Presumptive heterokaryons in Aspergillus fonsecaeus.

Raper and Fennell (1953) studied heterokaryon formation in Aspergillus fonsecaeus Thom and Raper, strain NRRL 67, a carbonaceous black species of the Aspergillus niger group (Thom and Raper, 1945) which produces large multinucleate conidia. Eleven various color, nutritional and morphological mutants (figure. 1) were produced by ultraviolet irradiation of the wild strain. The mutants were paired with the parent and with themselves in various combinations. Raper and Fennell (1948, 1953) examined 46 of the 66 possible combinations and obtained 30 heterokaryons (figure 2): 7 blend heterokaryons of which the pigmentation was intermediate between black (the color of STOCK and of the nitrateless mutant, THIN) and the color of the other component; 4 gray-brown heterokaryons, obtained when TAN and mutants derived from it were paired with CINNAMON and when PINK was paired with CLAY; and 19 large carbonaceous black heterokaryons which appeared in all the other heterokaryon producing combinations tested. Ten combinations, not originally studied by Raper and Fennell, also yielded black heterokaryons during this investigation and in a parallel study by Ciegler and Raper (1957) as summarized in figure 2. In these 3 types of heterokaryons, each head produced 3 categories of conidia: 2 of these contained only 1 or the other type of parent nuclei (homokaryotic), hence yielded colonies of only 1 mutant or STOCK, if this was implicated;