High genetic variability and low local diversity in a population of arbuscular mycorrhizal fungi

Abstract

Arbuscular mycorrhizal fungi (AMF) are ecologically important root symbionts of most terrestrial plants. Ecological studies of AMF have concentrated on differences between species; largely assuming little variability within AMF species. Although AMF are clonal, they have evolved to contain a surprisingly high within-species genetic variability, and genetically different nuclei can coexist within individual spores. These traits could potentially lead to within-population genetic variation, causing differences in physiology and symbiotic function in AMF populations, a consequence that has been largely neglected. We found highly significant genetic and phenotypic variation among isolates of a population of Glomus intraradices but relatively low total observed genetic diversity. Because we maintained the isolated population in a constant environment, phenotypic variation can be considered as variation in quantitative genetic traits. In view of the large genetic differences among isolates by randomly sampling two individual spores, <50% of the total observed population genetic diversity is represented. Adding an isolate from a distant population did not increase total observed genetic diversity. Genetic variation exceeded variation in quantitative genetic traits, indicating that selection acted on the population to retain similar traits, which might be because of the multigenomic nature of AMF, where considerable genetic redundancy could buffer the effects of changes in the genetic content of phenotypic traits. These results have direct implications for ecological research and for studying AMF genes, improving commercial AMF inoculum, and understanding evolutionary mechanisms in multigenomic organisms.

Fig. 1.

Procedure to isolate an AMF population and subsequent measurement of phenotypic and genetic variation. Soil from the field site is used to inoculate plants. Single spores from trap cultures were used to inoculate individual plants (single-spore isolates). Spores were used to inoculate sterile Ri tumor-inducing plasmid T-DNA-transformed carrot roots. The isolates were transferred two more times in an identical environment. Sixteen replicate plates were inoculated with each isolate with an addition of another 16 two-compartment plates for genetic analysis. AFLP fingerprint analysis of genomic DNA allowed measurement of genetic variation.

Fig. 2.

Mean final hyphal length (A), final spore number (B), maximal hyphal growth rate (C), maximum rate of spore production (D), and final spore number per hyphal length (E) of the 16 AMF isolates. Identical shading patterns indicate isolates originating from the same plot. Isolates A₁–B₄ and C₁–D₄ belong to no-tillage and tillage treatments, respectively. Only significant main effects from the ANOVA are given. Bars indicate standard error, and significance levels are as follows: (A) Isolate F_(12,46) = 19.48 (P ≤ 0.001), plot F_(2,12) = 8.12 (P ≤ 0.01); (B) Isolate F_(12,46) = 9.06 (P ≤ 0.001); (C) Isolate F_(12,46) = 7.21 (P ≤ 0.001), plot F_(2,12) = 5.72 (P ≤ 0.05); (D) Isolate F_(12,46) = 6.13 (P ≤ 0.001); and (E) Isolate F_(12,46) = 7.21 (P ≤ 0.001), plot F_(2,12) = 5.74 (P ≤ 0.05). Full ANOVA tables are given in Table 3.

Fig. 3.

Phylogenetic analyses based on binary data generated by using AFLP on 10 isolates of an AMF population from Switzerland. An unrooted consensus tree was obtained through a heuristic search procedure by using stepwise addition and tree bisection-reconnection branch swapping options (with 10 additions). Support values are indicated at branches when found in at least 90% of the 1,000 bootstrap trees. (A) The analysis was performed with AMF isolates from the Swiss population only. The Swiss isolate codes follow those described in Materials and Methods. The two independent DNA extractions are denoted with the lowercase letters a and b after the isolate letter and number code. (B) The analysis was performed on 10 isolates of the Swiss population and an isolate of Canadian origin. The Canadian isolate is designated Can.

Fig. 4.

(A) Relationship between the amount of genetic diversity and the number of isolates sampled from an AMF population. The estimation of genetic diversity was performed including (♦) and excluding (▾) an isolate from Canada. Data points indicate the mean number of polymorphic bands for a given number of isolates. Error bars represent ± 1 SD. The dotted line represents the threshold for 90% of the observed genetic diversity. (B) Relationship between the amount of phenotypic diversity and the number of isolates sampled from an AMF population. The procedure for estimation of phenotypic diversity was performed on the basis of different numbers [two (▪), three (•), and four (▴)] of equally long intervals (classes) that were introduced to describe the variation in a trait. For each of the three different classes, the threshold for 90% of the observed diversity is indicated with a dotted line.

Fig. 5.

Relationship between Q_st and F_st for all six possible combinations of pairs of plots. Q_st values are shown for the following phenotypic traits: final hyphal length (A), final spore number (B), maximal hyphal growth rate (C), maximum rate of spore production (D), final spore number per hyphal length (E), and a combination of all five variables (F). Data points are denoted with two letters describing which plots have been compared. All variance components of phenotypic traits used in the calculations are shown in Table 4.