Conservative ecological and evolutionary patterns in liverwort-fungal symbioses

Abstract

Liverworts, the most ancient group of land plants, form a range of intimate associations with fungi that may be analogous to the mycorrhizas of vascular plants. Most thalloid liverworts contain arbuscular mycorrhizal glomeromycete fungi similar to most vascular plants. In contrast, a range of leafy liverwort genera and one simple thalloid liverwort family (the Aneuraceae) have switched to basidiomycete fungi. These liverwort switches away from glomeromycete fungi may be expected to parallel switches undergone by vascular plants that target diverse lineages of basidiomycete fungi to form ectomycorrhizas. To test this hypothesis, we used a cultivation-independent approach to examine the basidiomycete fungi associated with liverworts in varied worldwide locations by generating fungal DNA sequence data from over 200 field collections of over 30 species. Here we show that eight leafy liverwort genera predominantly and consistently associate with members of the Sebacina vermifera species complex and that Aneuraceae thalloid liverworts associate nearly exclusively with Tulasnella species. Furthermore, within sites where multiple liverwort species co-occur, they almost never share the same fungi. Our analyses reveal a strikingly conservative ecological and evolutionary pattern of liverwort symbioses with basidiomycete fungi that is unlike that of vascular plant mycorrhizas.

Figure 1.

Phylogenetic placement of fungi associated with Aneuraceae liverworts within Tulasnellales based on neighbour-joining and bootstrap from an alignment of partial mitochondrial large subunit ribosomal DNA sequences generated for this study and retrieved from GenBank.

Figure 2.

Phylogenetic placement of fungi associated with liverworts within Sebacinales based on neighbour-joining and bootstrap from an alignment of partial nuclear large subunit ribosomal DNA sequences generated for this study and retrieved from GenBank. The clade designations A and B correspond to those used by Weiss et al. 2004.

Figure 3.

Scanning electron micrographs of basidiomycetes in Aneuraceae. (a–c) Sebacinoid-containing Aneura species from Navarino, Southern Chile. (a) General view showing up to five ventral thallus layers packed with hyphae (arrowed). (b) Cell packed with hyphal coils. (c) Cells with degenerating hyphal masses. (d) A fungus-free thallus of Aneura pellioides. Scale bars for (a) and (d) are 200 µm and for (b) and (c) are 20 µm.

Figure 4.

Scanning electron micrographs of basidiomycete fungi in Jungermanniales. (a–d) Lophozia incisa. (a) Hyphal coils in ventral stem cells with no evidence of fungal degradation. (b) Liverwort wall ingrowths preventing hyphal colonization of an uncolonised stem cell. (c–e) Hyphal entry/exit sites. (c) Hyphae extending from the base of a severed rhizoid. (d) Direct hyphal entry sites (arrowed) through ventral stem cells. (e) Saccogyna viticulosa showing numerous hyphae associated with a branched rhizoid apex. Scale bars for (a), (c) and (e) are 10 µm and for (b) and (d) are 5 µm.