Changing With the Times

High parasite diversity accelerates host adaptation and diversification (2018) Betts et al., Science. https://doi:10.1126/science.aam9974

Image Credit: Dr. Graham Beards, CC BY-SA 3.0

The Crux

Host-parasite relationships are often thought of or depicted in a pairwise structure. That is, one host is attacked by one parasite, without an acknowledgement or consideration of how complex the relationship can be. For example, hosts are often attacked by more than one type of parasite, and the parasites themselves have to compete with one another for resources from the host. Because parasites are costly for a host, the hosts benefit from evolving resistance to the parasites. It follows that the more parasites a host is attacked by, the higher the benefit of evolving resistance, so we’d expect to see more resistance in hosts that are attacked more often. This should then result in differential evolutionary rates among hosts, which would then result in greater evolutionary divergence (see Did You Know?)

To test this idea, the authors of today’s study used a bacterium (Pseudomonas aeruginosa) and five lytic viral parasites (hereafter bacteriophages). These bacteriophages reproduce within host cells until they eventually cause the host to burst, killing the host (think of the chestburster in Alien, but a LOT of them). Because their reproduction results in the death of the host, lytic parasites impose a very strong selection pressure on hosts, making this a perfect host-parasite system to test the above prediction.

Did You Know: Evolutionary Divergence

Evolutionary divergence is the process where different species become increasingly different over time. In the case of today’s study, evolutionary divergence was related to differing evolutionary rates among hosts, which in turn was due to different selection pressures. To give very simplified example of what that means, imagine two groups of giraffe ancestors. One group experienced strong selection for longer necks as the trees they grew up around were taller, while the other didn’t experience this strong pressure (obviously I’m simplifying – organisms usually experience a ton of selection from a variety of sources). The group experiencing strong pressure would (if they had the necessary mutations/genetic materials) change over a relatively short amount of time to have longer necks, while the group not experiencing this pressure would stay the same. This difference among the two groups is evolutionary divergence, and stronger selection (and subsequent evolution) should result in greater evolutionary divergence.

What They Did

To test for an effect of parasite diversity on host evolution, the authors simply exposed host bacteria to one of four treatments. 1) High diversity (five different bacteriophage parasites), 2) medium diversity (two different bacteriophages), 3) low diversity (a single bacteriophage), or 4) control (no bacteriophages). Differences between these treatments would tell them how increasing parasite diversity influenced the manner and rate of host evolution.

The authors then used time-shift assays to measure if host-parasite co-evolution took place. Time-shift assays are a super cool method where researchers expose a host to their past, present, and future parasites (in addition to exposing parasites to past, present, and future hosts). “Past” parasites/hosts simply mean those that have not had a chance to evolve with a host/parasite, while “future” parasites/hosts are those that have evolved with a host/parasite. This method is well-suited to bacteria hosts, as they have very short generation times, and as such evolution takes place over the course of days.

Evolutionary divergence was measured by screening for mutations in the host genome across the various treatments. The more mutations there were, the greater the divergence of the evolved populations from their ancestral populations.

Behold, the mighty P. aeruginosa! I am guilty of ignoring microorganisms like these bacteria, but they are incredibly useful for evolutionary studies. They experience selection just like larger and more charismatic organisms do, but their short generation times make them a lot easier to work with (Image credit: Microbe World, CC BY-NC-SA 2.0).

What They Found

The results of the time-shift assays showed that future hosts and future parasites were more resistant and more infective, respectively. This was clear evidence of a coevolutionary arms-race, where one organism evolves increasing offensive abilities in response to the increasing defensive ability of another. However, host resistance increased with parasite diversity, meaning that more diverse parasite communities imposed greater selection on their hosts.

In regards to evolutionary divergence, the authors found that the greatest amount of divergence occurred in the high diversity treatment, with evolutionary divergence decreasing as parasite diversity decreased.

Problems?

This study utilized an elegant experimental design that allowed the authors to conduct a variety of tests. While I have no criticisms against the methods they used, I am curious as to how the results may change when parasites (and increasingly diverse parasite communities) are exposed to increasingly diverse host populations. Host-parasite interactions do not occur in isolation, and parasites often infect multiple host species within the same communities. As such, we can make more predictions on how these coevolutionary dynamics may change.

So What?

By investigating the influence of parasite communities, and not single parasite species, the authors successfully demonstrated how parasite diversity shapes host evolution. The rapid evolution in today’s study has shown that parasites can form hot spots of antagonistic coevolution, which has implications for how we think about host evolution in natural communities.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is soooooo close to finishing his PhD. 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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