Chemically-Induced Instabilities in a Heterozygous Diploid of Penicillium Chrysogenum.

OMATIC sergregation from heterozygous diploids in molds has been analyzed S in detail for Aspergillus nidulans by PONTECORVO et al. (PONTECORVO, TARR GLOOR and FORBES 1954; PONTECORVO and KAFER 1956). Basically the same processes seem to take place in Penicillium chrysogenum ( SERMONTI 1956, 195 7), and probably also in Aspergillus oryzae ( ISHITANI, IKEDA and SAKAGUCHI 1956). Somatic segregation analysis is comparatively straightforward where it concerns visible or easily selected markers ( PONTECORVO and KAFER 1956; ROPER and KAFER 1957), but it becomes extremely laborious for invisible markers like nutritional deficiency, and impracticably difficult when the genes involved are those quantitatively controlling the production of antibiotics or other metabolites. Even segregation analysis of color markers is made laborious, in Penicillium chrysogenum, by the impossibility of microscopic identification of the color segregants as single yellow or white heads on the sporulating surface of heterozygous diploids (PONTECORVO 1953; SERMONTI 1957). Color segregants in this species appear, either as entire colonies or as macroscopic sectors, after plating conidia of the heterozygous diploid on complete medium. The rate of spontaneous appearance is sometimes so low that the collection of a modest number of segregants demands the observation of many thousands of colonies. Attempts to increase the rate of somatic segregation of heterozygous diploids were made by IKEDA, ISHITANI and NAKAMURA (1957) on Aspergillus oryzae by ultraviolet irradiation of heterozygous diploid conidia, and by MORPURGO and SERMONTI ( 1958) on Penicillium chrysogmum by ultraviolet irradiation of conidia and by treating them with various chemical mutagenic agents. Somatic segregation was strongly stimulated in almost every case by the action of the mutagenic agents. In MORPURGO and SERMONTI’S paper (1958), the delayed appearance of segregating sectors in colonies derived from conidia which had survived mutagenic treatment was very striking, suggesting a delayed effect of the mutagenic agent. An analogous delayed effect of chemical mutagenic agents had been noticed by AUERBACH ( 195 1 ) in Drosophila melanogaster. The present paper was begun as an analysis of this effect in the material under discussion.