And Interspecific Hybridiation in Agaric Fungi
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Mycologia, 105(6), 2013, pp. 1577–1594. DOI: 10.3852/13-041 # 2013 by The Mycological Society of America, Lawrence, KS 66044-8897 Evolutionary consequences of putative intra- and interspecific hybridization in agaric fungi Karen W. Hughes1 to determine the outcome of hybridization events. Ronald H. Petersen Within Armillaria mellea and Amanita citrina f. Ecology and Evolutionary Biology, University of lavendula, we found evidence of interbreeding and Tennessee, Knoxville, Tennessee 37996-1100 recombination. Within G. dichrous and H. flavescens/ D. Jean Lodge chlorophana, hybrids were identified but there was Center for Forest Mycology Research, USDA-Forest no evidence for F2 or higher progeny in natural Service, Northern Research Station, Box 137, Luquillo, populations suggesting that the hybrid fruitbodies Puerto Rico 00773-1377 might be an evolutionary dead end and that the Sarah E. Bergemann genetically divergent Mendelian populations from which they were derived are, in fact, different species. Middle Tennessee State University, Department of Biology, PO Box 60, Murfreesboro Tennessee 37132 The association between ITS haplotype divergence of less than 5% (Armillaria mellea 5 2.6% excluding Kendra Baumgartner gaps; Amanita citrina f. lavendula 5 3.3%) with the USDA-Agricultural Research Service, Department of presence of putative recombinants and greater than Plant Pathology, University of California, Davis, California 95616 5% (Gymnopus dichrous 5 5.7%; Hygrocybe flavescens/ chlorophana 5 14.1%) with apparent failure of F1 2 Rodham E. Tulloss hybrids to produce F2 or higher progeny in popula- PO Box 57, Roosevelt, New Jersey 08555-0057 tions may suggest a correlation between genetic Edgar Lickey distance and reproductive isolation. Department of Biology, Bridgewater College, Key words: biodiversity, Dobzhansky-Muller in- Bridgewater, Virginia 22812 compatibility, hybridization, speciation Joaquin Cifuentes Herbario FCME, Biologı´a, Facultad de Ciencias INTRODUCTION UNAM, Me´xico DF Interspecific hybrids are common in higher plants and play a significant role in plant evolution (Abbott 1992), but reports of such hybrids for fungi are Abstract: Agaric fungi of the southern Appalachian relatively rare (see Olson and Stenlid 2002, Schardl Mountains including Great Smoky Mountains Nation- and Craven 2003, Le Gac and Giraud 2008 for al Park are often heterozygous for the rDNA internal summaries). Among ascomycetes, non-orthologous transcribed spacer region (ITS) with .42% of ITS2 sequences, suggestive of ancient hybridization, collections showing some heterozygosity for indels have been identified in populations of plant patho- and/or base-pair substitutions. For these collections, genic Fusarium species (O’Donnell and Cigelnik intra-individual haplotype divergence is typically less 1997). In addition, Inderbitzin et al. (2011) reported than 2%, but for 3% of these collections intra- that the plant pathogen Verticillium longisporum was a individual haplotype divergence exceeds that figure. hybrid that originated at least three separate times. We hypothesize that high intra-individual haplotype Among basidiomycetes, hybridization as a mecha- divergence is due to hybridization between agaric nism to explain genetic variation was reported in fungi with divergent haplotypes, possibly migrants several populations. For example, rare interspecific from geographically isolated glacial refugia. Four hybrids were reported in Heterobasidion (Garbelotto species with relatively high haplotype divergence were examined: Armillaria mellea, Amanita citrina f. et al. 1998) and among cryptic species of Coniophora lavendula, Gymnopus dichrous and the Hygrocybe puteana in a region of geographical overlap (Kau- flavescens/chlorophana complex. The ITS region was serud et al. 2007, but see Skrede et al. 2012). sequenced, haplotypes of heterozygotes were resolved Newcombe et al. (2000) reported rare hybrids in the through cloning, and phylogenetic analyses were used rust genus Melampspora, while Morin et al. (2009) reported an apparent rust fungus hybrid between Submitted 30 Jan 2013; accepted for publication 7 May 2013. Puccinia lagenophorae and an unknown rust fungus 1 Corresponding author. E-mail: [email protected] on Senecio in Africa based on recovery of divergent 2 Research associate, New York Botanical Garden, Bronx. ITS haplotypes. Le Gac et al. (2007) determined the 1577 1578 MYCOLOGIA rates of pre- and post-syngamy isolation in pairings of thus potentially underestimating species numbers sympatric and allopatric sister species of anther smuts suggested by ITS barcoding. in the genus Microbotryum. Lindner and Banik (2011) We hypothesize that observed heterozygosity (ge- reported