Site of synthesis and phylogenetic distribution of a hemolymph trophic factor of the tobacco hornworm,Manduca sexta

Identification of fith instar larvalManduca sexta fat body and epidermis as sites of synthesis of a hemolymph protein (hemolymph trophic factor or HTF) was achieved using in vitro³H-leucine incorporation into protein and subsequent immunoprecipitation of tissue homogenates. Fat body is the primary site of HTF synthesis with a maximal rate on Day 1; epidermis is a secondary site with peak synthesis on Day 0. In vitro radiolabelling followed by TCA precipitation of general protein of fat body and epidermal homogenates suggest that fat body actively elaborates protein on Days 0–5 with peak rates on Days 1 and 4, while epidermis is active on Days 0–5 with a peak rate on Day 3. Based on Anti-HTF ELISA estimates, HTF 500 to 1000 μg/ml] was found in the hemolymph of representatives of the insect orders Blattodea, Hemiptera, Orthoptera, and Lepidoptera and in the class Crustacea, but not in the class Merostomata. These studies suggest a possible fundamental role for HTF among modern arthropods in cuticular deposition involving both epidermis and fat body. The physiological role of HTF is undetermined.