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Ten nutritionally deficient strains of Penicillium italicum with mutant colors were involved in 13 heterocaryons. Three heterocaryons yielded diploid strains. Not all possible genotypes were present among the segregants from the diploids. One case of presumptive linkage was detected. Seven nutritionally deficient strains of P. digitatum with mutant colors were involved in five combinations to obtain heterocaryons. Heterocaryosis could not be definitely demonstrated in this species.

Prototrophic color and auxotrophic mutants of Penicillium italicum and P. digitatum, causal agents of citrus fruit rot, were obtained by irradiating conidia with ultraviolet light. Avirulent mutants caused a necrosis but not an obvious rot at the site of inoculation in orange rind. Avirulence was not necessarily associated with a specific alteration in the color of sporulating colonies or with the tested nutritional requirements. Supplementing necrotic lesions with the compounds required by the avirulent auxotrophic mutants enhanced growth but did not cause an obvious rot. Although heterocaryons of P. italicum involving avirulent auxotrophic strains were weakly virulent, the corresponding diploid strains were as virulent as the haploid prototrophic parental strain. Prototrophic segregants from the diploid strains were virulent. Avirulence was not related to the inability of the avirulent mutants to grow at the site of inoculation. It is probable that more than one locus may be responsible for the loss of virulence.

parasexual cycle in filamentous fungi is initiated by the formation of a TFe:erocaryon, usually involving haploid strains differing in nutritional requirements and, if possible, in color or colonial morphology, Consequently, genetically dissimilar nuclei are intermingled in a common cytoplasm. A "fusion" of such nuclei may occur to give a heterozygous nucleus which usually may be captured by plating conidia from the heterocaryon on a defined medium. A non-sectoring prototrophic colony with the color or morphology of the wild-type haploid strain is diploid. During the vegetative multiplication of nuclei in a diploid strain, somatic crossing over or nondisjunction leading to haploidization or both may occur so that conidia eventually may be obtained which have either a diploid nucleus homozygous for certain gene markers or a haploid nucleus containing new combinations of gene markers. Disomic nuclei may also be involved either in somatic crossing over or in haploidization or both. The discovery of the parasexual cycle and its successful exploitation for genetic studies in Aspergillus nidulans ( PONTECORVO, ROPER, HEMMONS, MACDONALD and BUFTON * A-autonomous NA-nonautonomous. t bu-leucine, &-lysine, pan-pantothenic acid, cys-cysteine, ade-adenine, met-methionine, pab-paraminobenzoic add, arg-arginine, phe-phenylalanine, thi-thiamine, pro-proline, Val-valine, ex-exudate.