An instance of cytoplasmic inheritance in Aspergillus nidulans

Abstract

Aspergillus nidulans is a homothallic ascomycete normally reproducing by asexual conidia and sexual ascospores in perithecia, both types of spore being uninucleate (for other details see Pontecorvo (1953)). In a wild-type strain of this fungus obtained from the Commonwealth Mycological Institute (No. 16643), about 4% of spores whether conidia or ascospores, give rise under standard cultural conditions, to asexual colonies reproducing entirely by conidia (figure 29, plate 19). Numerous such asexual colonies were isolated by mass transfer first excluding the possibility of previous hyphal fusions with normal colonies in the same cultures. About one in twenty of these reverted to normal and produced perithecia, but only after prolonged incubation. The other isolates have remained asexual under a wide variety of cultural conditions and after many subcultures. The conidia and ascospores of normal colonies continue to segregate for normal and asexual type and frequently sectors of asexual type arise in normal colonies (figure 29). The normal perithecial colony invariably develops a reddish-brown pigment in the hyphal cells, the conidial colony being characterized by the absence of this pigment. A cross was made between a ‘true-breeding’ conidial form having white conidia ( w ) and being biotin-independent ( BI ) and a perithecial form having wild-type green conidia ( W ) and requiring biotin ( bi ). These nuclear markers for w and bi arose spontaneously and are the result of a single gene difference in each case (cf. Pontecorvo 1953). Following hyphal fusion of the two strains, perithecia of three types were obtained resulting respectively from the nuclear fusions (1) W bi x W bi , (2) W bi x w BI , and (3) w BI x w BI . There was thus in (3) recombination of the factor for perithecial formation without cross-fertilization. In other words the factor is not directly associated with the nucleus of the perithecial strain, but presumably with its cytoplasm (Jinks 1954; Sharpe 1956). Other crosses between perithecial and conidial forms gave similar results, but crosses between two conidial strains never produced perithecia although heterokaryons were established.