The insect immune system serves as a key defense against attack by parasitoids. Incompatible hosts often eliminate parasitoids by encapsulation, a process in which hemocytes form a multilayered envelope around the invading organism. Capsule formation involves cooperation between one or more classes of hemocytes and is likely mediated by cytokines and adhesion molecules. Reciprocally, parasitoids have evolved a variety of strategies for overcoming host immune responses. Some parasitoids passively avoid elimination by developing in locations inaccessible to host hemocytes or by possessing surface features that fail to elicit an immune response. Other species actively disrupt the host immune system by injecting specific factors into the host at oviposition. In particular, polydnaviruses associated with several taxa of parasitoids disrupt capsule formation by killing hemocytes or altering their ability to adhere to foreign surfaces. These symbionts have likely played a critical role in evolution of host range...

Immunological Basis for Compatibility in Parasitoid-Host Relationships

Recent studies suggest that insects use pattern recognition molecules to distinguish prokaryotic pathogens and fungi from “self” structures. Less understood is how the innate immune system of insects recognizes endoparasitic Hymenoptera and other eukaryotic invaders as foreign. Here we discuss candidate recognition factors and the strategies used by parasitoids to overcome host defense responses. We suggest that host–parasitoid systems are important experimental models for studying how the innate immune system of insects recognizes foreign invaders that are phylogenetically more closely related to their hosts. The strategies used by parasitoids suggest that insects may employ “hidden-self” recognition molecules for attacking foreign objects intruding the open circulatory system. BioEssays 23:344–351, 2001. © 2001 John Wiley & Sons, Inc.

Innate immunity and its evasion and suppression by hymenopteran endoparasitoids.

Insects have evolved many mechanisms that reduce their potential for serving as hosts for entomophagous species. Some of these mechanisms involve escape, mimicry, and repellancy, which are effective defense mechanisms against both predators and parasitoids. But, insects have a second line of defense against parasitoids and parasites. These may include repellancy and a cuticular barrier to invasion but they include several internal defenses that are collectively referred to as immune mechanisms.



The current understanding of insect immunity is reviewed as background to examining the ways in which insect parasitoids have evolved to successfully handle the immune system of the host. The various means that parasitoids utilize to handle the insect immune systems have been divided into five approaches. These five approaches are described and current knowledge of the mechanisms used by parasitoids to deal with the immune system of their host is explored.

How parasitoids deal with the immune system of their host: An overview

The four major biological strategies of ichneumonoid parasitoids, koinobiont and idiobiont, ecto-and endoparasitism, are discussed and the evolutionary radiations of the two families Ichneumonidae and Braconidae compared in an attempt to relate differences in patterns of host utilization to differences in evolutionary history. The most primitive members of both families are idiobiont ectoparasitoids of hosts concealed in plant tissue. Idiobiont ectoparasitic braconids are all still primarily associated with such hosts, but idiobiont ectoparasitic ichneumonids have radiated to attack hosts in other situations, such as in aculeate nests or in cocoons. A shift in emphasis between the behavioural steps, host habitat location and host location, is envisaged as being important in such evolutionary change. Idiobiont endoparasitism is postulated as having arisen amongst ectoparasitoids attacking cocooned hosts, as an adaptation that allows them to exploit pupae and puparia in relatively exposed positions; it is a fairly common strategy in the Ichneumonidae, but virtually unknown in the Braconidae. Koinobiosis is perceived as having evolved in association with hosts which feed in a relatively weakly concealed position, but pupate in a more secluded and safe location. The strategy is advantageous as it allows a parasitoid to oviposit on an easily discoverable host, but to use the host's pupation concealment to complete its own development. The evolution of koinobiosis has allowed parasitoids to exploit hosts that feed in exposed positions, and to attack hosts at a younger and numerically more common stage in the host's life cycle. Koinobiont ectoparasitism is envisaged, in some braconid and ichneumonid groups, to occupy an evolutionary transitional position between idiobiosis and endoparasitic koinobiosis; only in the Ichneumonidae have large radiations of koinobiont ectoparasitoids occurred. Endoparasitic koinobiosis is hypothesized as having arisen in the Braconidae in association with lepidopterous/coleopterous hosts, whilst in the major lineage of endoparasitic koinobiont ichneumonids, this habit is hypothesized as having arisen in association with symphytan hosts. The great majority of braconids are koinobiont endoparasitoids, but only about 50% of the Ichneumonidae have this habit. Very few koinobiont braconids develop as endoparasitoids of hymenopterous hosts, although many endoparasitic ichneumonids attack Hymenoptera. However, lineages of the Braconidae have radiated to exploit adult insects and exopterygote nymphs; ichneumonids do not utilize such hosts.

Evolutionary patterns of host utilization by ichneumonoid parasitoids (Hymenoptera: Ichneumonidae and Braconidae)*

Polydnaviruses are an unusual group of insect viruses that have an obligate mutualistic association with certain parasitic wasps. Polydnaviruses are unique both in terms of their association with parasitic Hymenoptera and because of their profusely segmented DNA genomes. These viruses are named on the basis of their unique polydisperse DNA genomes (i.e., poly -DNA-viruses), but their life cycles are equally distinctive (Stoltz et al., 1995). In this chapter, a general overview of polydnaviruses is provided but is biased by the preponderance of molecular studies performed in a single species, the Campoletis sonorensis polydnavirus (CsPDV). CsPDV is the type species member of the ichnoviruses, one of the two major polydnavirus groups. Bracoviruses, which comprise the other major group, have distinctive morphologies and appear to be evolutionarily unrelated to the ichnoviruses (Whitfield, 1997), making it likely that significant differences will continue to be found in the organization and function of the two polydnavirus genera. While attempting to provide comprehensive coverage of the field, I have deliberately focused on more recent work. For more comprehensive consideration of earlier developments, the reader should consult reviews by Stoltz and Vinson (1979a), Fleming (1992), Stoltz and Whitfield (1992), Beckage (1993), Fleming and Krell (1993), Strand and Pech (1995a), and Lavine and Beckage (1996).

D. Guzo

Papers

Apparent haemocytic transformations associated with parasitoid-induced inhibition of immunity in Malacosoma disstria larvae

Observations on cellular immunity and parasitism in the tussock moth