The games that mushrooms play 🍄
Written by Dr Petra Guy - Plant Data Scientist
All organisms exist in relationship to each other, but at first glance these relationships can sometimes seem risky or unbalanced. One partner can ‘cheat’ another.That is, take more and return less than another organism, and hence have an apparent evolutionary advantage. The question then is, how do these dependencies remain stable?
The mycorrhizal fungal symbiosis is an example. Either partner has the ability to ‘cheat’ the relationship, by receiving more goods than they supply. One mycorrhizal fungus could supply less nutrients than another, but receive the same or more carbon from the tree host. Why don't all mycorrhizal fungi ‘cheat’ the relationship and extract carbon without return? Or why aren't those that do cheat more successful in an ecosystem, outcompeting those that don't?
Evolutionary Game Theory gives an insight into how these interactions remain stable. To understand these interactions, we first need to know that not all ectomycorrhizal fungi (EMF) occur on tree roots with the same probability. Some fungi have a limited host range, that is, they tend to be found in relationships with some trees more than others. We call these specialists. Other species associate with a wide variety of trees, we call these generalists. Specialism and generalism can vary between different habitats. Some habitats show very low levels of specialists, for example willows in tundra or conifer dominant dunes. Early successional ecosystems can display a high species richness and abundance of specialists of the invading tree species. Mature woodland ecosystems tend to show greater species richness of EMF with some degree of host preference, but a greater abundance of generalists.
Photo Credit: Rhizocore
Specialists and generalists are defined by their physical rather than their functional traits. That is - specialists are defined as such due to data which suggests they have a limited host range. The functional parameters of specialism and generalism are not yet well known. It feels intuitive to think of these EMF functionally - that specialist EMF have evolved with their partner in such a way as to provide more nutrients and receive more carbon, in a reciprocal arrangement. But, we don't know that for sure, and, if it is reciprocal, why haven’t they outcompeted generalists? Why are generalists more abundant in many woodlands?
Specialists and generalists are defined by their physical rather than their functional traits. That is - specialists are defined as such due to data which suggests they have a limited host range. The functional parameters of specialism and generalism are not yet well known. It feels intuitive to think of these EMF functionally - that specialist EMF have evolved with their partner in such a way as to provide more nutrients and receive more carbon, in a reciprocal arrangement. But, we don't know that for sure, and, if it is reciprocal, why haven’t they outcompeted generalists? Why are generalists more abundant in many woodlands?
Game Theory provides an answer to this question. Game Theory describes the rate of change (over evolutionary time scales) of different proportions of populations which exhibit different behaviours. This change is related to the fitness (number of surviving offspring) conferred by that behaviour. For example, the proportion of specialists in an EMF community is related to the fitness of that proportion of the community when compared to the average fitness of all EMF.
The (extremely sophisticated) diagram below depicts one possible result of those equations. The arrows depict the direction of the fitness relationship and the size of the arrows depicts the relative size of the conferred fitness benefits. So the left hand panel describes the benefits conferred to the emf partners by trees. Game Theory tells us that the non-host trees tend to confer greater benefits to generalists rather than specialists, the converse being true for the specialised host tree.
The right hand panel depicts the benefits conferred by the fungi to their hosts, showing that specialists will tend to confer greater benefits to their non-partner trees. For example, while a specialist Suillus might receive more nutrients from its specialised host pine compared to that allocated by the pine to a generalist, the specialist emf will tend to confer greater benefits to a different host tree than it does to its specialist host.
Game Theory suggests that EMF relationships cannot be reciprocal, but are in fact asymmetric, and it is this asymmetry that leads to evolutionary stability and means that specialists and generalists both continue to exist.
This has been demonstrated in the lab with arbuscular mycorrhizal fungi (AMF). A particular species of AMF has been shown to be somewhat specialist on Plantago lanceolata. However, this specialist is not the best at promoting growth in the plant. The plant's growth is found to be greater with a non-specialized AMF.
In the context of EMF symbiosis, direct empirical validation of these equations remains incomplete. Nevertheless, there are suggestive indicators:
Specialists such as Rhizopogon and Suillus have demonstrated higher carbon storage compared to generalists when associated with the same host.
Certain Suillus species produce numerous sporocarps (mushrooms) despite their infrequent presence on tree roots, implying an efficient mechanism for extracting carbon from their host.
Trees possess mechanisms to eliminate EMF that contribute fewer nutrients, potentially explaining the predominance of generalists on roots.
Conflicting data also exist; for example, laboratory experiments have shown that different Suillus species exhibit varied effects on disease resistance in Pinus tubularformis.
So where does that leave us? There is likely to be an asymmetric relationship between specialists and generalists and their host trees; this has been seen in many other interactions of this type. It is probably true that we cannot assume that a specialist will confer greater benefits to its host tree and it might be a larger drain on the tree's resources than a generalist would be. This is unlikely to matter in mature woodlands where carbon supply to EMF is not limited, but it might be important for younger saplings. However, there is a lot we don't know about EMF functionality, how that varies within and between species, how that changes depending on habitat, and how that changes when EMF species richness on the host tree is very limited.
🌱 Next Steps: If you're curious about mycorrhizal fungi and their fascinating roles in forest ecosystems, consider exploring recent research papers or join a discussion group—this field is evolving rapidly, and there's much to discover!
Let's keep learning and uncovering the hidden games these mushrooms play! 🍄
