Years ago, it was perceived that facultative bacteria living inside other organisms (facultative endobacteria) were an intermediate, transitional stage leading to more obligate mutualisms. Facultative relationships are interactions between two species in which their functions benefit the others fitness but are not required for life cycle completion. On the other hand, obligate relationships are required for the species survival. Before this 2012 paper by Stephen Mondo and company, it was thought that over time, facultative endobacteria shift to playing more and more obligatory roles. Though, this research has upended this apparent evolutionary trajectory.
The most notorious endobacteria we should all be familiar with are the mitochondria within eukaryotic cells. These organelles supply energy to our cells and without them we could not function. The leading hypothesis describing mitochondrial origin is that tiny prokaryotes with complex oxidative mechanisms made their way into early eukaryote cells. They gained a secure place to live, more sheltered from the elements, while their symbiote received energy through these oxidative processes. After millions of years, these once facultative endobacteria became obligate mutualists. They don’t function as individual bacteria anymore either, now working seamlessly as cellular organelles, even though they have a separate genome from us.
Endobacteria are quite common, and Stephen Mondo and his team wanted to test if all facultative endobacteria follow this trend towards becoming obligate mutualists. To test this evolutionary prediction these researchers used a facultative endobacteria called Candidatus glomeribacter gigasporarum that are found within arbuscular mycorrhizal fungi (AMF). These endobacteria modify the metabolism of lipids within fungal spores, thereby enhancing spore germination and hyphal proliferation before AMF penetrate plant roots. These fungi can live without its endobacterial symbiote, but that doesn’t go both ways as the endobacteria solely depend on its fungal host to provide it with energy.
By examining the fossil record, the phylogenies of the two partners, and by measuring the rate of evolution in the endobacteria, these researchers provide us with results contrary to initial predictions. Firstly, they found that this symbiosis has been around for 400 million year-more than enough times for obligate relationships to evolve. Though, instead of evolving into more obligate relationships, they have remained facultative. Unlike the endobacteria that eventually became our mitochondria, some environmental conditions presented to AMF promote the expulsion of the endobacteria. Since these endobacteria promote hyphal growth pre-plant invasion, if AMF exist in soil dense with suitable plant roots, the cost of housing these endobacteria doesn’t outweigh their benefits. Once expelled, these bacteria can then remain outside of AMF for weeks and can become horizontally transmitted to other fungi. This environmental variability essentially locks this symbiosis in a facultative state.
Compared to endobacteria that have traversed past facultative mutualisms into obligate ones, the endobacteria involved with AMF have a slower evolution. There is simply more genetic drift in endobacteria that only are transmitted vertically, like the ancestors of eukaryotic mitochondria. Here, these researchers suggest that facultative mutualisms are not intermediate stages of a species evolutionary trajectory. Not all facultative endobacteria are on route to becoming an obligate mutualist. Instead, these facultative relationships can persist for millions of years, especially when their interactions vary greatly with different environmental conditions.