Introduction

The genetic nature of the ‘fungal individual’ has intrigued biologists for over 50 years. To some extent this is because fungi have an indeterminate growth form and the physical shape of a ‘fungus’ can not usually be described a priori in the same way that a cat or a tree is characterized and identified. Most filamentous fungi are cryptic organisms whose visible portion consists of ephemeral fruit-bodies that occur in a temporally and spatially discontinuous manner and the boundaries of individuals are, therefore, not immediately evident to the casual observer (see Chapter 1). An early view of the fungal individual was recognized in the ‘unit mycelium’ concept (Buller, 1958; Raper, 1966). Within this view, a basidiomyoete could be a mosaic of several different nuclear genotypes operating within a physiologically integrated individual. Studies that examine the distribution of genetic markers within fungal populations support an alternative view of fungal individualism (Todd 8:.Rayner, 1980; Rayner, 1991), in which mycelia occupy discrete territories in space, have cellular non-self recognition systems and are genetically distinct. Reports that basidiomycete genotypes can occur throughout extensive geographic areas (Adams, 1974; Shaw & Roth, 1976; Anderson et al., 1979; Dickman & Cook, 1989; Smith, Bruhn 3Anderson, 1992) are surprising since some fungi must now be thought of as large, and consequently ancient, individuals. Intrinsic to this notion, such individuals must be genetically stable, at least to the extent that they are recognizable as genetic units. possibly to the extent that they are potentially immortal. In this chapter, genetic stability and factors that may influence genetic stability in fungi will he discussed.