Why study insect immunity

The current great interest of researchers—not all of them entomologists—in insect immunity and infection has been driven by a variety of causes. Early attention mostly grew from economic concerns. For example, in 1835 Agostino Bassi was among the first to verify the germ theory of disease by showing that a fungus (Beauveria bassiana) was the cause of the white muscardine disease of the commercial silkworm (Bombyx mori). Later, Louis Pasteur spent the years 1865-1870 investigating pébrine (caused by a microsporidian) and flacherie (a viral disease), two further silkworm maladies that threatened to destroy the Provençal silk industry. Among the great microbiologist's discoveries was that the insects showed considerable variation in their resistance to these pathogens. In 1880, the pioneer immunologist Elie Metchnikoff was among the first to propose practical methods of microbial biological control of an insect crop pest, initiating trials of the fungus Metarhizium anisopliae against grain beetles (Lord, 2005).

Such applied interest continues today. Since it is estimated that insects destroy approximately 18% of the world annual crop production (Oerke and Dehne, 2004) and nearly 20% of stored food grains (Bergvinson and Garcia-Lara, 2004), the damage amounting to around US$100 billion every year (Carlini and Grossi-de-Sa, 2002), this interest is not surprising. Since populations of most insects are regulated by density dependent factors involving pathogens and parasites, to feed the world we need to understand, and if necessary manipulate, the interactions of pest insects with their natural enemies.

Insects also provide crucial ecosystem services as pollinators, as has been highlighted by recent transnational declines in the populations of both solitary and social bees (Biesmeijer et al., 2006) and concern over the mysterious Colony Collapse Disorder of honey bees in North America (Oldroyd,

2007). The estimated value of US pollination services alone is between $4.1 and 6.7 billion (Nabhan and Buchman, 1997). It is possible that pollinator declines are at least in part consequences of the compromised immunity of these insects to their usual parasites and pathogens (e.g. Gregory et al, 2005).

Furthermore, mosquitoes, sandflies, and many other insect vector species cause severe health and economic problems. To take only one example, of the 2.5 billion people at risk (40% of the world's population), more than 500 million become severely ill with malaria every year, and more than 1 million die from its effects (World Health Organization,

2008). To comprehend the biology of the disease, it is crucial to understand the ability of the parasite to survive the rigours of the vector's immune system.

However, it has also long been recognized (the case is summarized, for example, by Wigglesworth,

1971) that insects can be tractable models of vertebrate function, and so studies of insect immune function have been undertaken with the idea of making fundamental discoveries about the mechanisms of immune defences. Cuénot studied the phagocytic function of insect blood cells as early as 1895 (cited by Munson, 1953). A paradigmatic study of humoral immunity is that of Stephens (1962), who showed that a bacteria-killing factor appeared in the haemolymph of wax moth (Gallería mellon-ella) larvae that had been injected with Pseudomonas aerugínosa, and that this bactericidal activity was apparently able to confer protective immunity on the living insect.

It is a fact of scientific life, however, that the payoff from basic studies can often be long in coming. In the case of insect immunity, we might identify the work of Hans Boman and his colleagues as defining a crucial experimental moment. Boman et al. (1972) demonstrated that the fruit fly Drosophila mela-nogaster could be induced to synthesize new antibacterial defences when injected with bacteria. These discoveries quickly began to generate an understanding of the molecular nature of the responsible antimicrobial peptides. However, recognition of the importance of studying insect immunity only took off years later, after the discovery in 1996 (Lemaitre et al., 1996) that Toll receptors, formerly only known for their role in embryonic development, play a crucial role in the immune signalling of flies. The subsequent discovery of an immune role of Toll-like receptors (TLRs) in vertebrates (Medzhitov et al, 1997) followed swiftly. This has led to a spectacular, so-called Toll rush. As a consequence, D. mela-nogaster has become established as one of the prime models for studying innate immunity, and also as a model for studying human pathogens.

Evolutionary biologists have joined this field, because it provides an excellent means in which to study host-parasite co-evolution, principles of population genetics and trade-offs shaping life histories from population to molecular levels (Schmid-Hempel, 2005). Also, insects provide tractable model species for proof-of-principle studies in evolutionary biology, as they enable researchers to use powerful approaches, such as experimental evolution or quantitative genetics. Yet insects have much wider intrinsic importance, and studying them should not be because of, and should not be driven solely by, their convenience as model systems.

Beyond these directly applied considerations, insects form the most diverse metazoan taxon. Consequently, a better understanding of how they deal with the majority of their natural enemies, parasites, and pathogens has the potential to enhance our understanding of interactions in an ecological framework and at the community level. At still another level, some insects are also beautiful, and we simply don't know enough about them. Therefore, we feel that whatever the subject and speciality of a researcher are, natural curiosity, or 'the love of insects' (Eisner, 2004) will, and often should, take centre stage.

This book, never claiming to be a complete reflection of research on insect immunity, still covers a great diversity of research interests and approaches. The first part of the book focuses mainly on mechanistic views on insect immunity, whereas the second focuses on the level of the whole organism.

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