Host specificity in Fusarium oxysporum

Fusarium oxysporum is a fungal pathogen that can cause severe wilt disease and root rot in various plant species. Every individual strain is restricted to causing disease in only one or a few plant species.
In this thesis, we focused on identifying novel virulence factors (‘effectors’) secreted by the fungus to enhance successful plant colonization. We found that strains affecting the same plant possess a highly similar set of effector genes. Moreover, the nucleotide sequence of these genes is oftentimes identical between these strains.
We also identify the dedicated pathogenicity chromosome of a strain affecting cucurbits. In co-inoculation experiments, we successfully selected strains that had received this specific chromosome in a non-pathogenic background strain. Interestingly, these strains had also gained the capability to cause disease in cucurbits.
The fact that Fusarium oxysporum can so ‘easily’ transfer entire chromosomes between strains explains why strains affecting the same plant species also have similar (identical) effectors. This knowledge was applied in this thesis to develop molecular diagnostics markers based on these effector genes, allowing the differentiation of strains affecting different plant species from each other and from non-pathogenic strains.
Finally, we describe results supporting horizontal transfer of genetic material between F. oxysporum and other Fusarium species. Two exciting findings support this hypothesis: the presence of (i) so-called ‘mimps’ (transposable elements) and (ii) effector genes in the identified strains with very high sequence similarity.