Parasitic manipulation of hosts' phenotype, or how to make a zombie--an introduction to the symposium.

Nearly all animals in nature are infected by at least one parasite, and many of those parasites can significantly change the phenotype of their hosts, often in ways that increase the parasite's likelihood of transmission. Hosts' phenotypic changes are multidimensional, and manipulated traits include behavior, neurotransmission, coloration, morphology, and hormone levels. The field of parasitic manipulation of hosts' phenotype has now accrued many examples of systems where parasites manipulate the phenotypes of their hosts and focus has shifted to answering three main questions. First, through what mechanisms do parasites manipulate the hosts' phenotype? Parasites often induce changes in the hosts' phenotypes that neuroscientists are unable to recreate under laboratory conditions, suggesting that parasites may have much to teach us about links between the brain, immune system, and the expression of phenotype. Second, what are the ecological implications of phenotypic manipulation? Manipulated hosts are often abundant, and changes in their phenotype may have important population, community, and ecosystem-level implications. Finally, how did parasitic manipulation of hosts' phenotype evolve? The selective pressures faced by parasites are extremely complex, often with multiple hosts that are actively resisting infection, both in physiological and evolutionary time-scales. Here, we provide an overview of how the work presented in this special issue contributes to tackling these three main questions. Studies on parasites' manipulation of their hosts' phenotype are undertaken largely by parasitologists, and a major goal of this symposium is to recruit researchers from other fields to the study of these phenomena. Our ability to answer the three questions outlined above would be greatly enhanced by participation from individuals trained in the fields of, for example, neurobiology, physiology, immunology, ecology, evolutionary biology, and invertebrate biology. Conversely, because parasites that alter their hosts' phenotype are widespread, these fields will benefit from such study.