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 c ells homozygous for the rad3-1 mutation in Coprinus cinereus. Comparis on of 42 wild-type nuclei that had been spread, stained with silver, a nd viewed by electron microscopy with 30 mutant nuclei treated in the same manner revealed several defects in the mutant. Axial core formati on was defective in the mutant, although limited side-by-side associat ion of axial cores was observed. To detect any differences in three-di mensional architecture between the wild-type and mutant nuclei, we rec onstructed three of the former and six of the latter after serial sect ioning. It was not possible to trace the expected number of axial core s from section to section in the mutant, although some tripartite syna ptonemal complex was observed. Many axial core ends failed to terminat e in the nuclear envelope in the mutant. This spectrum of defects (inc omplete axial core assembly with some tripartite synaptonemal complex formation) had not been observed previously in either C cinereus or ot her systems. We conclude that this combination of spreading and sectio ning methods is very useful for analysis of meiotic mutants.