The evolution of heavy metal tolerance in plants: Adaptations, limits, and costs
Habitats contaminated with heavy metals provide natural laboratories for studies of evolution in plant populations. Understanding the genetic basis of tolerance is crucial for subsequent studies of evolutionary processes. Recent work on several species and metals suggests that tolerance is effected by a limited number of major genes, with modifiers that condition levels of tolerance rather than the presence of tolerance per se. Natura] selection on highly contaminated mine tailings is primarily stabilizing, whereas selection on plants in closely adjacent uncontaminated soils appears to be directional (againsl tolerant individuals). Selection can occur at multiple stages in the life cycle, including both the diploid sporophyte phase and haploid gametophytes. In mosses, selection occurs primarily on haploid gametophytes, which constitute the perennial stage of the life cycle. Theory and some empirical data suggest that selection among haploid pollen in the styles of flowering plants growing in contaminated habitats could be an important factor in tolerance evolution, but experimental work to date has not confirmed selection among pollen individuals. Rather, selection occurs among developing ovules in pistils enriched with metals. The general restriction of tolerant plants to contaminated habitats and steep clines in tolerance from mine tailings onto adjacent "normal" soils suggest that there are costs associated with tolerance and that tolerant individuals are at a disadvantage in the absence of metal contamination. The nature of such costs is poorly understood, however, and additional research is needed in this area. Copyright.
