Dilution of gene products in the cytoplasm of heterokaryons in Coprinus lagopus

(a) Coprinus lagopus has in nature both a monokaryon with one haploid nucleus per cell and a dikaryon with two haploid nuclei per cell. It also has in experiment both a heterokaryon with two types of nuclei distributed irregularly in the mycelium and a diploid monokaryon with one diploid nucleus per cell.

(b) Two pairs of linked recessive auxotrophic mutants, me-5 ad-8 and adhis-1 chol-1 have been combined in two different sets of multiple heterogenotes so that in the dikaryon and heterokaryon the + (active) alleles of the four genes are distributed equally in the two component nuclei. In one combination the linked genes are in trans and in the other they are in cis arrangement in diploid, dikaryon and heterokaryon.

(c) Complementation between the + genes, as measured by growth on minimal and complete media, was complete in the diploid and dikaryon but was only partial in the heterokaryon.

(d) A similar comparison of a recessive suppressor of the mutant me-1 revealed that the recessive gene was not expressed and therefore was fully recessive in the diploid and dikaryon but was only partially recessive in the heterokaryon.

(e) The exact similarity of complementation of the structural genes of the auxotrophic mutants and the recessiveness of the me-1 suppressor in diploids and dikaryons exclude a localization of gene interaction in the nucleus. In a heterokaryon cytoplasmic dilution of the product of a dominant gene can occur and has the effect of producing a phenotypic difference between heterokaryons and diploids.