recovery of divergent ITS genotypes in netic distance between haplotypes) that is greater clones of the bracket fungus genus Laetiporus and than 2% is the consequence of rare hybridization the effect of these on clade circumscription in between divergent populations. To further examine phylogenetic trees, but divergent sequences in that the relationship between degree of heterozygosity and study could have represented hybridization with thefateofhybridprogeny,weexaminedfour unresolved rRNA homogenization. exemplars with .2% base-pair heterozygosity: Ama- A probable hybrid, identified from a recombinant nita citrina f. lavendula (ectomycorrhizal), Armillaria ITS sequence between Flammulina rossica and F. mellea (plant pathogen and wood decay saprobe), velutipes, was reported from Argentina where it Gymnopus dichrous (saprobe) and the Hygrocybe might have been introduced (Hughes and Petersen flavescens/chlorophana complex (not ectomycorrhizal, 2001). Later, a European collection with an identical possible rhizosphere or moss symbiont). hybrid ITS sequence was identified suggesting that either this is an old hybridization that has survived MATERIALS AND METHODS and was propagated in distant locations or that this hybridization has occurred more than once on Collections.—Collections were documented, photographed, different continents (Ripkova´ et al. 2010). Baum- given a Tennessee Field Book (TFB) number, dried and gartner et al. (2012) presented evidence from single- accessioned into TENN (available at http://tenn.bio.utk. gene phylogenies of discordance between cytoplas- edu/) (TABLE I). For long-term storage of tissue, a small 3 mic and nuclear genes of Eurasian and an intro- piece of each basidiome (0.3 cm ) was placed in a microfuge vial containing silica beads and stored at 280 C. duced African homothallic Armillaria mellea sugges- tive of intra-lineage hybridization. Similarly hybrids DNA extraction.—DNA was extracted by grinding a 2–3 mm2 from heterothallic populations of A. ostoyae and A. dried tissue sample with a mortar and pestle in a small tabescens have been noted (Schnabel et al. 2005, volume of sterile sand at room temperature until powdered. Hanna et al. 2007). The sample was added to 750 mL Carlson lysis buffer The southern Appalachians, including Great (Carlson et al. 1991), vortexed 5 s and heated at 74 C for Smoky Mountains National Park (GSMNP), is a 30 min with brief vortexing at 15 and 30 min. The sample was centrifuged to precipitate sand and cell debris, and the region of exceptional biodiversity within North supernatant was mixed with an equal volume of 24:1 America (Stein 2000). We sampled the fruit bodies chloroform-isoamyl alcohol. The top layer containing of southern Appalachian agaric fungi over a 5 y DNA was removed and an equal volume of isopropanol period and obtained diagnostic nuclear ribosomal added to precipitate DNA. The DNA pellet was washed with ITS sequences for 2172 collections. We found that a 80% ice-cold ethanol and resuspended in 100 mL sterile significant number (42%) were heterozygous for Tris-EDTA (TE) buffer. The ITS region of the ribosomal indels (data concerning base-pair heterozygosity in RNA repeat was amplified with primers ITS1F (Bruns and sequences without indels were not collected for the Gardes 1993) and ITS4 (White et al. 1990). PCR products entire dataset). We evaluated both base-pair and indel were cloned (pGEM-T easy kit with M109 competent cells, heterozygosity for a random sampling of 100 hetero- Promega Corp., Madison, Wisconsin 53711). Between five zygous collections from this dataset and demonstrat- and 10 clones were sequenced for each basidiome. Clone ed that percent ITS sequence differences within an consensus sequences were not determined. Sequencing was performed with an automated ABI 3100 DNA sequencer individual basidiome varied from one base pair (bp) (ABI Prism Dye Terminator cycle sequencing, Perkin-Elmer % to more than 3.3 bp divergence, but the great Inc.) using primers ITSIF and ITS4 (White et al. 1990). For majority of these collections (97%) revealed less than G. dichrous, clade 1 and 2 reverse primers were designed 2% bp divergence among haplotypes within an from ITS sequences of both clades. These were used to test individual and 99% of the heterozygotes had less homogeneity of the ribosomal repeat in homozygotes. than 3% divergence (Hughes et al. 2009). We suggested that for a geographically localized region Determination of heterozygosity.—Base-pair heterozygosity was inferred from the presence of double peaks on a such as the southern Appalachians biological species chromatogram. Indels were inferred when peaks abruptly (individuals of an interbreeding population) could be went out of phase. For a simple 1–2 bp indel, a comparison defined for barcoding purposes by collections with