Trypan Blue-induced Deletions in Duplication Strains of Aspergillus nidulans

IN the filamentous fungus Aspergillus nidulans , strains with a chromosome segment in duplicate (one in normal position, one translocated to another linkage group) are unstable at mitosis 1,2 . This “mitotic non-conformity” is provoked by chromosomal imbalance and initial changes of genotype are confined largely to the segments carried in duplicate 3,4 . Duplication strains, which have a characteristic morphology and reduced linear growth rate, give sectors produced by nuclei which have undergone spontaneous deletions, of variable size, from one or other duplicate segment. Sectors can almost always be scored unequivocally by their improved morphology and relative growth advantage, determined by nuclei more nearly haploid in quantity than those of the duplication parent. In suitably marked strains, loss of one or more dominant alleles gives sectors differing from the parent in conidial colour and/or nutritional requirements (Fig. 1 a ). The approximate size of each deletion, and the segment involved, can be determined by genetic analysis or sometimes simply by the morphology, nutritional requirements and stability or instability of the sector carrying it. This system seems to offer a simple screening test for deletion-producing agents and for any selective activity they might show towards particular chromosome regions. In addition, success in modifying sector frequency might help in elucidating the processes of mitotic non-conformity. The first attempt to induce deletions, using trypan blue (TB), has been successful.