Diversity of phenotype and vegetative instability are often encountered with isolates from nature of the genus Aspergillus (Raper & Fennel, 1965). When conidia from strains previously cloned by plating for isolated colonies are cultivated on agar, variants frequently appear as distinct sectors, over-growths or localized areas of changed appearance. They may or may not retain their distinguishing characteristics when isolated. Yuill (I 939):isolated a buff-coloured conidial variant which arose spontaneously in a culture of Aspergillus fumigatus, and a white variant from a culture of Aspergillus nidulans.

Aspergillus niger has uninucleate conidia (Yuill, 1950). The form of colonies is characteristic, and variants occurring in vegetative growth can readily be observed. We report observations on such variation in a strain of A. niger isolated from the wild, and which has the properties expected of a heterozygous diploid. To test whether it was heterozygous, we sought answers to the following questions: Is the spontaneous variation in the strain under genetic control? Is this variation affected by the incorporation of the amino acid analogue p-fluorophenylalanine (pFA) in the growth medium? Is the effect of p F A onthe strain consistent with its known effect as an agent facilitating the haploidization of diploid strains?


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