Genetic polymorphism and taxonomic infrastructure of the Pleurotus eryngii species-complex as determined by RAPD analysis, isozyme profiles and ecomorphological characters.
The Pleurotus eryngii species-complex includes populations of choice edible mushrooms, growing in the greater Mediterranean area in close association with different genera of plants of the family Apiaceae. Their distinct host-specialization served as the principal criterion for the discrimination of several taxa; however, the genetic relationships among the various P. eryngii ecotypes remain ambiguous. In the present study, 46 Pleurotus strains with a wide range of geographical origins were isolated from Eryngium spp., Ferula communis, Cachrys ferulacea, Thapsia garganica and Elaeoselinum asclepium subsp. asclepium, and were subjected to isozyme and random amplified polymorphic DNA-PCR (RAPD) analysis. The 16 enzyme activities tested were controlled by 28 loci, 11 of which were monomorphic. Host-exclusive zymograms for the Aph (acid phosphatase) and Phe-1 (dopa-phenoloxidase) loci were obtained from Pleurotus strains associated with C. ferulacea. Allele frequencies, genetic diversity and mean diversity were high for isolates from Eryngium spp. and Ferula communis. In RAPD analysis, the use of five primers allowed the production of 45 (out of 48) polymorphic bands, while four molecular markers specific for the identification of Pleurotus strains growing on E. asclepium subsp. asclepium and C. ferulacea were obtained. The Pleurotus strains produced 35 distinct electrophoretic types and 42 RAPD patterns, which independently permitted the separation of the fungal populations into five clusters in accordance with their host-specificity. In addition, the evaluation of the principal ecological and morphological characters provided further evidence for discriminating between P. nebrodensis growing on C. ferulacea and the rest of the host-associated populations. The latter represent taxa at the varietal level: P. eryngii var. eryngii, P. eryngii var. ferulae and P. eryngii var. elaeoselini. The position of taxa of dubious validity, such as P. hadamardii and P. fossulatus, is discussed in relation to the new findings. All Mediterranean Pleurotus populations growing on umbellifers seem to have recently diverged through a sympatric speciation process, that is based on both intrinsic reproductive barriers and extrinsic ecogeographical factors.