ANCIENT AND RECENT PATTERNS OF GEOGRAPHIC SPECIATION IN THE OYSTER MUSHROOM PLEUROTUS REVEALED BY PHYLOGENETIC ANALYSIS OF RIBOSOMAL DNA-SEQUENCES

Evidence from molecular systematic studies suggests that many mushroom species may be quite ancient. Gene phylogenies were developed to exam ine the relationship between reproductive isolation, genetic divergenc e, and biogeography in oyster mushrooms (Pleurotus). Sequence data wer e obtained for two regions of DNA from populations belonging to eight intersterility groups (biological species). Phylogenetic analysis of s equences from the 5' portion of the nuclear encoded large subunit rDNA demonstrates an ancient origin for four intersterility groups of broa d geographic distribution (world-wide), with a more recent radiation o f several intersterility groups that are restricted to the Northern. H emisphere. An expanded analysis using sequence data from the more vari able rDNA internal transcribed spacer region also reveals a phylogenet ically based pattern of genetic divergence associated with allopatric speciation among populations from different continents in the Northern Hemisphere. The ability of rDNA sequences to resolve phylogenetic rel ationships among geographically isolated populations within intersteri lity groups illustrates the importance of biogeography for understandi ng speciation in Pleurotus. Patterns of geographic distribution among intersterility groups suggest that several species lineages evolved qu ite early, with recently evolved groups restricted to the Northern Hem isphere and older lineages occurring throughout the world. Based on ph ylogenetic evidence, analysis of historical biogeography using area cl adograms shows that multiple dispersal and vicariance events are respo nsible for patterns of speciation observed.