Changes in Manduca sexta hemolymph induced by the parasitoid Cotesia congregata
Abstract
In order for an insect endoparasitoid to develop successfully, it must survive the defense system of the host. In insects, response to foreign substances consists of two components, cellular and humoral. The humoral response consists of synthesis of antibiotic proteins induced in response to bacteria. The normal defense response against large objects like parasitoid eggs is encapsulation by hemocytes. In M. sexta larvae parasitized by C. congregata, the humoral component of the defense response remains intact, while the cellular component rapidly becomes dysfunctional, as measured by susceptibility to bacterial pathogens. Parasitism of M. sexta by C. congregata provides a system where the host cellular response to foreign substances is specifically blocked. Changes in hemocyte structure and function, and changes in the hemolymph plasma surrounding the hemocytes, occurring early in parasitoid development have been examined. We have shown that injection of both venom and calyx fluid are required to produce the increase in susceptibility to bacteria observed in naturally parasitized larvae. Injection of either component alone does not produce this effect. Previous observations of the behavior and morphology of hemocytes from parasitized M. sexta larvae have been confirmed and extended by phase and fluorescence microscopy and by flow cytometry. Hemocytes from parasitized larvae are slower to attach and spread on foreign surfaces in vitro than are hemocytes from non-parasitized larvae. The actin cytoskeleton of many spread plasmatocytes is altered in cells from parasitized larvae, with a higher proportion of cells displaying brightly-staining foci of filamentous actin in hemocytes. Monoclonal antibodies have been produced which stain hemocytes from parasitized and non-parasitized larvae differently. Some of the changes observed in morphology of hemocytes from parasitized animals can be produced by lowering the osmolality of the medium surrounding hemocytes from non-parasitized animals. Osmolality of hemolymph declines within four hours of parasitization, but increases in non-parasitized larvae during the same developmental time period. This decline continues for at least 48 hours and coincides with the decrease in hemocyte function. The major contributor to this decline is a decrease in the concentration of specific free amino acids in hemolymph plasma.
Degree
Ph.D.
Advisors
Dunn, Purdue University.
Subject Area
Entomology|Anatomy & physiology|Animals
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