Hemocytic aggregation in Schistocerca gregaria and Periplaneta americana as a response to injected substances of microbial origin

Abstract

The extent of hemocytic aggregation (“nodule formation”) induced in the locust, Schistocerca gregaria, and the cockroach, Periplaneta americana, by injecting suspensions of conidiospores from the entomopathogenic fungus, Metarhizium anisopliae, or solutions of molecules from microbial cell walls was quantified by flushing out and both counting and noting the size of the aggregates produced. In the locust, all the suspensions and solutions tested stimulated large numbers of nodules to form within 1 hr postinjection, compared with the saline-injected controls. The size of the nodules induced varied with the inducing agent. A fresh spore suspension stimulated the formation of large nodules, generally >100 μm in diameter. Nodules in the medium size range (40–100 μm) were induced by a solution of Laminarin, an algal β-1,3-glucan oligomer, whereas a solution of mixed β-1,3- and β-1,6-glucans from Zymosan (cell walls from the yeast, Saccharomyces cerevisiae) or solutions of bacterial lipopolysaccharides (LPS) induced nodules in the small size range (< 40 μm). A 24-hr-old spore suspension induced formation of both large and small nodules, and it was found that small nodules could be induced by injection of only the supernatant from this suspension. Since the locust is known to exhibit a poor cellular response, compared with the cockroach, to many types of injected biotic and abiotic particles, the effect of injecting LPS and Zymosan solutions into cockroaches was investigated and was found to induce the same order of response as in the locust. Nodule formation was not induced in insects injected with saline or a solution of dextran (α-1,6-linked glucans). The results are discussed in the context of recognition of non-self, and a hypothesis is proposed to account for the differences in size of the nodules stimulated by the different inducers.