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1984. Breeding relationships among several species of Agaricus. Can. J. Bot. 62: 1884-1889. We have examined the breeding behavior of several species of Agaricus including the cultivated A. brur~nescerls and the wild A. bitorquis, A. vaporarius, A. nrvensis, A. campestris, A. silvicoln, and A. placorn~~ces. For A. brunnescens, two homokaryons carrying auxotrophic mutations and compatible mating types were obtained from the American Type Culture Collection. A stable, prototrophic heterokaryon was recovered by nutritional selection from a pairing of the two auxotrophic strains. The two nuclear types were recovered from the heterokaryon by the formation and regeneration of protoplasts, many of which were homokaryotic. In A. birorquis, a distinct macroscopic interaction correlated with dikaryon formation was observed in compatible matings of single-spore isolates. Fluffy zones of dikaryotic mycelium appeared in compatible pairings where the two cultures met. Mating among monosporous isolates was specified by unifactorial heterothallism. These observations of A. bitorquis were similar to those reported by others. Pairings of isolates from different stocks indicated a minimum of eight incompatibility alleles among 10 stocks of A. bitorquis. Although migration of nuclei generally does not occur in Agaricus, one stock of A. bitorquis produced monosporous isolates with nuclei that migrated through the resident mycelium of some compatible mates. The migration was evident as a fluffy zone of dikaryotic hyphae that spread unilaterally in a pairing. Monosporous isolates of A. vapornrius showed distinct mating interactions similar to those in A. bitorquis. No mating interactions were observed in pairings of sibling monosporous isolates of any of the other species examined. When isolates of different taxonomic species were paired, no reactions suggestive of compatibility were observed between A. brunnescens, A. bitorquis, A. vnpornrius, or any of the other species.

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