Search for: All records

Abstract Given their ubiquity in nature and their importance to human and agricultural health, it is important to gain a better understanding of the drivers of the evolution of infectious disease. Across vertebrates, invertebrates, and plants, defence mechanisms can be expressed either constitutively (always present and costly) or induced (activated and potentially costly only upon infection). Theory has shown that this distinction has important implications to the evolution of defence due to differences in their impact on both individual fitness and the feedback of the population-level epidemiological outcomes such as prevalence. However, despite the fact that pathogens evolve in response to host immunity and that this can have important implications to the evolution of host defence, the implications of coevolution on constitutive and induced immunity have not been examined. Here we show theoretically how and when incorporating host-parasite coevolution between host defences and parasite growth strategies plays an important role in determining the optimum outcome. A key result is that whether the parasite affects host reproduction critically impacts host-parasite coevolution; when the parasite impacts fecundity, selection on the host is largely geared towards minimizing reproductive costs, through reducing investment in reproductively costly constitutive defence when the parasite prevalence is low, but also by investing in immunity to avoid infection or recover when prevalence is high. Our work emphasizes the importance of coevolution and epidemiological feedbacks to the coevolution of hosts and parasites and provides testable predictions of the determinants of constitutive verses induced defence. 

Abstract There is overwhelming evidence that the microbiome can be important to host physiology and fitness. As such, there is interest in and some theoretical work on understanding when hosts and microbiomes (co)evolve so that microbes benefit hosts and hosts favour beneficial microbes. However, the outcome of evolution likely depends on how microbes benefit hosts. Here, we use adaptive dynamics to investigate how host and symbiont evolution depend on whether symbionts increase host lifespan or host reproduction in a simple model of host and symbiont dynamics. In addition, we investigate 2 ways hosts release (and transmit) symbionts: by releasing symbionts steadily during their lifetime or by releasing them at reproduction, potentially increasing symbionts’ chances of infecting the host’s offspring. The former is strict horizontal transmission, whereas the latter is also a form of indirect or “pseudovertical” transmission. Our first key result is that the evolution of symbionts that benefit host fecundity requires pseudovertical transmission, while the evolution of symbionts that benefit host lifespan does not. Furthermore, our second key result is that when investing in host benefits is costly to the free-living symbiont stage, intermediate levels of pseudovertical transmission are needed for selection to favour beneficial symbionts. This is true regardless of fitness effects because release at reproduction increases the free-living symbiont population, which increases competition for hosts. Consequently, hosts could evolve away from traits that favour beneficial symbionts. Generally, our work emphasizes the importance of different forms of vertical transmission and fitness benefits in host, microbiome, and holobiont evolution as highlighted by our prediction that the evolution of fecundity-increasing symbionts requires parent-to-offspring transmission.