Strong phylogenetic congruence between Tulasnella fungi and their associated Drakaeinae orchids

Abstract

The study of congruency between phylogenies of interacting species can provide a powerful approach for understanding the evolutionary history of symbiotic associations. Orchid mycorrhizal fungi can survive independently of orchids making cospeciation unlikely, leading us to predict that any congruence would arise from host-switches to closely related fungal species. The Australasian orchid subtribe Drakaeinae is an iconic group of sexually deceptive orchids that consists of approximately 66 species. In this study, we investigated the evolutionary relationships between representatives of all six Drakaeinae orchid genera (39 species) and their mycorrhizal fungi. We used an exome capture dataset to generate the first well-resolved phylogeny of the Drakaeinae genera. A total of 10 closely related Tulasnella Operational Taxonomic Units (OTUs) and previously described species were associated with the Drakaeinae orchids. Three of them were shared among orchid genera, with each genus associating with 1-6 Tulasnella lineages. Cophylogenetic analyses show Drakaeinae orchids and their Tulasnella associates exhibit significant congruence (p < 0.001) in the topology of their phylogenetic trees. An event-based method also revealed significant congruence in Drakaeinae-Tulasnella relationships, with duplications (35), losses (25), and failure to diverge (9) the most frequent events, with minimal evidence for cospeciation (1) and host-switches (2). The high number of duplications suggests that the orchids speciate independently from the fungi, and the fungal species association of the ancestral orchid species is typically maintained in the daughter species. For the Drakaeinae-Tulasnella interaction, a pattern of phylogenetic niche conservatism rather than coevolution likely explains the observed phylogenetic congruency in orchid and fungal phylogenies. Given that many orchid genera are characterized by sharing of fungal species between closely related orchid species, we predict that these findings may apply to a wide range of orchid lineages.

Keywords: Tulasnella; Drakaeinae; cophylogeny; duplication; orchid mycorrhizal fungi (OMF); sexually deceptive orchids.

FIGURE 1

Drakaeinae orchid phylogeny obtained from 221 single‐copy orthologous genes (250 431 bp in total) developed for the Drakaeinae orchids (following the methodology of Peakall et al. (2021)). The numbers above the branches are bootstrap support (%). Only bootstrap values ≥70% are shown. The tree is rooted with Pyrorchis nigricans (pruned in this figure).

FIGURE 2

Midpoint rooted MrBayes phylogenetic tree for Tulasnella group IV associated with Drakaeinae, obtained for the internal transcribed spacer (ITS). Drakaeinae‐associating OTUs are shown in bold. OTU: Operational taxonomic unit. The numbers above/under the branches are Bayesian posterior probabilities / maximum likelihood bootstrap values (%). Only Bayesian posterior probabilities ≥0.70 / bootstrap values ≥70% are shown. Sequence details for each clade are shown in Table S1.

FIGURE 3

Interaction network and phylogenies of Drakaeinae orchids and Tulasnella fungi. The cophylogenetic analysis in PACo shows significant phylogenetic congruence (cophylogenetic signal m ² _XY = 42673.91; p < 0.001) between them. The interactions are weighted by their contribution to the overall phylogenetic congruence. The thicker lines show smaller residual distance or interactions that show stronger support for the hypothesis of phylogenetic congruence. Orchid genera abbreviations – D: Drakaea; P: Paracaleana; Cal: Caleana; C: Chiloglottis; A: Arthrochilus; S: Spiculaea. Fungal genus abbreviation – T: Tulasnella.

FIGURE 4

(a) The orange lines show the Procrustes residuals for the Tulasnella–Chiloglottis interactions that were strongly supported in Figure 3, and the distribution gives the values of the rest of the interactions in the Drakaeinae–Tulasnella network. (B) Comparison of the strongest interactions that are most phylogenetically congruent between Tulasnella–Chiloglottis compared with the rest of the network (rest), measured as the residuals from the Procrustes patristic distance correlation.

FIGURE 5

Tanglegram showing the Drakaeinae phylogenetic tree in blue and the Tulasnella phylogenetic tree in black. Analysis was performed using jane 4.0 with default cost settings based on Conow et al. (2010). Observed minimum total cost = 73, the number of occurrences of each event; cospeciations = 1, duplications = 35, host‐switches = 2, losses = 25 and failure to diverge = 9. A cospeciation event is marked by a hollow coloured circle, a duplication is marked by a solid coloured circle, a host‐switch is marked by a solid coloured circle with an arrow following the trajectory of the switching species, a loss is marked by a dashed line, and a failure to diverge is marked by a jagged line.