Virtually every organism will be attacked by parasites and/or infected by pathogens at some point in its life. The ubiquitous nature of parasites and pathogens means that selection pressures to evolve resistance mechanisms, in one form or another, will be common. The benefits of a resistance mechanism against parasites and pathogens (be it a physiological immune system or a behavioural mechanism) are obvious, as it will alleviate or prevent the fitness loss caused by the parasite or pathogen. However, resistance mechanisms can also bear costs and in such cases, the relative magnitudes of costs and benefits determine the strength of the overall selection pressure for, or against, a strong resistance mechanism.

Drosophila and its parasites and pathogens have been proven to be a valuable model system for understanding opposing selection pressures on resistance to parasites and pathogens. Most of the work has focused on the wide range of parasites and pathogens infecting Drosophila melanogaster, both in the field and in the laboratory, but other species (as detailed below) have also been included in several studies. Microbial pathogens such as fungi and bacteria are tackled by the humoral immune system, in which various antimicrobial peptides play a key role (Lemaitre et al., 1997). The cellular immune system does play a role against microbial pathogens via phagocytosis, but is mostly used against macro-parasites which are too large to be phagocytosed (such as parasitoids). Macroparasites are encapsulated by a two-stage process consisting of envelopment of the parasite by blood cells, followed by the deposition of melanin (Lavine and Strand, 2002).

Costs of resistance come in two forms. First, when a host is parasitized or infected and launches an immune response, this will require energy and resources. Assuming energy and resources are limiting, once used in the activation of the resistance mechanism, they cannot be used for other purposes and this may have fitness consequences to the individual. Second, energy and resources will be necessary for constructing and maintaining the resistance mechanism (e.g. the immune system) in anticipation of future parasitism or infection. These are the costs of the ability to resist parasites or pathogens. Whereas the first form of cost is only paid when the individual is actually parasitized or infected, the second is paid by the individual irrespective of parasitism or infection. These two forms of cost are analogous to the costs of maintaining a standing army (second form), and the costs of taking this army to war (first form).

The approach mostly used for identifying and quantifying the costs of actual resistance, involves comparing unparasitized hosts with those parasitized hosts which have successfully combated the parasite or pathogen, in a range of fitness parameters. One problem here is that it is often difficult (if not impossible) to separate the effect that activating the resistance mechanism has on the host's fitness from the pathogenic effect the parasite or pathogen may have on the host before being dealt with by the resistance mechanism. A powerful way to quantify and identify the costs of the resistance mechanism itself is artificial selection. Typically, a base population is subject to replicated selection for increased resistance. Selected lines are then compared, in the absence of parasitism, with the appropriate control lines in a range of fitness parameters. Differences observed are likely to be linked to the resistance mechanism, as the genetic background of control and selected lines coming from the same base population, are identical. Replication at the line level is essential in such experiments, to rule out an association between focus trait and correlated responses occurring due to hitchhiking or chance.

In the first section of this chapter, we focus on the parasites and pathogens known to attack or infect D. melanogaster (or other Drosophila species), from macro-parasites, such as parasitoids and mites to microbial (fungal and bacterial) pathogens to viruses. We concentrate on the main selection pressures for and against resistance against these parasites and pathogens. High resistance is selected for when the benefits it incurs outweigh its costs. For each parasite/pathogen, we summarize the existing knowledge on abundance in the field, fitness effects of parasitism/infection, and costs of resistance, where we will distinguish between costs of actual resistance and costs of the resistance mechanism.

In the second section of this chapter, we shift our focus to the genomic level. Over the last few years several papers have been published where micro-arrays have been used to investigate D. melanogaster resistance to parasites and pathogens. We ask what these studies are telling us about Drosophila resistance mechanisms and about the associated costs.

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