Biological Species in Pleurotus
RONALD H. PETERSEN, KAREN W. HUGHES, AND NADEZHDA PSURTSEVA
Definition of "Species"
The listing of species by their "intersterility group" (ISG) number already indicates that species are herein defined as populations of Pleurotus which are internally genetically coherent (i.e. collections of the same taxon are compatible with others of the same ISG), but extra-specifically intersterile (i.e. they are not compatible with collections from other intersterility groups). This definition conforms largely to that put forward by Mayr (1942, 1970) and others, but as pointed out by Weresub and Gibson (1963), compatibility (i.e. ability of monokaryotic, haploid isolates to form a dikaryon) is only an indicator of interfertility (i.e. the ability of monokaryon, haploid isolates to proceed through plasmogamy to form basidiomata with viable basidiospores), not a synonym. So compatibility is a phenomenon from which we extrapolate probable fertility. Because Pleurotus cultures can often be induced to fruit in vitro, it is common that both intercompatibility and interfertility can be tested.
In Pleurotus, workers are fortunate that there seems to be little "leakage" between ISGs. For example, no hybrid dikaryons have been produced between collections of P. ostreatus and P. djamor (but see under P. pulmonarius for a pan-specific strain). In other groups (notably Flammulina) there is low-level "inter-specific leakage" (i.e. hybridization) between certain "intersterility groups" (= species), and at least some of those "interspecific" hybrids are fertile (i.e. they produce basidiomata with viable basidiospores). This seems NOT to be the case in Pleurotus, which means that ISGs are confidently congruent to morpho-species (up to this time).