Protein and nucleic acid metabolism in insect fat body.

Abstract

Summary1. The appearance of larval fat body as seen under the light or electron microscope depends on the nutritional state of the larva and on the stage of larval development at which the fat body is observed.2. Early in the last larval instar the cells usually possess a well‐developed endo‐plasmic reticulum rich in ribosomes, numerous mitochondria, glycogen granules, a Golgi complex and fat droplets, while later in the instar the endoplasmic reticulum is much reduced and mitochondria are few, but glycogen and fat droplets are present in greater amount together with the appearance of large numbers of proteinaceous spheres.3. Early in the last instar the fat body synthesizes proteins and exports them into the blood, while later in the instar proteins are sequestered from the blood into the fat body.4. The rate of protein synthesis by the fat body is high in the early to mid part of the last instar, but then falls off rapidly to a low level, at which it remains until the larva pupates. In diapausing pupae, protein synthesis remains at this low level.5. The similarity between the electrophoretic patterns of proteins from the fat body and those from the blood provides strong evidence that the fat body is the site of synthesis of many of the blood proteins.6. Some of the blood proteins have been shown to possess enzymic properties, while others are thought to play a role in the transportation of various types of compounds.7. Ecdysone and juvenile hormone both stimulate the rate of protein synthesis by larval fat body. Protein synthesis in fat body from diapausing pupae is stimulated after injury to the pupae.8. The appearance of adult fat body and the amount of protein it contains is often closely linked with the nutritional and reproductive states of the insect.9. An important role of the fat body in the adult female insect is the synthesis of yolk proteins, which are released into the blood and then taken up by the developing oocytes. This synthesis and uptake are under the control of hormones secreted by the corpora allata and by the median neurosecretory cells of the pars intercerebralis.10. The RNA content of fat body in final‐instar larvae is not constant throughout the instar. In some larvae it is at its highest level early in the instar, falling to a low level as the instar progresses, while in other larvae (e.g. Calliphora) the level of RNA in fat body does not decrease as the instar progresses.11. In some dipterous insects the base composition of total RNA is DNA‐like in that the guanine + cytosine content is low, accounting for 40 % of the bases. A similar composition is seen in rapidly labelled RNA isolated from insects of other orders (Coleoptera and Lepidoptera), but the base content of total RNA from these latter insects resembles ribosomal RNA from vertebrate tissues in that it has a high (ca. 60 %) guanine + cytosine content.12. The RNA/DNA ratios in blowfly larval tissues are high compared with those found in any vertebrate tissue.13. In larval fat body, RNA synthesis is low at the time of a moult, increases during the early and mid‐instar period and subsequently falls during the latter part of the instar. During the pupal period, especially during pupal diapause, the rate of RNA synthesis is very low and then increases during the subsequent development of the pharate adult. Injury to diapausing pupae results in an increased rate of RNA synthesis in most of their tissues.14. Ecdysone and juvenile hormone both stimulate RNA and DNA synthesis in larval and adult fat body and in other tissues, although there is evidence that in some tissues these two hormones may act antagonistically to each other. The insecticide DDT also has been shown to stimulate RNA synthesis in tissues of adult insects.