that the pathogenicity of this fungus on the tomato lies in a particular chromosome. Rep showed afterwards that the chromosome which has codes to make tomatoes sick can jump to non-pathogenic Fusarium species. 'Mobile chromosomes!' says Kema.
Spontaneous gene mutation
The mechanism behind such processes is like a black box for Kema, but he can offer a suggestion. Kema is researching into the fungus Mycosphaerella graminicola, the major pathogen in wheat in Northwest Europe. Together with Mehrabi, he examined the workings of different genes by turning them off. After the gene Gβ was turned off, it seemed that fungus threads no longer grew neatly next to one another, but started to fuse together. 'You can therefore picture different fungi in nature fusing after a spontaneous gene mutation', says Kema.
Fungi are genetically untamed in nature, as shown by further research into graminicola. Kema juxtaposed the DNA-sequences of different fungi and discovered that 13 of the 21 chromosomes were always present, while the remaining 8 varied. 'Strange things are taking place in fungi at the chromosome level. Chromosomes duplicate or disappear. I think that the 8 have been transferred from other fungi species over time.'
Gene or chromosome transfer can therefore lead to new or more aggressive fungi. 'You can't prevent such processes from happening', says Kema. 'So you have to be alert and find out how to tackle these fungi and prevent their spread.'