The Dual Phenomenon In Imperfect Fungi

Abstract

When some 900 isolates from 30 genera of imperfect fungi were analyzed by the single-spore series method it was found that more than 50% of them were dual, i.e. were composed of two culturally distinct individuals. This condition is referred to as the “dual phenomenon.” Such dual fungi when single spored give rise to three culture types: One producing abundant mycelium and few conidia is called the M (mycelial) type, another producing many conidia and relatively less mycelium is termed the C (conidial) type and a third type in general intermediate between M and C in cultural characters is named MC suggesting that it is composed of the other two types. M and C when single-spored give rise to M and C types only, whereas when MC is single spored it gives rise to all three types. When the homotypes M and C are grown together in mixed culture they combine, presumably by the mechanism of anastomosis, and produce the MC type which upon single sporing again gives rise to all three types M, C and MC. The above indicates that the dual phenomenon is due to heterocaryosis i.e. that individual cells and individual spores of dual fungi contain two genetically distinct types of nuclei, this is further substantiated by the relative difficulty or ease with which the homotypes M and C are separated from MC heterotypes having multi, bi or mainly uninucleate spores. The frequency with which the dual phenomenon is observed to occur in imperfect fungi suggests that duality may be the natural condition for many of them.