Diploids of Armillaria mellea: synthesis, stability, and mating behavior

In nature, the vegetative cells of most species of Hymenomycetes are dikaryotic. The only species of this group thought to have a persistent vegetative diploid phase in nature is Armillaria mellea. Mating in this fungus is determined by bifactorial heterothallism, a two-locus (A and B) multiallelic system occurring commonly in the Hymenomycetes. Compatible matings of auxotrophic haploids of A. mellea, however, routinely yield prototrophic diploids. No selective system is needed to recover diploids in compatible matings. In this study, prototrophs were also recovered from hemicompatible (A ≠ B = and A = B ≠) and fully incompatible (A = B =) pairings of auxotrophs, but only when nutritional selection was applied to these pairings. The prototrophic cells recovered by selection were uninucleate and diploid, rather than heterokaryotic, regardless of whether the strains paired were compatible or not. All of the diploids were stable on nutritionally nonselective medium; only two somatic segregants carrying auxotrophic genotypes were detected in tests with 17 different diploid strains. Diploids were capable of mating with haploid strains and the results of diploid × haploid matings clearly indicated heterozygosity for mating-type loci in diploids originally recovered from compatible and hemicompatible pairings.