ABSTRACT
The concept of the biological species is often difficult to define with a reasonable degree of certainty, and this is perhaps especially true in fungi. The second class of incompatibility system found in fungi is heterogenically controlled. This incompatibility system acts primarily at the vegetative stage of the life cycle and as such has been variously referred to as vegetative, somatic, or heterokaryon incompatibility. An alternative explanation for the natural function of the vegetative incompatibility system has been suggested. Although the genetic analysis by conventional methods of vegetative incompatibility is possible within a sexual species such as A. nidulans, the use of protoplast fusion has greatly facilitated this analysis because it has permitted the more efficient parasexual methods to be used. Vegetative incompatibility in Aspergillus was first described in A. niger and subsequently in several other sexual and asexual species. An alternative explanation for the natural function of the vegetative incompatibility system has been suggested.
