Genetic analysis of amdS transformants of Aspergillus niger and their use in chromosome mapping.

The Aspergillus nidulans gene coding for acetamidase (amdS) was introduced into A. niger by transformation. Twelve Amd+ transformants were analysed genetically. The amdS inserts were located in seven different linkage groups. In each transformant the plasmid was integrated in only a single chromosome. Our (non-transformed) A. niger strains do not grow on acetamide and are more resistant to fluoroacetamide than the transformants. Diploids hemizygous for the amdS insert have the Amd+ phenotype. We exploited the opportunity for two-way selection in A. niger: transformants can be isolated based on the Amd+ phenotype, whereas counter-selection can be performed using resistance to fluoroacetamide. On this basis we studied the phenotypic stability of the heterologous amdS gene in A. niger transformants as well as in diploids. Furthermore, we mapped the plasmid insert of transformant AT1 to the right arm of chromosome VI between pabA1 and cnxA1, providing evidence for a single transformational insert. The results also show that the amdS transformants of A. niger can be used to localize non-selectable recessive markers and that the method meets the prerequisites for efficient mitotic mapping. We suggest the use of amdS transformants for mitotic gene mapping in other fungi.