Evolutionary stability in a 400-million-year-old heritable facultative mutualism

Area of Science:

• Microbiology
• Evolutionary Biology
• Mycology

Background:

• Eukaryotes often host heritable endobacteria for metabolic support.
• Facultative symbioses, where partners are non-essential, are typically viewed as transitional evolutionary stages.
• The evolutionary trajectory of facultative symbioses remains incompletely understood.

Purpose of the Study:

• To investigate the evolutionary stability and age of facultative endosymbioses.
• To test the prediction that facultative symbioses are transient stages towards obligate relationships.
• To examine the dynamics of the symbiosis between Ca. Glomeribacter gigasporarum and arbuscular mycorrhizal fungi.

Main Methods:

• Analysis of codivergence patterns between endosymbionts and host fungi.
• Integration of phylogenetic data with the fungal fossil record.
• Assessment of recombination and host-switching frequencies in endobacteria.

Main Results:

• The symbiosis between Ca. Glomeribacter gigasporarum and Glomeromycota is ancient, dating back at least 400 million years.
• Significant codivergence patterns suggest a long-standing, stable association, contrary to expectations for transient symbioses.
• Despite codivergence, Glomeribacter endobacteria exhibit recombination and host switching, indicating a lack of progression towards obligate interdependence.

Conclusions:

• Heritable facultative mutualisms can be ancient and evolutionarily stable.
• Codivergence in facultative symbioses does not invariably lead to reciprocal dependence.
• Low recombination rates and host switching may stabilize ancient facultative mutualisms over extended evolutionary periods.