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How do symbiotic relationships evolve between species?

Symbiotic relationships are everywhere in nature. Fungi enhance water and nutrient absorption for plants by feeding on their roots. Ants find food and shelter in tree structures, while insects defend the trees from being eaten. However, not all symbiotic relationships are created equal. 

Some interactions are specific, where one host pairs off with only one other species. Others are general and flexible. Specific relationships have advantages, allowing the two species to reap mutual benefits and co-evolve. But this type of partnership can also come with a cost, where one partner exploits the other and takes more than it offers in return.

To investigate the evolutionary dynamics of specialization, researchers at the University of Pennsylvania developed a model framework using game theory.

“Each interaction has very different dynamics,” said study senior author Erol Akçay. “Generalists may evolve to work better with more partners, specialists may be specializing to be more cooperative, and other specialists may be specializing to exploit better. Ours is the first study that gets at that dialectical nature of specialization, not just looking at specialization versus generalization.”

As an undergraduate student, Chris Carlson became interested in understanding why variations in generalism and specialism might persist. 

Working with Akçay and Bryce Morsky, a model was developed that imagines the interactions of designated “hosts” with either general or specialized preferences and “symbionts” that provide and receive different benefits and costs from their interactions with various hosts.

The experts found that a cooperative partnering strategy, where both parties benefit, could persist in populations with one specialized host and a corresponding specialized symbiont. Depending on the benefits or payoffs, a generalist partnering strategy could also persist.

On the other hand, in partnerships where the host increased benefits while the symbiont reaped less, a more dynamic outcome arose. Different hosts and symbionts rise and fall in the population over time. Generalists were driven to extinction if the payoff was lower than that of being a specialist. 

The researchers applied a spatial layer to their models to reflect that symbiotic relationships require that the two species involved encounter one another in space.

“Neighbors are going to be more likely to interact with neighbors, so who your neighbor is and how they interact can affect the outcome,” said Morsky

The team found that cooperative specialists could develop “patches” of dominance if they could move over short distances. If the payoff was high enough, generalists could persist around the periphery of these patches. If, however, hosts and symbionts could move over greater distances, just one host-symbiont pair could completely dominate a space.

“Let’s say you have a coral reef with different types of coral and different types of algae,” said Akçay. “If a host is a cooperative specialist, and the symbiont it specializes on gets fixed, the host does better, and therefore those hosts may expand. That creates specific spatial patterns that we were able to track.”

An overall takeaway is that cooperative specialization appears to be a valuable strategy, but it comes with risks. A host’s cooperating partner could go extinct, leaving the host without a symbiont.

The researchers hope their work lends insight into the variety of patterns of mutualisms that exist in nature and contribute to biodiversity.

“When people discuss specialization, this dualism is always in the background,” said Akçay. “This work helps bring it out and clarifies the tensions that exist between mutualisms.”

The research is published in the journal Evolution.

By Katherine Bucko, Staff Writer

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