Hormonal control of protein synthesis in the fat body of Leptinotarsa decemlineata

This thesis deals with the hormonal control of protein synthesis in the female fat body of Leptinotarsa decemlineata Say. The results are discussed in relation to reproduction (obtained by rearing under long day conditions) and the preparation for diapause (obtained by rearing under short day conditions).

The effect of the photoperiod on protein synthesis and storage in the adult fat body is described in Article 1. Protein synthesis is studied with a newlydeveloped in vitro system which measures the rate of lysine incorporation into protein. The synthesis of the released and retained proteins is measured separately. The rates of lysine incorporation into the retained proteins are not affected by the photoperiod. In addition, the behaviour of the RNA/DNA-ratio is the same in long day and short day females. However, the synthesis of the released proteins differs significantly. Fat bodies derived from females reared under long day conditions synthesize many more released proteins. Females reared under short day conditions start storing proteins in the fat body on day 6 after adult ecdysis, but protein storage in haemolymph is also observed.

Article 2 deals with the synthesis of five specific released proteins (two vitellogenins and three diapause proteins) which are measured simultaneously. The five proteins are separated by disc-electrophoresis. The relative incorporation of the five investigated proteins produced by the fat body in vitro is the same as in vivo. This demonstrates the important role of the fat body in haemolymph protein synthesis in the entire insect. The large increase in synthesis of released proteins in females reared under long day conditions is mainly attributed to vitellogenin synthesis; vitellogenin synthesis is low in prediapause females. This difference, however, is not expressed in the amount of vitellogenin present in haemolymph. A single increase in the synthesis of one diapause protein is observed in ovipositing females. Massive synthesis of three diapause proteins occurs during the second half of the prediapause period, which results in high levels of these proteins in the haemolymph. The fat body stores two diapause proteins, which are probably taken up from the haemolymph.

In Article 3, the histological properties of the female fat body are correlated with the results of Article 1 and 2. The fat body at adult ecdysis is subject to lysis and cell remodeling. All protein granules disappear. The fat body of long day females develops into a tissue with a high capacity for protein synthesis. Rough endoplasmic reticulum is observed around the nucleus and at the periphery of the cell. The rest of the cell is used for fat storage, and the cell surface is invaginated to allow large surface contact between haemolymph and fat body. The development of the fat body of females reared under short day conditions is not essentially different, between day 0 and day 2, from the development of long day females. However, massive storage of fat in short day females occurs during the first half of the prediapause period. At this stage in development, large invaginations at the periphery of the cell are also observed. The storage of protein starts at day 6 after adult ecdysis when the first electron-dense protein granules are seen around the nucleus. On day 10, the first autophagic granules appear. They contain isolated cell organelles, and their appearance correlates with the low rate of protein synthesis during diapause.

The hormonal control of vitellogenin synthesis is studied in Article 4. Vitellogenin synthesis increases after the administration of juvenile hormone to allatectomized females in vivo . The synthesis of diapause proteins is not stimulated, however, which shows that the effect of juvenile hormone is specific. Stimulation of the synthesis of vitellogenins and diapause proteins is also achieved in the absence of juvenile hormone. It seems that additional factors are involved in the regulation of vitellogenin synthesis. beta-ecdysone has been tested, but injections of 10 micro-g do not result in an increase of vitellogenin synthesis. The regulation of vitellogenin synthesis by juvenile hormone has also been studied in mated and virgin females. However, vitellogenin synthesis and the juvenile hormone titre do not differ in virgin and mated females. This is rather surprising since a large difference in oviposition rate is observed. Apparently, the female Colorado potato beetle can uncouple the rate of oviposition from the rate of vitellogenin synthesis.