Ecological genetics of heavy metal tolerant populations of Suillus luteus

Abstract

Genomic and transcriptomic aspects of the adaptation to increased environmental heavy metal concentrations were studied in relation to phenotypic observations of heavy metal tolerance in populations of the ectomycorrhizal basidiomycete Suillus lutev,s. S. fotwu8 sporocarps were collected at nine different locations in the Campine phytogeographic district (province of Limbmg, Bdgium). At six of these locations the soil is severely polluted with heavy metals due to past activities of several zinc smelters and phcnotypic characterization, by means of in vitro growth experiments, revealed an obvious increase of the heavy metal tolerance in the populations at these sites. However, evidence for natural selection was not found at the genomic level. AFLP and microsatellite marker analyses both indicated high levels of genetic diversity within the total sample and within geographic subpopulat ions, but limited population differentiation. No current or past reduction of the genetic diversity of populations inhabiting polluted environments was discovered, nor was population differentiation found to be related to the environmental heavy metal pollution. In accordance with the high level of genomic diversity revealed by the more-or-less neutral AFLP and microsat ellite markers, considerable diversity was found at the level of the transcriptorne by cDNA-AFLP analysis of a heavy metal tolerant and a nontolerant isolate. Although transcript profiling should be regarded as a tool for exploratory data analysis and some of the differences in expression level will be due to genetic hitchhiking and natural regulatory variation, some information was obtained about possible mechanisms that may explain the difference in tolerance level. Expression of a hydrophobin homologue, a water-repellent protein in the cell wall which is believed to protect against penetration of heavy metal cont aining solutions, may be partly responsible for the higher external ,'.;inc concentr ations required to induce modulation of transcription in the tolerant isolate and candidate genes that may play a role in the detoxification of and cellular defense against heavy metals were identified as encoding a heat shock protein (HSP60), a putative metal transporter and several proteins involved in ubiquitin-dependent proteolysis. The results of these different experiments show that heavy met al pollution has limited influence on the genetic diversity of S. luteus populations, a result that seems unlikely a priori but that has been described in several other organisms. In case of S . lnteus bottleneck events associated with the foundation of populations in polluted environments may have been prevented by the introduction of different tolerant genotypes, followed by sexual reproduction on the site and rapid evolution of the tolerance trait. Additionally, recurrent migration of tolerant genotypes that originate due to admixture in nonpolluted areas, which is very likely due to the high migration rates, may have attributed to the high levels of genetic diversity of populations in polluted habitats.