Evolutionary adaptation to Zn toxicity in populations of Suilloid fungi

Zn tolerance was investigated in populations of four ectomycorrhizal fungi: Suillus luteus, Suillus bovinus, Rhizopogon luteolus and Paxillus involutus. The fungi were collected in pioneer pine forests at 14 different locations, situated along a Zn pollution gradient. Genetic adaptation to Zn toxicity was previously presumed in a population of S. luteus. Mycelial biomass production was assessed for 235 isolates exposed to increasing Zn2+ stress. EC50 concentrations were determined. Adaptive Zn tolerance was found in the three species from the Suilloid clade and not in P. involutus. The Suilloid fungi collected within 5 km from a Zn smelter were highly Zn-tolerant, in contrast to isolates collected at least 15 km away from a pollution source. Mixed populations with tolerant and sensitive S. luteus isolates were found in a transition zone, between 5 and 15 km from the Zn smelters. The severe Zn pollution in the surroundings of the Zn smelters has clearly triggered the evolution of an increased Zn tolerance in the pioneer Suilloid fungi. With increasing distances from the Zn smelters, the frequency of Zn-tolerant genotypes decreases.