A shift in nuclear state as the result of natural interspecific hybridization between two North American taxa of the basidiomycete complex Heterobasidion.

A natural first generation hybrid fungus shows interspecific heterozygosity. The nuclear condition of a rare natural hybrid between two taxa of the Heterobasidion complex is investigated. Heterobasidion species are known to be either homokaryotic (haploid) or heterokaryotic (n+n), but heterokaryons are made up of both homokaryotic and heterokaryotic sectors. The natural hybrid appears to be either a heterokaryon undergoing a primary homothallic phase or a diploid with limited ability to exchange nuclei when mated with homokaryons. The natural hybrid is stable and long lived, suggesting hybridization may play an important role in the evolutionary history of this fungal complex.