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Accessory chromosomes and host-specific pathogenicity in Fusarium oxysporum

Martijn Rep: University of Amsterdam

<div>The <em>Fusarium oxysporum</em> species complex (FOSC) is notorious for harbouring plant-pathogenic clonal lines that cause wilt or root/bulb rot in crops and ornamentals. However, pathogenicity is rare in the FOSC. Colonization of roots by <em>Fusarium oxysporum</em> without negative consequences for plant health appears to be very common in most plant species investigated. It appears that monoculture has selected rare pathogenic clonal lines, providing a ‘substrate’ for continued spread of those lines, with Panama disease of banana as a dramatic example.</p> <p>We have discovered that host-specific pathogenicity resides in accessory ‘pathogenicity chromosomes’ and is at least partly encoded by genes for small secreted proteins (‘effectors’) on these chromosomes. On the basis on the repertoire of putative effector genes in a genome of a strain of <em>Fusarium oxysporum</em>, we can now predict host-specific pathogenicity with high confidence for an increasing number of plant hosts, starting with tomato and cucurbits. Pathogenicity chromosomes can be transferred horizontally between strains, explaining why many host-specific forms (<em>formae speciales</em>) consist of multiple clonal lines. We have indications that this horizontal chromosome transfer occurs in a heterokaryon emerging from hyphal fusion between ‘incompatible’ strains in which rare nuclear fusion is followed by selective loss of ‘core’ chromosomes from one parental strain.</div>

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