Many insects harbour bacterial symbionts, often in their guts. As Hurst and Darby review in Chapter 8, evidence is accumulating to suggest that bacterial symbionts provide protection against other natural enemies, including fungi, viruses, parasitoids, and even predators. If they provide resistance, how does the evolutionary ecology of symbiont-mediated protection differ from resistance through the host's own immune system? Although this is hardly studied, Hurst and Darby speculate that these symbi-onts are similar to constitutive defences: the insect always pays a metabolic cost. However, secondary symbionts can be lost easily if the selection pressure exerted by a parasitoid relaxes, for example. Aside from protection, there is another twist to the story. In most cases, these symbionts will be expressing pathogen-associated molecular patterns (PAMPs) similar to, or the same as, those of the pathogen. Moreover, the host needs to ensure that the symbionts co-operate. This establishes a very interesting perspective on the evolution of the insect's immune system: maintaining and managing symbionts could constitute a formidable selection pressure for the evolution of a policing system, such as immunity.
The interactions between hymenopteran para-sitoids and their lepidopteran hosts have long held the attention of applied entomologists, insect ecologists, and evolutionary biologists. For more than 30 years, it has been known that the female wasp coats her eggs with virus particles in order to interfere with both the immune status and life history of the caterpillar host. Several polydnavi-rus genome sequences are now available. Moreau et al. (Chapter 9) review how this information has emerged as an important experimental tool to reveal the mechanisms of host immunity. The viral genome is transferred to caterpillar host cells, where it directs the expression of a variety of factors perverting host function in favour of the wasp. Effectively, the viral DNA has been captured to become an external extension of the wasp genome. Analysis of viral genes not only reveals which host genes are most important in resisting parasitic attack, but simultaneously casts light on the co-evolutionary arms race that exists between pairs of interacting parasite and host genes.
Koella's chapter (Chapter 10) takes a theoretical perspective on the evolution of immune defences, and speculates on the conditions under which an organism can become resistant. Central to his argument are trade-offs between components of the immune system, the immune system and life history of the host. Moreover, understanding resistance evolution is made more difficult by the fact that host and parasite/pathogen genotypes interact, which might very well be genotype-specific. Using co-evolutionary models in which host and parasite are allowed to evolve, yields results different to those obtained from merely studying the host and the parasites separately. As they are tied together in a very intimate relationship, the presented theory strongly argues for empirical studies investigating both sides of the relationship.
The outcome of host-parasite interactions can also depend on the presence of other players, for example predators. Adamo's chapter (Chapter 11) shows how and why short-term immunosuppres-sion mediated by stress occurs. The best example is that of crickets, which show a flight-or-fight syndrome. In the presence of predators or other sources of stress, these insects show lowered resistance against infection. This seems to be mediated by the demand for lipids to fuel the flight. A specific protein, apoliphorin III, is usually involved in immunosurveillance, but has now been found to be essential for lipid transport. Such physiological trade-offs are not only a cause of concern when assessing experimentally the resistance of a particular organism, but they add a further layer of complexity to understanding the evolution of resistance in its ecological context, where encounters with predators might be rather frequent. This chapter resonates, of course, with the earlier one by Schneider (Chapter 7).
The majority of insect immunity research has been carried out on D. melanogaster. Yet, as clarified by Kraaijeveld and Wertheim in Chapter 12, knowledge about how flies resist parasites and pathogens is mostly limited to just a few standard pathogens. Flies, almost certainly like all other insects, encounter a variety of natural enemies in the wild, ranging from nematodes, parasitic wasps and flies, bacteria, and fungi to viruses. How flies resist common natural enemies, such as nematodes and microsporidia, has hardly been investigated. Kraaijeveld and Wertheim emphasize that if we are to understand the evolution of the insect immune system, we need to quantify the strength of selection. To achieve this, information on infection rates in wild populations is paramount. Moreover, we need to understand the costs of immune functions, which puts constraints on the evolution of resistance. Although these costs are well studied at the phenotypic level, especially for parasitoid attacks, at the genomic level hardly anything is known. The authors explore the use of post-genomic tools to explore the costs of resistance. So far, ideas about costs can only be inferred indirectly from changes in the carbohydrate metabolism of infected individuals, for instance. However, the combination of experimental evolution studies combined with genomic techniques holds great promise here.
Despite the advantage of being 'immunocompetent', i.e. showing resistance against a wide range of parasites and pathogens, natural populations of hosts show great variation in parasite resistance. Junjeja and Lazzaro (Chapter 13) dissect this observation by discussing the evolution of different components from a population genetic perspective. Looking at recognition, signal transduction, and immune effectors, they show how these differ in their evolutionary trajectories within and across species, making use of data generated from whole-genome studies in more than 15 species of insect. Interestingly, the components of the immune system differ not only in how fast they evolve, but also in the way in which they change. Signalling pathways, for example, show a high degree of amino acid divergence. Yet across species, the main proteins in signalling are highly conserved; they are orthologues. By contrast, antimicrobial peptides hardly show any signatures of adaptive evolution at the amino acid level, yet different taxa of insects seem to have their own groups of antimicrobial peptides. The authors then go on to discuss how these genotypes translate into phenotypes with varying degrees of resistance. They highlight the importance of understanding trade-offs within the immune system (see also Chapter 12 in this volume) and the way pathogens, here mostly bacteria and viruses, can interfere with the immune system. From the pathogen's perspective, the phenotype matters, because it constitutes their selective environment.
Hallmarks of vertebrate immunity are memory and specificity, and the mechanisms underlying these traits are well studied. As these components of the acquired immune system are confined to the jawed vertebrates, it has been inferred that the functional outcome, specificity and memory, must be limited to the jawed vertebrates. Pancer et al. (2004) recently demonstrated the existence of different diversifying mechanisms in the lampreys and hagfish, and thereby challenged this notion. In fact, transplantation experiments in the 1960s already indicated the existence of acquired immunity in lampreys. In their chapter, Sadd and Schmid-Hempel (Chapter 14) discuss examples of highly specific immune reactions in insects, as well as the nature of secondary response. Although the underlying mechanisms are elusive, the functional outcomes are clearly worth studying. They discuss a generic way of defining specificity and convincingly show that genetic diversity contributes to pathogen resistance. In social insects, this could be achieved through the mating system, and therefore at the colony level this is somewhat analogous to a simple somatic diversifying mechanism.
Wounding has frequently been assumed to be a major route for infections. As Siva-Jothy discusses in Chapter 15, it has only recently become clear that copulatory wounding occurs frequently in insects. This has implications for the evolution of wound repair, which is mediated by the immune system via clotting and melanization (Theopold et al., 2004). Moreover, Siva-Jothy puts a convincing argument forward that females are in control of the timing of mating and therefore, are in a position to 'predict' their infection risks, if mating bears the risk of wounding and infection, for example, by sexually transmitted diseases. All else being equal, this leads to the hypothesis that the investment and management of immunity is tailored to meet the demands of higher risks during mating.
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