Apparent Recombination or Gene Conversion in the Ribosomal ITS Region of a Flammulina (Fungi, Agaricales) Hybrid

Internally transcribed spacer (ITS) regions of ribosomal DNA are widely used in reconstructing phylogenies at lower taxonomic levels (see, e.g., Vilgalys and Sun 1994;Baldwin et al. 1995).Ribosomal genes are tandemly repeated, and the paralogs are usually identical due to a poorly understood process of concerted evolution.Proposed mechanisms of concerted evolution include unequal crossing over (Schlotterer and Tautz 1994) and biased gene conversion (Hillis et al. 1991;Linares, Bowen, and Dover 1994).Within-individual diversity for the ribosomal ITS region has been observed and has in some cases been attributed to hybridization (Sang, Crawford, and Stuessy 1995;Hugall, Stanton, and Moritz 1999).This report documents an apparent natural hybridization event between two species of Flammulina (F.velutipes and F. rossica) which resulted in a homogenized ribosomal repeat containing elements of both parents.A chromosome carrying the hybrid repeat has apparently introgressed into F. velutipes and persists in nature.Flammulina velutipes is predominantly found in temperate Northern Hemisphere forests but has also been collected in Argentina and New Zealand.Flammulina rossica is a more northern species, so far identified from Alaska, northern Canada, and eastern Russia.Approximately 200 collections of Flammulina worldwide were identified to species on the basis of morphology (Redhead and Petersen 1999), mating studies (Petersen et al. 1999), and restriction fragment length polymorphism (RFLP) patterns (Methven, Hughes, and Petersen 2000).DNA was extracted from geographically diverse collections of each species.The ribosomal ITS region was amplified and sequenced on both strands using primers ITS5 and ITS3 in the forward direction and primers ITS4 and ITS2 in the reverse direction as described by Hughes et al. (1999).This combination of primers provided unambiguous sequence data on both forward and reverse strands.Forward and reverse sequences were compared and corrected manually and aligned with other Flammulina sequences using the Genetics Computer Group (1999) family of programs, and all divergent bases were rechecked against the original sequence files.Sequence alignments were deposited with TreeBASE (accession M641;Hughes et al. 1999).Two of ca. 150 natural dikaryon (diploid) worldwide collections identified morphologically as F.