SUMMARYA quantitative survey has been made of conidiation mutants in Aspergillus nidulans. Conidiation mutants were 6 to I 3 times more numerous than auxotrophic mutants, but growth tests showed that the mutant defect in 85 % of the conidiation mutants was not confined to conidiation. The number of loci specifically involved in conidiation was estimated to be 45 to 150. Mutants were classified according to the stage of the developmental block and also into asporogenous and oligosporogenous types; 64 % were oligosporogenous, but in further tests, 84 % of the asporogenous mutants were found to be temperature sensitive or slightly leaky, suggesting that many of the loci involved act only as modifiers in conidiation. The largest group of mutants was blocked before conidiophore formation, while many others failed at ill-defined stages of conidiation, suggesting that failure was due to the gradual build-up of metabolic deficiencies. A class of conidial maturation mutants has also been identified.
I N T R O D U C T I O NBalassa (1969), in an extensive quantitative survey of sporulation mutants of Bacillus subtilis, concluded that about 800 genes were concerned in the sporulation process. Aspergillus nidulans is a much more complex organism than B. subtilis and yet Clutterbuck (I 969) was able to find only two genes essential for the later stages of conidiation. Hopwood, Wildermuth & Palmer (1970) have also found relatively few genes which control the production of sporulation septa in Streptomyces coelicolor. To investigate this apparent discrepancy, Balassa's procedure of comparing the numbers of auxotrophic and sporulation mutants has been applied to A. nidulans.Balassa used both visual and replication techniques for the identification of sporulation mutants. Only about 10 to 15 % of sporulation mutants were found to be totally asporogenous, the rest produced a genetically determined low frequency of spores and were termed oligosporogenous. Similarly, in the present work, conidiation mutants of Aspergillus nidulans have been identified by two criteria: the loss or alteration of the wild-type green spore colour, or failure of the mutants to be replicated by velvet. Oligosporogenous and asporogenous mutants have also been distinguished. An additional basis of classification of sporulation mutants was employed by Ryter, Schaeffer & Ionesco (1966), who defined seven stages of sporulation in Bacillus subtilis. A comparable series of stages for conidiation in Aspergillus nidulans has been put forward and used for the classification of mutants.
A series of strains, doubly mutant at conidiation loci, have been made. The phenotypes of these strains reflected the epistasy of earlier blocking mutants over later ones and confirmed the order of gene sequence predicted from the phenotypes of single mutants. Oligosporogenous mutants gave complex interactions, especially between brl and med mutants.These results indicated that (i) gene action overlapped in time, (ii) several parts of the conidial apparatus were interchangeable and (iii) nuclei leaving the vesicle were not irreversibly programmed. Structures produced by mutants were reminiscent of the conidial apparatus of other Aspergillus species and of related genera.
Three genotypically suppressible alleles, a1X4, alcA125, and niaD500, are phenotypically suppressed by aminoglycoside antibiotics. Unsuppressible alleles at these loci are unaffected as are known missense mutations at the yA and gdhA loci. This is consistent with the premise that the suppressible mutations are nonsense and that this highly-allele-specific phenotypic suppression can be used to distinguish nonsense from missense mutations of Aspergillus nidulans. Paromomycin and tobramycin are recommended for screening unknown mutations.
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