Genetic diversity patterns of arbuscular mycorrhizal fungi associated with the mycoheterotroph Arachnitis uniflora Phil. (Corsiaceae)

Abstract

Background and aims: Arachnitis uniflora is a mycoheterotrophic plant that exploits arbuscular mycorrhizal fungi of neighbouring plants. We tested A. uniflora 's specificity towards fungi across its large latitudinal range, as well as the role of historical events and current environmental, geographical and altitudinal variables on fungal genetic diversity.

Methods: Arachnitis uniflora mycorrhizas were sampled at 25 sites. Fungal phylogenetic relationships were reconstructed, genetic diversity was calculated and the main divergent lineages were dated. Phylogeographical analysis was performed with the main fungal clade. Fungal diversity correlations with environmental factors were investigated.

Key results: Glomeraceae fungi dominated, with a main clade that likely originated in the Upper Cretaceous and diversified in the Miocene. Two other arbuscular mycorrhizal fungal families not previously known to be targeted by A. uniflora were detected rarely and appear to be facultative associations. High genetic diversity, found in Bolivia and both northern and southern Patagonia, was correlated with temperature, rainfall and soil features.

Conclusions: Fungal genetic diversity and its distribution can be explained by the ancient evolutionary history of the target fungi and by micro-scale environmental conditions with a geographical mosaic pattern.

Keywords: Andean–Patagonian forest; Arachnitis uniflora; Arbuscular mycorrhizal fungi; genetic diversity; mycoheterotrophy; phylogeography.

>Fig. 1.

Bayes„ian in„fer„ence tree based on the mid„dle frag„ment (604 pb) of the nu„clear 18S rDNA gene se„quences of the glomeromycotan in„di„vid„uals found in Arachnitis uniflora roots and other fun„gal lin„eages used as iden„ti„fiers, which are described in Supplementary Data Table S1. Terminal nodes denoted with„out GenBank ac„ces„sion num„bers cor„res„pond to fungi found in this work. Node sup„port is shown as Bayes„ian pos„ter„ior prob„abil„ity (BPP) over each node. Nodes with BPP <0·5 were collapsed to polytomies. Black boxes under nodes in„di„cate main di„ver„gence times (in Mya); all Roman nu„merals cor„res„pond to the node ages shown in Table 2, to„gether with their 95 % HPD inter„vals. (Inset) Image of A. uniflora plants and de„tails of flower and roots.

>Fig. 1.

Bayes„ian in„fer„ence tree based on the mid„dle frag„ment (604 pb) of the nu„clear 18S rDNA gene se„quences of the glomeromycotan in„di„vid„uals found in Arachnitis uniflora roots and other fun„gal lin„eages used as iden„ti„fiers, which are described in Supplementary Data Table S1. Terminal nodes denoted with„out GenBank ac„ces„sion num„bers cor„res„pond to fungi found in this work. Node sup„port is shown as Bayes„ian pos„ter„ior prob„abil„ity (BPP) over each node. Nodes with BPP <0·5 were collapsed to polytomies. Black boxes under nodes in„di„cate main di„ver„gence times (in Mya); all Roman nu„merals cor„res„pond to the node ages shown in Table 2, to„gether with their 95 % HPD inter„vals. (Inset) Image of A. uniflora plants and de„tails of flower and roots.

Fig. 2.

Geographical distribution patterns of genetic diversity. (A) nucleotide diversity of all Arachnitis uniflora-associated fungal tax at each site. (B) Number of glomeromycotan clades per site. (C) Haplotype diversity using only those sequences belonging to the Arachnitis clade (Fig. 1). Colour scales indicate genetic diversity values based on a 33·3 × 33·3 km grid cell.

Fig. 3.

Bayesian clustering of the AMF belonging to the Arachnitis clade. (A) Most probable population membership arrangement. Each colour indicates a cluster. (B–D) Maps of each cluster selected by Geneland; colours indicate the Bayesian posterior probability (BPP), white representing the maximum value and red the lowest. Black dots show each studied site. (B) COR group. (C) Most Patagonian sites and Bolivian representatives. (D) Southernmost Patagonian sites, associated with Tierra del Fuego.

Fig. 4.

Geographical distribution and genealogical relationship of the nuclear 18S rDNA haplotypes found in fungi associated with Arachnitis uniflora roots and particularly those belonging to the Arachnitis clade. Colour correspondence exists between panels. (A) Haplotype distribution. The pie charts reflect the frequency of occurrence of each haplotype in each site. Site codes are referenced in Table 1. (B) Haplotype network. Each short line on the bar between haplotypes represents one mutational step. Black dots represent median vectors and are represented as non-sampled or extinct ancestors. (C) Haplotype lineages found within the Arachnitis clade and their respective divergence times (over each node).

Fig. 5.

Genetic landscape shape interpolation analysis using a 50×50 grid size and a distance weighting parameter of a = 1. The x- and y-axes correspond to geographic locations; the z-axis shows genetic distances. Positive peaks show genetic discontinuities or possible barriers to gene flow, and are referenced with latitude coordinates. The analysis was conducted with fungal sequences belonging to the Arachnitis clade (Fig. 1).

Fig. 6.

Relationship between nucleotide diversity of Arachnitis uniflora fungi and environmental factors at each site, listed in Table 1. Only significant regressions are shown. (A) Annual temperature. (B) Mean temperature during the wettest season. (C) Mean temperature during the warmest season. (D) Mean temperature during the coldest season. (E) Precipitation during the driest season. (F) Soil pH.