Mycelial Pigmentation Research Articles

Heterkaryon Formation and Genetic Analyses of Color Mutants in Aspergillus heterothallicus

Heterokaryon compatibility between strains of Aspergillus heterothallicus was tested by using prototrophic mutants of contrasting conidial head colors obtained by ultraviolet radiation. Heterokaryons were formed between mutants only when two conditions were fulfilled: The mutants had to be of the same mating type, and the isolates from which the mutants were derived had to be of similar macroscopic growth characteristics and mycelial pigmentation. Heterokaryotic heads were conspicuously striate, being composed of spore chains pigmented as the parental types, and representing, in most cases, examples of an “autonomous” nuclear action. Heterokaryotic heads of unusual interest, resulting from “autonomous-nonautonomous” type of gene actions, were produced in combinations of a pinkish tan (PT) mutant. The nuclear action of the PT mutant was nonautonomous regardless of the type of mutant with which it was paired, whereas the nuclear action of the other head-color mutants was autonomous. Genetic analyses of head-color mutants were conducted by using all possible crosses between mutants derived from strains WB 4982 (A mating type) and WB 5086 (a mating type). It was found that similar color mutations of these two strains were allelic. All mutations responsible for different head colors, however, were found to be nonallelic to each other. The interactions between the different head-color mutants were studied in double mutants formed in the aforementioned crosses. Epistatic-hypostatic relationships between the different mutations were determined, and such relationships were used in the construction of a possible biosynthetic sequence leading to the production of the wild-type (green) pigment of conidial heads. The main sequential steps for the biosynthesis of conidial pigmentation appear to proceed from white to yellow, to brown, to brownish green, and to green.

SEXUALITY AND CULTURAL CHARACTERISTICS OF ASPERGILLUS HETEROTHALLICUS

Aspergillus heterothallicus K., F. and R., isolated from Costa Rican soils, represents the first species in this genus that is truly heterothallic. Each of the eleven strains investigated is functionally hermaphroditic but self‐sterile and falls into one of two cross‐mating classes, A or a. The mating‐type factors A and a appear to be an allelic pair segregating independently of the locus for pigmentation of mycelium which varies from yellow to pinkish orange in different isolates. The striking variation in the development of hülle cell masses that occurred among the progeny from the cross WB 5096(A) × WB 5097(a) was found to be genetically controlled. The gene responsible for a delay in the formation and maturation of cleistothecia appeared to be loosely linked to the a mating‐type locus and could be recombined into the A mating type. The mechanism of fertilization has not been completely elucidated. Coiled ascogonia were found within young hülle cell masses developed in cultures where two isolates of opposite mating types were crossed; such coils have not been observed, thus far, within hülle cell masses in unmated cultures. Although no recognizable male structure has been found, the fertilizing element appears to be mycelial in form rather than conidial. Interspecific mating did not occur when strains of Aspergillus heterothallicus were paired with other members of the A. nidulans group.