Meiotic chromosome synapsis in a haploid yeast.

An extensive synaptonemal complex (SC) is found at pachytene in whole mount spread preparations of a haploid yeast, Saccharomyces cerevisiae, strain. Whereas unsynapsed axial elements are present only in a few nuclei, in others non-homologous synapsis involves virtually the whole chromosome set. This suggests that homology is not an indispensable precondition for SC formation in yeast but that chromosomes engage in non-homologous synapsis if no homologous partner is available. Recent evidence that in the sporulation deficient yeast mutants rad50 and mer1 axial elements do form but remain unsynapsed in the majority of nuclei is discussed in the light of the above findings.