A new biotype of Fusarium oxysporum f. sp. lycopersici race 2 emerged by a transposon-driven mutation of avirulence gene AVR1

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Emergence of races in Fusarium oxysporum f. sp. lycopersici (Fol) is caused by loss or mutation of at least one avirulence (AVR) gene. The product of AVR1 is a small protein (Avr1) secreted by Fol in tomato xylem sap during infection. This protein triggers Fol race 1 specific resistance (I) in tomato, indicating that AVR1 is an AVR gene. Deletion of AVR1 in race 1 resulted in the emergence of race 2, and an additional mutation in AVR2 generated race 3. Previously, we reported a new biotype of race 3, KoChi-1, in which AVR1 was truncated by a transposon Hormin, which suggested a new route to evolution of races in Fol. However, to date no race 2 isolate carrying Hormin-truncated AVR1 has been reported. In this report, we describe such isolates, represented by Chiba-5, in which Hormin insertion occurred in AVR1 at a position different from that in KoChi-1. AVR1 truncation in both isolates resulted in production of defective Avr1 proteins. Chiba-5 and KoChi-1 belong to different phylogenetic clades, A1 and A2, respectively, suggesting that insertion of Hormin in AVR1 in Chiba-5 and KoChi-1 occurred as independent evolutionary events.

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