Three dimensional reconstructions of 15 pachytene nuclei of Schizophyllum commune have revealed in each nucleus I 1 distinct synaptonemal complexes, demonstrating a haploid chromosome number of 11. Each bivalent contained one short region of condensed chromatin marking the position of the centromere. The synaptonemal complex of each bivalent contained from 0 to six recombination nodules. A high correlation was found between the total length of the synaptonemal complexes in a nucleus and the number of recombination nodules. Total synaptonemal complex length correlated only weakly with the nuclear volume.

SUMMARYHydroxyurea (HU) effectively inhibits meiosis in Schizophyllum. The predominant cytological stage in inhibited fruit bodies is fusion. The inhibition is reversible and makes possible synchronization of a naturally nonsynchronous system. Microphotometric determinations of the DNA content in prefusion nuclei treated with HU suggest that premeiotic DNA replication occurs in prefusion nuclei. Synaptinemal complexes are not completed in HU-treated nuclei, suggesting that this event is dependent on premeiotic DNA replication.

In a search for an experimental procedure to synchronize meiosis in fruit bodies of Schizophyllum commune, the effect of hydroxyurea on sporulation was tested. Results indicate that hydroxyurea has an immediate and reversible effect on basidiospore sporulation, germination, and nuclear number. A tentative time schedule for meiosis is presented.