This thesis focuses on the evolution of races within Fusarium oxysporum f.sp. lycopersici (Fol). Fol is a soil-borne fungal
pathogen causing wilt disease on tomato (Solanum lycopersicum L.). So far three races (race 1, 2 and 3) have been identified
based on their ability to infect tomato cultivars carrying monogenic, race-specific resistance genes. Fol race 1 isolates
are characterized by the presence of AVR1 in their genome. The product of this gene, Avr1, triggers resistance in tomato cultivars
carrying resistance gene I. In Fol race 2 and race 3 isolates, AVR1 is absent and hence they are virulent on tomato cultivars
carrying I. In this research, we analyzed an approximately 100 kb genomic fragment containing the AVR1 locus of race 1 isolate
Fol004, and compared it to the sequenced genome of Fol race 2 isolate 4287 (Fol4287). A genomic fragment of 31 kb containing
AVR1 was found to be missing in Fol4287. Further analysis suggests that race 2 evolved from race 1 by deletion of this 31
kb fragment due to a recombination event between two Helitron transposable elements bordering the fragment. A worldwide collection
of 71 Fol isolates from five different geographical origins was subjected to PCR analysis of the AVR1 locus, including the
two bordering transposable elements and avirulence genotypes. We show that Fol races evolved in a stepwise manner by the loss
of function of avirulence genes in a number of alternative ways.
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