The rad3-1 mutant is defective in axial core assembly and homologous chromosome pairing during meiosis in the basidiomycete Coprinus cinereus

We have utilized spreading methods as well as serial sectioning three-dimensional reconstruction to examine meiotic chromosome behavior in cells homozygous for the rad3-1 mutation in Coprinus cinereus. Comparison of 42 wild-type nuclei that had been spread, stained with silver, and viewed by electron microscopy with 30 mutant nuclei treated in the same manner revealed several defects in the mutant. Axial core formation was defective in the mutant, although limited side-by-side association of axial cores was observed. To detect any differences in three-dimensional architecture between the wild-type and mutant nuclei, we reconstructed three of the former and six of the latter after serial sectioning. It was not possible to trace the expected number of axial cores from section to section in the mutant, although some tripartite synaptonemal complex was observed. Many axial core ends failed to terminate in the nuclear envelope in the mutant. This spectrum of defects (incomplete axial core assembly with some tripartite synaptonemal complex formation) had not been observed previously in either C. cinereus or other systems. We conclude that this combination of spreading and sectioning methods is very useful for analysis of meiotic mutants. © 1993 Wiley-Liss, Inc.