Genetic analysis of Aspergillus niger: Isolation of chlorate resistance mutants, their use in mitotic mapping and evidence for an eighth linkage group

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Abstract
This paper describes the use of chlorate resistant mutants in genetic analysis of Aspergillus niger. The isolated mutants could be divided into three phenotypic classes on the basis of nitrogen utilization. These were designated nia, nir and cnx as for Aspergillus nidulans. All mutations were recessive to their wild-type allele in heterokaryons as well as in heterozygous diploids. The mutations belong to nine different complementation groups. In addition a complex overlapping complementation group was found. Evidence for the existence of eight linkage groups was obtained. Two linked chlorate resistance mutations and two tryptophan auxotrophic markers, which were unlinked to any of the known markers (Goosen et al. 1989), form linkage group VIII. We used the chlorate resistance mutations as genetic markers for the improvement of the mitotic linkage map of A. niger. We determined the linear order of three markers in linkage group VI as well as the position of the centromere by means of direct selection of homozygous cnxA1 recombinants. In heterozygous diploid cultures diploid chlorate resistant segregants appeared among conidiospores with a frequency of 3.9×10−2 (cnxG13 in linkage group I) to 2.1 × 10−2 (cnxD6 in linkage group 111). The mean frequency of haploid chlorate resistant segregants was 1.3 × 10−3. The niaD1 and niaD2 mutations were also complemented by transformation with the A. niger niaD + gene cloned by Unkles et al. (1989). Mitotic stability of ten Nia+ transformants was determined. Two distinct stability classes were found, showing revertant frequencies of 5.0 × 10−3 and 2.0 × 10−5 respectively.