Bifactorial Heterothallism and Vegetative Diploidy in Clitocybe Tabescens

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Abstract
The sexual incompatibility system and nuclear cycle of the root-rot fungus Clitocybe tabescens were investigated. Mating among monosporous progeny of a single fruiting body was clearly determined by bifactorial heterothallism. Compatible pairings of monosporous isolates were distinguishable from incompatible pairings on the basis of macroscopic colony morphology and presence or absence of clamp connections and dikaryotic cells. Dikaryotic cells isolated from compatible pairings, however, were unstable and always produced uninucleate, presumably diploid cells lacking clamp connections after prolonged culture without transfer. Diploidy in uninucleate cells was confirmed in another experiment. Compatible pairings of auxotrophic mutants derived from monosporous strains yielded uninucleate, prototrophic cells. The simplest explanation for the observed prototrophy is that complementation occurred within diploid nuclei. The diploid cells found in compatible pairings produced cultures similar in every respect to isolations of naturally occurring vegetative material of C. tabescens. The only other hymenomycete known to have a prolonged vegetative diploid phase is Armillaria mellea, a species which is closely related to C. tabescens. The possibility of interfertility between these two species was examined by pairing representative monosporous isolates. Isolates of C. tabescens were intersterile with all representatives of A. mellea tested.