MUTATIONS OF ALPHA-MATING-TYPE GENE IN NEUROSPORA-CRASSA

Abstract

In Neurospora, the mating-typing locus controls both mating (A + a is fertile) and heterokaryosis (A + a is incompatible). The 2 alleles appear stable, no novel fertility reactions have ever been reported, and attempts to seperate fertility and heterokaryon incompatibility functions by recombination have been unsuccessful. In the present approach the locus was studied through a mutational analysis of heterokaryon incompatibility function. A selection system was used that detects vigorous (A + a) heterokaryotic colonies against a background of inhibited growth. Twenty-five mutants of an a strain were produced following mutagenic treatment with UV and NG nitrosoguanidine; 15 were viable as homokaryons and 10 were not. All but one were infertile, but most showed an abortive mating reaction involving the production of barren, well-developed perithecia with A and a testers. None of the mutants complement each other to restore fertility. Seven mutants were mapped to the mating-type locus region of chromosome 1. Restoration of fertility was used to detect revertants, and these were found in 5 out of the 8 mutants tested. (A dose response was observed). In 4 cases incompatibility was fully restored and in 1 case it was not. The results suggest 2 positive actions of the locus when in heterozygous (A/a) combination (the stimulation of some stage of ascus production and the inhibition of vegetative heterokaryosis), and 1 positive action in homozygous combination (the production of a perithecial inhibitor).