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Abstract Predators can strongly influence disease transmission and evolution, particularly when they prey selectively on infected hosts. Although selective predation has been observed in numerous systems, why predators select infected prey remains poorly understood. Here, we use a mathematical model of predator vision to test a long‐standing hypothesis about the mechanistic basis of selective predation in aDaphnia–microparasite system, which serves as a model for the ecology and evolution of infectious diseases. Bluegill sunfish feed selectively onDaphniainfected by a variety of parasites, particularly in water uncolored by dissolved organic carbon. The leading hypothesis for selective predation in this system is that infection‐induced changes in the transparency ofDaphniarender them more visible to bluegill. Rigorously evaluating this hypothesis requires that we quantify the effect of infection on the visibility of prey from the predator's perspective, rather than our own. Using a model of the bluegill visual system, we show that three common parasites,Metschnikowia bicuspidata,Pasteuria ramosa, andSpirobacillus cienkowskii, decrease the transparency ofDaphnia, rendering infectedDaphniadarker against a background of bright downwelling light. As a result of this increased brightness contrast, bluegill can see infectedDaphniaat greater distances than uninfectedDaphnia—between 19% and 33% further, depending on the parasite.PasteuriaandSpirobacillusalso increase the chromatic contrast ofDaphnia. These findings lend support to the hypothesis that selective predation by fish on infectedDaphniacould result from the effects of infection onDaphnia's visibility. However, contrary to expectations, the visibility ofDaphniawas not strongly impacted by water color in our model. Our work demonstrates that models of animal visual systems can be useful in understanding ecological interactions that impact disease transmission.
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Virulence evolution during a naturally occurring parasite outbreak
Abstract Virulence, the degree to which a pathogen harms its host, is an important but poorly understood aspect of host-pathogen interactions. Virulence is not static, instead depending on ecological context and potentially evolving rapidly. For instance, at the start of an epidemic, when susceptible hosts are plentiful, pathogens may evolve increased virulence if this maximizes their intrinsic growth rate. However, if host density declines during an epidemic, theory predicts evolution of reduced virulence. Although well-studied theoretically, there is still little empirical evidence for virulence evolution in epidemics, especially in natural settings with native host and pathogen species. Here, we used a combination of field observations and lab assays in theDaphnia-Pasteuriamodel system to look for evidence of virulence evolution in nature. We monitored a large, naturally occurring outbreak ofPasteuria ramosainDaphnia dentifera, where infection prevalence peaked at ~ 40% of the population infected and host density declined precipitously during the outbreak. In controlled infections in the lab, lifespan and reproduction of infected hosts was lower than that of unexposed control hosts and of hosts that were exposed but not infected. We did not detect any significant changes in host resistance or parasite infectivity, nor did we find evidence for shifts in parasite virulence (quantified by host lifespan and number of clutches produced by hosts). However, over the epidemic, the parasite evolved to produce significantly fewer spores in infected hosts. While this finding was unexpected, it might reflect previously quantified tradeoffs: parasites in high mortality (e.g., high predation) environments shift from vegetative growth to spore production sooner in infections, reducing spore yield. Future studies that track evolution of parasite spore yield in more populations, and that link those changes with genetic changes and with predation rates, will yield better insight into the drivers of parasite evolution in the wild.
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- Award ID(s):
- 1748729
- PAR ID:
- 10396237
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Evolutionary Ecology
- Volume:
- 37
- Issue:
- 1
- ISSN:
- 0269-7653
- Format(s):
- Medium: X Size: p. 113-129
- Size(s):
- p. 113-129
- Sponsoring Org:
- National Science Foundation
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