Cytogenetic Behavior of Spore Killer Genes in Neurospora

Crosses heterozygous and homozygous for Sk-1, Sk-2 and Sk-3 were examined by light microscopy. All three Spore killers behave similarly. In heterozygous killer x sensitive crosses, meiosis and ascospore development are normal until after the second postmeiotic mitosis when four of the eight ascospores in each ascus stop developing and degenerate. The four surviving ascospores carry the killer. Death of sensitives thus occurs only after killer and sensitive alleles, Sk(K) and Sk(S), have segregated into separate ascospores. Homozygous killer x killer crosses do not show such a pattern of degeneration. Either all ascospores are normal or, if some fail to mature, they do not resemble the degenerating sensitive ascospores in heterozygous asci.--With Sk-2, it was shown that Sk(S) nuclei do not abort when both Sk(K) and Sk(S) are present in the same ascospore. Mutants affecting ascus development were used to obtain large ascospores enclosing both Sk(K) and Sk(S) meiotic products in a common cytoplasm. Sk(S) nuclei do not then undergo the degeneration that would be seen if they were sequestered into separate ascospores, and viable Sk(S) progeny are recovered in undiminished numbers when the mixed multinucleate large ascospores are germinated. In a four-spored mutant, where each ascospore encloses a single nucleus following meiosis, degeneration of Sk(S) ascospores nevertheless occurs, even though the third nuclear division is omitted. Cycloheximide and temperature treatments do not affect the expression of Sk(K).