All auxotrophs of Aspergillus foetidus and all but two auxotrophs of A. niger which we isolated yield glucoamylase and citric acid, respectively, at levels below that of the prototrophic strain from which they were derived. Results of representative heterokaryon tests suggest that the nucleus was principally responsible for the inheritance of citric acid or glucoamylase production. Most somatic diploid strains of A. foetidus gave rise to higher yields of glucoamylase when compared to their haploid component strains. Both heterokaryons and somatic diploid strains of A. niger synthesized between auxotrophs which were simultaneously reduced in citric acid yields also gave rise to enhanced yields when compared with their haploid components. The yields of a heterokaryon and somatic diploid synthesized between two high producers of citric acid were not higher than those of respective haploid components. We concluded from these results that gene dosage (or ploidy) does not increase the yield of citric acid. The apparent enhancement in yields observed in diploids or heterokaryons synthesized between auxotrophs with reduced yields in both species can be interpreted as resulting from intergenic complementation.
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