Evidence from molecular systematic studies suggests that many mushroom species may be quite ancient. Gene phylogenies were developed to examine the relationship between reproductive isolation, genetic divergence, and biogeography in oyster mushrooms (Pleurotus). Sequence data were obtained for two regions of DNA from populations belonging to eight intersterility groups (biological species). Phylogenetic analysis of sequences from the 5' portion of the nuclear encoded large subunit rDNA demonstrates an ancient origin for four intersterility groups of broad geographic distribution (world-wide), with a more recent radiation of several intersterility groups that are restricted to the Northern Hemisphere. An expanded analysis using sequence data from the more variable rDNA internal transcribed spacer region also reveals a phylogenetically 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 relationships among geographically isolated populations within intersterility groups illustrates the importance of biogeography for understanding speciation in Pleurotus. Patterns of geographic distribution among intersterility groups suggest that several species lineages evolved quite early, with recently evolved groups restricted to the Northern Hemisphere and older lineages occurring throughout the world. Based on phylogenetic evidence, analysis of historical biogeography using area cladograms shows that multiple dispersal and vicariance events are responsible for patterns of speciation observed.
Mating compatibility studies using strains from 170 North American collections in the Pleurotus ostreatus complex reveal three intersterile species of oyster mushrooms in the United States and Canada. Initial pairings among monokaryotic single spore isolates were performed to determine the number of intersterile groups. Dikaryon – monokaryon pairings using monokaryotic tester strains were then used to screen additional dikaryotic cultures of the oyster mushroom. Pairings of tester strains from Europe indicate that two European taxa, P. ostreatus and Pleurotus pulmonarius, are also widely distributed in North America. A third intersterility group occurring primarily on aspen in North America was intersterile with all other known species, and is described as a new species, Pleurotus populinus. All three species differ from one another in their morphology, growth characteristics, geographic distributions, and host ranges. Other than mating compatibility tests, however, no single type of character was found to be completely reliable for distinguishing these species in the field or under laboratory conditions. Furthermore, morphological characters used to distinguish European species of the P. ostreatus complex were not useful when applied to corresponding mating-compatible groups from North America. Key words: oyster mushroom, species concept, mating incompatibility.
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