Non-random distribution of multiple mitotic crossing over among nuclei of heterozygous diploid <i>Aspergillus</i>

SOMATIC, or mitotic, crossing-over1 occurs in nuclei of the mycelium of the homothallic ascomycete Aspergillus nidulans 2,3, made diploid and heterozygous for known markers by means of a new technique2. The asexual Aspergillus niger has now been found to behave in a similar manner; that is, diploids heterozygous for known markers give mitotic recombinants homozygous for one or more of these markers but usually still heterozygous for the others, and therefore undergoing further mitotic recombination. Detailed analysis of mitotic recombinants in A. nidulans further shows that mitotic crossing-over is not distributed at random between the nuclei of the mycelium but tends to be concentrated in a small proportion of them. The kind of evidence on which this conclusion is based is exemplified in the following paragraphs