Perithecial distribution patterns in standard and variant strains of Neurospora crassa

When crosses are made homozygous for the mutation fl, which completely eliminates conidiation, the patterns of sexual morphogenesis may be observed unobscured by effects due to conidia and conidial dispersal. In one cross, arbitrarily chosen as the standard, a striking double line of perithecia is seen, bordering a distinct clear central zone. Other crosses show a complex array of mating reactions, some involving an apparent invasion of one culture by nuclei of the opposite mating type, either through heterokaryotization or through overgrowth of one culture by the other. The width of the clear zone and the direction of invasion are variable and strain specific. The extent of heterokaryotization in the standard and variant crosses is discussed in relation to the situation found in other species. A complex genetic control of these reactions is indicated from inheritance patterns.