The consequences of immune priming within and across generations

The demonstration in insects of specific immune priming and trans-generational immunity suggest that while they rely on different mechanisms, the immune systems of both vertebrates and invertebrates have been selected by parasites and pathogens to perform similar functions. It is likely that a particular level of spatial and temporal variation in exposure to parasite types would give a benefit of immunity that is acquired depending on prior experiences. For immune priming to be beneficial variation in the exposure to parasite types should be rapid enough to stay ahead of changes in innate defence levels, but slow enough that secondary exposure to a distinct parasite type is likely once it has already been encountered.

The existence of immune priming both within individuals and across generations will have implications beyond that of understanding the evolution of immune system function. Once they have evolved in hosts, these phenomena of immune priming would have a set of consequences for the further evolution of hosts and parasites, and the co-evolutionary interactions between the two. While this area has received little attention, it is possible to make some intuitive predictions. The postulated examples and potential outcomes given below are generalizations and are by no means exhaustive. However, their aim is to demonstrate that specific immune priming will have far-reaching consequences for the evolution of host and parasite traits, and their co-evolution.

Immune priming in a basic sense can be seen as an acquired element of host resistance. This acquired resistance will only be present in a host individual if it, or its mother, previously encountered a particular parasite type. This is in contrast to the genetically encoded innate levels of resistance that can be found even in immuno-logically naive individuals. It can reasonably be assumed that the presence of these two immune strategies will have certain impacts on the effectiveness of the other, and thus on each other's evolution. Logically, acquired resistance will have no value in a system where the innate defence of individuals does not allow them to overcome an initial infection. However, it is more plausible that immune priming will be found in a host population that exhibits a spectrum of genetically encoded defences from susceptible through to resistant. Where this is the case, and not all susceptible individuals are removed from the population on an initial exposure, immune priming may maintain more innately susceptible genotypes within the population. This will be the result of immune priming turning susceptible phenotypes into resistant phenotypes. As a consequence this would probably slow down the evolution of genetically based resistance against parasites. This has been considered for the case of vertebrates, where modelling the evolutionary dynamics of host resistance traits showed that while acquired resistance has benefits, it decreases the rate of evolution in innate resistance traits (Harding et al., 2005). However, to understand how the evolution of immune priming and innate resistance traits are linked in insects, it will be necessary to understand more about their mechanistic basis. It is possible that they employ similar pathways, and as such, levels of acquired resistance mediated by immune priming will be tied to innate levels.

Virulence evolution is a topic that has received a great deal of attention and controversy in evolutionary biology (Bull, 1994; Frank, 1996). If immune priming in insects affects the fitness of parasites, then it must have had implications for the evolution of parasite virulence. At a basic level, it can be imagined that heightened immune capabilities of a primed host will select for faster and more efficient immune evasion strategies in parasites; immune evasion strategies that may contribute considerably to virulence (Frank and Schmid-Hempel, 2008). Borrowing results derived from models based on vertebrate immunity and vaccination, it can be suggested that immune priming will enable the co-existence of inferior parasite types by mediating competition (Wodarz, 2003), and promote the evolution of faster-replicating, more virulent strains (Fenton et al., 2006; André and Gandon, 2006). This latter consequence has been confirmed empirically in a mouse/malaria system that investigated the effect of immune selection via vaccination. Parasite lines transferred through mice with acquired resistance (via immunization) were found to be more virulent than lines transferred through naïve mice (Mackinnon and Read, 2004).

In the prior two paragraphs we have considered how host and parasite traits may evolve in response to the presence of immune priming in the system. However, it is clear that it is not possible to separate the two protagonists, and what we observe in nature will be ongoing co-evolutionary dynamics. Such dynamics between hosts and parasites are important for a number of evolutionary theories. As touched on above in a different regard, these include the Red Queen hypothesis that invokes the role of parasites in the maintenance of sexual reproduction and recombination (Peters and Lively, 1999; Salathe et al, 2008). This hypothesis is based on the existence of oscillations in the frequency of host and parasite genotypes. These oscillations are produced by negative frequency-dependent selection, where rare genotypes are at a fitness advantage. Based principally on the idea that more susceptible host genotypes will be maintained in a population with immune priming, it has been suggested that immune priming will dampen the amplitude of co-evolutionary oscillations (Little and Kraaijeveld, 2004). While this may have complications for theories that depend on co-evolutionary dynamics, the issue clearly needs to be given greater consideration both theoretically and empirically. It is possible that, as in vertebrates (Borghans et al., 2004), immune priming is achieved by highly polymorphic loci, and as such will add more material for co-evolution to act upon.

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