Nucleo-cytoplasmic control of perithecial formation in Aspergillus nidulans

Abstract

A spontaneous mutant colony alba , being aperithecial and having colourless mycelium, arises from spores of four wild-type strains of Aspergillus nidulans . Its analysis gives a clear-cut demonstration of non-Mendelian inheritance. The frequency of alba under standard conditions is different in each strain, the difference being under the control of a nuclear gene f , three alleles of which have been identified and its position mapped on the y chromosome. Two possible models explaining the experimental results are presented, the first assuming the presence of a replicating cytoplasmic particle interacting with the f gene. Where the relative amount of particles is below a certain threshold value (this being determined by f ), alba colonies result under standard conditions. These revert to normal on appropriate treatment. Stable alba mutants arise when the particle is lost to the cell altogether. The second postulates a reaction cycle in which the heterocatalytic action of each member of the cycle insures the reproduction of the catalysts themselves in a pseudo-autocatalytic system. Loss of one of the constitutent units would cause the breakdown of the system and this would be reflected in the production of alba colonies. The f gene is seen as having a regulatory function in the system, different f alleles giving different reaction rates. Restoration of cytoplasmic deficiency can be made only by a ‘transfusion’ of normal cytoplasm following hyphal fusion. Two genes affecting other aspects of perithecial formation are described, one determining a condition of self-sterility and the other controlling perithecial density.