Always There for You

Evolution and maintenance of microbe-mediated protection under occasional pathogen infection (2020) Kloock et al., Ecology and Evolution, https://doi.org/10.1002/ece3.6555

Image Credit: Zeynep F. Altun, CC BY-SA 2.5, Image Cropped

The Crux

Microbes are everywhere in nature, and I don’t just mean out in the wild. They live inside of every plant and animal, including humans. These microbes can be harmful, beneficial, or do nothing to their hosts. When they help us, microbes take part in what’s called “defensive mutualism”, which is where they help their hosts fight off parasites. Benefiting from this mutualistic relationship depends on whether or not there are parasites around to defend against, as microbial defense mechanisms can harm not only the parasite but also the host itself.

For this symbiotic relationship to continue and not be selected against over time, the benefits of hosting the microbe must outweigh the costs. This is all well and good when there are always a lot of parasites to defend against, but that is not always the case. Today’s authors wanted to test how changes in parasite pressure over time affected the relationship between a defensive microbe and its host.

What They Did

The authors paired the host and symbiont together and allowed them to evolve with one another for 20 generations. The host was the worm Caenorhabditis elegans, the symbiont was Enterococcus faecilis, and the parasite was Staphylococcus aureus. To test how temporal variation in parasitism pressure affected this coevolutionary relationship, they used five different treatments. The first treatment involved having the parasite present for all 20 generations of host-symbiont coevolution. The second and third treatments involved having alternating generations of parasite pressure, either at the beginning of the two-generation cycle (this means the parasite was introduced in generation one, absent in generation two, present in generation three, absent in generation four, etc.) or at the end (this means the parasite was introduced in generation two, absent in generation three, present in generation four, absent in generation five, etc.). The fourth and fifth treatments were similar to the third and fourth, but instead of alternating every other generation they alternated every fifth generation, either at the beginning or end of it like in treatments two and three.

Protective benefits of the symbiont were measured by comparing the survival of the various coevolutionary treatments to the survival of the ancestral worm. If the survival of the coevolutionary treatments increased relative to the ancestor, that would indicate that coevolving with the symbiont increased the protective ability of the symbiont itself.

Did You Know: The Parasitism-Mutualism Continuum

Parasites are simply organisms that get what they need to survive from the host that they live on/in. Mutualists are a little different, in that they can also live on/in a host organism, but they are more of a benefit to the host than a cost. Although the host still pays a cost by hosting the mutualist, the benefits outweighs this cost. However, some mutualists are known to cheat, taking more than they give to the host. These “cheaters” tend to be selected against, because the host wants nothing to do with that kind of relationship. 

What They Found

The defensive mutualist symbiont conferred protective benefits to their hosts in all treatments in which the parasite was present, regardless of the temporal variation. This means that, as long as the host and symbiont evolved with one another, it did not matter if the parasite was there at the beginning of the host-symbiont coevolutionary history, showed up multiple generations later, or was always present.

Although the researchers didn’t use these exact plates, studies like this one are amazing in that one can actually observe evolution happening in relatively short time periods in small, plastic plates (Image credit: J.N. Eskra, CC BY-SA 4.0).

Problems?

Although the host and symbiont were both allowed to evolve with one another, the parasite was not allowed to evolve in response to the host-symbiont pair. This means that as the host and symbiont evolved over the course of the experiment, they were exposed to the same parasite that their ancestors were. Logistically, it may have been impossible to account for/allow the evolution of three separate organisms, but I wonder how these coevolutionary dynamics may have changed if the parasite was allowed to evolve, as well.

So What?

Theory says that hosts get the most from their defensive symbionts when parasites are always present. Interestingly, this study showed that this is not the case. Varying the presence of the parasite showed that hosts still benefit from their symbionts even when parasite infection was a rare occurrence. These results mean there are exciting new opportunities to study how this transgenerational immunity may limit the spread of disease in host populations, even when those host populations are rarely exposed to disease pressures.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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