Introduction

Germinating meiospores of Schizophyllum commune grow into primary mycelia which are monokaryons, i.e. mycelia in which each hyphal compartment contains one nucleus only. When two primary mycelia aregroum together so that hyphal contacts can occur, hyphae may fuse, irrespective of mating type, and produce heterokaryotic cells. The fate of these heterokaryotic cells depends on the mating-type genes carried by the opposing mycelia. If they carry different MATA and different MATB genes, a stable heterokaryotic dikaryon (the secondary myoelium) is formed (Fig. 13.1). First, nuclei from each mate, rapidly travel into the other mate, inducing the dissolution of the complex dolipore septa which otherwise block nuclear translocation (Raper, 1966; Wessels, 1978). This process of septal dissolution and nuclear migration is apparently controlled by the presence of different MATB genes in the heterokaryon, because it occurs when the MATB genes only are different (MATA=MATB ≠). However, if the MATA genes are also different (MATA ≠MATB ≠) the foreign nucleus associates with a resident nucleus in u tip cell and from now on septal dissolution is switched off and the two nuclei(carrying different MATA and MATB genes) remain closely associated with each other and divide synchronously. During this division the spindle of one of the dividing nuclei is oriented obliquely so that one of its daughter nuclei is positioned in a small lateral branch, the hook cell. This branch is subsequently cut off from the apical compartment by a septum.