Lethal Mutations and Balanced Lethal Systems in Aspergillus nidulans

SUMMARY The frequency of spontaneous and ultraviolet-induced, recessive, lethal mutations was estimated in two diploid strains of Aspergillus nidulans. The parent diploids were heterozygous for recessive and semi-dominant markers; each recessive lethal resulted in failure to recover haploid strains carrying the allele linked in coupling to that lethal. Diploids 1 and 2 carried markers permitting direct assay of lethals on 4 and 8 chromosomes, respectively, out of the total diploid complement of 16. No lethals were detected in 100 isolates from each untreated diploid. One hundred isolates from each treated parent yielded 6 and 16 lethals, respectively, suggesting a frequency for the whole genome, at 5 % survival, of about 28 % lethals. The lethals were nutritionally irreparable and were not temperature-sensitive. Several of them have been located roughly by mitotic crossing-over. One isolate was heterozygous for two unlinked lethals. It segregated to give a stable heterokaryon bearing two classes of non-viable haploid conidia. Meiotic analysis, via a three-component heterokaryon, showed that the stable heterokaryon was balanced by the non-allelic lethals, one in each haploid component. Another isolate had a balanced lethal system resulting from linked, non-allelic lethals in trans. Six isolates from treatment were stable diploids which produced no haploid sectors.