Fungal Vegetative Compatibility

Heterokaryon fonnation between different fungal individuals is an important component of many fungal life cycles and may serve as the first step in the parasexual cycle and the transmission of hypovirulent factors such as dsRNAs. Heterokaryosis also is a means by which nonnally haploid fungi may enjoy the benefits of functional diploidy, such as complementation or heterosis. In plant pathogenic fungi, the entity that emerges following heterokaryosis may differ from its constituents in aggressiveness or host range; some of these aspects have been reviewed in previous volumes in this series (9, 28, 45, 62, 72, 112, 114, 117, 143, 154, 158, 160). In most heterothallic fungi, the fonnation of a heterokaryon between two genetically different haploid strains is an essential part of the life cycle. Such heterokaryons may be quite stable and persist for an indefinite period of time or may last only long enough for the component haploid nuclei to fuse and then immediately undergo meiosis. In many fungi, sexual and vegetative heterokaryons are quite distinct from one another. Strains capable of fonning a successful sexual heterokaryon may be unable to fonn a successful vegetative heterokaryon and vice versa. Strains that are capable of fonning these types of heterokaryons are referred to as sexually or vegetatively compatible, respectively. Strains that are vege­ tatively compatible with one another are frequently described as members of the same vegetative compatibility group, or VCG. Sexual compatibility is usually governed by one or more mating-type loci that may have two or more alleles (58, 62). Vegetative compatibility may be governed by the mating-type loci in some fungi, e.g. many basidiomycetes, but there also are examples in which a separate set of genes controls the fonnation and stability of these vegetative heterokaryons.