Abstract
BackgroundAnimal and plant species can harbour microbes that provide them with protection against enemies. These beneficial microbes can be a significant component of host defence that complement or replaces a repertoire of immunity, but they can also be costly. Given their impact on host and parasite fitness, defensive microbes have the potential to influence host-parasite interactions on an evolutionary timescale.ResultsUsing a phenotypic framework, we explore the evolutionary and coevolutionary dynamics of a host-parasite interaction in the presence of defensive microbes. We show that costs of host-defensive microbe systems are critical in determining whether a defensive microbe based system or an immune system provides better host protection investment. Partitioning the coevolutionary dynamics yields testable predictions. The density of defensive microbes influences the strength of selection resulting from host - defensive microbe - parasite coevolutionary interactions. We find that they lessen the negative effects of infection on hosts and reduce infectivity by directly competing with parasites.ConclusionsDefensive microbes might thus play a central role in host-parasite interactions, by outright replacing host-based defences, engaging in within-host competition with parasites, and ultimately driving tripartite coevolutionary dynamics.
Highlights
Animal and plant species can harbour microbes that provide them with protection against enemies
Contrasting dynamics occur in the presence of defensive microbes; the dynamical effects of the parasite are reduced and the host dynamics show sustained growth (Fig. 2)
We investigate the determinants of host fitness in the presence of defensive microbes
Summary
Animal and plant species can harbour microbes that provide them with protection against enemies These beneficial microbes can be a significant component of host defence that complement or replaces a repertoire of immunity, but they can be costly. The inherited bacteria Hamiltonella in aphids produces toxins which directly kill developing wasp parasitoid larvae [13], and in grasses, fungal endophytes produce alkaloids in grass host tissue making them toxic to herbivores [14]. Possessing these defensive microbes can be costly [15, 16]. This form of protection provided by microbes can be a significant component of host defence [17,18,19]
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