Meiotic drive in fungi: Chromosomal elements that cause fratricide and distort genetic ratios

Fungal Spore killers (Sk), studied most extensively in Neurospora and to a lesser extent in Podospora, Gibberella and Cochliobolus, cause the death of ascospores (= meiospores) that do not contain the killer (Sk(K)) element. When a Spore killer is heterozygous (Sk(K) x Sk(S)) in Neurospora, every ascus (= meiocyte) contains four normal-sized, black, viable ascospores (Sk(K)), and four ascospores that are tiny, unpigmented and unviable (Sk(S)). Killing of sensitive nuclei is expressed postmeiotically, and results in gross distortion of segregation ratios for Sk-linked genes. A sensitive nucleus that would otherwise die is rescued if a killer nucleus is also enclosed in the same ascospore. In Neurospora, Sk is centromere-linked (linkage group III), and when heterozygous, shows a recombination block in a 30-map-unit region spanning the centromere of linkage group III. There is no ascospore death or recombination block in killer x killer or sensitive x sensitive crosses. Spore killers are Fairly common in Gibberella fujikuroi and Neurospora sitophila but extremely rare in N. intermedia, and have not yet been found among natural isolates of N. crassa.