Within-population genetic variability in mycorrhizal interactions

Abstract

The geographic mosaic theory of coevolution hypothesizes that natural selection on species interactions varies among ecosystems, partly because the genes involved in species interactions differ in their fitness effects among environments. This selection mosaic may be expressed, at the extreme, as ecological outcomes ranging from mutualism to parasitism among environments. In a recent laboratory experiment on the interaction between a plant, bishop pine (Pinus muricata), and a root-symbiotic ectomycorrhizal fungus, Rhizopogon occidentalis, we demonstrated the potential for selection mosaics in that interaction, and the existence of substantial within-population genetic variation for symbiotic compatibility in the interaction. Here, we present the results from a second experiment on the interaction between the same ectomycorrhizal fungus and a different plant, shore pine (Pinus contorta var. contorta), designed to test for the presence of genetic variation for symbiotic compatibility in another similar system, and also to test whether such variation might be generated in part by adaptation of fungal lineages to individual trees. In this experiment, we found no genetic variation among plant lineages for compatibility with the fungal symbiont, and no evidence for adaptation of fungal lineages to individual plants, but the two fungal genotypes differed greatly in their compatibility with the plant hosts. Specifically, one of the two fungal genotypes not only colonized host plants less intensively than the other, but also had a negative effect on plant growth. Altogether, these results suggest the potential for ongoing natural selection on the ectomycorrhizal fungus, R. occidentalis, for different levels of symbiotic compatibility with particular pine hosts, but the mechanisms generating and maintaining genetic variation for symbiotic compatibility remain unclear. Such results will aid in efforts to develop realistic models of how plants and their symbionts coevolve over broad geographic ranges in which they co-occur.

A meta‐analysis of context‐dependency in plant response to inoculation with mycorrhizal fungi

“…2004; Piculell et al. 2008; Hoeksema et al. 2009), may have substantial effects on how plants respond to mycorrhizal inoculation, and yet are almost never manipulated explicitly in experiments or reported in publications.…”

Geographic divergence in a species‐rich symbiosis: interactions between Monterey pines and ectomycorrhizal fungi

“…), a more northerly coastal species. Using crossinoculation experiments with multiple genotypes of fungi and pines, we found that within populations, fungal and pine seedling performance frequently varied with fungal genotype, pine genotype, and abiotic factors, suggesting the potential for ongoing coevolution (G 3 G interactions) mediated by selection mosaics (G 3 G 3 E interactions) among populations (Piculell et al 2008, Hoeksema et al 2009). We also found evidence for local adaptation of fungal populations to nearby pine populations at a large geographic scale, suggesting pines are exerting geographically variable selection on fungal populations (Hoeksema and Thompson 2007).…”

Strategy Diversity Stabilizes Mutualism through Investment Cycles, Phase Polymorphism, and Spatial Bubbles

“…Many studies suggest that microbial populations and communities are often structurally and genetically more diverse [67], [69], considering both type or strain richness and/or genetic diversity [68], than what can be explained by local host diversity [70]. Also the effectiveness of rhizobia, such as their ability to form nodules and their capacity to fix nitrogen, varies greatly within species, and naturally, between species [27], [58], [71], [72]; similar conclusions hold for the performance of mycorrhizal interactions [26], [65], [73]. This diversity amounts to a high variety of investment strategies; in other words, less mutualistic types coexist with more beneficial mutualists in natural communities [6], [19], [21], [22], [27], [34], [36], [71].…”

“…For example, many nutritional mutualisms, including mycorrhizal or rhizobial mutualisms [13], are highly beneficial for host plants as long as the resource provided (e.g., phosphorus, nitrogen, or copper) is absent from the environment, but can become harmful (implying that costs exceed benefits) when that resource no longer is a limiting factor [2], [38], [19], [76]. This not only underscores the importance of reactive strategies for modeling mutualism, but also offers one explanation for the spatial mosaic structures observed that involve different genotypes, as well as the different local coevolutionary states shaped by different local selective forces [45], [65], [77]–[79]. Our findings highlight that spatial environmental heterogeneity is not required for the creation of such mosaics, as the mechanisms unraveled here provide a testable alternative explanation of these empirical observations, even in the complete absence of spatial environmental heterogeneity.…”