Abstract

Importance of horizontal genetic transfer in eukalyotic organisms in the evolution is recently discussed although the evidence remains anecdotal (Kidwell 1993). In fungus the possible evidences of horizontal genetic transfer from fungi or other organisms have been presented (Yang et al. 1996; Walton 2000; Johnson et al. 2001; Rosewich & Kistler 2002; Hatta et al. 2002). Here we found a partial cucumber mosaic virus (CMV)-coat protein (CP) sequence in the genome of a plant pathogenic fungus Fusarium oxysporum Schlechtend.:Fr. f. sp. lycopersici (Sacc.) W.C. Snyder & Hans. (FOL; the tomato wilt pathogen) race 1. Restriction enzyme mediated integration (REMI) of FOL race 1 (NBRC 6531) generated a transformant X-83 which had a dramatic change in host range. X-83 gained pathogenicity on tomato cultivars (cvs.) carrying a resistant gene I (resistant to FOL race 1), showing the same trait of FOL race 2. This result may be explained by the gene-for-gene hypothesis (Flor 1955) in which a putative avirulence gene (AVR1) is knocked out in X-83. Plasmid rescue recovered an ~ 7 kb genomic DNA sequence of FOL NBRC 6531 including the transformation vector-integration site in X-83. In this sequence, we found a locus of 1089 bp in size including the plasmid-integration site and terminal inverted-repeats (TIRs; 26 and 27 bp) at both ends. Insertional mutants of the locus in NBRC 6531 reproduced the same phenotype of X-83, confirming that the locus determines the avirulence in NBRC 6531. BLAST searches of the GenBank DNA Databases revealed that part of the locus had a very high similarity (98% at the nucleotide level) to a 265 bp region of the CMV-CP RNA. This is not only the first report of a plant RNA virus sequence present in a fungal genome but also horizontal genetic transfer may be concerned in evolution of (a)virulence of eukalyotic plant pathogens.