The Making of a Mutualism

Mutualistic relationships can arise from the coevolution of organisms initially involved in parasitism, enslavement or commensalism. But what about the formation of mutualisms without a history of coevolution? Recently, Science published a paper, Niche engineering demonstrates a latent capacity for fungal-algal mutualism, by Erik Hom and Andrew Murray that demonstrates how mutualistic relationships can immediately arise following changes in environmental conditions—by “ecological fitting”.

Hom and Murray tested whether they could spontaneously (sans genetic change) establish a mutualistic relationship between the yeast Saccharomyces cerevisiae and the alga Chlamydomonas reinhardtii. The idea was to create an obligate mutualism between the two species with glucose and nitrite as the only carbon and nitrogen sources— obligate, because the alga is unable to use glucose as a carbon source but can use the carbon dioxide resulting from yeast glucose metabolism; and similarly, the yeast is unable to use nitrite for nitrogen but can use the ammonia produced by the alga’s nitrite metabolism.

Hom and Murray succeeded in creating this obligate mutualism, independent of the amount of nutrient input. They also succeeded with multiple other yeast and many Chlamydomonas species (diverging over 300 million years ago). They did find variation in the success of these mutualisms, likely resulting from species-specific traits. This raises interesting questions about which species-specific traits may influence the success and possible persistence of the mutualism, about how phylogenetically broad these mutualisms could be, and how easily these relationships could be established.

What else? The researchers added atmospheric CO2 to the equation to find that C. reinhardtii would become a facultative mutualist- no longer depending solely on S. cerevisiae for carbon (but with the yeast remaining dependent upon the alga). On the other hand, if the experiment is altered to allow the yeast to get what it needs from the environment (ammonia added), the yeast outproliferates the alga, driving it to near extinction. This raises questions about the importance of generation times or growth patterns of the two mutualistic species and about how particular the environmental conditions must be in order to spark one of these relationships.

The work suggests that mutualisms can form without coevolution if two species have previously evolved traits that allow for them to be complementary in some given environment, or after some ecological change. The chart below from “The birth of cooperation” (also published in Science) by Duur Aanen and Ton Bisseling demonstrates some possible origins of mutualistic relationships.

Science Mutualism Cooperation

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