Inhibitory effects of parasitism by the gregarious endoparasitoidCotesia congregata on host testicular development

Abstract

Cotesia congregata is a gregarious larval endoparasitoid of the tobacco hornworm, Manduca sexta. Parasitized larvae exhibit a variety of physiological and developmental aberrations, the most obvious of which is the induction of developmental arrest characterized by the absence of wandering behavior and suppression of pupation. This arrest appears attributable to continued maintenance of an elevated titer of juvenile hormone and reduced levels of hemolymph juvenile hormone esterase activity. Injection of the wasp's polydnavirus into nonparasitized larvae also causes arrest and the larvae eventually form larval-pupal intermediates instead of normal pupae, indicating the virus may be partially responsible. Aside from causing arrested host development, parasitism also inhibits the normal development and differentiation of testes in male host larvae, so that the testes atrophy instead of growing synchronously with other larval tissues. Here we report that parasitism has pronounced disruptive cytological effects on the developing reproductive organs of male hosts, in addition to causing them to atrophy. Parasitism results in a reduction in testicular volume attributable to a reduction in the number of developing germ cells. Microscopy revealed that the structural integrity of the sheaths surrounding the testicular follicles also is disrupted, so that the tissues appear grossly abnormal compared to those of nonparasitized larvae. Intrahemocoelic injection of purified C. congregata polydnavirus in combination with venom into nonparasitized fourth instar larvae, or topical application of 100 μg of methoprene to fourth instar larvae, also alters sheath integrity and reduces the numbers of developing germ cells, but not to the same degree as the pattern observed in truly parasitized hosts. The occurrence of cell death in the male gonad was documented using the vital dyes acridine orange and ethidium bromide. Arch. Insect Biochem. Physiol. 36:95–114, 1997. © 1997 Wiley-Liss, Inc.