Genetics of Phytopathogenic Fungi. Iv. Experimentally Caryons of Fusarium Oxysporum F. Pisi1,’ Induced Alterations in Nuclear Ratios of Hetero-

HE parasexual cycle in fungi is initiated by the formation of a heterocaryon. Consequently, genetically dissimiliar nuclei are intermingled within a common cytoplasm in which nuclear “fusion” may occur. When genotypically different haploid nuclei are involved, a heterozygous diploid nucleus results. During the multiplication of such nuclei, mitotic crossing over or haploidization or both may occur and spores may eventually be obtained which have either diploid nuclei homozygous for certain genetic markers or haploid nuclei containing new combinations of genetic markers. In either event, methods are now available for genetic studies using fungi lacking a perfect stage (PONTECORVO 1953). BUXTON (1956) suggested that a genetic analysis of factors controlling or determining pathogenicity in the imperfect fungus Fusar ium oxysporum f. pisi might be accomplished by using the parasexual cycle. TUVESON and GARBER (1959b) have shown that certain heterocaryons and diploid strains of this species yielded spores with nuclei carrying markers characteristic of only one of the component strains. These results were interpreted by assuming that selective forces favored either multiplication of one nuclear type or the incorporation of one nuclear type in the conidia. Genetics studies involving the parasexual cycle are based on the assumption that the inclusion of different nuclear types in spores is random (PONTECORVO and KAFER 1958). Situations in which certain nuclear types do not become incorporated in the spores do not lend themselves to a rational analysis. This paper presents data indicating that it is possible to alter the presumptive selective force responsible for the anomalous situation in heterocaryons of F. oxysporum f . pisi.