Box 141 Concepts and caveats

Immunity in the evolutionary ecology sense is an organism-level trait

Generally in this chapter, immunity and specific immunity are discussed in an evolutionary ecology sense. This sets them apart from any particular mechanism. Immunity here refers to any trait that leads to resistance and the prevention or reduction of parasite infection.

There is a spectrum of specificity and the level on which it occurs is important

Specificity is a quantitative measure, and discrimination of parasites by the immune system can take place on different levels of parasite relatedness. Specificity can be coarse-grained, for example between general classes of microbes, such as bacteria and fungi. This coarse specificity can be explained by differential effectiveness of pathways that are already relatively well understood (e.g. Toll and Imd). However, at the discriminative extreme of the specificity spectrum is the differentiation between strains of the same parasite species. Some of the examples given in this chapter suggest that the insect immune system can act on this level of specificity.

This suggests that it is necessary to look beyond the currently well-studied mechanisms for the mechanistic basis of immune specificity in insects (see Chapter 5 in this volume). The evolutionary consequences of immune specificity will also depend on the level at which it acts. For example, selection for diversity of parasite genotypes will only occur if these genotypes are differentiated by the immune system.

Resistance is not the only means of coping with parasites

Resistance will limit infection intensity, whereas tolerance mechanisms will limit the fitness impact of a particular infection (Schneider and Ayres, 2008). Taken together, resistance and tolerance will determine the fitness loss that is incurred by an individual on parasite infection. It is plausible that there may be specific tolerance, just as there is specific resistance. The study in insects of these aspects of coping with parasite infection, and their consequences, is still in its infancy. As a result, this fascinating topic will not be discussed further in this chapter, but interested readers are directed to a relevant review by Schneider and Ayres (2008).

evolutionary and behavioural ecology. Hamilton and Zuk started this process with their influential paper on parasite-mediated sexual selection (Hamilton and Zuk, 1982). The study of sexual selection and the role of parasites has been increasingly affected by the search for evolutionary tradeoffs between the investment into attracting mates (fertility selection/sexual selection) and other components of fitness (viability selection/natural selection). Similarly, the general framework of evolutionary life history theory is based on the existence of various trade-offs between different fitness components, such as reproduction and survival, the latter being very closely associated with immune defence. The idea that a less-than-perfect immune response and widespread susceptibility of hosts towards their parasites should be the result of unavoidable trade-offs has been a fruitful concept (Sheldon and Verhulst, 1996; Folstad and Karter, 1992), whose validity was documented early in the history of the field (Gustafsson et al., 1994; Norris et al., 1994; K├Ânig and Schmid-Hempel, 1995). Further discussions on trade-offs involving the immune system can be found in Chapters 10, 11, and 12 in this volume. However, regardless of the usefulness of the trade-off concept, the element of specificity has not been a major consideration in trade-off discussions (see Schmid-Hempel and Ebert, 2003). Specificity has been touched on in terms of trade-offs within the immune system, for example between non-specific and specific defence (Moret, 2003; Mallon et al, 2003), but has rarely been incorporated into any other framework in this branch of the field.

This chapter will approach the question of immune specificity from an evolutionary ecology perspective. For the sake of clarity, immune specificity will be addressed on two levels. First, immune specificity will be considered in the light of evidence for specific interactions between hosts and parasites (Figure 14.1a). The importance of these specific interactions for questions concerning genetic diversity will then be discussed. The second level on which immune specificity will be addressed is in the context of immune priming (Figures 14.1b and 14.1c). It must be stressed that these two phenomena are almost certainly not mutually exclusive. For instance, the level of

(a) Specific host-parasite interactions

Parasite: Host:

I II

P1 P2 H1

P1 P2 H2

(b) Immune priming (within a host type against a single parasite)

Prior parasite exposure: Yes

(c) Specific immune priming (within a host type)

I ll

Current parasite exposure: P1 P2 P1 P2 Prior parasite exposure: P1 P1 P2 P2

Figure 14.1 (a) Specific interactions. (b) Immune priming. (c) Specific immune priming.

primed defences may be constrained by the innate defence capacity of an individual. Consequently, immune priming may play a role in the formation of specific interactions between hosts and parasites when re-infections are persistent or infections are chronic. Prior to concluding, the chapter will consider sociality, and in particular immune defence within social insects. Social insects have been important study organisms in elucidating the evolutionary ecology of immune defence. Furthermore, as they are particularly sensitive to issues relating to specific immune defence, empirical examples from the social insects are found throughout the chapter.

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