Effects of a fat body extract on larval midgut cells and growth of lepidoptera.

Treatment with fat body extract (FBX) from pupae of the tobacco hornworm, Manduca sexta, caused mortality in larvae of two pest lepidopterans, the gypsy moth, Lymantria dispar, and the cotton leafworm, Spodoptera littoralis. In FBX-treated larvae, the feeding rate was depressed, causing reduced weight gain and then larval death. Their midgut showed formation of multicellular layers of midgut epidermis, indicating stem-cell hyperplasia. Hence, the integument of FBX-treated larvae had a double cuticle, indicating induction of premature molting. But radioimmunoassay measurements confirmed that the amount of ecdysteroids in FBX was too low to be responsible for the molt-inducing effects observed after treatment with FBX. With midgut stem cell cultures in vitro, addition of FBX to the culture medium stimulated cell proliferation and differentiation in a concentration-dependent manner. This effect was compared with those of insect molting hormones, ecdysone and 20-hydroxyecdysone; an ecdysteroid agonist, RH-2485; and a purified protein from FBX (multiplication factor). This article describes the mode of action of FBX and possible interplay between fat body factor(s) and insect hormones in the development and metamorphosis of the insect midgut.