Lepidoptera peritrophic membrane structure and effects of wheat germ agglutinin on formation

T.L. Hopkins & M.S. Harper

Dept. Entomology, Kansas State Univ. Manhattan, KS 66506 USA

Peritrophic membrane (PM) structure and the effects of dietary wheat germ agglutinin (WGA) on its formation was studied in the larvae of the European corn borer, (ECB) Ostrinia nubilalis and the tobacco hornworm, (THW) Manduca sexta by light and electron microscopy techniques. Growth and development of ECB is strongly inhibited by low amounts of WGA in the diet (0.05%), whereas THW is not affected by amounts of up to 2 %. ECB larvae secrete a chitinous meshwork in the anterior midgut where it becomes embedded in a proteinacous matrix to form a thin single-layered PM that quickly becomes multilayed as it moves posteriorly. Chitin microfibrils, as visualized by binding with WGA-gold, form an orthogonal meshwork within the outer region of the microvilli, which then moves to the tips of the microvilli where proteinacious matrix is added. The PM then delaminates from the surface of the brush border into the lumen where it encloses ingested food material. WGA at 0.05% w/w in the diet causes hypersecretion of unorganized PM in the anterior midgut lumen, disintegration of microvilli and cessation of feeding. Large amounts of WGA became localized in the PM as indicated by staining with anti-WGA. WGA was shown to bind to both the the chitinous meshwork with WGA-gold, and to several PM proteins by western blotting. Voids in the chitinous meshwork and sparse matrix material are indicated to be caused WGA binding to these PM components. This allows the passage of plant cell wall fragments and bacteria through a defective PM into the ectoperitrophic space and penetration into the brush border. Stimulation of PM secretion and cessation of feeding maybe in response to damage to the brush border by abrasive food particles. Unlike ECB, the chitinous meshwork of THW is a felt-like structure with no apparent geometric pattern, and it is embedded in a proteinaceous matrix that is very stable to solubilization. The PM is secreted as a thin single layered structure in the most anterior region of the midgut, but becomes multilayered as it moves posteriorly in the lumen. THW tolerates high amounts of WGA in its diet with no disruption of PM formation or inhibition of feeding, growth or development. WGA does accumulate in large masses embedded in the PM perhaps by binding to certain components, but does not cause any abnormalities or voids that allow the penetration of food particles and subsequent damage to the brush border. Therefore, large differences in PM formation or structure between two lepidopterous species greatly affects WGA interaction with chitinous and proteinaceous PM components and inhibition of larval growth.

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