Most basidiomycete species are outcrossing, producing variable offspring from their fruit bodies. However, not all species are outcrossing, and without a fully functional sexual process the spores produced in a basidiome will either contain identical nuclei (homothallic species) or two sister nuclei already mated in the basidium (pseudohomothallic species). Occasionally, both outcrossing and non-outcrossing lines may be present within the same species (Hallenberg, 1991). Similar genotypes can also be the result of mitotic spores, conidia or oida produced on mycelia (Kendrick and Watling, 1979). These spores can, for example be picked up by insects and spread over vast areas. Such clonality can be said to be dispersive while vegetative spread through a resource or set of spatially discontinuous resource gives rise to territorial clones (Anderson and Kohn, 1995).
For a while there were divergent views on whether 'the mycelium acted as an ''individual'' or as a ''social unit'''. The suggestion with the latter was that individual mycelia can pool their resources and collaborate in the colonization and composition of a resource. This was first described by Buller (1931) for Coprinus sterquilinus colonizing dung balls (Rayner and Todd, 1979). The idea of the individualistic mycelium, where each genotype builds its reproductive output and fitness by monopolizing resources for its mycelium seemed to be in contrast with the unit mycelium idea, until it was realized that C. sterquilinus is a homothallic species (Rayner, 1991b). Thus all spores in the dung ball had the same genotype, hence the cooperation between mycelia originating from different spores can be regarded as fusing of ramets of the same genet.
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