Location, but not defensive genotype, determines ectomycorrhizal community composition in Scots pine (Pinus sylvestris L.) seedlings

Abstract

For successful colonization of host roots, ectomycorrhizal (EM) fungi must overcome host defense systems, and defensive phenotypes have previously been shown to affect the community composition of EM fungi associated with hosts. Secondary metabolites, such as terpenes, form a core part of these defense systems, but it is not yet understood whether variation in these constitutive defenses can result in variation in the colonization of hosts by specific fungal species.We planted seedlings from twelve maternal families of Scots pine (Pinus sylvestris) of known terpene genotype reciprocally in the field in each of six sites. After 3 months, we characterized the mycorrhizal fungal community of each seedling using a combination of morphological categorization and molecular barcoding, and assessed the terpene chemodiversity for a subset of the seedlings. We examined whether parental genotype or terpene chemodiversity affected the diversity or composition of a seedling's mycorrhizal community.While we found that terpene chemodiversity was highly heritable, we found no evidence that parental defensive genotype or a seedling's terpene chemodiversity affected associations with EM fungi. Instead, we found that the location of seedlings, both within and among sites, was the only determinant of the diversity and makeup of EM communities.These results show that while EM community composition varies within Scotland at both large and small scales, variation in constitutive defensive compounds does not determine the EM communities of closely cohabiting pine seedlings. Patchy distributions of EM fungi at small scales may render any genetic variation in associations with different species unrealizable in field conditions. The case for selection on traits mediating associations with specific fungal species may thus be overstated, at least in seedlings.

Keywords: Scots pine (Pinus sylvestris); community composition; ectomycorrhizal fungi; evolution; mutualism; secondary metabolites.

FIGURE 1

Various ectomycorrhizal root tips observed in the experiment, showing some of the various morphotypes observed. Molecular IDs (from top left, left to right, and top to bottom): 1. unidentified Cantharellales sp. 2. Cenococcum geophilum; 3. Suillus variegatus; 4. Piloderma olivaceum

FIGURE 2

Map showing the locations of the six populations used in the study. Labels: 1 = Abernethy; 2 = Rothiemurchus; 3 = Glen Derry; 4 = Beinn Eighe; 5 = Loch Clare; 6 = Shieldaig

FIGURE 3

Summary graphs: EM species abundance and the terpene composition of individual seedlings. (a) Heatmap showing the total number of root tips of each EM species found at each of the six sites. (b) Plot of proportional α‐pinene concentration against proportional Δ³‐carene concentration, showing a strong linear decrease in Δ³‐carene concentration with increasing α‐pinene concentration. Colors indicate increasing terpene chemodiversity. (c) Histogram of terpene chemodiversity, showing two distinct peaks, indicating high/low Δ³‐carene phenotypes

FIGURE 4

Variation in EM fungal community composition between sites and genotypes. (a) NMDS ordination of seedling EM communities showing variation between sites. Each point represents the mean NMDS score ± SE for one of the 12 maternal families at a particular site. Circles represent eastern sites, and triangles represent western sites. Plots of all other axes are available in Figure A3. (b) Percentage variance explained by each component in the maternal family turnover model (Equation 4) and Δ³‐carene turnover model (Equation 5), calculated from the estimates of the posterior modes. Site = turnover between sites, Grid = turnover between grids, OTU = unexplained variation in the abundance of fungal species. Genotype effects are not shown in the legend as no effects were found

FIGURE A1

Maximum‐likelihood trees of sequences for each genus, showing consistent taxonomic identification of sequences. Where tips are colored, colors represent different species as identified by BLAST

FIGURE A2

Species accumulation curves for each of the six sites, showing that most fungal species present in the grids at each site were recovered

FIGURE A3

The full NMDS ordination with all four calculated axes plotted against each other. Each point represents the mean NMDS score ± SE for one of the 12 maternal families at a particular site. Circles = eastern sites, Triangles = western sites