IMMUNOLOGICAL BASIS FOR COMPATIBILITY IN PARASITOID HOST RELATIONSHIPS

The insect immune system serves as a key defense against attack by par asitoids. Incompatible hosts often eliminate parasitoids by encapsulat ion, 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 a nd adhesion molecules. Reciprocally, parasitoids have evolved a variet y of strategies for overcoming host immune responses. Some parasitoids passively avoid elimination by developing in locations inaccessible t o host hemocytes or by possessing surface features that fail to elicit an immune response. Other species actively disrupt the host immune sy stem by injecting specific factors into the host at oviposition. In pa rticular, polydnaviruses associated with several taxa of parasitoids d isrupt capsule formation by killing hemocytes or altering their abilit y to adhere to foreign surfaces. These symbionts have likely played a critical role in evolution of host range and in defining parasitoid-ho st compatibility.