A new dawn – the ecological genetics of mycorrhizal fungi

Abstract

A new dawn -the ecological genetics of mycorrhizal fungi Many human activities, such as ore mining and smeltering, sewage sludge treatment and fossil fuel consumption, result in toxic soil concentrations of ' heavy metals ' (Al, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Ti, Zn and others) (Gadd, 1993). There are also natural soils, such as serpentine, with levels of heavy metals that inhibit or preclude the growth of many plants and soil microorganisms. However, certain plants and microorganisms do grow in these metalliferous sites. Understanding the physiology, ecology and evolution of tolerance to elevated soil metal concentrations is important in an applied setting, and is also of interest in theoretical biology. Applied importance relates to the improvement of forest health in areas subject to increasing pollution, rehabilitation of severely polluted sites by phytostabilization of metals, and metal removal using hyperaccumulating plants (Kra$ mer, 2000 ;Ernst, 2000). Areas of theoretical interest include the evolution of local adaptation (Sork et al., 1993) and how it is shaped by the combined influences of natural selection, gene flow and genetic architecture, as well as metal influences on various species interactions (Pollard, 2000). A paper appears on pages 367-379 in this issue by Jan Colpaert and coworkers which adroitly combines the disparate fields of physiology, genetics and ecology to answer several outstanding questions concerning heavy metal tolerance in mycorrhizal fungi.Mycorrhizal fungi, which interact mutualistically with the majority of plant species, are well known for improving the P status of their hosts (Smith & Read, 1997). Some mycorrhizal fungi are also able to mobilize N and P from organic substrates and to provide plants with improved micronutrient and water acquisition, pathogen resistance, and a variety of other benefits (Smith & Read, 1997). One of these additional benefits is the amelioration of toxicity in metalliferous soils. Surprisingly widespread toleranceSeveral negative impacts of heavy metal pollution on mycorrhizal communities are well documented. Extremely polluted sites typically have lower rates of mycorrhizal colonization, fewer fungal propagules, and lower fungal species diversity (Gadd, 1993 ;Hartley et al., 1997 ;Leyval et al., 1997). Evolutionary impacts of metalliferous soils on plants have received much more attention than impacts on fungi. Some of the steepest selection gradients yet recorded in plants occur across the transitions from clean to heavily polluted sites (Antonovics et al., 1971 ;McNeilly, 1979). Evolutionary adaptation of these plants to the polluted conditions can be accepted based on numerous lines of evidence, including genetic variation in tolerance, heritability of tolerance, higher fitness of tolerant individuals on polluted sites, and higher fitness of non-tolerant individuals on unpolluted sites. One might expect such sites to exert similarly strong selection on mycorrhizal fungi, and that it might result in genetic differentiation among fungal p...

Variation in mycorrhizal performance in the epiphytic orchid Tolumnia variegata in vitro: the potential for natural selection

“…There are examples of the former but not the latter. For example, the nonphotosynthetic orchids Cephalanthera austinae and Corallorrhiza maculata are associated with ectomycorrhizal fungi in the families Thelephoraceae and Russulaceae, respectively (Taylor andBruns, 1997, 1999;see Rasmussen, 2002 for similar examples). These fungi are very different than Rhizoctonia-like fungi with which most orchids associate, suggesting that their ancestors switched from their original mycorrhizal symbionts.…”

“…They occur in 80% of plant species and affect plant competition and diversity (Van der Heijden et al, 1998). Despite the importance and ubiquity of mycorrhizae, little is known of their evolution (Taylor, 2000). We would expect that traits affecting mycorrhizal interactions have been shaped by natural selection, as has been shown in other symbiotic interactions (e.g., Schemske and Horvitz, 1984;Schemske, 1986, 1990;Doebeli and Knowlton, 1998;Alexandersson and Johnson, 2002).…”

Evidence for mycorrhizal races in a cheating orchid

“…Mineral absorption from the soil is usually mediated by soil microbes, upon which wild plants depend heavily. Little is known about the genetics and microevolution of traits governing resource acquisition in wild plants, especially traits related to interactions with soil microbes (Taylor 2000). Mycorrhizal symbioses, which occur in ca.…”

Divergence in mycorrhizal specialization within Hexalectris spicata (Orchidaceae), a nonphotosynthetic desert orchid

“…Mycorrhizal symbioses with soil fungi are ancient, possibly dating to the emergence of plants on land (Heckman et al, 2001) and are still critical to the mineral nutrition of most wild plants (Smith and Read, 1997). While mycorrhizal interactions have certainly influenced the evolution of nutrient uptake strategies in plants, most plants do not interact specifically with narrow taxonomic groups of fungi (Molina et al, 1992), making pairwise coevolution between plant and fungal species unlikely (Taylor, 2000). In stark contrast, some members of the species-rich Orchidaceae display marked mycorrhizal specificity toward particular taxonomic groups of basidiomycete fungi (reviewed in Taylor et al, 2002).…”