The origin and functions of the insect peritrophic membrane and peritrophic gel

There is a a fluid (peritrophic gel) or membranous (peritrophic membrane, P M) film surrounding the food bolus in most insects. The PM is composed of c hitin and proteins, of which peritrophins are the most important. It is pro posed here that, during evolution, midgut cells initially synthesized chiti n and peritrophins derived from mucins by acquiring chitin-binding domains, thus permitting the formation of PM. Since PM compartmentalizes the midgut , new physiological roles were added to those of the ancestral mucus (prote ction against abrasion and microorganism invasion). These new roles are rev iewed in the light of data on PM permeability and on enzyme compartmentaliz ation, fluid fluxes, and ultrastructure of the midgut. The importance of th e new roles in relation to those of protection is evaluated from data obtai ned with insects having disrupted PM. Finally, there is growing evidence su ggesting that a peritrophic gel occurs when a highly permeable peritrophic structure is necessary or when chitin-binding molecules or chitinase are pr esent in food. (C) 2001 Wiley-Liss, Inc.