Nuclear State

Defining the fungal individual has been the topic of several reviews (Rayner et al., 1984; Rayner, 1991a, 1991b; Malik and Vilgalys, 1999; Glass and Dementhon, 2006)). Basically the problems lie in the multitude of genetic constitutions present in Kingdom Fungi and in the fact that fungal vegetative mycelia may fragment, so the genetic entity—the genet—becomes composed of several ramets.

In basidiomycetes, genetically, the vegetative mycelium is typically a dikaryon where two nuclei with dissimilar mating types are maintained in a pair wise manner in each compartment throughout the whole mycelium (Burnett, 2003). In some species, however, the number of copies of each nuclear type is not under strict control. A more inclusive description of the nuclear state is, therefore, to use the term heterokaryon for a mated mycelium and homokaryon for a mycelium harbouring only one nuclear type. There is increasing evidence that the homokaryotic phase in certain cases might be prolonged, especially in protected substrata such as inside the wood of a large log or a living tree (Stenlid, 1994a; Garbelotto et al., 1999; Redfern et al., 2001). Such mycelia are able to act physiologically as functional units. From an evolutionary standpoint, homokaryons can be regarded as dead ends if they do not find a mate. On the other hand, they can function as facilitators for the establishment of subsequent genotypes in the resource and increase the possibilities for a second spore to establish in their vicinity (Kemp, 1975). The domain occupied by a homokaryon is a selective substratum for conspecific spores capable of mating, but acts to exclude other competing species. In the phase of early colonization of a resource, this can extend and reinforce the spatiotemporal window for spore germination and increase the chances to establish a fertile heterokaryon. Based on this knowledge, species-specific spore traps have been developed using homokaryotic mycelia to catch conspecific spores in the environment (Adams et al., 1984; James and Vilgalys, 2001; Edman et al., 2004a, 2004b).

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