Specific interactions and the maintenance of genetic polymorphism

The presence of specific immune responses and consequently specific host-parasite interactions not only has implications for the kind of immuno-logical mechanisms operating in insects that might achieve such specificity, but also has many ramifications for a number of basic and important problems in ecology and population and evolutionary biology. Foremost among those is the expectation that the selective processes associated with specific interactions between hosts and parasites should lead to the maintenance of diversity, genetic and otherwise, in natural populations. This discussion was started by Haldane (1949) and has become a more intensively studied research topic over the last two decades. Theory has indeed shown that host-parasite co-evolution is likely to generate negative frequency-dependent selection that is able to maintain polymorphisms in host (or parasite) populations (Hamilton, 1982; Seger, 1988; Hamilton et al., 1990). The topic is closely linked to the question of how sex and recombination can evolve and be maintained (Peters and Lively, 1999; Salathe et al., 2008). One prominent hypothesis for the widespread occurrence of sex and recombination, the Red Queen hypothesis, requires antagonistic co-evolution between species resulting in genotype-frequency-dependent selection. The idea is that sex and recombination might allow hosts to escape parasites by creating rare offspring genotypes (Salathe et al., 2008). Genetically encoded host-parasite interactions are a requisite to achieve the necessary co-evolutionary dynamics. As discussed above, these interactions are most likely present due to the existence of immune responses specific to parasite types. However, in all these discussions relating to genetic diversity and co-evolution, the level of specificity is key (see Box 14.1).

The exact way in which specific responses influence dynamics and evolution in host-parasite systems will greatly depend on the genetic architecture that underlies these traits. Work on this area has focused mainly on organisms and interactions that are of economic or medical importance, but general patterns have begun to emerge. Resistance seems to be achieved by one or a few major effect loci, including where specificity has been considered (Carton et al., 2005). In a meta-analysis of published QTL studies it has been found that between one and six (mean 2.47) main effect loci are responsible for resistance to parasites in animal hosts (Wilfert and Schmid-Hempel, 2008). Relevant for theories linking host-parasite interactions with the evolution and maintenance of sex and recombination (Salathe et al., 2008), it seems that epistatic i nteractions between loci contribute substantially to the observed variance (Wilfert and Schmid-Hempel, 2008). For example, for the Red Queen hypothesis to work epistasis between parasite-resistance loci is necessary (Salathe et al., 2008). Empirical evidence for genetic diversity versus parasitism

That specific responses exist, and genetically distinct individuals show—depending on their genotypes—differential immune responses will mean that the resistance of a population will cover a greater range of parasite types than the resistance of a single individual within that population. Clearly, as genetic diversity increases the discrepancy increases. The same principle of genetic diversity will apply to diploid individuals, and individuals heterozygous at loci related to specific responses will cover a greater range of parasite types than those that are homozygous. It has been demonstrated that in vertebrates individual heterozygos-ity at resistance-related loci, but not neutral loci, is negatively correlated with parasite burden (Luikart et al, 2008). For the major histocompatibility complex in vertebrates, a relationship between locus allelic diversity and parasite load was found, showing a minimum parasite load at an intermediate rather than maximum number of alleles (Madsen and Ujvari, 2006; Kurtz et al., 2004). However, limited data exist for insects or other invertebrates. In Drosophila bacterial infections were shown to induce more damage in more inbred individuals (Spielman et al, 2004), but no corresponding effect was found for the immune response in bumblebees (Gerloff et al., 2003), for example. This is an interesting area of research that deserves further attention in insects. However, it also requires a more in-depth understanding of the host genetics behind specific host-parasite interactions.

The relevance of group diversity will be particularly pertinent for those animals living socially, such as the social insects. Indeed a variety of studies in social insects have shown the benefit of diversity when it comes to defence against parasites and pathogens (Table 14.2). It is the very fact that responses are specific that increasing genetic diversity has such an effect. However, it has also been suggested that diversity and higher heterozygosity may increase the efficiency of social antiparasite behaviour and in this way reduce infection levels, rather than having implications for individual immune defence (Calleri et al., 2006). Yet, it is hard to imagine that individual immunity does not play any role at all in generating the observations documented in Table 14.2. As such, the interplay of host diversity, specific responses, and parasites will have a number of consequences in social groups. For example, these factors are likely to have played a role in the evolution of particular mating strategies in social insects (Sherman et al., 1988; Schmid-Hempel and Crozier, 1999; Brown and Schmid-Hempel, 2003). Polyandry, mating with multiple males, has been proposed to have benefits in that it will increase genetic variation in offspring, thus reducing parasite infection (Brown and Schmid-Hempel, 2003).

The focus here has been the maintenance and benefits of genetic diversity of hosts. However, parasite genetic diversity is also likely to be influenced by the presence of genetically encoded specific responses that discriminate between parasite strains. Host selection of parasite genotypes will maintain diversity when specific responses occur and host populations or communities are made up of individuals that differ in these responses (Hitchman et al, 2007).

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