Experiments beyond evolution, a primer on coevolution in the laboratory

A review paper on coevolution published online ahead of the print version of Trends in Ecology and Evolution caught my eye a couple of weeks ago. Brockhurst and Koskella (2013) review that state of affairs of experimental coevolution research. Both of these researchers have a rich experience in this field and present a concise review of the field.

The major contributions of experimental coevolution thus far have been to provide direct evidence of the tempo and mode of antagonistic coevolutionary dynamics, the role of antagonistic coevolution in increasing diversity within and among populations, including the role of parasitism in maintaining sexual recombination, and the structure of specificity in coevolving antagonistic interactions.

Within the article, the authors summarize the expected outcomes or results from common experimental coevolution studies.

Approaches to quantifying reciprocal adaptation.
(Modified figure 2 from Brockhurst and Koskella, 2013). 

The process of rapid reciprocal adaptation inherent to antagonistic coevolution can be driven by at least two contrasting modes of reciprocal selection: ‘Fluctuating Selection Dynamics’ (FSD) where changing allele frequencies in host and parasite populations are driven by parasite-mediated selection against common host resistance alleles or ‘Arms Race Dynamics’ (ARD) where recurrent selective sweeps of novel host resistance and parasite infectivity alleles occur through time, leading to increases in the host range of the parasite and the subsequent host resistance traits. Experimental coevolution has revealed evidence for the operation of both of these modes of reciprocal selection.

The authors also do a good job of pointing to a path rich in research aims for understanding coevolutionary interactions. To date, most experimental coevolution studies have focused on single pair, antagonistic interactions. Beyond the common critique of laboratory experiments (a need to increase “reality”), they suggest that studying more complex communities as well as different forms of interactions.

Don’t wait for the article to show up in print, check out the review online now. The Brockhurst lab website can be found here. You can also see what Dr. Koskella is up by reading her blog, Nature's Microcosm.


Reference

Brockhurst MA, Koskella B (2013) Experimental Coevolution of Species Interactions. Trends in Ecology & Evolution: DOI: http://dx.doi.org/10.1016/j.tree.2013.02.009

Parasites can maintain host diversity

In their recent paper, Morgan et al (2009) look at the role of an antagonistic interaction in promoting coexistence among different hosts. Using a bacteria and phage system (bacterium: Pseudomonas fluorescens and bacteriophage: SBW25Φ2), they determined that in the presence of a coevolving phage, a slower growing, but phage resistant host would persist with a susceptible faster growing host. Without the phage, the better competitor became fixed in experimental lines. This paper did not explicitly demonstrate coevolution between the phage and the bacterial host, however there is previous evidence in this system for reciprocal selection (Buckling and Rainey 2002). The point of this experiment was to demonstrate the cost of parasite resistance.

The authors also presented a second hypothesis that was a little less explicit: "the probability of coexistence would alter through time." This general hypothesis was supported and the authors provided several explanations for the fitness of the resistant mutant changing over time with respect to wild type. They narrow down the field to changes in the cost of resistance and compensatory mutations. Their evidence comparing growth rates from the beginning to the end of the experiment support a change in the cost, but I wasn't completely convinced that this ruled out compensatory mutations.

A disappointing portion of this system is a lack of understanding of the mechanism of phage resistance. This is no fault of the authors, as the paper is just the beginning of an investigation. They have some details about a general reaction (production of "cellulose-like polymer"). It would be very interesting to take this system to the next step and start targeting some genes

References

Morgan, A. D., R. C. Maclean, and A. Buckling. 2009. Effects of antagonistic coevolution on parasite-mediated host coexistence. J Evolution Biol 22:287-292.

Buckling, A. and Rainey, P.B. 2002. Antagonistic coevolution between a bacterium and a bacteriophage. Proc. R. Soc. Lond. B Biol. Sci. 269: 931–936.

MORGAN, A., CRAIG MACLEAN, R., & BUCKLING, A. (2009). Effects of antagonistic coevolution on parasite-mediated host coexistence Journal of Evolutionary Biology, 22 (2), 287-292 DOI: 10.1111/j.1420-9101.2008.01642.x