Evolutionary consequences of putative intra-and interspecific hybridization in agaric fungi

Abstract

Agaric fungi of the southern Appalachian Mountains including Great Smoky Mountains National Park are often heterozygous for the rDNA internal transcribed spacer region (ITS) with >42% of collections showing some heterozygosity for indels and/or base-pair substitutions. For these collections, intra-individual haplotype divergence is typically less than 2%, but for 3% of these collections intra-individual haplotype divergence exceeds that figure. We hypothesize that high intra-individual haplotype divergence is due to hybridization between agaric fungi with divergent haplotypes, possibly migrants from geographically isolated glacial refugia. Four species with relatively high haplotype divergence were examined: Armillaria mellea, Amanita citrina f. lavendula, Gymnopus dichrous and the Hygrocybe flavescens/chlorophana complex. The ITS region was sequenced, haplotypes of heterozygotes were resolved through cloning, and phylogenetic analyses were used to determine the outcome of hybridization events. Within Armillaria mellea and Amanita citrina f. lavendula, we found evidence of interbreeding and recombination. Within G. dichrous and H. flavescens/chlorophana, hybrids were identified but there was no evidence for F₂ or higher progeny in natural populations suggesting that the hybrid fruitbodies might be an evolutionary dead end and that the genetically divergent Mendelian populations from which they were derived are, in fact, different species. The association between ITS haplotype divergence of less than 5% (Armillaria mellea = 2.6% excluding gaps; Amanita citrina f. lavendula = 3.3%) with the presence of putative recombinants and greater than 5% (Gymnopus dichrous = 5.7%; Hygrocybe flavescens/chlorophana = 14.1%) with apparent failure of F₁ hybrids to produce F₂ or higher progeny in populations may suggest a correlation between genetic distance and reproductive isolation.

• biodiversity
• Dobzhansky-Muller incompatibility
• hybridization
• speciation

Acknowledgments

We thank Great Smoky Mountain National Park for providing the necessary permissions to collect in the Park and numerous individuals who helped with collections and documentation of materials from the southern Appalachians including Vince Hustad, Nadezhda Psurtseva (Komarov Botanical Institute) and Rick Kerrigan. Research was supported by NSF DEB 6338699 and NSF DEB1144974. We thank Dan Lindner and Ned Klopfenstein for presubmission comments and two anonymous reviewers. We thank undergraduates Samuel Morris, Ana Reboredo-Segovia and Whit Hoskins for help with sequencing.