antipodarum consist of asexual clones and sexual individuals and therefore can be used to study the evolution and advantages of sexual reproduction. There is a high correlation between the presence of parasites and the frequency of sexual individuals within the different populations. This result is consistent with the favouring of sexual reproduction proposed in the Red Queen hypothesis. Trade-off between transmission and virulence Tribolium castaneum, the red flour beetle, is a host for the microsporidian Nosema whitei. This parasitoid kills its host for transmission, so the host's lifespan is important for the parasite's success. In turn, parasite fitness most likely depends on a trade-off between transmission (spore load) and virulence. A higher virulence would increase the potential for the production of more offspring, but a higher spore load would affect the host's lifespan and therefore the transmission rate. This trade-off is supported by coevolutionary experiments, which revealed the decrease of virulence, a constant transmission potential and an increase in the host's lifespan over a period of time. Further experiments demonstrated a higher recombination rate in the host during coevolutionary interactions, which may be selectively advantageous because it should increase diversity of host genotypes. Resources are generally limited. Therefore, favouring one trait, like virulence or immunity, limits other life-history traits, such as reproductive rate. Moreover, genes are often pleiotropic, having multiple effects. Thus, a change in a pleiotropic immunity or virulence gene can automatically affect other traits. There is thus a trade-off between benefits and costs of the adaptive changes that may prevent the host population from becoming fully resistant or the parasite population from being highly pathogenic. The costs of gene pleiotropy have been investigated in coevolving Escherichia coli and bacteriophages. To inject their genetic material, phages need to bind to a specific bacterial cell surface receptor. The bacterium may prevent injection by altering the relevant binding site, e.g. in response to point mutations or deletion of the receptor. However, these receptors have important functions in bacterial metabolism. Their loss would thus decrease fitness (i.e. population growth rate). As a consequence, there is a trade-off between the advantages and disadvantages of a mutated receptor, leading to polymorphism at this locus. Model systems for research The nematode Caenorhabditis elegans and the bacterium Bacillus thuringiensis have become established as a model system for studying host–parasite coevolution. Laboratory evolution experiments have provided evidence for many of the basic predictions about these coevolutionary interactions,