Emerging themes

As with any edited volume, the current one has some obvious gaps. Different readers may miss different things. Whereas these gaps come down in part to decisions by the editors as well as practical considerations and problems, there are clearly areas that have not been touched upon because the body of research is rather limited. In this section, we will briefly discuss a number of areas that we feel deserve attention by researchers, ideally garnering interest from different disciplines. These areas have rarely been studied (we also readily admit that these are subjective choices and do not pretend to give a complete coverage).

First, in parallel to speculations about the evolution of immunity in vertebrates (Hedrick, 2004; McFall-Ngai, 2007; Rolff, 2007), it is not entirely clear how the different components of the insect immune system have evolved. In Chapter 8, Hurst and Darby suggest an important role for symbi-onts and the need of the host to control its sym-bionts. Also, pathways such as the melanization pathway are involved in wound repair, and concurrent selection on this function will almost certainly have shaped the evolution of the proPO system (Cerenius et al., 2008). Identifying the key selection pressures that led to the evolution of particular components of the immune system will be important, as well as understanding how selection moulded their integration into existing resistance mechanisms. To better perceive the starting origins of the insect immune system, it may be beneficial to take a closer look at homologies between the immune genes and signalling pathways of insects and those of other invertebrates, rather than just comparing them with mammalian systems. The highly diversified form of Dscam, for example, is restricted to insects and crustaceans, whereas Dscam itself is probably very old and is found in vertebrates as well (Brites et al., 2008). The implications of this for understanding the insect and crustacean immune systems still need to be explored.

Second, whereas many insect parasites have been described, mechanistic understanding of insect immunity against pathogens and parasites is rather limited to a few groups (see Chapter 12 by Kraaijeveld and Wertheim). Many of the pathogens commonly used in experiments are actually gen-eralists, and are used to elicit and study immune responses mostly for the sake of convenience. Frequently, the parasite is injected into the host, circumventing the first line of immune defence offered by the cuticle. In order to understand the natural dynamics of infections, natural pathogens and manipulations of parasite loads via natural routes of infections are required. Moreover, insect immune genes must be presumed to have co-evolved with the virulence genes of the most frequently encountered, often specialist, parasites and pathogens. Studies of bacterial infections in Drosophila only very rarely use natural pathogens (but see Vodovar et al, 2005; Lazzaro et al, 2006). Macroparasites of insects are hardly studied, yet insects are hosts for parasites with complex life cycles such as tapeworms (Hurd, 1998), as well as numerous species of nematodes.

Third, it is often assumed that wounds breach the cuticle, allowing pathogens to invade. Although wound repair is quite well studied (Theopold et al., 2004), it is not self-evident that most microbial infections normally occur through this route. We are not aware of any data quantifying the frequencies of wounds in natural populations of insects, which at least would offer an estimate of the opportunities for infection. This has important implications, since septic systemic infections imposed on experimental insects through injection or body-wall piercing must often be necessarily 'unnatural'. Under such circumstances, the choice of the model pathogen becomes important. The virulence genes of pathogens that access the insect haemocoel directly are likely to have evolved differently from those of pathogens that enter from the gut, for example.

Yet, in the light of copulatory wounding, as discussed by Siva-Jothy (Chapter 15), wounding could well turn to be a frequent event in an insect's lifetime, and hence contribute to the selection for resistance. We need to know the identities of the pathogens concerned. Perhaps another exception here is the case of the specialist insect-pathogenic bacteria of the genera Photorhabdus and Xenorhabdus, which are symbiotic partners of entomopathogenic nematodes, and are indeed literally injected into their hosts by their worm partners (Goodrich-Blair and Clarke, 2007).

Fourth, an additional argument for studying the interactions of the insect immune system with 'natural' pathogens and parasites, as discussed by Moreau et al. (Chapter 9), is that those immune genes that are crucial to host defence can often be revealed by identifying virulence genes in the genomes of co-evolved parasites. While a serious start has been made on this approach with viruses, less has been done with the larger genomes of insect-pathogenic bacteria and (even less) fungi. A start on this has now been made by screening cos-mid libraries of Photorhabdus spp. (e.g. Waterfield et al., 2008), and other bacterial pathogens will surely follow. The larger and often less experimentally tractable genomes of fungi pose a bigger challenge, however.

Finally, the issue of the specificity of immune responses emerges in several of the chapters of this book. To what extent are specific immune responses adaptive (Sadd and Schmid-Hempel, 2006)? How many and how much of different defensive response types should be deployed? How specific should they be? As pointed out by Sadd and Schmid-Hempel in Chapter 14, specificity can take two different forms in insects: immune responses that are specific to pathogen species/strains, and the phenomenon of immune priming, whereby the response to a second infection by the same pathogen strain is more successful. This is likely to be mediated by haemocytes (Pham et al., 2007). Moreover, resistance, as discussed by Koella (Chapter 10), is an outcome of the interaction between host and parasite. Whereas Dscam (see Das et al., Chapter 5) is considered to be a candidate for specific immune reactions in insects, specificity does not need to be restricted to the recognition level. Recognition, transduction, and effectors all show different signatures of adaptive evolution (Juneja and Lazzaro, Chapter 13). One could speculate that specificity is also determined by the interactions between these different levels of the immune system during a course of an infection (Haine et al., 2008).

In writing this chapter, it again became clear to us how seemingly arcane or irrelevant research can unexpectedly become important. Under the current funding climate (Braben, 2008) it would be almost impossible to secure funding to investigate a peculiar physiological trait in an insect of no economic importance, which had nevertheless attracted the curiosity of researchers. Yet, Osama Shimomura set out in the early 1960s to study the fluorescence of the jellyfish Aequorea victoria (Shimomura, 1995), an investigation that ultimately led to the development of what is now an essential tool for cell biology and infection biology. In fact, most researchers that contributed to the current volume use green fluorescent protein (GFP)-labelled bacteria routinely in their research.

We hope that the inter-disciplinary nature of this volume will lead 'willing scholars' into some quite unfamiliar territory. We are confident that over the frontier lie many undiscovered human and scientific benefits, as well as much enjoyable biology. We also hope that not only insect immun-ologists, but scientists everywhere, will continue to insist on the importance of applying the highest scientific standards to problems of great intrinsic scientific interest, but with no apparent immediate applicability.

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