Teratocytes are cells derived from the extra-embryonic membrane (serosa) of some braconid and scelionid endoparasites. The cells of the serosa dissociate after the parasite egg hatches, releasing teratocytes into circulation where they become greatly enlarged, synthetically hyperactive and secrete large amounts of biologically active protein. Physiological doses of teratocytes, or the proteins secreted by them, from Microplitis croceipes, persistently inhibit host larval growth and/or cause host (Heliothis virescens) death while parasite larvae are unaffected. Results from physiological studies of teratocyte function in vivo and in organ culture bioassays show that teratocytes inhibit host cell protein synthesis, apparently at the level of translation. These physiological, behavioral and developmental characteristics are similar to those associated with parasitized hosts. A cDNA encoding one teratocyte secreted protein has been cloned from a biologically active fraction of teratocyte secreted products. This cDNA, when expressed and purified from a recombinant baculovirus, inhibits protein synthesis in host tissues at the level of translation. Details related to the localization and function of this protein in vivo along with information on tissue specificity, species specificity, in vivo half-life in the absence of actively secreting teratocytes, and mode of action of the secreted protein will be discussed. Index terms: Microplitis croceipes, Heliothis virescens, parasitoid

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