SUMMARY: A spontaneous change, possibly involving a mating-type switch, occurred during storage of a homokaryotic culture (F2.4) of the basidiomycete and resulted in a dramatic transformation of its cultural and interactive properties. Subcultures grown on malt agar rapidly produced farinaceous hymenial surfaces if exposed to light, and were not receptive to nucleus migration when paired with compatible homokaryons. These properties were transmissible to other homokaryons by direct pairing, the recipients becoming transformed regardless of whether they were sexually or somatically compatible or incompatible with F2.4. With the incompatible sib F2.9, transformation apparently occurred without nuclear transfer.

Progeny monobasidiospore cultures from transformed F2.4 and F2.9 strains consistently belonged to two interaction groups, and , the former reacting similarly to the corresponding untransformed strains against a range of sib homokaryons, the latter interacting in exactly the opposite fashion. pairings resulted in all cases in an unusual ‘quasi-compatible’ reaction, characterized by development of an asymmetrically migrating farinaceous region.

Progeny from transformed F2.9 were dimorphic, being divisible into () slow-growing, dense and () fast-growing, effuse colony types, the former reverting spontaneously to the latter. In several cases mycelial ‘mounds’ developed on reverted slow, dense colonies. Basidiospore progeny and mycelial subcultures from mounds produced colonies with further mounds. These and further observations indicate pleiotropic effects initiated or mediated by a cytoplasmically transmissible, possibly insertional factor on recognition and developmental processes in .


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