Entomology at the Land Grant University: Perspectives from the Texas A&M University Department Centenary

13. Coevolution of Resistance and Virulence in Host-Parasitoid Interactions

C  Coevolution of Resistance and Virulence in Host-Parasitoid Interactions
. . . , . . . , . .  Centre for Population Biology Department of Biology Imperial College at Silwood Park Ascot, Berkshire, United Kingdom The study of the coevolutionary interaction between organisms and their natural enemies, particularly their parasites and pathogens, is an extremely active area of contemporary evolutionary biology. Because hosts and prey are often under strong natural selection to avoid predation and disease, and because their natural enemies in turn are under strong selection to counter any such adaptation, the resulting interaction is likely to be dynamic at the genetic level, and also to impact upon the population dynamics of the two species (Godfray, ). These considerations have led to intense research on how natural selection will mold optimum levels of resistance and virulence (Frank, ), how the population dynamic interaction between a species and its natural enemies may be influenced by reciprocal adaptation (Thompson, ), and serious discussion about whether parasites and pathogens may be responsible for sexual reproduction (Ryan, ) and spectacular adaptations such as extreme secondary sexual characters (Hamilton and Zuk, ; Andersson, ). From being a backwater of evolutionary research twenty years ago, it is now very much in the mainstream. Progress in evolutionary research, as in many other branches of science, comes from a combination of research that utilizes the huge diversity of animals , plants, and microbes available for study on the planet, and studies of chosen model systems that are subject to concerted effort by numerous labo- ratories. Until recently, evolutionary biologists have used model systems rather sparingly, at least compared to those working in genetics and development. However, the ever-increasing information that is available for the classic model systems of genetics and developmental biology makes them more and more attractive for evolutionary study (Powell, ). Of course, if one’s area of research is predator-prey or host-pathogen interactions , then choosing a model organism as a subject only provides half the solution to many problems. Nevertheless, there are a growing number of studies that use Escherichia coli and its pathogenic bacteriophages, Arabidopsis and its fungal pathogens, and mice and their protozoan and helminth parasites to address evolutionary questions. In this chapter we want to explore the value of Drosophila and its parasitoids as a model system for investigating general questions in coevolution, as well as specific issues concerning host-parasitoid coevolution. We begin with a brief description of how hosts and parasitoids interact behaviorally and physiologically, and in the context of the present volume it is a pleasure to acknowledge the major contributions of the Entomology Department at Texas A&M to this field. In the second section, we review some of the main strands of research that have utilized Drosophila and its parasitoids, and then go on to discuss in a little more detail recent work from our laboratory. We finish by speculating on further questions that studies on Drosophila and its parasitoids may help resolve. -  Parasitoids are insects that lay their eggs in or on the bodies of other insects, their hosts; the eggs hatch and the parasitoid larvae develop on their hosts, eventually killing it (Godfray, ; Quicke, ). Each larva obtains all the resources it needs to become an adult from a single host, like most parasites, but invariably kills the host, like a predator. The majority of parasitoids are wasps in the order Hymenoptera but a substantial minority are flies (Diptera) with smaller numbers found amongst the beetles (Coleoptera) and other orders . Parasitoids are ubiquitous in nearly all terrestrial ecosystems, and are thought to have a significant effect on the population dynamics of many of their hosts, often being the chief regulatory factor (Hassell, ). Estimates of the number of species of parasitoid on earth vary widely, with most workers going for figures in the order of . to  million. Much research on parasitoids is driven by their importance in pest management, and they have been       used successfully on many occasions as classical or inundative biological control agents (Holt and Hochberg, ). Different species of parasitoid attack the host egg, larval (nymphal) or pupal stages (or combinations of these), with adult hosts being subject to parasitism by a relatively small fraction of species. One major division in parasitoid life histories is between species that feed as ectoparasitoids from outside the host and those that feed internally as endoparasitoids (Godfray, ). A second major division concerns those species that attack growing stages of the host: those that kill or permanently paralyze...