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ABSTRACT
The gradual genetic isolation which precedes speciation can be readily observed in the genus Pleurotus. Strains of P. ostreatus originating from different parts of the world and growing on different substrates display 100% compatibility in crossing experiments. The resulting hybrid dikaryons are viable without exception and develop fully fertile fruitbodies. On the basis of their lacease isoenzyme spectra and other characters, the variability of their F1-monokaryons is striking. Compatibility is reduced, on the other hand, when hybridization is attempted between the allopatric morphovariants P. abalonus and P. cystidiosus. The same is true for the three ecotypes of P. eryngii. There are total reproduction barriers between the accepted species of Pleurotus which are sometimes morphologically barely distinguishable. Results of crossing experiments with P. pulmonarius and P. ostreatus are in agreement with the idea that genetic reproduction barriers are erected early in the speciation of fungi. Measurements of the relative content of DNA of cell nuclei indicate the presence of different levels of ploidy with different chromosome numbers in various species.