Characteristics of chromosomally located Spore killer elements have been summarized by Turner et al. (Am. Nat. 137:416-429, 1991; Fungal Genet. Newsl. 34:59-62, 1987) and will only be summarized briefly here. In crosses heterozygous for killer and sensitive (SkK SkS), four ascospores are usually killed in each 8-spored ascus and the survivors are SkK. All eight ascospores survive in crosses homozygous for the same killer element. Killer elements have been found in natural populations of N. intermedia (Sk-2K and Sk-3K, both rare) and N. sitophila (Sk-1K, common). Sk-2K and Sk-3K have been introgressed from N. intermedia into N. crassa for convenience of genetic analysis. Both are haplotypes presumed gene complexes in a centromere-spanning segment of linkage group III within which meiotic recombination is repressed in the killer/sensitive heterozygotes. Sk-2K and Sk-3K are similar in behavior except that each is sensitive to killing by the other. The two also differ in their reaction to genes conferring resistance to killing. Genes conferring resistance to Sk-2K or to Sk-3K are present in some populations of N. intermedia. Resistance to Sk-2K is found at low frequency throughout the range of N. crassa. The resistance genes, symbolized r(Sk)-2 and r(Sk)-3, have been mapped in wild type sequence in N. crassa. They are linked to loci within the killer complex. Linkage relations of Sk-1K are not known, nor is it known whether this killer element is associated with a complex.
Sk-2K strains have been found only in N. intermedia, and only in four localities: Brunei (B), Java (J), Papua New Guinea (P), and Sabah (SA). Sk-3K is known solely from Papua New Guinea. All commonly used N. crassa laboratory wild-type strains and their derivatives are sensitive to killing both by Sk-2K and by Sk-3K.
Strains containing the aconidiate mutation fluffy (fl) are conveniently used a female parents in test crosses for scoring killer vs. sensitive. The fl testers are highly fertile, and because conidia are absent, ascospores ejected to the sides of the tube can be seen clearly. With N. crassa, tests are made by fertilizing the testers on 10 75 slants of synthetic cross medium with 1% sucrose and examining shot ascospores after 10 days at 25x. With N. intermedia, N. sitophila, and N. discreta, tests are best made on 13 100 mm slants using synthetic cross medium with filter paper as sole carbon source. If this medium is employed, stocks without the fluffy mutation can be used, because few conidia are produced. Standard stocks of N. tetrasperma are also satisfactory as testers because they make few conidia at 25x, even on sucrose medium.
Sk-2S Sk-3S strains are sensitive to killing both by Sk-2K and by Sk-3K. The double symbol is used to specify phenotype, and does not imply that Sk-2K and Sk-3K necessarily represent two genes at separate loci. It has not been determined how many loci are involved in determining sensitivity vs. resistance to either or both Spore killers.