The Geosiphon-Nostoc symbiosis: recent elaboration, or remnant of an enduring association?

Abstract

Background and aims: Fungal associations with photosynthetic microbes have regularly been invoked as precursors to the evolution of land plants and their mycorrhizal associations. The fungus Geosiphon pyriformis (Glomeromycotina) deviates from its arbuscular mycorrhizal (AM)-forming relatives through the presence of an intracellular symbiosis with Nostoc cyanobacteria, and is frequently viewed as the only extant representative of an ancient and formerly widespread association between fungi and cyanobacteria that occupied early terrestrial ecosystems. Here we add to growing evidence suggesting the Geosiphon-Nostoc-like associations are not ancestral to AM associations and did not occupy landscapes prior to the evolution of land plants. In addition, we discuss the underlying drivers contributing to the origination and persistence of this argument and discuss other ways in which Geosiphon has been misunderstood.

Methods: We inferred timescaled phylogenies of Glomeromycotina and utilized ancestral state reconstruction to both evaluate the plausibility of a Geosiphon-like ancestral state, and highlight the additional complexity required to maintain support for a Geosiphon-like ancestral state.

Key results: Our analyses overwhelmingly recovered the Geosiphon-like state as being derived from AM associations.

Conclusions: Our work illustrates the diverse ways in which Geosiphon has been misunderstood and adds to a growing body of evidence suggesting the Geosiphon-like ecology is derived from AM-forming ancestors and did not occupy terrestrial ecosystems prior to the evolution of land plants. We conclude by discussing outstanding questions pertaining to the ecology and evolution of Glomeromycotina fungi.