Abstract

In heterothallic ascomycete fungi, isolates of opposite mating type are required to initiate the sexual cycle. Mating-type of each isolate is determined by the MAT1 locus carrying one of two highly divergent alleles known as idiomorphs that are designated MAT1-1 or MAT1-2. Fusarium oxysporum is considered to be an asexual ascomycete lacking a sexual phase in its life cycle. However, MAT1 idiomorphs were identified in the F. oxysporum genome which were homologous to those in Gibberella fujikuroi, a sexual relative of F. oxysporum, and the MAT1 genes were expressed. A recent phylogenetic study of F. oxysporum f. sp. lycopersici (FOL), the tomato wilt fungus, revealed three well-supported clades, each of which contained isolates of only one mating type. Moreover, whole-chromosome PFGE analysis revealed that electrophoretic karyotypes were highly variable among clades. These data suggest that F. oxysporum may be unable to complete the sexual cycle due to chromosomal divergence among clades and lack of a compatible mate within each clade. To address this hypothesis, we generated a transformant of FOL in which the MAT1-2 idiomorph was replaced with the MAT1-1 idiomorph. The electrophoretic karyotype of the transformant was identical to that of the parental isolate. When the original isolate (MAT1-2) and the transformant (MAT1-1) were paired under mating conditions, unknown structures which did not show up in wild type were observed.