Abstract

FFusarium oxysporum f. sp. lycopersici (FOL) is the pathogen of soilborne wilt of tomato. In the pathogen, three races 1, 2, and 3 have been determined based on the specific pathogenicity to tomato cultivars. The compatible or incompatible relationships between races and cultivars can be explained by the interactions between avirulence gene(s) carried by FOL and resistance genes carried by tomato according to gene-for-gene theory (Flor 1956); for example, race 1 carrying AVR1 is avirulent to tomato cultivars having a resistance gene I, and race 2 and 3 having no AVR1 are virulent to the tomato cultivars having I. Houterman (2008) found the race 1-specific genome locus SIX4, which determines the avirulence to I; in other words, SIX4 is the AVR1 in FOL. In 2008, a strain of FOL which overcomes the races 1-resistance due to I emerged in Kochi, Japan. The strain was phylogenetically classified in to the Japanese race 1-clade (Kawabe 2005; Hirano 2006) based on rDNA-IGS region sequence. Real time-PCR using the specific primer-probe set (Yoshioka 2009) revealed that the strain carried SIX4 genomic locus, indicating that the strain was race 1. However, we found that the amplicon from the strain was about 770 bp longer than that from race 1 using a SIX4-specific primer set. Sequencing of the amplicon revealed that in the strain SIX4 was truncated by a putative transposable element belonging to the hAT family (Kempken 2001). All together, we assume that the new strain of FOL found in this study had overcome I by transposon insertion in the corresponding avirulence gene. As far as we know, this is the first report of an avirulence gene disrupted by a transposable element in nature which may give new insight in studying evolution of pathogenicity in plant pathogenic fungi.