Heterokaryosis is the condition in which one cell of a mycelium contains two or more genetically different nuclei. Hansen and Smith (1932), working with Botrytis cinerea, which has multi-nucleate spores, showed that single-spore cultures of this fungus sometimes contained two types of nuclei and that such heterokaryotic types could readily be induced by hyphal fusion between different homokaryotic types. These authors analysed individuals from thirty genera of imperfect fungi and found more than half of them to be heterokaryotic. It seems, therefore, that heterokaryosis is a widespread phenomenon among such fungi.

N. crassa is a heterothallic fungus with an eight-spored ascus and a pair of well-defined mating-type factors which have been very generally called sex factors. Perithecia are formed only when cultures of the opposite mating types are brought into contact. Both mating types can produce ascogonia, generally conceived to be female organs, and microconidia, sometimes called spermatia (Dodge, 1935), which may correspond to male gametes. Therefore it is doubtful whether the mating-type factors are true sex factors.

A non-committal term such as “mating type” or “incompatibility factor” is preferable. However, because of the convenience of the terms associated with sex, such as “unisexual” and “bisexual”, I shall follow previous authors (Dodge, Lindegren, Beadle) in using these terms with reference to the mating-type factors, at the same time emphasising that these factors may not be sex factors in the generally accepted sense of the term.